Bearings

Faculty of Engineering, University of Kragujevac

Seminar paper work

- Bearings-

- Engleski jezik 2-

Student:

Professor:

Gari Slobodan 324/2013 Stefanovi SandraTable of Contents3.1. Introduction

3.2. Plain bearings

5.3. Antifriction bearings

5.4. Bearings loads

6.5. Ball bearings

6. Roller bearings7.

8.7. Bearing selection

8.8. Bearing classifications

9.9. Shaft and housing fits

10.10. Symbols

11.BIBLIOGRAPHY

1. INTRODUCTION

Bearings permit smooth, low-friction movement between two surfaces. The movement can be either rotary (a shaft rotating within a mount) or linear (one surface moving along another). Bearings can employ either a sliding or a rolling action. Bearings based on rolling action are called rolling-element bearings. Those based on sliding action are called plain bearings.2. PLAIN BEARINGS

A plain bearing is any bearing that works by sliding action, with or without lubricant. This group encompasses essentially all types other than rolling-element bearings.

Plain bearings are often referred to as either sleeve bearings or thrust bearings, terms that designate whether the bearing is loaded radially or axially.

Lubrication is critical to the operation of plain bearings, so their application and function is also often referred to according to the type of lubrication principle used. Thus, terms such as hydrodynamic, fluid-film, hydrostatic, boundary-lubricated, and self-lubricated are designations for particular types of plain bearings.

Mostly bearings are oil-lubricated. The designs shown in Fig. 1 illustrate simple, effective arrangements for providing supplementary lubrication.

Fig. 1 Common methods of lubricating plain bearings.

Journal or Sleeve Bearings

These are cylindrical or ring-shaped bearings designed to carry radial loads. See Fig. 2. The terms sleeve and journal are used more or less synonymously since sleeve refers to the general configuration while journal pertains to any portion

of a shaft supported by a bearing. In another sense, however, the term journal may be reserved for two-piece bearings used to support the journals of an engine crankshaft.

The simplest and most widely used types of sleeve bearings are cast-bronze and porous-bronze (powdered-metal) cylindrical bearings. Cast-bronze bearings are oil-, or grease-lubricated. Porous bearings are impregnated with oil and often have an oil reservoir in the housing.

Plastic bearings are being used increasingly in place of metal. Originally, plastic was used only in small, lightly loaded bearings where cost saving were the primary objective. More recently, plastics are being used because of functional advantages, including resistance to abrasion, and they are being made in large sizes.

Thrust Bearings

This type of bearing differs from a sleeve bearing in that loads are supported axially rather than radially. See Fig. 3. Thin, disk like thrust bearings are called thrust washers.

Bearing Materials

Babbitts Tin and lead-base babbitts are among the most widely used bearing materials. They have an ability to embed dirt and have excellent compatibility properties under boundary-lubrication conditions.

In bushings for small motors and in automotive engine bearings, babbitt is generally used as a thin coating over a steel strip. For larger bearings in heavy-duty equipment, thick babbitt is cast on a rigid backing of steel or cast iron.

Bronzes and Copper Alloys

Dozens of copper alloys are available as bearing materials. Most of these can be grouped into four classes: copper-lead, lead-bronze, tin-bronze, and aluminum-bronze.

Aluminum

Aluminum bearing alloys have high wear resistance, load-carrying capacity, fatigue strength, and thermal conductivity; excellent corrosion resistance; and low cost. They are used extensively in connecting rods and main bearings in internal-combustion engines; in hydraulic gear pumps, in oil-well pumping equipment, in roll-neck bearings in steel mills; and in reciprocating compressors and aircraft equipment.

Porous Metals

Sintered-metal self-lubricating bearings, often called powdered-metal bearings, are simple and low in cost. They are widely used in home appliances, small motors, machine tools, business machines, and farm and construction equipment.

Common methods used when supplementary lubrication for oil-impregnated bearings is needed are shown in Fig. 4.

Plastics

Many bearings and bushings are being produced in a large variety of plastic materials. Many require no lubrication, and the high strength of modern plastics lends to a variety of applications.

3. Antifriction Bearings

Ball, roller, and needle bearings are classified as antifriction bearings since friction has been reduced to a minimum. They may be divided into two main groups: radial bearings and thrust bearings. Except for special designs, ball and roller bearings consist of two rings, a set of rolling elements, and a cage. The cage separates the rolling elements and spaces them evenly around the periphery (circumference of the circle). The nomenclature of an antifriction bearing is given in Fig. 5.4. BEARING LOADS

Radial Load

Loads acting perpendicular to the axis of the bearing are called radial loads. See Fig. 6(A). Although radial bearings are designed primarily for straight radial service, they will withstand considerable thrust loads when deep ball tracks in the raceway are used.

Thrust Load

Loads applied parallel to the axis of the bearing are called thrust loads. Thrust bearings are not designed to carry radial loads. See Fig. 6(B)

Combination Radial and Thrust Loads When loads are exerted both parallel and perpendicular to the axis of the bearings, a combination radial and thrust bearing is used. See Fig. 6(C). The load ratings listed in the manufacturers catalogs for this type of bearing are for either pure thrust loads or a combination of both radial and thrust loads.

