Unit 2. Applied Mechanics, Test and Measurements

2–1. Aerospace Drawings................................................................................................................. 2–1 211. Types of drawings .............................................................................................................................2–1 212. Drawing management........................................................................................................................2–5

2–2. Aerospace Hardware and Tools .............................................................................................. 2–9 213. Hardware ...........................................................................................................................................2–9 214. Hand tools........................................................................................................................................2–19

2–3. Pneumatic and Hydraulic Principles .................................................................................... 2–46 215. Theory and operation of pneumatic and hydraulic systems.............................................................2–46 216. Major hydraulic components ...........................................................................................................2–51 217. Other hydraulic system components................................................................................................2–55

2–4. Inspection System ................................................................................................................... 2–64 218. Quality control.................................................................................................................................2–64 219. Plate inspections ..............................................................................................................................2–66

EGARDLESS OF vocation or position, the exchange of ideas is essential to everyone. Normally, this exchange is carried on by the oral or written word; but under some conditions the use of these alone is impractical. Industry discovered that it couldn’t depend entirely upon

written or spoken words for the exchange of ideas because misunderstanding and misinterpretation arose frequently. A written description of an object can be changed in meaning just by misplacing a comma; the meaning of an oral description can be completely changed by the use of a wrong word. To avoid these possible errors, industry uses drawings to describe objects. For this reason, drawing is called the draftsman’s language.

2–1. Aerospace Drawings Drawing, as we use it, is a method of conveying ideas concerning the construction or assembly of objects. This is done with the help of lines, notes, abbreviations, and symbols. The dimensions used in a drawing are used to show the ideal size of parts. It’s very important that the technician who is to make or assemble an object understand the meaning of the different lines, notes, abbreviations, and symbols that are used in a drawing.

211. Types of drawings All drawings of objects are composed of one or a combination of triangles, circles, cubes, cylinders, cones and spheres. Drawings are grouped into three types: detail, assembly, and installation. In addition to the three types, there’s also the orthographic projection drawing, which can show all six sides of an object. We will take a closer look at all three groups as well as the orthographic projection.

Detail drawing A detail drawing is a description of a single part, given in such a manner as to describe by lines, notes, and symbols the specifications as to size, shape, material, and methods of manufacture that are used in making the part. Detail drawings are usually rather simple; and, when single parts are small, several detail drawings may be shown on the same sheet or print. (An example is shown at the top of fig. 2–l.)

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Assembly drawing An assembly drawing is a description of an object made up of two or more parts. Examine the assembly drawing in the center of figure 2–1. It describes the object by giving, in a general way, the size and shape. Its primary purpose is to show the relationship of the various parts. An assembly drawing is usually more complex than a detail drawing, and is often accompanied by detail drawings of various parts.

Installation drawing An installation drawing includes all necessary information for a part or an assembly of parts in the final position in the component. It shows the dimensions necessary for the location of specific parts with relation to the other parts and reference dimensions that are helpful in later work in the shop. (An example is at the bottom of fig. 2–1.)

Orthographic projection drawings In order to show the exact size and shape of all the parts of complex objects, a number of views are necessary. This is the system used in orthographic projection. All objects have six sides; thus, in orthographic projection there are six possible views of an object:

They are as follows:

1. Front. 2. Top. 3. Bottom. 4. Rear. 5. Right side. 6. Left side.

View A on figure 2–2 shows an object placed in a transparent box, hinged at the edges. The projections on the sides of the box are the views as seen looking straight at the object through each side. If the outlines of the object are drawn on each surface and the box opened as shown in view B, then laid flat as shown in view C, the result is a six-view orthographic projection. It’s seldom necessary to show all six views to portray an object clearly; therefore, only those views necessary to illustrate the required characteristics

of the object are drawn. One-view, two-view, and three-view drawings are used the most, with the three view being the most common. In regards to the three views, the front, top and right sides are the views included most often. Regardless of the number of views used, the arrangement is generally as shown in figure 2–2, with the front being the principal view. If the right-side view is shown, it will be to the right of the front view. If the left-side view is shown, it will be to the left of the front view. The

Figure 2–1. Drawings.

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top and bottom views, if included, will be shown in their respective positions relative to the front view.

Figure 2–2. Orthographic projection.

Figure 2–3. One-view drawing.

One-view drawings are commonly used for objects of uniform thickness, such as gaskets, shims, and plates. A dimensional note gives the thickness as shown in figure 2–3. One-view drawings are also commonly used for cylindrical, spherical, or square parts if all the necessary dimensions can be properly shown in one view.

When space is limited and two views must be shown, symmetrical objects are often represented by half views, as illustrated in figure 2–4.

Figure 2–4. Symmetrical object with exterior half view.

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Exploded view The exploded view shows how all the pieces of a component or system fit together. This drawing is most commonly used in illustrated parts manuals as shown in figure 2–5.

Figure 2–5. Exploded view.

Detail view A detail view shows only a part of the object but in greater detail and to a larger scale than the principal view. The part that’s shown in detail elsewhere on the drawing is usually encircled by a heavy line on the principal view. Figure 2–6 is an example of the use of detail views. The principal view shows the complete control wheel, while the detail view is an enlarged drawing of a portion of the control wheel.

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Sectional views A section or sectional view is obtained by cutting away part of an object to show the shape and construction at the cutting plane. The part or parts cut away are shown by the use of section (crosshatching) lines. Sectional views are used when the interior construction or hidden features of an object can’t be shown clearly by exterior views.

Half sections In a half section, the cutting plane extends only halfway across the object, leaving the other half of the object as an exterior view. Half sections are used to advantage with symmetrical objects to show both the interior and exterior.

Revolved sections A revolved section drawn directly on the exterior view shows the shape of the cross section of a part, such as the spoke of a wheel.

Removed sections Removed sections illustrate particular parts of an object. They’re drawn like revolved sections, except that they’re placed at one side and, to bring out pertinent details, are often drawn to a larger scale than the view on which they’re indicated. These sectional views are drawn to the same scale as the principal view; however, as already mentioned, they’re often drawn to a larger scale to bring out pertinent details.

212. Drawing management Every print contains some means of identification and additional areas that give more information about the drawing. The title block and revision information identify the print, while notes, zone numbers, finish marks, and tolerances provide additional information that enable the technician to read the drawing better. In the following lessons below, we will look at the previously mentioned items.

Title block The title block consists of a drawing number and certain other data concerning the drawing and the object it represents. This information is grouped in a prominent place on the print, usually in the lower right-hand corner. Sometimes the title block (fig. 2–7) is in the form of a strip extending almost the entire distance across the bottom of the sheet. Although title blocks don’t follow a standard form, insofar as layout is concerned, all of them will present essentially the following information:

1. A drawing number to identify the print for filing purposes and to prevent confusing it with any other print.

2. The name of the part or assembly. 3. The date.

Figure 2–6. Detail view.

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4. The name of the firm. 5. The name of the draftsman, the

checker, and the person approving the drawing.

In addition to the above information, three more important factors are involved when you’re working with title blocks. They are as follows:

1. Drawing or print numbers. 2. Reference and dash numbers. 3. Universal numbering system.

Drawing or print numbers All prints are identified by a number, which appears in a number block in the lower right-hand corner of the title block. It may also be shown in other places such as near the top borderline, in the upper right-hand corner, or on the reverse side of the print at both ends so that the number will show when the print is folded or rolled. The purpose of the number is for quick identification of a print. If a print has more than one sheet and each sheet has the same number, this information is included in the number block, indicating the sheet number and the number of sheets in the series.

Reference and dash numbers Reference numbers that appear in the title block refer you to the numbers of other prints. When more than one detail is shown on a drawing, dash numbers are used. Both parts would have the same drawing number plus an individual number, such as 40267–1 and 40267–2. In addition to appearing in the title block, dash numbers may appear on the face of the drawing near the parts they identify. Dash numbers are also used to identify right-hand and left-hand parts. The left-hand part is always shown in the drawing. The right-hand part is called for in the title block.

Above the title block, you’ll find a notation such as: 47024–1LH shown; 47024–2RH opposite. Both parts carry the same number, but the part called for is distinguished by a dash number. Some prints have odd numbers for left-hand parts and even numbers for right-hand parts.

Universal numbering system The universal numbering system provides a means of identifying standard drawing sizes. In the universal numbering system, each drawing number consists of six or seven digits. The first digit is always 1, 2, 4, or 5, and indicates the size of the drawing. The remaining digits identify the drawing. Many firms have modified this basic system to conform to their particular needs. In some cases, letters may be used instead of numbers. The letter or number depicting the standard drawing size may be prefixed to the number—separated from it by a dash (fig. 2–7). Other numbering systems provide a separate box preceding the drawing number for the drawing size identifier. In other modifications of this system, the part number of the depicted assembly is assigned as the drawing number.

Revision block Revisions to a drawing are necessitated by changes in dimensions, design, or materials. The changes are usually listed in ruled columns either adjacent to the title block or at one corner of the drawing.

NOTE: All changes to approved drawings must be carefully noted on all existing prints of the drawing.

When drawings contain such corrections, attention is directed to the changes by lettering or numbering them and listing those changes against the symbol in a revision block (fig. 2–8). The revision block contains the following data:

Figure 2–7. Title block.

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• Identification symbol. • Date of the revision. • Nature of the revision. • Authority for the change. • Name of the draftsman who made the change.

To distinguish the corrected drawing from its previous version, many firms are including (as part of the title block) a space for entering the appropriate symbol to designate that the drawing has been changed or revised.

Figure 2–8. Revision block.

Notes Notes are added to drawings for various reasons. Some of these notes refer to methods of attachment or construction. Others give alternatives so that the drawing can be used for different styles of the same object. Still others list modifications that are available. Notes may be found alongside the item to which they refer. If the notes are lengthy, they may be placed elsewhere on the drawing and identified by letters or numbers. Notes are used only when the information can’t be conveyed in the conventional manner or when it’s desirable to avoid crowding the drawing. When the note refers to a specific part, a light line with an arrowhead leads from the note to the part. If it applies to more than one part, the note is so worded that no mistake can be made as to the parts to which it pertains. When there are several notes, they’re generally grouped together and numbered consecutively. A general note will typically be located at the bottom of the drawing.

Zone numbers Zone numbers on drawings are similar to the numbers and letters printed on the borders of a map. They’re there to help locate a particular point. To find a point, mentally draw horizontal and vertical lines from the letters and numerals specified; the point where these lines would intersect is the area sought. Use the same method to locate parts, sections, and views on large drawings, particularly assembly drawings. Parts numbered in the title block can be located on the drawing by finding the numbers in squares along the lower border. Zone numbers read from right to left.

Finish marks Finish marks are used to indicate the surface that must be machine finished. Such finished surfaces have a better appearance and allow a closer fit with adjoining parts. During the finishing process the required limits and tolerances must be observed.

NOTE: Don’t confuse machined finishes with those of paint, enamel, chromium plating, and similar coating.

Tolerances When a given dimension on a print shows an allowable variation, the plus (+) figure indicates the maximum, and the minus (–) figure indicates the minimum allowable variation. The sum of the plus and minus allowance figures is called tolerance. Tolerance is the difference between extreme

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permissible dimensions in which the part or component will still be acceptable. For example, using .225 + .0025 – .0005, the plus and minus figures indicate the part will be acceptable if it isn’t more than .0025 larger than the .225 given dimension or not more than .0005 smaller than the .225 dimension. Tolerance in this example is .0030 (.0025 max. plus .005 min.). If the plus and minus allowances are the same, you’ll find them presented as .225 ± .0025. The tolerance would then be .0050. Allowance can be indicated in either fractional or decimal form. When very accurate dimensions are necessary, decimal allowances are used. Fractional allowances are sufficient when close dimensions aren’t required. Standard tolerances of –.010 or –1/32 may be given in the title block of many drawings, to apply throughout the drawing. Some terms associated with tolerances are as follows:

• Interference fit happens when the tolerances or the limits of size are so prescribed that an interference always results when mating parts are assembled.

• Clearance fit happens when the tolerances or the limits of size are defined such that a clearance always results when mating parts are assembled.

• Regardless of feature size applies to a position tolerance for a size feature shown in a feature control frame without a modifier.

• Maximum material condition (MMC) is that condition of a part feature wherein it contains the maximum amount of material within the stated limits of size. For example, the minimum hole size would determine the maximum shaft size. MMC is used in the following geometric tolerancing symbols; parallelism, perpendicularity, and flatness.

• A diameter symbol indicates a circular feature when used on the field of a drawing. It indicates that the tolerance is a cylinder or diametrical when used in a feature control frame. The symbol will precede all diameters on the drawing and can be used when a datum target area is round.

Self-Test Questions After you complete these questions, you may check your answers at the end of the unit.

211. Types of drawings 1. What is a detail drawing?

2. What is a detail view?

3. What is a removed section?

212. Drawing management 1. What information is contained in the title block?

2. The universal numbering is used to identify what type of drawing sizes?

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3. What is a tolerance?

2–2. Aerospace Hardware and Tools Aerospace maintenance involves using various types of aerospace hardware. Many jobs in this field depend on you, the technician, to be knowledgeable about how to properly use these items.

213. Hardware This section gives you information on how to correctly use, along with the knowledge on how to identify, a variety of aerospace hardware. Much of the equipment and components you are asked to maintain are very expensive and sensitive. As a result, the hardware you use must be carefully selected to ensure the proper match for the application. You will be using many types of hardware such as bolts, screws, washers, nuts, and rivets, to name a few. Knowing how to identify and use this hardware will make your job much easier.

Hardware items are identified by their size, type, and material of construction. This material may be identified by markings, weight, codes, and other practical methods. This type of hardware is identified by the description of the item. The common hardware items are bolts, nuts, rivets, screws, and washers. The size of an item of hardware is determined by measuring it. The diameter of the item may be measured with a pocket slide caliper, the length with a calibrated scale, and the number of threads per inch with a screw-pitch gauge.

Bolts Aerospace bolts are fabricated from the following metals:

• Cadmium- or zinc-plated corrosion-resistant steel. • Unplated corrosion-resistant steel. • Anodized aluminum alloys.

Most bolts used in aerospace structures are either general-purpose bolts (AN bolts), close-tolerance bolts (NAS bolts) or internal-wrenching bolts (MS bolts). AN refers to Air Force–Navy, NAS refers to National Aerospace Standards, and MS refers to military standard. In certain cases, manufacturers make bolts of different dimensions or greater strength than the standard types. Such bolts are made for a particular application, and it’s of extreme importance you use like bolts in replacement.

AN bolts come in these three head styles:

1. Hex head. 2. Clevis. 3. Eyebolt.

NAS bolts are available in these three head styles:

1. Hex head. 2. Internal wrenching. 3. Countersunk.

MS bolts come in these two head styles:

1. Hex head. 2. Internal wrenching.

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NOTE: Special bolts are usually identified by the letter “S” stamped on the head. The different head styles are shown on figure 2–9.

Figure 2–9. Bolt identification.

General-purpose bolts The hex-head bolt (AN–3 through AN–20) is an all-purpose structural bolt used for general applications involving tension or shear loads where a light-drive fit is permissible. Tension load is a pulling action exerted lengthwise; shear load is a cutting or shearing action exerted crosswise. An AN Clevis bolt is for shear applications only. This type of bolt is used only where shear loads occur and never in tension. It is often inserted as a mechanical pin in a control surface. They have a slotted head. General-purpose bolts are made of either steel or aluminum alloy. Alloy steel bolts smaller than no. 10–32 and aluminum-alloy bolts smaller than 1/4 in. In diameter aren’t used in primary structures. Aluminum-alloy bolts and nuts aren’t used where they’ll be repeatedly removed for purposes of maintenance and inspection. The AN–73 drilled-head bolt is similar to the standard hex bolt but has a deeper head which is drilled to receive wire for safetying. The drilled-head bolt needs no nut; it is secured with safety wire.

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Close-tolerance bolts This type of bolt is machined more accurately than the general-purpose bolt. Close-tolerance bolts may be hex headed (AN–173 through AN–186) or have a 100-degree-countersunk head (NAS–80 through NAS–86). A bolt head with a triangle, recessed or not, is a “close tolerance” bolt and must be driven into a hole. They’re used in applications where a tight-drive fit is required (the bolt will move into position only when struck with a 12–14-ounce hammer).

Internal-wrenching bolts These bolts (MS–20004 through MS–20024 or NAS–495) are fabricated from high-strength steel and are suitable for use in both tension and shear applications. When they’re used in steel parts, the bolt hole must be slightly countersunk to seat the large corner radius of the shank at the head. In dural material, a special heat-treated washer must be used to provide an adequate bearing surface for the head. The head of the internal-wrenching bolt is recessed to allow the insertion of an internal wrench when installing or removing the bolt. Special high-strength nuts are used on these bolts. Replace an internal-wrenching bolt with another internal-wrenching bolt. Standard AN hex-head bolts and washers can’t be substituted for them as they don’t have the required strength.

Identification and coding Bolts are manufactured in many shapes and varieties. A clear-cut method of classification is difficult; however, bolts can be identified by the shape of the head, method of securing, material used in fabrication, or the expected usage. AN-type aerospace bolts can be identified by the code markings on the bolt heads. The markings generally denote the bolt manufacturer, the material of which the bolt is made, and whether the bolt is a standard AN-type or a special-purpose bolt. For example:

• AN standard steel bolts are marked with either a raised dash or asterisk. • Corrosion-resistant steel bolts are indicated by a single raised dash. • AN aluminum-alloy bolts are marked with two raised dashes.

Additional information, such as bolt diameter, bolt length, and grip length, may be obtained from the bolt part number. For example, let’s say the bolt part number is AN3DD5A. In this case:

• The “AN” designates that it’s an Air Force–Navy standard bolt. • The “3” indicates the diameter in sixteenths of an inch (3/16). • The “DD” indicates the material is 2024 aluminum alloy. The letter “C” in place of the “D

would indicate corrosion-resistant steel, and the absence of the letters would indicate cadmium-plated steel.

• The “5” indicates the length in eighths of an inch (5/8). • The “A” indicates that the shank is undrilled. If the letter “H” preceded the “5” in addition to

the “A” following it, the head would be drilled for safetying.

Close-tolerance NAS bolts are marked with either a raised or recessed triangle. The material markings for NAS bolts are the same as for AN bolts, except that they may be either raised or recessed. Bolts inspected magnetically (Magnaflux) or by fluorescent means (Zyglo) are identified by means of colored lacquer or a head marking of a distinctive type.

Screws Another common type of threaded fastener used in the aerospace field is the screw. The primary differences between screws and bolts are that screws usually have a lower material strength, a looser thread fit, and the shanks are threaded along their entire length. The heads are shaped for use with screwdrivers. The coding system used to identify screws is similar to that used for bolts. There are AN and NAS screws. NAS screws are structural screws. In general, screws are classed as structural screws, machine screws, and self-tapping screws.

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Structural screws Structural screws have a definite grip and have the same shear strength as a bolt of the same size. Shank tolerances are similar to AN hex-head bolts, and the threads are National Fine. Common types of structural screws are the flat head (countersunk) and the washer head.

Machine screws Machine screws do not have a definite grip length and may be threaded their entire length. Common types of machine screws are the round head, button head, and fillister head. The type of safetying

device you use on a screw depends on the type and purpose of the screw. The drilled-head fillister screw is secured with safety wire. In most other cases, lock washers are the safetying device.

Self-tapping screws Self-tapping sheet-metal screws are blunt on the end. They’re used in the temporary attachment of sheet metal for riveting and in the permanent assembly of nonstructural assemblies. Self-tapping screws shouldn’t be used to replace standard screws, nuts, bolts, or rivets.

Washers There are many types of washers used today. They include plain washers, lock washers, and special washers. They come in various aluminum and steel alloys. In addition, they’re available in a wide variety of shapes and sizes.

Plain washers Both the AN960 and AN970 plain washers are used under hex nuts (fig. 2–10). They provide a smooth bearing surface and act as a shim in obtaining the

correct grip length for a bolt-and-nut assembly. They’re used to adjust the position of castellated nuts in respect to drilled cotter-pin holes in bolts. Plain washers should be used under lock washers to prevent damage to the surface material.

Aluminum and aluminum-alloy washers may be used under bolt heads or nuts on aluminum-alloy or magnesium structures where corrosion caused by dissimilar metals is a factor. When used in this manner, any electric current flow will be between the washer and the steel bolt. However, it’s common practice to use a cadmium-plated steel washer under a nut bearing directly against a structure because this washer will resist the cutting action of a nut better than an aluminum-alloy washer. The AN970 steel washer provides a greater bearing area than the AN960 washer and is used on wooden structures under both the head and the nut of a bolt to prevent crushing the surface.

Lock washers Both the AN935 and AN936 lock washers are used with machine screws or bolts where the self-locking or castellated-type nut isn’t appropriate. These washers are shown in figure 2–10. The spring action of the washer (AN935) provides enough friction to prevent loosening of the nut from vibration. Lock washers should never be used under the following conditions:

• With fasteners to primary or secondary structures. • With fasteners on any part of the aerospace component where failure might result in damage

or danger to the structure or personnel. • Where failure would permit the opening of a joint to the airflow.

Figure 2–10. Various types of washers

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• Where the screw is subject to frequent removal. • Where the washers are exposed to the airflow. • Where the washers are subject to corrosive conditions. • Where the washer is against soft material without a plain washer underneath to prevent

gouging the surface.

Shakeproof lock washers Shakeproof lock washers are round washers designed with tabs or lips that are bent upward across the sides of a hex nut or bolt to lock the nut in place. There are various methods of securing the lock washer to prevent it from turning. One method is the use of an external tab bent downward 90º into a small hole in the face of the unit. Another method is an internal tab which fits a keyed bolt. Shakeproof lock washers can withstand higher heat than other methods of safetying and can safely be used under high-vibration conditions.

NOTE: Shakeproof lock washers should be used only once because the tabs tend to break when bent a second time.

Special washers The AC950 and AC955 ball-seat-and-socket washers are special washers used where a bolt is installed at an angle to a surface or where perfect alignment with a surface is required. These washers are used together. They’re shown in figure 2–11. The NAS143 and MS20002 washers are used for internal wrenching bolts of the NAS144 through NAS158 series. This washer is either plain or countersunk. The countersunk washer (designated as NAS143C and MS20002C) is used to seat the bolt-bead shank radius, and the plain washer is used under the nut.

Aerospace nuts Aerospace nuts are made in a variety of shapes and sizes. They’re made of the following metals:

• Cadmium-plated carbon steel. • Stainless steel. • Anodized 2024T aluminum alloy.

Nuts may be obtained with either right- or left-handed threads. No identifying marking or lettering appears on nuts. They can be identified only by the characteristic metallic luster or color of the aluminum, brass, or the insert when the nut is of the self-locking type. They can be further identified by their construction.

Aerospace nuts can be divided into these two general groups:

1. Non-self-locking nuts. 2. Self-locking nuts.

Non-self-locking nuts are those that must be safetied by external locking devices, such as cotter pins, safety wire, or locknuts. In contrast, self-locking nuts contain the locking feature as an integral part.

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Figure 2–11. Non-self-locking nuts.

Non-self-locking nuts Most of the familiar types of nuts are shown on figure 2–11. They include the following:

• Plain nut. • Castle nut. • Castellated shear nut. • Plain hex nut. • Light hex nut. • Plain check nut. • Wing nut.

Plain nut This nut, AN316, is used as a locking device for plain nuts, setscrews, threaded rod ends, and other devices.

Castle nut The castle nut, AN310, is used with drilled-shank AN hex-head bolts, clevis bolts, eyebolts, drilled head bolts, or studs. It’s fairly rugged and can withstand large tensional loads. Slots (called castellations) in the nut are designed to accommodate a cotter pin or lockwire for safety.

Castellated shear nut The castellated shear nut, AN320, is designed for use with devices (such as drilled clevis bolts and threaded taper pins) which are normally subjected to shearing stress only. Like the castle nut, it’s castellated for safetying. Note, however, that the nut isn’t as deep or as strong as the castle nut, also that the castellations aren’t as deep as those in the castle nut.

Plain hex nut The plain hex nut, AN315 and AN335 (fine and coarse thread), is of rugged construction. This makes it suitable for carrying large tensional loads. However, since it requires an auxiliary locking device, such as a check nut or lock washer, its use on aerospace structures is somewhat limited.

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Light hex nut The light hex nut, AN340 and AN345 (fine and coarse thread), is a much lighter nut than the plain hex nut and must be locked by an auxiliary device. It’s used for miscellaneous light-tension requirements.

Plain check nut The plain check nut, AN316, is employed as a locking device for plain nuts, setscrews, threaded rod ends, and other devices.

Wing nut The wing nut, AN350, is intended for use where the desired tightness can he obtained with the fingers and where the assembly is frequently removed.

Self-locking nuts As their name implies, self-locking nuts need no auxiliary means of safetying; instead, they have a safetying feature included as an integral part of their construction. Many types of self-locking nuts have been designed and their use has become quite widespread. Self-locking nuts are used on components to provide tight connections that won’t shake loose under severe vibration (fig. 2–12).

The two general types of self-locking nuts currently in use are as follows:

1. All-metal type. (stainless steel self-locking nut) 2. Fiber-lock type. (elastic stop nut)

Stainless steel self-locking nut The stainless steel self-locking nut may be spun on and off with the fingers, as its locking action takes place only when the nut is seated against a solid surface and tightened. The nut consists of two main parts—a case with a beveled locking shoulder and key. In addition, it has a threaded insert with a locking shoulder and slotted keyway. Because the threaded insert is the proper size for the bolt, it spins on the bolt easily until the nut is tightened. However, when the nut is seated against a solid surface and tightened, the locking shoulder of the insert is pulled downward and wedged against the locking shoulder of the case. This action compresses the threaded insert and causes it to clench the bolt tightly.

Elastic stop nut The elastic stop nut is a standard nut in which the height has been increased to accommodate a fiber-locking collar. This fiber collar is very tough and durable and is unaffected by immersion in hot or cold water or ordinary solvents such as ether, carbon tetrachloride, oils, and gasoline. It won’t damage bolt threads or plating. The fiber-locking collar isn’t threaded, and it’s inside diameter is smaller than the largest diameter of the threaded portion or the outside diameter of a corresponding bolt. When the nut is screwed onto a bolt, it acts as an ordinary nut until the bolt reaches the fiber collar. When the bolt is screwed into the fiber collar, however, friction (or drag) causes the fiber to be pushed upward. This creates a heavy downward pressure on the load-carrying part and automatically throws the load-carrying sides of the nut and bolt threads into positive contact. After the bolt has been forced all the way through the fiber collar, the downward pressure remains constant. This pressure locks and securely holds the nut in place even under severe vibration. Nearly all elastic stop nuts are steel or

Figure 2–12. Self-locking nuts.

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aluminum alloy. However, such nuts are available in practically any kind of metal. Aluminum-alloy elastic stop nuts are supplied with an anodized finish. Steel nuts are cadmium plated. Normally, elastic stop nuts can be used many times with complete safety and without detriment to their locking efficiency. When reusing elastic stop nuts, be sure the fiber hasn’t lost its locking friction or become brittle. If a nut can be turned with the fingers, replace it. After the nut has been tightened, make sure the rounded or chamfered end of the bolts, studs, or screws extends at least the full round (or chamfer) through the nut. Flat-end bolts, studs, or screws should extend at least 1/32 in. through the nut.

Bolts of 5/16 in. diameter and over with cotter-pin holes may be used with self-locking nuts but only if free from burrs around the holes. Bolts with damaged threads and rough ends aren’t acceptable. Don’t tap the fiber-locking insert. The self-locking action of the elastic stop nut is the results of having the bolt threads impress themselves into the untapped fiber. Don’t install elastic stop nuts in places where the temperature is higher than 250° F because the effectiveness of the self-locking action is reduced beyond this point.

Installation of nuts and bolts There are three important considerations you must keep in mind when you’re installing bolts. They are as follows:

1. Bolt and hole sizes. 2. Proper installation practices. 3. Torque.

Bolt and hole sizes Slight clearances in bolt-holes are permissible wherever bolts are used in tension and aren’t subject to reversal of load. Bolt-holes are to be normal to the surface, involved to provide full bearing surface for the bolt head and nut, and must not be oversized or elongated. A bolt in such a hole will carry none of its shear load until parts have yielded or deformed enough to allow the bearing surface of the oversized hole to contact the bolt. In this respect, remember that bolts don’t become swaged to fill up the holes as do rivets. Usually, such factors as edge distance, clearance, or load factor must be considered. Many bolt-holes, particularly those in primary connecting elements, have close tolerances. Generally, it’s permissible to use the first lettered drill size larger than the normal bolt diameter, except where the AN hexagon bolts are used in light-drive fit (reamed) applications and where NAS close-tolerance bolts or AN clevis bolts are used. Light-drive fits for bolts (specified on the repair drawings as .0015 in. maximum clearance between bolt and hole) are required in places where bolts are used in repair or where they’re placed in the original structure. The fit of holes and bolts can’t be defined in terms of shaft and hole diameters; instead, it’s defined in terms of the friction between bolt and hole when sliding the bolt into place. A tight-drive fit, for example, is one in which a sharp blow of a 12-ounce or 14-ounce hammer is required to move the bolt. A bolt that requires a hard blow and sounds tight is considered to fit too tightly. A light-drive fit is one in which a bolt will move when a hammer handle is held against its head and pressed by the weight of the body.

Proper installation practices To install nuts and bolts properly, you must follow certain guidelines. These include the following:

• Examine bolt-head markings. • Use the proper washer. • Use proper bolt positioning. • Ensure the bolt grip length is correct. • Use proper safetying.

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Examine bolt-head markings Examine the markings on the bolt head to determine that each bolt is of the correct material. It’s of extreme importance to use like bolts in replacement. In every case, refer to the applicable Maintenance Instructions Manual and Illustrated Parts breakdown.

Use the proper washer Be sure that washers are used under both the heads of bolts and nuts unless their omission is specified. A washer guards against mechanical damage to the material being bolted and prevents corrosion of the structural members. An aluminum-alloy washer should be used under the head and nut of a steel bolt securing aluminum-alloy or magnesium-alloy members. Any corrosion that occurs then attacks the washer rather than the members. Steel washers should be used when joining steel members with steel bolts.

Use proper bolt positioning Whenever possible, the bolt should be placed with the head on top or in the forward position. This positioning tends to prevent the bolt from slipping out if the nut is accidentally lost.

Ensure the bolt grip length is correct Be certain that the bolt grip length is correct. Grip length is the length of the unthreaded portion of the bolt shank. Generally speaking, the grip length should equal the thickness of the material being bolted together. However, bolts of slightly greater grip length may be used if washers are placed under the nut or the bolt head. In the case of plate nuts, add shims under the plate.

Safetying of bolts and nuts It’s very important that all bolts or nuts, except the self-locking type, be safetied after installation. This prevents them from loosening due to vibration in flight. Methods of safetying are discussed later in this unit.

Rivet A rivet is a metal pin used to hold two or more metal sheets, plates, or pieces of material together. A head is formed on one end when the rivet is manufactured. The shank of the rivet is placed through matched holes in two pieces of material, and the tip is then upset to form a second head to clamp the two pieces securely together. The shop head of a rivet is one and one-half (1½) times larger than the shank, and the bucked end is also one and one-half (1½) times larger than the shank diameter. Solid shank rivets are identified by the kind of material of which they’re made, their head type, size of shank, and their temper condition. Let’s take a look at a couple of common rivets.

1100 rivet The 1100 rivet, which is composed of 99.45 percent pure aluminum, is very soft. It’s for riveting the softer aluminum alloys that are used for nonstructural parts (all parts where strength isn’t a factor).

2117-T rivet The 2117-T rivet, known as the field rivet, is used more than any other for riveting aluminum-alloy structures. The field rivet is in wide demand because it’s ready for use as received; it has been heat-treated by the manufacture and can be driven when used. It requires no further heat treatment or annealing. It also has a high resistance to corrosion.

2017-T and 2024-T rivets The 2017-T and 2024-T rivets are used in aluminum-alloy structures where more strength is needed than is obtainable with the same size 2217-T rivet. These rivets are annealed and must be kept refrigerated until they’re to be driven. The 2017-T rivet should be driven within approximately one hour and the 2024-T rivet within 10 to 20 minutes after removal from refrigeration.

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5056 rivet The 5056 rivet is used for riveting magnesium-alloy structures because of its corrosion-resistant qualities in combination with magnesium. Mild steel rivets are used for riveting steel parts. The corrosion-resistant steel rivets are for riveting corrosion-resistant steels. Monel rivets are used for riveting nickel-steel alloys. They can be substituted for those made of corrosion-resistant steel in some cases. Copper rivets can be used only on copper alloys. If copper rivets are used on aluminum alloy, two dissimilar metals are brought in contact with each other and, in the presence of moisture, cause an electrical current to flow between them and chemical by-products to be formed, resulting in the deterioration of one of the metals.

Identification Markings on the heads of rivets are used to classify their characteristics. These markings may be a raised teat, two raised teats, a dimple, a pair of raised dashes, a raised cross, a single triangle, or a raised dash—some other heads have no markings. The different markings indicate the composition or specific alloy used in the manufacturing of the rivet stock. As explained previously, the rivets have different colors to identify the protective surface coating used by the manufacturers. The round-head rivet has a deep, rounded top surface. The head is large enough to strengthen the sheet around the hole and, at the same time, offer resistance to tension. The flat-head rivet, like the round-head rivet, is used on interior structures. It’s used where maximum strength is needed and where there isn’t sufficient clearance to use a round-head rivet. It’s seldom, if ever, used on external surfaces. The brazier-head rivet has a head of large diameter, which makes it particularly adaptable for riveting thin sheet stock (skin). The brazier-head rivet offers only slight resistance to the airflow, and because of

Figure 2–13. Rivet identification chart.

this factor, it’s frequently used for riveting skin on exterior surfaces. It’s used for riveting thin sheets exposed to the slipstream. A modified brazier-head rivet is also manufactured; it’s simply a brazier head of reduced diameter. Blind rivets are used where access to both sides of a riveted structure or

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structural part is impossible or where limited space won’t permit the use of a bucking bar. They are also used in the attachment of many nonstructural parts. The universal-head rivet is a combination of the round head, flat head, and brazier head. When replacement is necessary for protruding-head rivets––round head, flat head, or brazier head––they can be replaced by universal-head rivets. The counter-sunk-head rivet is flat topped and beveled toward the shank so that it fits into a countersunk or dimpled hole and is flush with the material’s surface. The angle at which the head slopes may vary from 78° to 120°. The 100° rivet is the most commonly used. These rivets are used to fasten sheets over which other sheets must fit.

The markings on the heads of rivets indicate the material of which they’re made and, therefore, their strength. Figure 2–13 identifies the rivet-head markings and the materials indicated by them. Although there are three materials indicated by a plain head, it’s possible to distinguish their difference by color. The 1100 is aluminum color; the mild steel is a typical steel color; and the copper rivet is a copper color. Any head marking can appear on any head style of the same material. Each type of rivet is identified by a part number so that the user can select the correct rivet for the job. The type of rivet head is identified by AN or MS standard numbers. The numbers selected are in series, and each series represents a particular type of head (fig. 2–13).

Rivet installation/removal When a technician installs a rivet, they use a center punch to mark the hole, select the appropriate drill bit for the size of the rivet, and drill the pilot hole. When installing a 100° rivet, countersink the hole for proper installation. A flat-rivet set is used for installing countersunk rivets. The rivet must be flush with material being riveted. When using or replacing a universal rivet, use a slightly greater radius rivet set to install the rivet. One factor that determines the minimum space between rivets is the diameter of the rivets being used. Rivets should be spaced about 3x rivet-shank diameter apart. They

should be installed a minimum of 2x rivet-shank diameter from the edge of materials being riveted together. Remember, when installing a rivet, always use a slightly larger drill than the rivet diameter. When removing a rivet, center punch the head first to indent it, then drill the center of rivet with one size smaller drill bit than the rivet shank. Drill off the rivet head only. Knock the rivet out with a ball-peen hammer using a pin punch slightly smaller than the rivet diameter. The technician must be careful not to change the hole’s diameter.

214. Hand tools There’s no doubt that the uses of tools may vary, but good practices for the safety, care, and storage of tools remain the same. Although many technicians don’t give much thought to the proper use of their tools, there are certain basic rules that apply to the use of each type. To be successful, a sound knowledge is required of these basic rules and of the situations in which they apply.

General-purpose tools Many types and classifications of tools fall under the broad category of general-purpose tools. In this lesson, we’ll cover a wide variety of the tools that fall under this category. They are as follows:

1. Hammers and mallets. 2. Screwdrivers. 3. Pliers and plier-type cutting tools. 4. Punches. 5. Wrenches. 6. Special wrenches.

Hammers and mallets Figure 2–14 shows some of the hammers that technicians may be required to use.

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Metal-head hammers Most hammers are constructed the same and used in the same fashion. The large striking surface of a hammer is called the face. The hammer head is attached to the handle by means of a wedge. The ball-peen hammer is most commonly used by machinists. Metal-head hammers are usually sized according to the weight of the head without the handle.

Soft-faced hammers Occasionally it’s necessary to use a soft-faced hammer, which has a striking surface made of wood, brass, lead, rawhide, hard rubber, or plastic. These hammers are intended for use in forming soft metals and striking surfaces that are easily damaged. Soft-faced hammers shouldn’t be used for rough work. Striking punch heads, bolts, or nails will quickly ruin this type hammer.

Mallet A mallet is a hammer-like tool with a head made of hickory, rawhide, or rubber. It’s handy for shaping thin metal parts without denting them. Always use a wooden mallet when pounding a wood chisel or a gouge. When using a hammer or mallet, choose the one best suited for the job. Ensure that the handle is tight. When striking a blow with the hammer, use the forearm as an extension of the handle. Swing the hammer by bending the elbow, not the wrist. Always strike the work squarely with the full face of the hammer. Always keep the faces of hammers and mallets smooth and free from dents to prevent marring the work.

Screwdrivers A screwdriver can be classified by its shape,

type of blade, and blade length. Any type of screwdriver is made for only one purpose—loosening or tightening screws or screw-head bolts. A screwdriver should not be used for chiseling or prying. Do not use a screwdriver to check an electric circuit since an electric arc will burn the tip and make it useless. In some cases, an electric arc may fuse the blade to the unit being checked. When using a screwdriver on a small part, always hold the part in the vise or rest it on a workbench. Don’t hold the part in the hand, as the screwdriver may slip and cause serious personal injury. There are several types of screwdrivers, but those used most frequently are as follows:

1. Common. 2. Phillips and Reed and Prince. 3. Offset. 4. Ratchet/spiral.

Common screwdriver The common screwdriver is used only where slotted-head screws or fasteners are found. When using the common screwdriver, select the largest screwdriver blade that makes a good fit in the screw being turned. A common screwdriver must fill at least 75 percent of the screw slot. If the screwdriver is the wrong size, it cuts and burrs the screw slot, making it worthless. A screwdriver with the wrong size blade may slip and damage adjacent parts of the structures.

Figure 2–14. Hammers.

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Phillips and Reed and Prince screwdrivers The two common types of recessed-head screws in use are the Phillips and the Reed and Prince. Both the Phillips and Reed and Prince recessed heads are optional on several types of screws. The Reed and Prince recessed head forms a perfect cross. The screwdriver used with this screw is pointed on the end. Since the Phillips screw has a slightly larger center in the cross, the Phillips screwdriver is blunt on the end. (NOTE: The Phillips screwdriver isn’t interchangeable with the Reed and Prince. The use of the wrong type screwdriver results in mutilation of the screwdriver and the screw head. When turning a recessed-head screw, use only the proper recessed-head screwdriver of the correct size.)

Offset screwdriver An offset screwdriver may be used when vertical space is limited. Offset screwdrivers are constructed with both ends bent 90° to the shank handle. By using alternate ends, most screws can be seated or loosened even when the swinging space is limited. Offset screwdrivers are made for both standard- and recessed-head screws.

Ratchet/spiral screwdriver The ratchet or spiral screwdriver is fast acting in that it turns the screw when the handle is pulled back and then pushed forward. It can be set to turn the screw either clockwise or counterclockwise, or it can be locked in position and used as a standard screwdriver. The ratchet screwdriver isn’t a heavy-duty tool and should be used only for light work.

CAUTION: When using a spiral or ratchet screwdriver, extreme care must be used to maintain constant pressure and prevent the blade from slipping from the slot in the screw head. If this occurs, the surrounding structure is subject to damage.

Pliers and plier-type cutting tools In essence, pliers are a tool that consists of two handles, two grasping jaws, and a hinge or pivot. They’re designed to hold small objects for operation such as cutting and bending. In addition, special types are designed to cut or shape wire. There are several types of pliers, but those used most frequently are as follows:

1. 6 in. slip joint. 2. Flat nose. 3. Round nose. 4. Needle-nose. 5. Duckbill. 6. Water pump. 7. Diagonal.

The size of pliers indicates their overall length, usually ranging from 5 to 12 in.

6 in. slip-joint pliers The 6 in. slip-joint plier is the preferred size for use in repair work. The slip-joint permits the jaws to be opened wider at the hinge for gripping objects with large diameters. Slip-joint pliers come in sizes from 5 to 10 in. The better grades are drop-forged steel.

Flat-nose pliers Flat-nose pliers are very satisfactory for making flanges. The jaws are square, fairly deep, and usually well matched, and the hinge is firm. These are characteristics that give a sharp, neat bend.

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Round-nose pliers Round-nose pliers are used to crimp metal. They aren’t made for heavy work because too much pressure will spring the jaws, which are often wrapped to prevent scarring the metal.

Needle-nose pliers Needle-nose pliers have half-round jaws of varying lengths. They’re used to hold objects and make adjustments in tight places.

Duckbill pliers Duckbill pliers resemble a “duck’s bill” in that the jaws are thin, flat, and shaped like a duck’s bill. They’re used exclusively for twisting safety wire.

Water-pump pliers Water-pump (channel locks) pliers are slip-joint pliers with the jaws set at an angle to the handles. The most popular type has the slip-joint channeled, hence the name channel locks. These are used to grasp packing nuts, pipes, and odd-shaped parts.

Diagonal pliers Diagonal pliers are usually referred to as diagonals or “dikes.” The diagonal is a short-jawed cutter with a blade set at a slight angle on each jaw. This tool can be used to cut wire, rivets, small screws, and cotter pins and is practically indispensable in removing or installing safety wire. The duckbill

pliers and the diagonal pliers are used extensively for the job of safety wiring.

Two important rules for using pliers are as follows:

1. Don’t make pliers work beyond their capacity. Long-nosed pliers are especially delicate. It’s easy to spring or break them, or nick the edges. If this occurs, they’re practically useless.

2. Don’t use pliers to turn nuts. In just a few seconds, a pair of pliers can damage a nut more than years of service.

Punches Punches are used to locate centers for drawing circles, start holes for drilling, punching holes in sheet metal, transferring location of holes in patterns, and removing damaged rivets, pins, or bolts. Solid or hollow punches are the two types

generally used. Solid punches are classified according to the shape of their points. Figure 2–15 shows several types of punches. There are several types of punches, but those used most frequently are as follows:

• Center. • Drive. • Pin. • Prick.

• Transfer.

Figure 2–15. Punches.

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Center punch When accurate hole locations need to be marked, use the center punch. Large indentations in metal that are necessary to start a twist drill are made with a center punch. The punch should never be struck with enough force to dimple the material around the indentation or to cause the metal to protrude through the other side of the sheet. A center punch has a heavier body than a prick punch and is ground to a point with an angle of about 60°.

Drive punch The drive punch, often called a tapered punch, is used for driving out damaged rivets, pins, and bolts that sometimes bind in holes. The drive punch is therefore made with a flat face instead of a point. The size of the punch is determined by the width of the face, which is usually 1/8 in. to 1/4 in.

Pin punch Pin punches, often called drift punches, are similar to drive punches and are used for the same purposes. The difference in the two is that the sides of a drive punch taper all the way to the face while the pin punch has a straight shank. Pin punches are sized by the diameter of the face, in 30 seconds of an inch, and range from 1/16 in. to 3/8 in. in diameter. In general practice, a pin or bolt, which is to be driven out, is usually started and driven with a drive punch until the sides of the punch touch the side of the hole. A pin punch is then used to drive the pin or bolt the rest of the way out of the hole. Stubborn pins may be started by placing a thin piece of scrap copper, brass, or aluminum directly against the pin and then striking it with a hammer until the pin begins to move. (NOTE: Never use a prick punch or center punch to remove objects from holes. If you do, the point of the punch will spread the object and cause it to bind even more.)

Prick punch Prick punches are used to place reference marks on metal. This punch is often used to transfer dimensions from a paper pattern directly on the metal. To do this, first place the paper pattern directly on the metal. Then go over the outline of the pattern with the prick punch, tapping it lightly with a small hammer and making slight indentations on the metal at the major points on the drawing. These indentations can then be used as reference marks for cutting the metal. A prick punch should never be struck a heavy blow with a hammer because it may bend the punch or cause excessive damage to the material being worked.

Transfer punch The transfer punch is usually about 4 in. long. It has a point that tapers and then turns straight for a short distance in order to fit a drill-locating hole in a template. The tip has a point similar to that of a prick punch. As its name implies, the transfer punch is used to transfer the location of holes through the template or pattern to the material.

Wrenches A wrench is a tool used to tighten or loosen nuts or bolts on components or structures. The wrenches most often used are classified as follows:

• Open end. • Box end. • Combination. • Socket. • Adjustable. • Special.

NOTE: One of the most widely used metals for making wrenches is chrome-vanadium steel. Wrenches made of this metal are almost unbreakable.

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Open-end wrench Solid, nonadjustable wrenches with open parallel jaws on one or both ends are known as open-end wrenches. These wrenches may have their jaws parallel to the handle or at an angle up to 90°; most are set at an angle of 15°. Basically, the wrenches are designed to fit a nut, bolt head, or other object that makes it possible to exert a turning action.

Box-end wrench Box-end wrenches are popular tools because of their usefulness in close quarters. They’re called box wrenches since they box, or completely surround, the nut or bolt head. Practically all box-end wrenches are made with 12 points, so they can be used in places having as little as 15° swing. Although box-end wrenches are ideal to break loose tight nuts or pull tight nuts tighter, time is lost turning the nut off the bolt once the nut is broken loose. Only when there’s sufficient clearance to rotate the wrench in a complete circle can this tedious process be avoided.

Combination wrench After a tight nut is broken loose, it can be completely backed off or unscrewed more quickly with an open-end than with a box-end wrench. In this case, a combination wrench is needed, which has a box end on one end and an open end of the same size on the other. Both the box-end and combination wrenches are shown in figure 2–16.

Socket wrench A socket wrench is made of these two parts:

1. The socket, which is placed over the top of a nut or bolt head.

2. A handle, which is attached to the socket.

Sockets are made with either fixed or detachable handles. Socket wrenches with fixed handles are usually furnished as an accessory to a machine. They have either a four-, six- or 12-sided recess to fit a nut or bolt head that needs regular adjustment. Many types of handles, extensions, and attachments are available to make it possible to use socket wrenches in almost any location or position. Sockets with detachable handles usually come in sets and fit several types of handles, such as the T, ratchet, screwdriver grip, and speed handle. Socket-wrench handles have a square lug on one end that fits into a square recess in the socket head. The two parts are held together by a light spring-loaded poppet. Two

types of sockets, a set of handles, and an extension bar are shown in figure 2–17. Speed handles (fig. 2–17) can be used with socket wrenches or various attachments for rapid removal and/or installation of nuts or bolts out in the open. The speed handle has a brace-type shaft with a revolving grip on the top.

Figure 2–16. Box-end and combination

wrenches.

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Adjustable wrenches The adjustable wrench is a handy utility tool that has smooth jaws and is designed as an open-end wrench. One jaw is fixed, but the other may be moved by a thumbscrew or spiral screw-worm adjustment in the handle. The width of the jaws may be varied from 0 to 1/2 in. or more. The angle of the opening to the handle is 22½° on an adjustable wrench. One adjustable wrench does the work of several open-end wrenches. Use the adjustable wrench to loosen an odd-size square nut. Although versatile, they aren’t intended to replace the standard open-end, box-end, or socket wrenches.

NOTE: When using any adjustable wrench, always exert the pull on the side of the handle attached to the fixed jaw of the wrench.

Figure 2–17. Socket wrench set.

Special The category of special wrenches includes the following:

• Spanner. • Torque. • Allen.

Spanner There are five types of spanner wrenches. They are as follows:

1. Hook spanners. 2. U-shaped hook spanners. 3. End spanners. 4. Pin spanners. 5. Face-pin spanners.

Hook spanners The hook spanner is for a round nut with a series of notches cut in the outer edge. This wrench has a curved arm with a hook on the end that fits into one of the notches on the nut. The hook is placed in one of these notches with the handle pointing in the direction the nut is to be turned. Some hook-spanner wrenches are adjustable and will fit nuts of various diameters.

U-shaped hook spanners U-shaped hook spanners have two lugs on the face of the wrench to fit notches cut in the face of the nut or screw plug.

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End spanners End spanners resemble a socket wrench but have a series of lugs that fit into corresponding notches in a nut or plug.

Pin spanners Pin spanners have a pin in place of a lug, and the pin fits into a round hole in the edge of a nut.

Face-pin spanners Face-pin spanners are similar to the U-shaped hook spanners except that they have pins instead of lugs.

Torque wrenches There are three types of torque wrenches:

1. Deflecting beam. 2. Rigid case, dial indicating. 3. Impulse feel, micrometer

adjustable.

Torque is the product of the pounds of force exerted on a tool multiplied by the distance to the center of the fastener from where the force is applied. Think of torque as a turning or twisting force. A force of 30 pounds applied to an 8 in. wrench handle exerts 240 inch pounds of torque on the fastener. Over torquing can damage threads, cause deteriora-tion, and make fasteners fail. However, the fastener must be tight enough to hold joined parts and keep them from loosening. The amount of torque you apply to a fastener is specified in the procedures for the task being

performed. The torque handles (commonly called the torque wrenches, fig. 2–18), are types of hand tools with which you must become proficient. They are designed to limit the torque applied to fasteners (nuts, bolts, screws, etc.).

Deflecting-beam wrench The deflecting-beam torque wrench consists of four basic parts––the drive head, deflective beam, self-centering scale, and the handle.

Rigid-case wrench The rigid-case and dial-indicating wrenches have a rigid body, drive head, dial-indicating scale, and a handle. These types of torque wrenches do just what they imply; they indicate the amount of torque or twisting action being applied using a deflective beam or dial indicator. The applied torque is read directly by a pointer as it moves over the indicating scale or dial.

Figure 2–18. Torque wrenches.

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Micrometer-adjustable wrench The micrometer-adjustable and impulse-feel wrenches are breakaway types and are the most common type used by the Air Force. A breakaway-type torque wrench automatically releases when it reaches a predetermined torque value. This torque wrench consists of an adjustable grip, a locking ring, a micrometer-type adjustment incorporating a thimble marked for secondary increments, a spring tube marked for primary increments, and a drive assembly.

To set the breakaway-type torque wrench to the selected value, you first unlock the handle grip by turning the lock ring located on the handle grip. Then turn the handle grip either clockwise or counterclockwise until the graduation on the thimble aligns with the desired graduation on the tube. The handle grip and the thimble are normally machined from one piece of steel so they turn as a unit. After the adjustment is made, relock the handle grip by turning the locking ring.

When using the breakaway-type torque wrench, always pull the wrench in a clockwise direction and apply a smooth and steady motion. While doing this, have one hand centered over the drive assembly and the other hand covering the adjustable handle grip. When the applied torque reaches the predetermined torque setting, the wrench automatically releases or “breaks,” producing approximately 5° to 10° of free travel. This release is distinct and indicates complete torquing action on the fastener.

Using torque tools is not complicated, but you must observe several precautions and approved torquing practices.

Selecting a torque wrench A torque wrench generally has no accuracy requirement for the lower 20 percent of its capacity. Therefore, you must select a torque wrench so the desired torque or range of torque is not within the lower 20 percent of the wrench’s capacity for impulse-feel torque wrenches. In addition, the desired torque or range of torque must not exceed the rated capacity of the selected torque wrench. For example, an impulse-feel torque wrench with a range of 5 to 150 inch pounds has a capacity of 150 inch pounds. The lower 20 percent is 30 inch pounds (20 percent of 150). Therefore, you would select this wrench only when the desired torque is from 30 to 150 inch pounds.

An exception to this rule is when the torque wrench was specially calibrated to a value or range of values in its lower 20 percent of capacity. The torque wrench is tagged or marked to indicate this special calibration, and you can only use the wrench for the special values.

Preparing an impulse-feel torque wrench Before using an impulse-feel torque wrench, you must cycle the wrench through the breakaway torque as recommended in the manufacturer’s handbook. If the manufacturer’s handbook is not available, set the torque device at the maximum setting and cycle the torque wrench through the breakaway torque at least six times. This allows lubricant to recoat internal parts, thus eliminating internal resistance and increasing accuracy.

One way to cycle the wrench is to clamp the square-drive tang in a smooth-jawed vise. Then, pull the handle through six breakaway cycles. The breakaway exercise is not required more than once each eight-hour shift for each torque wrench used.

Other reminders At times, you may need to use an extension with a torque wrench. Always place an extension on the drive end of a torque wrench, never on the handle end.

There are a few other points to remember when using a torque wrench:

• Do not try to change the setting when the grip is locked. • Do not try to apply more torque than the rated capacity of the tool.

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• Always store torque tools at the lowest torque value. • If you drop a torque tool, it must be verified before further use. • Before using a torque tool, make sure the calibration is current. • Do not use a torque tool to break loose a bolt unless procedures specifically require it. • Install an adapter at a 90° angle to the wrench handle.

Allen wrench The Allen wrench, although seldom used, is required on one special type of recessed screw. Most headless setscrews are the Allen type and must be installed and removed with an Allen wrench. Allen wrenches are six-sided bars in the shape of an L. They range in size from 3/64 in. to 1/2 in. and fit into a hexagonal recess in the set screw.

Metal-cutting tools There are several types of metal-cutting tools available, but those used most frequently are as follows:

• Hand snips. • Hacksaws. • Chisels. • Files. • Drills. • Reamers. • Countersinks.

Hand snips There are several kinds of hand snips, each of which serves a different purpose. Examples are shown in figure 2–19.

The hand snips most frequently used are as follows:

1. Straight. 2. Aviation.

Straight snips Straight snips are used for cutting straight lines when the distance isn’t great enough to use a squaring shear and for cutting the outside of a curve. The other types shown in figure 2–19 are used for cutting the inside of curves or radii. (NOTE: Snips should never be used to cut heavy sheet metal.)

Aviation snips Aviation snips are designed especially for cutting heat-treated aluminum-alloy and stainless steel. They’re also adaptable for enlarging small holes. The blades have small teeth on the cutting edges and are shaped for cutting very small circles and irregular outlines. The handles are the compound-leverage type, making it possible to cut material as thick as 0.051 in. Aviation snips are available in two types: those which cut from right to left and those which cut from left to right. Unlike the hacksaw (discussed next), snips don’t remove any material when the cut is made, but minute fractures often occur along the cut. Therefore, cuts should be made about 1/32 in. from the layout line and finished by hand filing down to the line.

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Hacksaws The common hacksaw is a tool that uses a fine-toothed blade (held under tension in a frame). The hacksaw can be used to cut metal and other hard materials. The common hacksaw has a blade, a frame, and a handle. The handle can be obtained in two styles: pistol grip and straight. Hacksaw blades have holes in both ends; they’re mounted on pins attached to the frame. When installing a blade in a hacksaw frame, mount the blade with the teeth pointing forward, away from the handle. Blades are made of high-grade tool steel or tungsten steel and are available in sizes from 6 in. to 16 in. in length. The 10 in. blade is most commonly used.

There are two types of blades:

1. All-hard. 2. Flexible.

An all-hard blade is best for sawing brass, tool steel, cast iron, and heavy cross-section materials. In flexible blades, only the teeth are hardened. Selection of the best blade for the job involves finding the right type and pitch. A flexible blade is usually best for sawing hollow shapes and metals having a thin cross section. The pitch of a blade indicates the number of teeth per inch. Pitches of 14, 18, 24, and 32 teeth per inch are available.

• A blade with 14 teeth per inch is preferred when cutting machine steel, cold-rolled steel, or structural steel.

• A blade with 18 teeth per inch is preferred for solid stock aluminum, bearing metal, tool steel, and cast iron.

• A blade with 24 teeth per inch is used when cutting thick-walled tubing, pipe, brass, copper, channel, and angle iron.

• A blade with 32 per inch is used for cutting thin-walled tubing and sheet metal.

When using a hacksaw, observe the following procedures:

1. Select an appropriate saw blade for the job. 2. Assemble the blade in the frame so that the cutting edge of the teeth points away from the

handle. 3. Adjust tension of the blade in the frame to prevent the saw from buckling and drifting. 4. Clamp the work in the vise in such a way that will provide as much bearing surface as

possible and will engage the greatest number of teeth. 5. Indicate the starting point by nicking the surface with the edge of a file to break any sharp

corner that might strip the teeth. This mark will also aid in starting the saw at the proper place.

6. Hold the saw at an angle that will keep at least two teeth in contact with the work at all times. Start the cut with a light, steady, forward stroke just outside the cutting line. At the end of the stroke, relieve the pressure and draw the blade back. (The cut is made on the forward stroke.)

7. After the first few strokes, make each stroke as long as the hacksaw frame will allow. This will prevent the blade from overheating. Apply just enough pressure on the forward stroke to

Figure 2–19. Snips.

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cause each tooth to remove a small amount of metal. The strokes should be long and steady with a speed not more than 40 to 50 strokes per minute.

8. After completing the cut, remove chips from the blade, loosen tension on the blade, and return the hacksaw to its proper place.

Chisels A chisel is a hard-steel cutting tool that can be used for cutting and chipping any metal softer than the chisel itself. It can be used in restricted areas and for such work as shearing rivets or splitting seized or damaged nuts from bolts. Chisels are usually made of eight-sided tool-steel bar stock, carefully hardened and tempered. Since the cutting edge is slightly convex, the center portion receives the greatest shock when cutting, and the weaker corners are protected. The cutting angle should be 60° to 70° for general use, such as for cutting wire, strap iron, or small bars and rods. When using a chisel, hold it firmly in one hand. With the other hand, strike the chisel head squarely with a ball-peen hammer.

The chisels often used in maintenance are classified as follows:

• Flat cold. • Cape. • Round nose. • Diamond point.

Flat-cold chisel The size of a flat-cold chisel is determined by the width of the cutting edge. Lengths will vary, but chisels are seldom under 5 in. or over 8 in. long.

Cape chisel When cutting square corners or slots, a special cold chisel called a cape chisel should be used. It’s like a flat chisel except the cutting edge is very narrow. It has the same cutting angle and is held and used in the same manner as any other chisel.

Round-nose chisel Rounded or semicircular grooves and corners that have fillets should be cut with a round-nose chisel. This chisel is also used to recenter a drill that has moved away from its intended center.

Diamond-point chisel The diamond-point chisel is tapered square at the cutting end, and then ground at an angle to provide the sharp diamond point. It’s used for cutting V-grooves and inside sharp angles.

Files In the most basic sense, a file is a steel bar or rod with cutting teeth on its surface. The file is used to square ends, file rounded corners, remove burrs and slivers from metal, straighten uneven edges, file holes and slots, and smooth rough edges. Most files are made of high-grade tool steels that are hardened and tempered. Files are manufactured in a variety of shapes and sizes. They are known by

1. Cross section. 2. General shape. 3. Particular use.

The cuts of files must be considered when selecting them for various types of work and materials.

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Distinguishing features Files have three distinguishing features. They are as follows:

1. Length. 2. Kind or name. 3. Cut.

Length The length of the file is measured from the tip to the heel of the file. The tang is never included in the length.

Shape The shape of the file refers to the physical configuration of the file (circular, rectangular, triangular, or a variation thereof).

Cut Refers to both the character of the teeth or the coarseness; rough, coarse, and bastard for use on heavier classes of work and second cut, smooth, and dead smooth for finishing work.

File cuts Files are usually made in two types of cuts. They are as follows:

1. Single cut. 2. Double cut.

Single cut The single-cut file has a single row of teeth extending across the face at an angle of 65° to 85° with the length of the file. The size of the cut depends on the coarseness of the file.

Double cut The double-cut file has two rows of teeth which cross each other. For general work, the angle of the first row is 40° to 45°. The first row is generally referred to as “overcut,” and the second row as “upcut”; the upcut is somewhat finer and not as deep as the overcut.

Most commonly used files Some of the most commonly used files are shown on figure 2–20. The most frequently used files are as follows:

• Hand file. • Flat file. • Mill file. • Square file. • Round or rattail file. • Triangular and three-square files. • Half-round file. • Lead-float file. • Warding file. • Knife file. • Wood file. • Vixen (curved-tooth) file.

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Hand files These are parallel in width and tapered in thickness. They have one safe edge (smooth edge) that permits filing in corners and on other work where a safe edge is required. Hand files are double cut and used prin-cipally for finishing flat surfaces and similar work.

Flat files Flat files are slightly tapered toward the point in both width and

thickness. They cut on both edges as well as on the sides. They’re the most common files in use. Flat files are double cut on both sides and single cut on both edges.

Mill files Mill files are usually tapered slightly in thickness and in width for about one-third of their length. The teeth are ordinarily single cut. These files are used for draw filing and, to some extent, for filing soft metals.

Square files Square files may be tapered or blunt and are double cut. They’re used principally for filing slots and key seats and for surface filing.

Round or rattail files.

Round or rattail files are circular in cross section and may be either tapered or blunt and single or double cut. They’re used principally for filing circular openings or concave surfaces.

Triangular and three-square files Triangular and three-square files are triangular in cross section. Triangular files are single cut and are used for filing the gullet between saw teeth. Three-square files, which are double cut, may be used for filing internal angles, clearing out corners, and filing taps and cutters.

Half-round files Half-round files cut on both the flat and round sides. They may be single or double cut. Their shape permits them to be used where other files would be unsatisfactory.

Lead-float files These are especially designed for use on soft metals. They’re single cut and are made in various lengths.

Warding files Warding files are rectangular in section and taper to narrow points as to width. They’re used for narrow-space filing where other files can’t be used.

Knife files The knife file is used by tool and die makers on work having acute angles.

Figure 2–20. Types of files.

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Wood files Wood files are in the same category as flat and half-round files but have coarser teeth and are especially adaptable for use on wood.

Vixen (curve-tooth) files Curved-tooth files are especially designed for rapid filing and smooth finish on soft metals and wood. The regular cut is adapted for tough work on cast iron, soft steel, copper, brass, aluminum, wood, slate, marble, fibber, rubber, and so forth. The fine cut gives excellent results on steel, cast iron, phosphor bronze, white brass, and all hard metals. The smooth cut is used where the amount of material to be removed is very slight but where a superior finish is desired.

Using files The following methods are recommended for using files:

• Cross filing. • Draw filing. • Rounding corners. • Removing burred or slivered edges. • Lathe filing.

NOTE: Before attempting to use a file, place a handle on the tang of the file. This is essential for proper guiding and safe use.

Cross filing In moving the file endwise across the work (commonly known as cross filing), grasp the handle so that its end fits into and against the fleshy part of the palm with the thumb lying along the top of the handle in a lengthwise direction. Grasp the end of the file between the thumb and first two fingers. To prevent undue wear, relieve the pressure during the return stroke.

Draw filing A file is sometimes used by grasping it at each end, crosswise to the work, then moving it lengthwise with the work. When done properly, work may be finished somewhat finer than when cross filing with the same file. In draw filing, the teeth of the file produce a shearing effect. To accomplish this shearing effect, the angle at which the file is held with respect to its line of movement varies with different files, depending on the angle at which the teeth are cut. Pressure should be relieved during the backstroke.

Rounding corners The method used in filing a rounded surface depends upon its width and the radius of the rounded surface. If the surface is narrow or only a portion of a surface is to be rounded, start the forward stroke of the file with the point of the file inclined downward at approximately a 45° angle. Using a rocking chair motion, finish the stroke with the heel of the file near the curved surface. This method allows use of the full length of the file.

Removing burred or slivered edges Practically every cutting operation on sheet metal produces burrs or slivers. These must be removed to avoid personal injury and to prevent scratching and marring of parts to be assembled. Burrs and slivers will prevent parts from fitting properly and should always be removed from the work as a matter of habit.

Lathe filing Lathe filing requires that the file be held against the work revolving in the lathe. The file shouldn’t be held rigid or stationary but should be stroked constantly with a slight gliding or lateral motion along

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the work. A standard mill file may be used for this operation, but the long-angle lathe file provides a much cleaner shearing and self-clearing action. Use a file with “safe” edges to protect work with shoulders from being marred.

Care of files There are several precautions that any good craftsman will take in caring for his files. They include the following:

1. Choose the right file for the material and work to be performed. 2. Keep all files racked and separated so they don’t bear against each other. 3. Keep the files in a dry place. If you don’t, rust will corrode the teeth points. 4. Keep files clean. Tap the end of the file against the bench after every few strokes to loosen

and clear the filings.

Particles of metal collect between the teeth of a file and may make deep scratches in the material being filed. When these particles of metal are lodged too firmly between the teeth and can’t be removed by tapping the edge of the file, remove them with a file card or wire brush. Draw the brush across the file so that the bristles pass down the gullet between the teeth.

NOTE: Remember, a dirty file is a dull file.

Drills In aerospace work, there are generally four types of portable drills and one stationary drill used for holding and turning twist drills. They are as follows:

1. Hand drill. 2. Breast drill. 3. Electric drill. 4. Pneumatic drill. 5. Drill Press

Hand drill Holes 1/4 in. in diameter and under can be drilled using a hand drill. This drill is commonly called an “egg beater.”

Breast drill The breast drill is designed to hold larger size twist drills than the hand drill. In addition, a breast plate is affixed at the upper end of the drill to permit the use of body weight to increase the cutting power of the drill.

Electric drill Electric-power drills are available in various shapes and sizes to satisfy almost any requirement. The portable electric drill is basically an electric motor in a metal housing. The housing is fitted with a “chuck” into which a bit or other attachment can be inserted. The portable electric drill is basically designed for drilling; however, it can be adapted for different jobs.

Pneumatic drill Pneumatic drills are also available in various shapes and sizes to satisfy almost any requirement. They’re preferred for use around flammable materials, since sparks from an electric drill are a fire or explosion hazard.

Drill press A variety of drill presses are available; the most common type is the upright drill press. The drill press is a precision machine used for drilling holes that require a high degree of accuracy. It serves as an accurate means of locating and maintaining the direction of a hole that’s to be drilled and provides the

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operator with a feed lever that makes the task of feeding the drill into the work an easy one. The degree of accuracy that’s possible to attain when using the drill press will depend to a certain extent on the condition of the spindle hole, sleeves, and drill shank. Therefore, special care must be exercised to keep these parts clean and free from nicks, dents, or warping. Always be sure that the sleeve is securely pressed into the spindle hole. Never insert a broken drill in a sleeve or spindle hole. Be careful never to use the sleeve-clamping vise to remove a drill since this may cause the sleeve to warp. When using a drill press, the height of the drill press table is adjusted to accommodate the height of the part to be drilled. When the height of the part is greater than the distance between the drill and the table, the table is lowered. When the height of the part is less than the distance between the drill and the table, the table is raised. After the table is properly adjusted, the part is placed on the table and the drill is brought down to aid in positioning the metal so that the hole to be drilled is directly beneath the point of the drill. The part is then clamped to the drill press table on the left-hand side to prevent it from slipping during the drilling operation.

Twist drills A twist drill is a pointed tool that’s rotated to cut holes in material. It’s made of a cylindrical hardened steel bar having spiral flutes (grooves) running the length of the body and a conical point with cutting edges formed by the ends of the flutes. The flutes allow an entry point for cutting fluid and allow chips and material to escape.

NOTE: Twist drills are commonly called drill bits. Twist drills are made of carbon steel or high-speed alloy steel. Carbon steel twist drills are satisfactory for the general run of work and are relatively inexpensive. The more expensive high-speed twist drills are used for the tough materials such as stainless steels. Twist drills have from one to four spiral flutes. Drills with two flutes are used for most drilling—those with three or four flutes are used principally to follow smaller drills or to enlarge holes.

Principal parts The principal parts of a twist drill are illustrated in figure 2–21. They are (1) the shank, (2) the body, and (3) the heel.

Drill shank The drill shank is the end that fits into the chuck of a hand or power drill. The two shank shapes most commonly used in hand drills are (1) straight shank and (2) square or bit-stock shank.

The straight shank generally is used in hand, breast, and portable electric drills. Straight-shank bits are generally manufactured up to a diameter of 1/2 in. In contrast; the square shank is made to fit into a carpenter’s brace. You may also encounter a drill bit with a tapered shank. These are generally used in machine shop drill presses.

Drill body The metal column forming the core of the drill is the body. The body clearance area lies just back of the margin, slightly smaller in diameter than the margin, to reduce the friction between the drill and the sides of the hole.

Figure 2–21. Twist drill.

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Drill heel The angle at which the drill heel is ground is the lip clearance angle. On standard drills used to cut steel and cast iron, the angle should be 59° from the axis of the drill. For faster drilling of soft materials, sharper angles are used. The drill heel is often referred to as the drill point.

Twist drill diameter The diameter of a twist drill may be given in one of these three ways: fractions, letters, and numbers.

Fractions Fractionally, twist drills are classified by sixteenths of an inch (from 1/16 to 3½ in.), by thirty-seconds (from 1/32 to 2½ in.), or by sixty-fourths (from 1/64 to 1¼ in.).

Letters For a more exact measurement, a letter system is used with decimal equivalents: A (0.234 in.) to Z (0.413 in.).

Numbers The number system of classification is most accurate: no. 80 (0.0314 in.) to no. 1 (0.228 in.).

Drilling When using a drill and when possible, the technician should use a vise to hold the material. Flat work should be mounted on parallels. If a hole is being drilled, the use of a pilot hole prevents the drill bit from creeping and makes for a very accurate hole. When drilling the material, keep a steady pressure on the drill and before the point is about to break through the work piece, decrease the pressure. A slow cutting speed will not dull the drill. On the other hand, a drill that is ran faster than recommended does not cut more efficiently. The drill size does not affect the speed at which they must be operated.

Sharpening For most drilling, a twist drill with a cutting angle of 118° (59° on either side of center) will be sufficient; however, when drilling soft metals, a cutting angle of 90° may be more efficient. In any case, the twist drill should be sharpened at the first sign of dullness. There are a few common practices when sharpening a drill. They are as follows:

1. Adjust the grinder tool rest to a convenient height for resting the back of the hand while grinding.

2. Hold the drill between the thumb and index finger of the right or left hand. Grasp the body of the drill near the shank with the other hand.

3. Place the hand on the tool rest with the center line of the drill making a 59° angle with the cutting face of the grinding wheel. Lower the shank end of the drill slightly.

4. Slowly place the cutting edge of the drill against the grinding wheel. Gradually lower the shank of the drill as you twist the drill in a clockwise direction. Maintain pressure against the grinding surface only until you reach the heel of the drill.

5. Check the results of grinding with a gauge to determine whether the lips are the same length and at a 59° angle.

Reamers Reamers are used to smooth and enlarge holes to exact size. Hand reamers have square end shanks so that they can be turned with a tap wrench or similar handle. The various types of reamers are illustrated in figure 2–22. Reamers are made of either carbon-tool steel or high-speed steel. The cutting blades of a high-speed steel reamer lose their original keenness sooner than those of a carbon-steel reamer; however, after the first super keenness is gone, they’re still serviceable. The high-speed reamer usually lasts much longer than the carbon-steel type. Reamers are available in any standard size. The straight-fluted reamer is less expensive than the spiral-fluted reamer, but the spiral type has

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fewer tendencies to chatter. Both types are tapered for a short distance back of the end to aid in starting. Bottoming reamers have no taper and are used to complete the reaming of blind holes.

For general use, an expansion reamer is the most practical. This type is furnished in standard sizes from 1/4 in. to 1 in., increasing in diameter by 1/32 in. increments. Taper reamers, both hand- and machine-operated, are used to smooth and true tapered holes and recesses. (NOTE: A cut that removes more than 0.007 in. places too much load on the reamer and shouldn’t be attempted.)

Also, be aware that reamer blades are hardened to the point of being brittle. Consequently, the reamer must be handled carefully to avoid chipping the blades. When reaming a hole, rotate the reamer in the cutting direction only. Turn the reamer steadily and evenly to prevent chattering, or marking and scoring, of the hole walls.

Countersink A countersink is a tool that cuts a cone-shaped depression around the hole to allow a rivet or screw to set flush with the surface of the material. Countersinks are made with various angles to correspond to the various angles of the countersunk rivet and screw heads. The angle of the standard countersink shown in figure 2–23 is 100°. Special stop countersinks are available. For example, the stop counter-sink shown in figure 2–23 is adjustable to any desired depth. Also the cutters are interchangeable so that holes of various countersunk angles may be made. Some stop countersinks have a micrometer-set arrangement (in increments of 0.001 in.) for adjusting the cutting depths. When using a countersink, care must be taken not to remove an excessive amount of material since this reduces the strength of flush joints. In order to succeed in today’s highly technical aerospace environment, technicians must be well versed in some of the fundamental equipment that’s associated with the machine tool industry. Although technicians aren’t expected to be expert machinist or tool and die makers, they should be capable of accurate use and care for precision-measuring instruments. In this section, we’ll present the information you need to become proficient in the use of layout and measuring tools.

Figure 2–23. Countersinks.

Figure 2–22. Reamers.

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Taps and dies A tap is used to cut threads on the inside of a hole, while a solid die is for cutting external threads on round stock as well as chasing and recutting damaged threads. Taps and dies are made of hard-tempered steel and ground to an exact size. There are four types of threads that can be cut with standard taps and dies. They are as follows:

1. National coarse. 2. National fine. 3. National extra fine. 4. National pipe.

Layout and measuring tools There’s no doubt that layout and measuring devices are precision tools. During manufacture, they’re carefully machined, accurately marked, and, in many cases, made up of very delicate parts. When using layout and measuring tools, be careful not to drop, bend, or scratch them. Also strive to be proficient in their use. After all, the finished product will be no more accurate than the measurements or the layout; therefore, it’s very important that you understand how to read, use, and care for these tools. Due to the scope of this special course, we can’t cover all of the various types of layout and measuring tools available to the aerospace industry. Instead, we’ll cover those used most frequently. They are as follows:

• Rules. • Thickness gauges. • Combination sets. • Scriber. • Dividers and pencil compasses. • Dial indicators. • Calipers.

Rules A rule may be used either as a measuring tool or as a straightedge. Rules are made of steel and are either rigid or flexible. The flexible-steel rule will bend, but it shouldn’t be bent intentionally as it may be broken rather easily.

Rule divisions In aerospace work, the unit of measure most commonly used is the inch. The inch may be divided into smaller parts by means of either common or decimal fraction divisions. Rules are manufactured in two basic styles, those divided or marked in common fractions (fig. 2–24) and those divided or marked in decimals or divisions of one one-hundredth of an inch. The fractional divisions for an inch are found by dividing the inch into equal parts as follows:

• Halves (1/2). • Quarters (1/4). • Eighths (1/8). • Sixteenths (1/16). • Thirty-seconds (1/32). • Sixty-fourths (1/64).

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These equal parts are shown in figure 2–24. The fractions of an inch may be expressed in decimals, called decimal equivalents of an inch; for example, 1/8 in. is expressed as 0.0125 (one hundred twenty-five ten-thousandths of an inch).

Thickness gauges A thickness gauge (also called a feeler gauge) is used when measuring the clearance between two surfaces. You use the thickness gauge when you adjust such items as turbine-bearing races and turbine-bucket clearances. A thickness gauge resembles a pocket knife with many blades. All the blades have the same shape, but each is accurately ground to a definite thickness, which is marked on the blade. Before you use the thickness gauge, always wipe the blades with a clean cloth to remove the film of oil, grease, or dirt. When you use the thickness gauge to check the gap between two surfaces, always remember that the accuracy of the gauge depends, to a large extent, on your ability to determine “by feel” when there’s the correct tension or pressure on the blade or blades. You can develop the “feel” of the thickness gauge by practicing measuring the gaps between two surfaces of a known dimension.

Combination sets As the name implies, the combination set is a tool that has several uses. It can be used for the same purposes as an ordinary tri-square (another common tool technicians use), but it differs from the tri-square in that the head slides along the blade and can be clamped at any desired place. Combined with the square or stock head are a level and scriber. The head slides in a central groove on the blade or scale, which can be used separately as a rule.

The spirit level in the stock head makes it convenient to square a piece of material with a surface and at the same time tell whether one or the other is plumb or level. The head can be used alone as a simple level. The combination of square head and blade can also be used as a marking gauge to scribe lines at a 45° angle, as a depth gauge, or as a height gauge. A convenient scriber is held frictionally in the head by a small brass bushing. The center head is used to find the center of shafts or other cylindrical work. The protractor head can be used to check angles and also may be set at any desired angle to draw lines.

Scriber This tool is designed to serve the technician in the same way a pencil or pen serves a writer. In general, it’s used to scribe or mark lines on metal surfaces. The scriber is made of tool steel, 4 in. to 12 in. long, and has two needle-pointed ends. One end is bent at a 90° angle for reaching and marking through holes. Before using a scriber, always inspect the points for sharpness.

Dividers and pencil compasses Dividers and pencil compasses have two legs joined at the top by a pivot. They’re used to scribe circles and arcs and for transferring measurements from the rule to the work. Pencil compasses have

Figure 2–24. Rules.

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one leg tapered to a needle point; the other leg has a pencil or pencil lead inserted. In contrast, dividers have both legs tapered to needle points. When using pencil compasses or dividers, the following procedures are suggested:

1. Inspect the points to ensure they’re sharp. 2. To set the dividers or compasses, hold them

with the point of one leg in the graduations on the rule. Turn the adjustment nut with the thumb and forefinger; adjust the dividers or compasses until the point of the other leg rests on the graduation of the rule which gives the required measurement.

3. To draw an arc or circle with either the pencil compasses or dividers, hold the thumb attachment on the top with the thumb and forefinger. With pressure exerted on both legs, swing the compass in a clockwise or counterclockwise direction and draw the desired arc or circle.

4. The tendency for the legs to slip is avoided by inclining the compasses or dividers in the direction in which they’re being rotated. In working on metals, the dividers are used only to scribe arcs or circles that will later be removed by cutting. All other arcs or circles are drawn with pencil compasses to avoid scratching the material.

5. On paper layouts, the pencil compasses are used for describing arcs and circles. Dividers should be used to transfer critical measurements because they’re more accurate than a pencil compass.

Dial indicators Dial indicators are mechanical motion-amplifying devices. A dial indicator produces easily visible indications of any small movement imparted to its

contact point by an object held against it. The contact point may be one end of a pivoted lever linked to the indicating hand, or it may be screwed on a rod, called the “rack,” which can be used to check rotor-shaft alignment or the plane of rotation of a disk. Dial indicators can’t be used alone for measurement or gauging because they’re rigidly mounted on any of a number of types of supports. Although many variations in construction are possible, a common feature of all dial indicators is the ability to greatly amplify movement. This amplification of movement enables you to detect extremely small displacement of the contact points caused by irregularity in a surface and minute differences in size.

Calipers Calipers are used for measuring diameters and distances or for comparing distances and sizes. They are most commonly used for outside, inside, and depth measurements.

Common types The three common types of calipers are outside, inside, and hermaphrodite (fig. 2–25).

Figure 2–25. Calipers.

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Outside calipers Outside calipers are used for measuring outside dimensions, for example, the diameter of a piece of round stock.

Inside calipers Inside calipers have outward curved legs for measuring inside diameters, such as diameters of holes, the distance between two surfaces, the width of slots, and other similar jobs. If an accurate reading is required, the inside-caliper setting should be checked with an outside caliper.

Hermaphrodite caliper A hermaphrodite caliper (such as gear-tool calipers) is generally used as a marking gauge in layout work. It shouldn’t be used for precision measurement.

Micrometer calipers The smallest measurement that can be made with the use of the steel rule is one sixty-fourth of an inch in common fractions, and one one-hundredth of an inch in decimal fractions. To measure more closely than this (in thousandths and ten-thousandths of an inch), a micrometer is used. If a dimension given in a common fraction is to be measured with the micrometer, the fraction must be converted to its decimal equivalent. Micrometers are available in a variety of sizes, 0 to 1/2 in., 0 to 1 in., 1 to 2 in., 2 to 3 in., 3 to 4 in., 4 to 5 in., or 5 to 6 in. sizes. There are four types of micrometer calipers, each designed for a specific use. The four types are as follows:

1. Outside micrometer. 2. Inside micrometer. 3. Depth micrometer. 4. Thread micrometer.

All four types of micrometers are read in the same way. The method of reading an outside micrometer is discussed later in this unit. The outside micrometer shown in figure 2–26 is used by the technician more often than any other type. When a technician needs to measure the diameter of a 1/4 in. hole, use a small-hole gauge and a micrometer. Micrometers may be used to measure the outside dimen-sions of shafts, thickness of sheet-metal stock, diameter of drills, and for many other applications.

Micrometer parts As you can see in figure 2–26, the fixed parts of a micrometer are the frame, barrel, and anvil. The movable parts of a micrometer are the thimble and spindle. The thimble rotates the spindle which moves in the threaded portion inside the barrel. Turning the thimble provides an opening between the anvil and the end of the spindle where the work is measured. The size of the work is indicated by the graduations on the barrel and thimble.

Figure 2–26. Outside micrometer.

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Reading a micrometer The lines on the barrel marked 1, 2, 3, 4, and so forth, indicate measurements of tenths, or 0.100 in., 0.200 in., 0.300 in., 0.400 in., respectively (fig. 2–27). Each of the sections between the tenths divisions (between 1, 2, 3, 4, etc.) is divided into four parts of 0.025 in. each. One complete revolution of the thimble (from zero on the thimble around to the same zero) moves it one of these divisions (0.025 in.) along the barrel, thus increasing or decreasing the distance between the

measuring faces by 0.025. The bevel edge of the thimble is divided into 25 equal parts. Each of these parts represents 1/25 of the distance the thimble travels along the barrel in moving from one of the 0.025 in. divisions to another. Thus, each division on the thimble represents one one-thousandth (0.001) of an inch. These divisions are marked for convenience at every five spaces by 0, 5, 10, 15, and 20. When 25 of these graduations have passed the horizontal line on the barrel, the spindle (having made one revolution) has moved 0.025 in. The micrometer is read by first noting the last visible figure on the horizontal line of the barrel representing tenths of an inch. Add to this the length of barrel between the thimble and the previously noted number. (This is found by multiplying the number of graduations by 0.025 in.) Add to this the number of divisions on the bevel edge of the thimble that coincides with the line of the graduation. The total of the three figures equals the measurement. (Fig. 2–28 shows several sample readings).

Vernier scale Some micrometers are equipped with a vernier scale that makes it possible to read directly the fraction of a division that may be indicated on the thimble scale. Typical examples of the vernier scale as it applies to the micrometer are shown in figure 2–29. All three scales on a micrometer aren’t fully visible without turning the micrometer; but the examples shown in figure 2–29 are drawn as though the barrel and thimble of the micrometer were laid out flat so that all three scales can be seen at the same time. The barrel scale is the lower horizontal scale; the thimble scale is vertical on the right; and the long horizontal lines (0 through 9 and 0) make up the vernier scale.

Figure 2–27. Reading a micrometer.

Figure 2–28. Sample readings.

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In reading a micrometer, an excellent way to remember the relative scale values is to remember the following:

• The 0.025 in. barrel scale graduations are established by the lead screw (40 threads per inch). • The thimble graduations divide the 0.025 in. into 25 parts, each equal to 0.001 in. • The vernier graduations divide the 0.001 in. into 10 equal parts, each equal to 0.0001 in.

By remembering the values of the various scale graduations, the barrel scale reading is noted. The thimble scale reading is added to it; then the vernier scale reading is added to get the final reading. The vernier scale line to be read is always the one aligned exactly with any thimble graduation. In the first example in figure 2–29, the barrel reads 0.275 in. and the thimble reads more than 0.019 in. The no. 1 graduation on the thimble is aligned exactly with the no. 4 graduation on the vernier scale. Thus, the final reading is 0.2944 in. In the second example in figure 2–29, the barrel reads 0.275 in., and the thimble reads more than 0.019 in. and less than 0.020 in. On the vernier scale, the no. 7 graduation coincides with a line on the thimble. This means that the thimble reading would be 0.0197 in. Adding this to the barrel reading of 0.275 in. gives a total measurement of 0.2947 in. The third and fourth examples in figure 2–29 are additional readings that would require use of the vernier scale for accurate readings to ten-thousandths of an inch.

Figure 2–29. Vernier scale readings.

Using a micrometer The micrometer is a delicate instrument, and as such, it must be handled carefully. If it’s dropped, its accuracy may be permanently affected. Continually sliding work between the anvil and spindle may wear the surfaces. If the spindle is tightened too much, the frame may be sprung permanently and inaccurate readings will result. A gauge block is used to check the accuracy or calibration of a micrometer. To measure a piece of work with the micrometer, hold the frame of the micrometer in the palm of the hand with the little finger or third finger, whichever is more convenient. This allows the thumb and forefinger to be free to revolve the thimble for adjustment.

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Self-Test Questions After you complete these questions, you may check your answers at the end of the unit.

213. Hardware 1. Where is an AN clevis bolt used?

2. A bolt with a single-raised dash on the head is classified as what?

3. How are commonly used screws classified?

4. What should be used under a lock washer to prevent damage to the surface material?

5. How many times can a shakeproof lock washer be used?

6. Aircraft nuts can be divided into what two groups?

7. Under what environmental conditions should elastic stop nuts not be used?

8. What factors need to be considered before drilling or reaming a bolt hole to a larger size?

9. Where can you look to determine if a replacement bolt is the correct type?

10. How are solid-shank rivets identified?

11. What will happen if a copper rivet is installed in an aluminum-alloy structure?

12. Where are blind rivets used?

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214. Hand tools 1. How are screwdrivers classified?

2. How much of a screw slot must a common screwdriver blade fill?

3. Which type of pliers would be used for twisting safety wire?

4. On which side of an adjustable wrench should pulling force be exerted?

5. What are the three most commonly used types of torque wrenches?

6. Why should cuts made with snips be made 1/32 in. away from the layout line?

7. In which direction should the teeth face when installing the blade in a hacksaw frame?

8. Which type of file would be used to file an internal angle?

9. What’s the angle of the standard countersink?

10. Which tool is used to measure the clearance between a surface plate and a relatively narrow surface being checked for flatness?

11. Which tool is used to find the center of a shaft or other cylindrical work?

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2–3. Pneumatic and Hydraulic Principles This section covers the fundamentals of hydraulic and pneumatic principles used throughout the aerospace maintenance career field. This information is important to you since a considerable amount of support and facility equipment used is hydraulically and pneumatically operated. Your duties as a technician include working on and with this type equipment on a daily basis. Therefore, this section is designed to familiarize you with both hydraulic and pneumatic principles.

Some ancillary or support systems found in aerospace maintenance use hydraulic (liquid) pressure for mechanical operations. We use hydraulic systems because they are relatively more reliable, cheaper, and easier to maintain than electromechanical systems. Other systems, however, use pneumatic (air) pressure to perform operations.

215. Theory and operation of pneumatic and hydraulic systems Man has devised methods to make work easier and less physically demanding throughout history. The development of hydraulic and pneumatic systems has certainly changed the way work is accomplished. One must only take a short look around our world today to realize the significant impact these systems have on a daily basis. From simply loosening the lug nuts on our car tires to moving huge objects, these systems have made life easier and processes to go much quicker. We’ll start by examining the fundamentals of pneumatics.

Pneumatic systems An important part of the aerospace field deals with air pressure (pneumatics). A fluid can be a liquid or a gas. What is the difference between the two? First, a gas fills its container completely, while a liquid remains at the bottom. As an example, pour a quart of water into a gallon bucket. Because the quart of water cannot fill the bucket completely, the water takes the shape of the bottom of the bucket. Now take the same amount of gas (air) and put it into the bucket. It completely fills the bucket and may even overflow the rim of the bucket.

Second, gases are lighter than equal amounts of liquids. If a bucket is filled with a liquid, we know it is heavier than the bucket that is filled with a gas. Third, gases are highly compressible, but for all practical purposes, liquids are not. Only under extremely high pressures can a liquid be compressed and then only slightly.

Characteristics of gases When it is subjected to an applied force, a gas (such as air or nitrogen) acts in a manner similar to a spring. It yields but pushes back with as much force as is applied to it. This characteristic of gases makes them useful in missile systems. In fact, some components are designed to use a gas even though a spring would work. This is because gases weigh less than metal springs and are not subject to metal fatigue.

Air is the gas that is most commonly used in pneumatic systems. It is used in accumulators, shock struts, and emergency systems and for pressurizing system reservoirs. In terms of compressibility, almost any gas could be used, but many gases are dangerous because they are flammable or explosive. Pure nitrogen is the only safe substitute for atmospheric air in pneumatic systems. It is the only substitute that is authorized.

Properties of gases When we speak of gases, we could be referring to any number of different gases. Air is the most common gas. However, air is actually a combination of several gases, mainly oxygen and nitrogen. Pressure, temperature, and volume are factors that determine the condition of gases. Changing one factor has some effect on the others.

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Boyle’s law The English scientist Robert Boyle (1627–91) was among the first to study the compressibility of gases, which he called the “springiness of air.” According to Merriam-Webster’s Collegiate Dictionary, 11th edition, Boyle’s law states that the volume of a gas at constant temperature varies inversely with the pressure, exerted on it (p. 148). This law can be demonstrated by confining a quantity of gas in a cylinder that has a tight-fitting piston (fig. 2–30, C). Notice that the molecules of gas are far apart, much like a bunch of balloons floating in the air. When the force applied to the piston is doubled, the gas is compressed to one-half its original volume, as shown in figure 2–30, D. Of course, as the force on the piston is increased, the pressure also increases. If the force pushing down (fig.2–30, E) creates so much pressure that it exceeds the design capabilities of the container, then the container bursts.

Charles’ law As noted in the Britannica Online Encyclopedia, a

Frenchman, Jacques Charles (1746–1823), was another scientist who experimented with gases. He found that all gases expand and contract in direct proportion to the change in the temperature, provided the pressure is constant. When the temperature of a substance is increased, the volume of the substance changes but the weight remains constant. This law is shown in figure 2–30, F, and G. Let’s assume that the container in figure 2–30, F, is at room temperature. As the temperature is increased (simulated by the lighted candle), the gas expands as shown in figure 2–30, G; however, the pressure remains the same. The reason for this is that the piston was pushed up along with the expanding gases.

Pneumatic controls Before we get into pneumatic controls, there are some basic facts that we must cover. Other than using a gas versus a liquid, pneumatic and hydraulic systems use essentially the same

basic opponents, except no return lines are used on pneumatic systems. The return pressure on a pneumatic system is vented to the atmosphere. Pneumatic systems utilize relief vales for venting. Next, the controls, pumps, and motors all work similar to hydraulic systems. Pneumatics use tanks for

Figure 2–30. Boyle’s and Charles’ laws.

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storage of gases that are under pressure. A nitrogen bottle, or K-bottle, is one type of storage device that serves as a receiver. In a hydraulic system, the reservoir acts as a receiver. On a compressor system, the storage tank is usually labeled to dictate its storage capacity in gallons.

Control valves Pneumatic valves control, regulate, and protect air-operated systems. A control valve is anything from a simple manual valve to a complicated electrically controlled, pneumatically actuated on/off valve. Hand-operated pneumatic selector valves control the flow of air. Solenoids and electric motors normally operate low-pressure pneumatic control valves. The solenoid usually operates a plunger-type valve, while a motor operates a butterfly-type valve.

A high-pressure pneumatic system that requires a large volume of airflow uses an electrically controlled, pneumatically actuated control valve. Figure 2–31 shows the operating principles of such a device that turns a high-pressure system on and off. This assembly consists of a pilot solenoid valve and a slave valve. The slave-valve plunger has a control poppet and a control piston, which are attached to the same shaft. The piston working area is larger than the poppet face (right side). This gives the control piston a mechanical advantage over the control poppet during valve operation.

Figure 2–31. Pneumatic control valve.

Figure 2–31 shows the control valve in the OFF (closed) position. The solenoid spring (not shown) holds the solenoid poppet closed. Spring pressure and system air pressure act on the face of the control poppet to hold it closed. When the pilot solenoid valve is energized, high-pressure air enters the piston chamber. Since the working area of the control piston is large enough to overcome the force on the control-poppet face and spring pressure, the plunger moves to the right. This opens the valve and lets high pressure into the system. Once the control poppet moves off its seat, the piston-chamber pressure must overcome only the spring pressure. De-energizing the pilot solenoid valve closes the solenoid poppet and shuts off high-pressure air to the piston chamber. The air in the piston chamber can then escape to the atmosphere through the bleed hole in the stem of the solenoid poppet. This lets the control-poppet spring move the slave-valve plunger to the closed position (left).

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Regulators A pneumatic regulating valve works like a hydraulic pressure regulator: it keeps the pressure constant for a given system. Both regulators regulate the quantity of fluid flow in their respective systems.

However, its design and operation vary from those of hydraulic regulators. Figure 2–32 is a schematic of a typical pneumatic regulator. The high-pressure air enters the regulator, passes around the poppet, and goes into the pressure outlet chamber. In addition, air enters the upper diaphragm control chamber through the bleed hole. As the pressure builds in the system, it is likewise building in the chamber above the diaphragm. This continued buildup causes the pressure to move the diaphragm downward. This lets the poppet spring move the poppet downward. This shrinks the size of the opening through which air can enter the regulator. If no pneumatic systems are operating, and there are no leaks, the poppet closes all the way. The pressure at which the poppet closes depends on the pressure applied by the control spring. However, when a component operates, the outlet pressure begins to

drop and the poppet spring moves the poppet back to its seat. In actual operation, if a subsystem needs a constant flow of air, the poppet opens enough to maintain the required pressure.

A small bleed hole rather than a large one keeps pressure surges in either the inlet or the outlet from damaging the diaphragm. The built-in relief valve serves the same purpose as any other relief valve: it protects the system from excess pressures in case the regulator fails.

Back off the adjustment screw to decrease compression of the control spring and reduce outlet pressure. Turn this screw further into the valve to increase the outlet pressure.

Hydraulic system Hydraulics studies the physical behaviors of all liquids at rest and in motion. It’s the transfer of fluid power from one location to another. Basically, hydraulic systems change power into fluid pressure, move the pressurized fluid a set distance, and change its pressure into power again to make it do work. This fluid is pressurized and distributed with pumps and motors. Liquid rather then gas is used in hydraulic systems because they are less compressible. Another way of stating this is negligible volume change under high pressure is best defined as incompressibility.

Unlike most pneumatic systems, hydraulic systems have a return system; usually the return lines have low or no pressure applied to them. In an actual system, return lines are usually larger than the supply lines. The volume of a system is the cubic inches of fluid contained in a closed system.

If you look at the power steering system on your car, the high-pressure (or supply) line has special fittings on the ends to contain the pressure. The return hose has clamps that hold it onto a pipe or nipple, which sticks out of the pump housing. If you look at the hoses themselves, the supply hose is labeled as a high-pressure hose and the return is labeled as low-pressure. Let’s look at some associated terms.

Associated terms To understand hydraulic principles and their application to hydraulic systems, you must learn the meanings and relationship of several hydraulic terms.

Figure 2–32. Pneumatic regulating valve.

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Area Area is the measurement of a surface. In this course, we express area in square inches. In a hydraulic system, for instance, you are concerned with the areas of piston heads.

Force Force is the amount of push on an object. Here we measure it in pounds. The pressure acting on the area of a piston head produces the force in a hydraulic system.

Pressure Pressure is the amount of force on one unit of area, usually one square inch. Thus we have pounds per square inch, or psi. This pressure acts on the piston head of an actuator to produce the force that operates a mechanism. While measuring pressure, the most important aspect is that it should be consistent.

Pascal’s law This law of physics, the basis for hydraulic calculation, has its practical application in working systems. As found in the Britannica Online Encyclopedia, Blaise Pascal (1623–62), a noted French mathematician and philosopher, stated his law as when a force is exerted on a confined fluid, the pressure is transmitted equally and undiminished in all directions. Dwell on the words “force” and “pressure.” From now on, you must use them and other terms according to their strict meanings. Note that Pascal’s law applies only to confined fluids. In figure 2–33, you see Pascal’s law illustrated. When a force acts on piston 1, the pressure transmitted through the confined fluid is exactly the same at all points throughout the system and is applied at right angles to the inner surfaces of the system.

Let’s consider the interrelationship of area, force, and pressure. Knowing any two of these factors, you can easily calculate the third one. A simple aid to use with hydraulic problems is the triangle in figure 2–34. For example, suppose you exert a force of 100 pounds on a piston that has an area of 4 square inches. Each square inch of the piston area bears its share of applied force; the force is evenly distributed over the area of the piston. In our example, each of the 4 square inches of the area bears one-fourth of the 100-pound force, or 25 pounds. The pressure on the face of the piston, then, is 25 psi. Refer again to Pascal’s triangle and cover the P, and you have F/A. Now work the problem; that is, divide F (100-pound force) by A (4-square-inch area) and note that P (pressure) equals 25 psi. To find force, cover F in the triangle and multiply A by P. Always express F in pounds, P in pounds per square inch, and A in square inches. Now let’s try these formulas on some examples.

Example 1 If a pressure of 50 psi acts on a piston surface of 5 square inches, what is the force on the surface? Using the triangle in figure 2–34, cover F and multiply P (50 psi) times A (5 square inches). The product is 250 pounds of force on the surface of the piston.

Example 2 Exert a force of 480 pounds on a surface whose area is 12 square inches. What is the pressure on the surface? Use the triangle once again. Cover P and the formula is F/A. Divide 480 pounds (F) by 12 square inches (A) and you find P, a pressure of 40 psi. To find the surface area when you know the pressure and force, cover A in Pascal’s triangle and divide F by P.

Figure 2–33. Application of Pascal’s law.

Figure 2–34. Force, area, and pressure

relations.

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216. Major hydraulic components Hydraulic pumps pressurize and provide flow to the system. Motors change this fluid pressure into rotating mechanical force. Your daily tasks are only to test,

replace, and troubleshoot these components. Your job is easier if you know how these components work in the system.

Fluid The liquids in hydraulic systems are used primarily to transmit and distribute forces to various units to be actuated. Liquids are able to do this because they’re almost incompressible. Pascal’s law states that pressure applied to any part of a confined liquid is transmitted with undiminished intensity to every other part. Thus, if a number of passages exist in a system, pressure can be distributed through all of them by means of the liquid. If a system is contaminated, liquid samples should be taken from the reservoir and various other locations in the system. Fluid contamination can occur when any action places foreign matter in it.

Manufacturers of hydraulic devices usually specify the type of liquid best suited for use with their equipment, in view of following factors:

• Working conditions. • Service required. • Temperatures expected inside and outside the systems. • Pressures the liquid must withstand. • Possibilities of corrosion. • Any other conditions that must be considered.

If incompressibility and fluidity were the only qualities required, any liquid that’s not too thick might be used in a hydraulic system. But a satisfactory liquid for a particular installation must possess a number of other properties. Four of the properties and characteristics that must be considered when selecting a satisfactory liquid for a particular system are as follows:

1. Viscosity. 2. Chemical stability. 3. Flash point. 4. Fire point.

Viscosity One of the most important properties of any hydraulic fluid is its viscosity. Viscosity is internal resistance to flow. A liquid such as gasoline flows easily (has a low viscosity) while a liquid such as tar flows slowly (has a high viscosity). Viscosity increases with decrease in temperature. A satisfactory liquid for a given hydraulic system must have enough body to give a good seal at pumps, valves, and pistons; but it must not be so thick that it offers resistance to flow, leading to power loss and higher operating temperatures. These factors will add to the load and to excessive wear of parts. A fluid that’s too thin will also lead to rapid wear of moving parts, or of parts that have heavy loads. The viscosity of a liquid is measured with a viscosimeter or viscometer. There are several types, but the instrument most often used by engineers in the US is the Saybolt universal viscosimeter. This instrument measures the number of seconds it takes for a fixed quantity of liquid (60 cc. [cubic centimeters]) to flow through a small orifice of standard length and diameter at a specific tempera-ture. This time of flow is taken in seconds, and the viscosity reading is expressed as SSU (seconds, Saybolt universal). For example, a certain liquid might have a viscosity of 80 SSU at 130° F.

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Constant-volume hydraulic pumps Constant-volume pumps have many applications. They differ from variable-volume pumps by the simple fact that the speed of the drive unit controls the output. Pump performance can be expressed in gallons per minute and cubic inches per revolution.

Gear-type pumps A gear-type pump may be used in hydraulic systems that require pressure below 1,500 psi. This type pump delivers a constant volume of fluid at a given revolution per minute (rpm). It contains two steel gears and is driven by an engine or a motor. Pumping is done by the rotation of the two gears. As the gear teeth disengage on the suction side of the pump, a low-pressure area forms. This lets fluid enter the pump and be trapped between the gear teeth and the pump housing. As the gears continue to turn, the fluid travels around the gears and is forced out the pressure side of the pump. The pump is lubricated and cooled by controlled internal seepage. This seepage inside the pump builds up a pressure that is known as case pressure. To prevent excessive case pressure, the hubs of both gears are made hollow and are connected to the intake side of the pump through a very weak relief valve.

Vane-type pumps This particular pump has four sliding vanes that are mounted in an eccentric rotor with springs. As the vanes turn, the springs behind the vanes force the vanes outward during the first half turn and increase the space (volume) between the vanes. This draws fluid into the pump. During the last half turn, the volume between the vanes decreases and fluid leaves the pump. If we change the turning direction, we must reverse the line connections. The vane-type pumps are usually used in low-pressure systems. In high-pressure systems, piston-type pumps are almost always used.

Piston-type pumps Constant-volume piston-type pumps, like vane-and-gear-type pumps, put out a constant flow of fluid for a given rpm. Usually there are seven or nine pistons that are fastened by a universal linkage to the drive shaft. A universal link drives the cylinder block, which the housing holds at an angle to the drive shaft. Everything within the housing turns with the drive shaft. The piston is turned to the upper position; its movement forces fluid out of the port. Since each piston is always somewhere between the upper and lower positions, constant intake and output of fluid results. Pumps are available with different angles between the drive shaft and cylinder block; a large angle provides more volume output per revolution by increasing the length of the piston stroke. As the drive shaft turns, it rotates the cylinder block and the piston assemblies. The pistons are always the same distance from their attachment points on the drive shaft. Although the pistons appear to move within the cylinders, it is the cylinders that actually move back and forth around the pistons; therefore, the block and piston assembly rotates.

Like the gear-type pump, the piston type uses case pressure for cooling and lubrication. Fluid seeps by the pistons in the cylinder block and fills all the space inside the pump. A seal around the drive shaft keeps fluid from escaping through the pump drive end. Excessive case pressure within the housing is prevented by routing the fluid back to the intake side of the pump drive end through a relief valve called a foot valve. This valve keeps the case pressure from rising above about 15 psi. In many of the operating parts of the pump, drilled passageways let the fluid circulate more freely throughout the pump.

The accessory drive rotating direction determines the pump flow direction. An arrow on the pump head shows the direction of rotation for which the pump is set up. To change the direction of rotation for models with two foot valves, simply switch the suction and pressure lines—don’t turn the head. The power pumps discussed thus far have been the constant-volume type; that is, for any set rpm, the volume output is constant. Pressure regulators or unloading valves keep the system pressure at the right point.

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Variable-volume pumps One advantage of variable-volume pumps is that they don’t need pressure regulators or unloading valves. Integral flow-control valves regulate the pressure according to the demands made on the system. This keeps the pressure more stable, thus reducing pressure surges. A system using a variable-volume pump has an accumulator only to aid the pump during peak loads. The ratings of most hydraulic pumps are determined by the volumetric output at a given pressure.

Hydraulic motor The hydraulic motor shown in figure 2–35 is a variable-displacement, axial-piston, rotating-cylinder-block unit, which delivers at least 16.6 horsepower (hp) at 8,000 rpm. Since this particular motor is used to drive an AC/DC generator at a constant speed, it has a very sensitive flyweight-type speed control governor.

Operation When the hydraulic system is pressurized (fig. 2–35), pressure goes to the pistons of the cylinder block and the closed starting valve of the motor. Normal system pressure can maintain cylinder and generator rotation; however, the cylinder block must first be started by positioning of the starting valve. The starting valve moves to the right when the system pressure reaches about 1,800 to 2,200 psi. This aligns a passage that lets the pressure move the control piston and the bottom of the wobbler plate to the right with sudden force. Movement of the wobbler plate forces the pistons to give the cylinder block an initial spin. Thereafter, the system pressure keeps the cylinder block spinning.

As the hydraulic pressure rises, motor speed increases and makes the governor flyweights begin to pivot outward. This moves the governor control valve to the left and slowly blocks the passage to the control piston. An “on speed condition” occurs when the passage is completely blocked, pressure is removed from the control piston, and the motor speed is no longer increasing.

Figure 2–35. Hydraulic motor.

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The load on the motor varies with the electrical load on the generator; the motor must change its torque to keep its normal operating speed. For example, say the generator load is reduced and the motor tends to overspeed. This moves the governor flyweights outward to pull the governor control valve slightly to the left. Movement to the left from the blocked position vents the control piston passage to return system pressure. Since system pressure on the upper pistons is greater than the return system pressure in the control piston passage, the upper pistons in the rotating cylinder block move the wobbler plate to the right. This reduces the angle of the wobbler plate, which then reduces the torque output and, thus, the speed of the motor.

Wobbler-plate movement also briefly activates the preact piston of the flyweight governor to prevent overtravel of the governor or control valve and to quicken the motor’s response to load change. The preact piston is activated when the wobbler plate moves to the right and the control piston moves to the left. This forces some of the fluid in the feedback line into the chamber. The preact piston moves to the right to increase the governor spring pressure and oppose governor control valve movement to the left. The preact piston helps control the governor of the motor. Then, as the motor speed decreases and the fluid in the preact piston chamber bleeds off through the restrictor bleed hole, the governor control piston moves back to the right. This blocks the control piston passage and holds the wobbler plate in its new position. This series of events occurs almost instantly and matches the torque requirement with the load.

Safety features A back-pressure valve in the discharge line of the lower piston prevents the lower piston from “floating” and “chattering.” Chattering occurs when the relief valve opens and closes rapidly as it hunts above and below a set pressure. If the valve were not installed in the return line, return pressure would exist on both sides of the lower pistons, and they would tend to float. The back-pressure valve in the discharge line vents all pressure that is 100 psi greater than system return pressure to the back side of the lower pistons. This holds these pistons against the wobbler plate and prevents chattering.

You can increase or decrease the speed of a hydraulic motor with an adjustment screw that is located on the back of the governor.

Hydraulic actuators The purpose of actuators is to change fluid or air pres-sure into mechanical force or action. This force can be in a straight line or in a rotary direction, depending on the type of actuators used. An actuating cylinder is gen-erally used for straight-line work, whereas motors are used for rotary actions. Even though we discuss hydrau-lic actuators, keep in mind that pneumatic actuators work on the same principle. Actuators can be used to lift and move heavy loads. Now that you know what they can be used for, let’s take a look at how they operate

Single-acting actuating cylinders The top drawing in figure 2–36 is a single-port, single-acting cylinder. Fluid under pressure enters the port and moves the piston toward the other end of the cylinder against the force of the spring. The distance the piston travels in a cylinder is known as the stroke. When the operation is done and the pressure is released, the spring returns the piston to its original position. The chamber behind the piston is vented to let air move in either direction.

Figure 2–36. Types of actuators.

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Double-acting, unbalanced-type actuating cylinders The center drawing in figure 2–36 shows the most common of two-port actuating cylinders. Fluid that is under pressure enters the right-hand port, forces the piston to the other end of the cylinder, and moves the mechanism that is attached to the rod. At the same time, the fluid ahead of the piston leaves the cylinder and returns through the selector valve to the reservoir. If fluid under pressure goes into the left-hand port, the direction of piston movement is reversed. Fluid ahead of the piston is again forced out to the cylinder and returned to the reservoir. This type of actuating cylinder can move a mechanism in either direction by using a selector valve to change the direction of fluid flow.

When it takes more force to move a mechanism in one direction than in the other, the movement of the actuating piston relates to the movement of the mechanism. Assuming that the pressure entering either side of the cylinder is the same, the greater force is on the full face of the piston.

Double-acting, balanced-type actuating cylinders The double-acting, balanced-type actuating cylinder (fig. 2–36, bottom) is like the unbalanced type except the piston rod extends through the piston and both ends of the cylinder. One or both of these piston rods may be attached to a mechanism; in either case, the area on each side of the piston is equal. Thus, the amount of fluid and force required to move the piston a set distance in one direction is the same as the amount needed to move it an equal distance the other way.

Rotary-type actuators Use this hydraulic actuator when rotating power is required. A selector valve controls the turning direction. The motor works like the constant-volume, piston-type pump except the hydraulic motor operation is reversed. The hydraulic motor receives fluid under pressure from the main hydraulic system to move the pistons and turn the shaft. The shaft is geared to the mechanism; the turning direction is determined by the selector valve position.

Accumulator The accumulator serves a twofold purpose: (1) it acts as a cushion or shock absorber by maintaining an even pressure in the system and, (2) it stores enough fluid under pressure to provide for emergency operation of certain actuating units. Accumulators are designed with a compressed air chamber or pressure gas component that is separated from the fluid by a flexible diaphragm or movable piston.

217. Other hydraulic system components As a technician, you may have to diagnose problems in various hydraulic systems. You can do this only if you understand the operation of the different components in the system. The following paragraphs describe different types of valves found in hydraulic systems. These valves act as traffic control and safety officers in hydraulic systems by determining when and where the system fluid is directed and the amount of pressure to be used in the system.

NOTE: The figures in this lesson are used to supplement the discussion and are not intended to be detailed drawings of actual components.

Relief valves Relief valves prevent pressure from rising above a predetermined level that could damage the system. How could this pressure build up when we have a pressure regulator or pump compensator to control pressure? It couldn’t, normally, but if one of the regulating units should fail, the relief valve is there to limit the maximum pressure in the system. Thus, the main-system relief valve must be large enough to relieve the pump’s full output back to the return manifold. This does not, however, unload the motor-driven pump because the pump must still maintain enough pressure to keep the relief valve open.

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Two-port relief valve Figure 2–37 shows a typical two-port relief valve. The pressure from the system enters the pressure port and pushes upward on the ball. If this pressure is high enough to overcome the force of the compression spring, the ball leaves its seat, and fluid from the system can go out the return port.

The adjusting screw controls the force of the spring and, in turn, requires more or less pressure to unseat the ball. The adjusting screw is usually covered by a protective cap. To increase the relieving pressure, turn the adjusting screw clockwise. The pressure at which the ball is slightly unseated and lets a few drops of fluid be relieved is called cracking pressure. If more fluid needs to be relieved, the ball rises higher and lets full-pump output be relieved. This is called the valve’s full-flow output. It is about 10 percent higher than cracking pressure because the spring must be compressed further. After the valve relieves fluid and the pressure drops, the ball must stop fluid flow around it. Since the fluid tends to go on flowing once it starts, the spring cannot force the ball on its seat until pressure drops to about 10 percent below cracking pressure. This pressure at which the return flow is cut off is called the reseating pressure. Most two-port relief valves are adjusted to the cracking pressure.

Thermal valves There are small thermal relief valves in isolated parts of all hydraulic systems. They are made

like those in the main system, only smaller. These valves are used where a check valve or selector valve stops pressure from relieving through the system relief valve. They are also used to relieve pressure that is produced when heat expands the fluid. The amount of fluid to be relieved is small; thus, a valve can be small and still do its job.

Relief valves need little maintenance, but sometimes they do fail. The trouble is usually internal-valve leakage. The hydraulic shop, if it has the equipment, repairs relief valves. Some relief valves are not repairable, so they must be replaced with new valves. When you get a replacement relief valve, the hydraulic shop must set its operating pressure before you install it.

Figure 2–37. Two-port relief valve.

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System control valves Every system must have some sort of controls. Hydraulics is no different. A selector valve is used to direct the flow of fluid in the system.

Figure 2–38. Three-way selector valve.

Figure 2–39. Four-way selector valve.

Three-way selector valve A poppet-type, three-way selector valve is shown in figure 2–38. This type valve operates a mechanism hydraulically in one direction. The poppet is a valving element that permits either a spring or the load on the mechanism to return the unit to its original position, thus achieving directional control.

Figure 2–38 (top drawing) shows hydraulic pressure forcing the piston outward against a load. The upper poppet (2) is unseated by the inside cam (4) and lets fluid flow from the pressure line (3) into the cylinder to actuate the piston supplying directional control. The lower poppet (1) is seated and seals the return manifold.

In the bottom illustration, the selector valve is in the opposite setting. The upper poppet (2) is seated and seals the fluid from the pressure manifold. The lower poppet (1) is unseated by the outside cam (5) so that the fluid forced from the cylinder can flow through the selector valve into the return manifold.

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Four-way selector valve Figure 2–39 shows a four-way selector valve that is also a poppet type. The cam-and-poppet arrangement of the four-way poppet-type selector valve is like the three-way valve except that it has two sets of poppets.

This selector moves a mechanism hydraulically in two directions:

• It lets the actuator extend or retract. • It provides a path to return the fluid to the manifold.

It works about the same as the three-way poppet type in that a cam turns to unseat the poppets and lets fluid flow to the actuator. In position 1, the left-hand upper poppet valve is closed and its lower valve is opened. You can see how this allows fluid to flow from the left side of the cylinder to the return manifold. At the same time, the right-hand upper poppet valve is opened and its lower valve is closed. This allows fluid to flow from the supply line to the right side of the cylinder, thus forcing the piston to the left. Position 2 shows the opposite arrangement of valves. You can see how supply side pressure is now routed to the left side of the cylinder. This forces the piston to the right and fluid on the right side of the cylinder to the return manifold.

Slide- or piston-type selector valves Use the slide- or piston-type selector valve where a short movement of the selector valve positioning mechanism is desired. One advantage of this valve is its metering ability. It does not have to be positioned all the way open unless such action is desired. Thus, a mechanism can be moved at varying speeds by controlling the fluid flow through the valve. The slide- or piston-type selector valve is often called a metering valve.

Figure 2–40, position 1, shows the metering valve in the neutral position. Fluid under pressure encircles the piston spool, but because the alternating ports are closed, the fluid is trapped. Position 2 shows the piston is moved to the right; this lets pressure from the pressure port enter the right alternating line. The other alternating port connects to the return manifold through the return port. Position 3 shows the piston is moved to the left and lets pressure enter the left alternating line and connect the other alternating line to the return port through the drilled passageway across the top.

Check valves Check valves in a hydraulic system trap fluid under pressure in some part of the system. They let fluid flow in one direction and block flow in the other. Two types of check valves with either a ball- or cone-valving element are shown in figure 2-41. The left one is the cone- or sleeve-type check valve.

Figure 2–40. Slide-type selector valve.

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Fluid pressure applied in the direction of the arrow pushes the cone off its seat. This lets fluid flow through the drilled openings of the cone. The right valve is a ball-type check valve. Applied pressure moves the ball off its seat, thus letting fluid flow through the valve. The internal spring holds the ball or cone on its seat when no fluid pressure is applied. The spring is very weak, and the pressure difference for full flow is only about 8 psi. When fluid tries to flow the wrong way, the system back pressure helps to hold the ball or cone on its seat and makes a tighter seal.

Orifice check valves An orifice check valve is a combination of an orifice and a check valve. It allows normal flow in one direction and limited flow in the other. Two orifice check valves are shown in figure 2–42.

The top illustration shows the cone-type orifice check valve. The cone moves off its seat when enough fluid pressure enters the inlet port, thus letting fluid flow through the valve. Flow the other way is limited by the size of the orifice in the center of the cone. The bottom illustration shows a ball-type orifice check valve. Flow through the valve from left to right is normal, whereas flow the other way is restricted by the orifice in the valve housing.

Snubbers In most hydraulic systems, system pressure is shown only on a pressure gauge. A snubber in the line to the gauge keeps pressure surges from giving erratic readings. The snubber is merely a tiny restrictor that is made by inserting a pin into a large hole. This restrictor dampens (smoothes) the changes in pressure to the gauge. When you install a snubber in a gauge line, you must remove the air from the line between the snubber and the gauge. If you do not do this, the gauge readings will not be right.

Line-disconnect valves These valves in hydraulic lines prevent fluid loss and save labor and time when you remove hydraulic units from the system. They are also in the pressure and suction lines just before and after the power pump. These valves let us connect hydraulic test stands into the main system. A quick disconnect is two interconnecting sections that are coupled together by a nut. Each section has a piston-and-poppet assembly that is spring-loaded closed when the unit is disconnected.

Figure 2–41. Check valves.

Figure 2–42. Orifice check valves.

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Figure 2–43. Line-disconnect valves.

The top drawing of figure 2–43 shows the valve in the line-disconnected position. The springs hold both poppets closed as shown, thus preventing any loss of fluid through the disconnected line. The bottom drawing shows the valve in the line-connected position. When the valve is being connected and the coupling nut draws the two sections together, the protrusion on one of the pistons pushes the other piston against its spring. This moves the poppet off its seat to permit flow through that section of the valve. As the nut is tightened, one poppet hits a stop; now the other poppet must move against its spring and off its seat. Now fluid can flow through the valve and on to the system.

Proper connection of the line-disconnect valve is very important. Hydraulic pumps have been ruined because the line disconnects were not properly connected to let hydraulic fluid flow through the system. Make sure they are properly connected!

Hydraulic filters Hydraulic and pneumatic filters are alike since both remove contaminants from the fluid and air. The difference between hydraulic and pneumatic filters is the type of filter element. Though the filter is not a complex component, it is very important because dirt, metal particles, or other foreign contaminants are the worst foes of any hydraulic system.

Micronic filters Micronic filters (fig. 2–44), which are used in most hydraulic systems, have four main parts:

1. Case, 2. Head, 3. Filter element 4. Relief valve.

Figure 2–44. Micronic filter.

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The head contains the in port, the out port, and the relief valve. The filter element is put into the case, and the case is screwed onto the head. Normal fluid flow is through the in port, around the elements outside, through the element to the inner chamber, and to the out port. The filter element is made of treated cel-lulose paper and is known as a micronic element. However, this element can be sintered bronze; woven wire; one-piece corrugated wire mesh; or corrugated, sintered stainless-steel mesh. A magnetic element can be used with any of these elements to make a dual-element filter. The above elements usually are rated between 10 and 25 microns (1 micron equals 0.00004 of an inch). Each filter has a relief valve that opens to let fluid bypass the element and keeps the system working if the element gets clogged.

In-line filter restrictors These are used in hydraulic subsystems. One looks like a large check valve with two wire-mesh finger screens on either side of the fixed orifice or orifice plate. If in-line filter restrictors do not have relief valves, the subsystem could fail if the filter gets clogged.

Pressure switches Hydraulic systems with electric pumps use pressure switches to maintain the right pressure. At the high-pressure limit, the pressure switch opens the circuit to stop the pump. As pressure drops to the lower limit, the pressure switch closes the circuit to start the pump again.

Figure 2–45 shows a Bourdon tube pressure switch that controls electric hydraulic pumps. The arc-shaped Bourdon tube is formed from spring-tempered steel alloy, which has been flattened to present an oval cross section. Shaping the tube in an arc provides more area on the outside circumference than on the inside. As a result, when pressure is applied, more of it acts outward than inward, and the tube tends to straighten. The flexible steel finger that is attached to the small end of the Bourdon tube moves outward as the tube starts to uncoil. This finger presses against the toggle plate until the right pressure is reached. Then it makes the toggle plate pivot to open and close the contact points rapidly. This breaks the circuit to the motor. When the pressure drops to a certain point, the tube begins to recoil, and the finger pushes on the opposite inner surface of the toggle plate. The toggle plate pivots rapidly, letting the contact points close and complete the circuit to the motor. The contact points open and close rapidly to reduce burning of the points.

Pressure gauges The pressure of liquids and gases, for our purposes, is measured in pounds per square inch. The Bourdon tube (fig. 2–46) is the basic mechanism in a pressure gauge because it gives direct pressure readings.

Figure 2–45. Bourdon tube pressure switch.

Figure 2–46. Pressure gauge components.

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While you study figure 2–46, note that one end of the tube is fastened to the intake socket and is stationary, but the other end can move and has an eyelet that is attached to the hinged linkage. When pressure is applied through the intake socket into the tube, the outward deflection of the tube pulls the hinged link in the direction of the arrow. This motion is transmitted to the sector gear, which is meshed with the pinion. The sector gear multiplies this motion and turns the pinion and attached pointer. The combined movements indicate a pressure increase. The hairspring takes up the backlash and play between the gauge components. Stop screws (not shown) stop the Bourdon tube’s motion when the pointer reaches full-scale deflection.

The Bourdon tube tends to straighten out when internal pressure is greater than atmospheric pressure. The higher the internal pressure, the more is the gauge deflection. Because of its spring-like qualities, the tube returns to its normal position when the pressure is released.

Self-Test Questions After you complete these questions, you may check your answers at the end of the unit.

215. Theory and operation of pneumatic and hydraulic systems 1. What happens to air when a force is applied to it?

2. What three factors determine the condition of gases?

3. What is Boyle’s law?

4. What happens to a confined gas if the temperature is increased?

5. What normally operates low-pressure pneumatic control valves?

6. What normally controls the high-pressure pneumatic systems?

7. What is the state of the poppet of a regulator when no pneumatic systems are operating?

8. What feature keeps pressure surges from damaging a regulator’s diaphragm?

9. In what quantity do we express area?

10. Describe Pascal’s law.

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11. How much force do you have if 100 psi is applied to a 50-square-inch piston?

12. How much pressure must be applied to a 5-square-inch piston to get 8,000 pounds of force?

216. Major hydraulic components 1. What moves the hydraulic fluid inside a gear-type pump?

2. What factor determines the output of a constant-volume piston-type pump?

3. What is one advantage of variable-volume pumps?

4. What happens to a hydraulic motor as pressure rises?

5. What occurs when the load on the motor varies with the electrical load on the generator?

6. What safety feature in a hydraulic motor prevents the lower piston from floating and chattering?

7. How do you increase the speed of a hydraulic motor?

8. What returns a single-acting actuator to its original position?

9. What does the double-acting, unbalanced-type actuator require to control the direction of travel?

217. Other hydraulic system components 1. What is the purpose of a relief valve in a hydraulic system?

2. Which selector valve hydraulically moves a mechanism in two directions?

3. What is the purpose of check valves?

4. Which type of valve permits normal flow in one direction and limited flow in the other?

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5. What function does the line-disconnect valve perform?

6. What are the four main parts of a micronic filter?

7. What device is used to maintain the correct pressure in hydraulic systems with electric pumps?

8. For our purposes, how is the pressure of liquids and gases measured?

2–4. Inspection System Inspections are visual examinations and manual checks to determine the condition of aerospace vehicles or components. An inspection can range from a casual walk around to a detailed inspection involving complete disassembly and the use of complex inspection aids.

An inspection system consists of several processes that include the following:

1. Reports made by technicians. 2. Regularly scheduled inspections.

An inspection system is designed to maintain an aerospace vehicle in the best possible condition. Thorough and repeated inspections must be considered the backbone of a good maintenance program. Irregular and haphazard inspections will invariably result in gradual and certain deterioration. The time that must eventually be spent in repairing often totals far more than any time saved in hurrying through routine inspections and maintenance.

218. Quality control It has been proven that regularly scheduled inspections and preventive maintenance assures guaranteed reduction in defects and system maintainability. Operating failures and malfunctions of equipment are appreciably reduced if excessive wear or minor defects are detected and corrected early. The importance of inspections and the proper use of records concerning these inspections can’t be overemphasized.

The time intervals for the inspection periods vary with the models involved and the types of operations being conducted.

Inspection intervals NOTE: The manufacturer’s instructions should be consulted when establishing inspection intervals.

Generally, inspection intervals are established in two ways. They are as follows:

1. Calendar inspection system. 2. Inspection system based on system usage.

Calendar inspection system Under the calendar inspection system, the appropriate inspection is performed on the expiration of a specified number of calendar weeks. The calendar inspection system is an efficient system from a maintenance-management standpoint. Scheduled replacement of components with stated hourly operating limitations is normally accomplished during the calendar inspection falling nearest the hourly limitation.

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System-usage inspection This inspection is conducted when a specified number of hours are accumulated. Components with stated hourly operating limitations are normally replaced during the inspection that falls nearest the hourly limitation.

Quality control Quality control is a system for ensuring the maintenance of proper standards in manufactured goods, especially by periodic, random inspection of the product. Quality control is a process of measuring performance, comparing it to the standard, determining any differences, and taking corrective actions when necessary. Seven processes are used to accomplish this.

Check sheet The function of a check sheet is to present information in an efficient, graphical format. This may be accomplished with a simple listing of items. However, the utility of the check sheet may be significantly enhanced, in some instances, by incorporating into the form a depiction of the system under analysis.

Pareto chart Pareto charts are extremely useful because they can be used to identify those factors that have the greatest cumulative effect on the system, and thus screen out the less significant factors in an analysis. Ideally, this allows the user to focus attention on a few important factors in a process.

Pareto charts are created by plotting the cumulative frequencies of the relative frequency data (event-count data) in descending order. When this is done, the most essential factors for the analysis are graphically apparent and in an orderly format.

Flowchart Flowcharts are pictorial representations of a process. By breaking the process down into its constituent steps, flowcharts can be useful in identifying where errors are likely to be found in the system.

Cause and Effect Diagram This diagram, also called an Ishikawa diagram (or fishbone diagram), is used to associate multiple possible causes with a single effect. Thus, given a particular effect, the diagram is constructed to identify and organize possible causes for it.

The primary branch represents the effect (the quality characteristic that is intended to be improved and controlled) and is typically labeled on the right side of the diagram. Each major branch of the diagram corresponds to a major cause (or class of causes) that directly relates to the effect. Minor branches correspond to more detailed causal factors. This type of diagram is useful in any analysis, as it illustrates the relationship between cause and effect in a rational manner.

Histogram Histograms provide a simple, graphical view of accumulated data, including its dispersion and central tendency. In addition to the ease with which they can be constructed, histograms provide the easiest way to evaluate the distribution of data.

Scatter Diagram Scatter diagrams are graphical tools that attempt to depict the influence that one variable has on another. A common diagram of this type usually displays points representing the observed value of one variable corresponding to the value of another variable.

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Control Chart The control chart is the fundamental tool of statistical process control, as it indicates the range of variability that is built into a system (known as common-cause variation). Thus, it helps determine whether a process is operating consistently or if a special cause has occurred to change the process mean or variance.

The bounds of the control chart are marked by upper and lower control limits that are calculated by applying statistical formulas to data from the process. Data points that fall outside these bounds represent variations due to special causes, which can typically be found and eliminated. On the other hand, improvements in common-cause variation require fundamental changes in the process.

Summary The tools previously listed are ideally utilized in a particular methodology, which typically involves either reducing the process variability or identifying specific problems in the process. However, other methodologies may need to be developed to allow for sufficient customization to a certain specific process. In any case, the tools should be utilized to ensure that all attempts at process improvement include:

• Discovery. • Analysis. • Improvement. • Monitoring. • Implementation. • Verification.

Furthermore, it is important to note that the mere use of the quality control tools does not necessarily constitute a quality program. Thus, to achieve lasting improvements in quality, it is essential to establish a system that will continuously promote quality in all aspects of its operation.

219. Plate inspections All components need to be produced to exacting dimensions. Sometimes it’s necessary to check the production measurements on parts and components to ensure they are being produced properly. Technicians can use several methods to check the components, but the one we will discuss is covered below.

Surface-plate inspection Virtually every production environment will contain at least one surface plate. A surface plate––basically a large granite table that’s both flat and level to a predetermined amount––provides a base or datum of measurement for different part types. Surface plates, gauge pins, and ring gauges are all examples of simulated datums. Matched V-blocks are accessories that can be used to contact multiple datum diameters. Primary datum surfaces are contacted by mounting them on the plate because the three or more highest points are contacted. The primary measurement made on a surface plate is height. If the plate is not flat, the datum for part measurement will be incorrect, and all of the parts will be measured improperly. This is a precision inspection instrument.

Precision is the ability to produce the same value or result, given the same input conditions and operating in the same environment. While working on the table, technicians use gauge block sets to assist in measuring. A gauge block set consists of a range of varying size blocks, along with two wear blocks. In use, the blocks are removed from the set, cleaned of their protective coating (petroleum jelly or oil), and wrung together to form a stack of the required dimension with the minimum number of blocks. Comparing the height of the stack to the height of the part is called a transfer measurement. The wear pieces are included at each end of the stack whenever possible as they provide protection

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against damage to the lapped faces of the main pieces. After use, the blocks are reoiled or greased to protect their faces from corrosion.

Self-Test Questions After you complete these questions, you may check your answers at the end of the unit.

218. Quality control 1. What’s the purpose of an inspection system?

2. What are the two types of inspection systems?

3. What are the seven processes used in quality control?

219. Plate inspections 1. What does a surface plate provide?

2. What are matched V-blocks used for during surface plate inspections?

3. What is a gauge block set?

Answers to Self-Test Questions

211 1. A drawing that describes a single part. 2. It shows only a part of the object but in greater detail and to a larger scale than the principal view. 3. Illustrates particular parts of an object.

212 1. A drawing number to identify the print for filing purposes and to prevent confusing it with any other print;

the name of the part or assembly; the scale to which it’s drawn; the date; the name of the firm; and the name of the draftsman, the checker, and the person approving the drawing.

2. Identifying standard drawing sizes. 3. The difference between extreme permissible dimensions in which the part or component will still be

acceptable.

213 1. Only for shear load applications. 2. AN corrosion resistant or standard steel bolt. 3. Structural screws, machine screws, and self-tapping screws. 4. A plain washer. 5. Only once.

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6. Non-self-locking and self-locking. 7. In places where the temperature is higher than 250° F. 8. Edge distance, clearance, and load factor. 9. The markings of the bolt head, the Maintenance Instruction Manual, and the Illustrated Parts Breakdown. 10. By the kind of material of which they’re made, their head type, size of shank, and their temper condition. 11. Two dissimilar metals are brought in contact with each other and in the presence of moisture cause an

electrical current to flow between them and chemical by-products to be formed, resulting in the deterioration of one of the metals.

12. Where access to both sides of a riveted structure or structural part is impossible, or where limited space won’t permit the use of a bucking bar. Also, in the attachment of many nonstructural parts.

214 1. By shape, type of blade, and blade length. 2. At least 75 percent of the slot. 3. Duckbill pliers. 4. On the side of the handle attached to the fixed jaw of the wrench. 5. The deflecting beam;, the rigid case, dial indicating; and the impulse feel, micrometer adjustable type. 6. Because minute fractures often occur along the cut, which requires hand filing down to the layout line. 7. With the teeth pointing forward, away from the handle. 8. A triangular and three-square file. 9. 100°. 10. A thickness gauge. 11. A combination set.

215 1. It acts like a spring: it yields but pushes back. 2. Pressure, temperature, and volume. 3. The volume of an enclosed dry gas varies inversely with its pressure, provided the temperature remains

constant. 4. The gas expands. 5. Solenoids and electric motors. 6. Electrically controlled, pneumatically actuated control valves. 7. The poppet is closed. 8. A small bleed hole. 9. Square inches. 10. When a force is exerted on a confined fluid, the pressure is transmitted equally and undiminished in all

directions. 11. 5,000 pounds of force. 12. 1,600 pounds per square inch (psi).

216 1. Rotation of the two gears. 2. The angle between the drive shaft and the cylinder block. 3. Variable-volume pumps don’t need pressure regulators or unloading valves. 4. Motor speed increases. 5. The motor must change its torque to keep its normal operation speed. 6. A back-pressure valve in the lower piston’s discharge line. 7. With an adjustment screw that is located on the back of the governor. 8. A spring.

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9. A selector valve.

217 1. To prevent the pressure from building high enough to damage the system. 2. Four-way selector valve. 3. To trap system fluid under pressure in some part of the system and let fluid flow in one direction only. 4. An orifice check valve. 5. Prevents fluid loss and saves labor and time when you remove hydraulic units from the system. 6. The case, head, filter element, and relief valve. 7. Pressure switch. 8. Pounds per square inch.

218 1. To maintain an aerospace vehicle or component in the best possible condition. 2. The hourly and calendar inspection systems. 3. Check Sheet, Pareto Chart, Flowchart, Cause and Effect Diagram, Histogram, Scatter Diagram, and Control

Chart.

219 1. A base or datum of measurement for different part types. 2. Used to contact multiple datum diameters. 3. Range of varying size blocks, along with two wear blocks.

Do the unit review exercises before going to the next unit.

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Unit Review Exercises

Note to Student: Consider all choices carefully, select the best answer to each question, and circle the corresponding letter.

49. (211) All drawings of objects are composed of one or a combination of which of the following shapes? a. Triangle, circle, cube, cylinder, cone and sphere. b. Triangle, circle, polygon, cylinder, cone and sphere. c. Octagon, circle, cube, cylinder, cone and sphere. d. Octagon, circle, polygon, cylinder, cone and sphere.

50. (211) Which drawing shows the final position of parts or an assembly of parts within a component? a. A detail drawing. b. A block drawing. c. An installation drawing. d. An orthographic projection.

51. (211) A three view orthographic projection most commonly consists of which views? a. Front, back and left side. b. Front, top and right side. c. Bottom, back and left side. d. Bottom, top and right side.

52. (211) Which drawings are most often used in illustrated parts manuals? a. Block drawings. b. Detail drawings. c. Sectional view drawings. d. Exploded view drawings.

53. (212) Which types of notes are typically located at the bottom of a drawing? a. Foot. b. Local. c. Special. d. General.

54. (212) What does it mean when a dimensional tolerance is given for a part? a. The tightest permissible fit for mating parts. b. The dimensional difference between two parts for a loose fit. c. The culmination of the extreme permissible dimensions of the part that can be accepted. d. The difference between the extreme permissible dimensions of the part that can be accepted.

55. (212) Which type of fit occurs when a shaft’s tolerance will always be slightly larger then the hole? a. Clearance. b. Impediment. c. Interference. d. Compression.

56. (212) Which of the following statements about diameter symbols are true? a. The symbol precedes all diameters on the drawing. b. The symbol is used when the tolerance zone is a cylinder. c. The symbol can be used when a datum target area is round. d. All the above.

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57. (213) Aerospace bolts with a cross or asterisk marked on the bolthead are a. standard steel bolts. b. close tolerance bolts. c. made of aluminum alloy. d. made of nickel cadmium alloy.

58. (213) How are alloy 2117 rivets heat treated? a. To a temperature of 910 to 930 degrees. b. To a temperature of 1010 to 1030 degrees. c. By the manufacturer but requires heat treatment before being driven. d. By the manufacturer and do not require heat treatment before being driven.

59. (214) The flutes of a twist drill a. admits cutting fluid. b. forms the cutting edge. c. allows chips to escape. d. All the above.

60. (214) Which tool is used to check the setting of an inside caliper when an accurate reading is required? a. Dividers. b. Dial indicators. c. Outside caliper. d. Hermaphrodite caliper.

61. (214) On a standard 1 inch micrometer, one complete revolution of the thimble increases or decreases the distance between the measuring faces by a. 0.001 in. b. 0.010 in. c. 0.020 in. d. 0.025 in.

62. (215) What statement best describes the similarities between hydraulic and pneumatic systems? a. The basic components of the system are essentially the same. b. Both systems depend on internal lubrication by the fluid. c. Both systems utilize return lines for fluid delivery. d. The basic components of the system are identical and interchangeable.

63. (215) Pneumatic systems utilize a. return lines. b. relief valves. c. snubber valves. d. diluter valves.

64. (215) Hydraulics studies the physical behavior of which state of matter? a. Gases. b. Solids. c. Liquids. d. Plasma.

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65. (215) Pascal’s law states that a. when force is applied to a confined fluid the pressure is transmitted equally and undiminished in all directions. b. when force is applied to a confined gas the pressure is transmitted equally and undiminished in all directions. c. when force is applied to a confined plasma the pressure is transmitted equally and undiminished in all directions. d. when force is applied to a confined solid the pressure is transmitted equally and undiminished in all directions.

66. (216) When choosing a hydraulic liquid for a particular system which of the following properties are considered during selection? a. Viscosity. b. Flash point. c. Chemical stability. d. All of the above.

67. (216) Which statement best describes stroke when discussing hydraulic actuators? a. Amount of force applied to a piston. b. Amount of force applied to the spring. c. Distance a piston travels in a cylinder. d. Distance the spring travels in a cylinder.

68. (216) Which of the following do accumulators contain? a. Fixed piston. b. Fixed diaphragm. c. Pressure gas component. d. Uncompressed air chamber.

69. (217) Relief valves are used for which of the following functions? a. To prevent thermal expansion of the fluids. b. To maintain fluid flow below a predetermined rate. c. To maintain pressure above a predetermined level. d. To prevent pressure from rising above a predetermined level.

70. (217) Which component is used to direct fluid flow in a hydraulic system? a. Check valve. b. Relief valve. c. Selector valve. d. Diluter valves.

71. (217) A poppet is used as the valving element for which of the following valve applications? a. Flow control. b. Spring control. c. Pressure control. d. Directional control.

72. (217) Check valves contain what types of valving elements? a. Ball and cone. b. Ball and poppet. c. Sleeve and cone. d. Sleeve and poppet.

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73. (217) What type of check valves permits free flow of fluid in one direction and a limited flow of fluid in the other? a. Ball. b. Orifice. c. Vertical. d. Snubbers.

74. (217) Besides the case what are the other main parts of a Micronic filter? a. head, filter element, and relief valve. b. head, filter element, and check valve. c. adjusting screw, relief valve, and check valve. d. adjusting screw, pressure port, and spring.

75. (218) What statement best describes quality control? a. Measures process performance and take corrective action if necessary. b. Measures individual performance and take corrective action if necessary. c. Compare it to goals and determine differences. d. Compare it to substandard and search for areas for improvement.

76. (219) Surface plates, gauge pins, ring gauges, are all examples of what kind of datums? a. Targets. b. Primary. c. Physical. d. Simulated.

77. (219) What is the measurement called when using a gauge block stack on the surface plate to compare the height of a part to the height of the stack? a. Transfer. b. Inferential. c. Differential. d. Circumferential

Please read the unit menu for unit 3 and continue