5. BALL BEARINGS

Ball bearings fall roughly into three classes: radial, thrust, and angular-contact. Angular-contact bearings are used for combined radial and thrust loads and where precise shaft location is needed. Uses of the other two types are described by their names: radial bearings for radial loads and thrust bearings for thrust loads. See Fig. 7.

Radial Bearings Deep-groove bearings are the most widely used ball bearings. In addition to radial loads, they can carry substantial thrust loads at high speeds, in either direction. They require careful alignment between shaft and housing.

Self-aligning bearings come in two types: internal and external. In internal bearings, the outer-ring ball groove is ground as a spherical surface.

Externally self-aligning bearings have a spherical surface on the outside of the outer ring, which matches a concave spherical housing.

Double-row, deep-groove bearings embody the same principle of design as single-row bearings. Double-row bearings can be used where high radial and thrust rigidity is needed and space is limited. They are about 60 to 80 percent wider than comparable single-row, deep-groove bearings, and they have about 50 percent more radial capacity.

Angular-contact thrust bearings can support a heavy thrust load in one direction combined with a moderate radial load. High shoulders on the inner and outer rings provide steep contact angles for high thrust capacity and axial rigidity.

Thrust Bearings

In a sense, thrust bearings can be considered to be angular-contact bearings. They support pure thrust loads at moderate speeds, but for practical purposes their radial load capacity is nil. Because they cannot support radial loads, ball thrust bearings must be used together with radial bearings.

Flat-race bearings consist of a pair of flat washers separated by the ball complement and a shaft-piloted retainer, so load capacity is limited. Contact stresses are high, and torque resistance is low.

One-directional, grooved-race bearings have grooved races very similar to those found in radial bearings.

Two-directional, groove-race bearings consist of two stationary races, one rotating race, and two ball complements.

6. ROLLER BEARINGS

The principal types of roller bearings are cylindrical, needle, tapered, and spherical. In general, they have higher load capacities than ball bearings of the same size and are widely used in heavy-duty, moderate-speed applications. However, except for cylindrical bearings, they have lower speed capabilities than ball bearings. See Fig. 8.

Cylindrical Bearings

Cylindrical roller bearings have high radial capacity and provide accurate guidance to the rollers. Their low friction permits operation at high speed, and thrust loads of some magnitude can be carried through the flange-roller end contacts.

Needle BearingsNeedle bearings are roller bearings with rollers that have high length-to-diameter ratios. Compared with other roller bearings, needle bearings have much smaller rollers for a given bore size.

Loose-needle bearings are simply a full complement of needles in the annular space between two hardened machine components, which form the bearing raceways. They provide an effective and inexpensive bearing assembly with moderate speed capability, but they are sensitive to misalignment.

Caged assemblies are simply a roller complement with a retainer, placed between two hardened machine elements that act as raceways. Their speed capability is about 3 times higher than that of loose-needle bearings, but the smaller complement of needles reduces load capacity for the caged assemblies.

Thrust bearings are caged bearings with rollers assembled like the spokes of a wheel in a wafer like retainer.

Tapered Bearings

Tapered roller bearings are widely used in roll-neck applications in rolling mills, transmissions, gear reducers, geared shafting, steering mechanisms, and machine-tool spindles. Where speeds are low, grease lubrication suffices, but high speeds demand oil lubrication, and very high speeds demand special lubricating arrangements.

Spherical Bearings

Spherical roller bearings offer an unequaled combination of high load capacity, high tolerance to shock loads, and self-aligning ability, but they are speed-limited.

Single-row bearings are the most widely used tapered roller bearings. They have a high radial capacity and a thrust capacity about 60 percent of radial capacity.

Two-row bearings can replace two single-row bearings mounted back-to-back or face-to-face when the required capacity exceeds that of a single-row bearing.

7. BEARING SELECTION

Machine designers have a large variety of bearing types and sizes from which to choose. Each of these types has characteristics, which make it best for a certain application. Although selection may sometimes present a complex problem requiring considerable experience, the following considerations are listed to serve as a general guide for conventional applications.

1. Generally, ball bearings are the less expensive choice in the smaller sizes with lighter loads, while roller bearings are less expensive for the larger sizes with heavier loads.

2. Roller bearings are more satisfactory under shock or impact loading than ball bearings.

3. If there is misalignment between housing and shaft, either a self-aligning ball or spherical roller bearing should be used.

4. Ball thrust bearings should be subjected to pure thrust loads only. At high speeds, a deep-groove or angular-contact ball bearing will usually be a better choice even for pure thrust loads.

5. Self-aligning ball bearings and cylindrical roller bearings have very low friction coefficients.

6. Deep-groove ball bearings are available with seals built into the bearings so that the bearing can be pre-lubricated and thus operate for long periods without attention.8. BEARING CLASSIFICATIONS

Because of standardization of boundary dimensions, it is possible to replace a bearing by another bearing produced by a different manufacturer without any modification to the existing assembly.

Ball and roller bearings are classified into various series:

rigid ball journals

self-aligning ball journals

rigid roller journals, etc.

Each series is subdivided into typesextra light, light, medium, and heavyto meet varying load requirements. Each type is manufactured to a range of standard sizes, which are usually represented by the diameter of the bore. Therefore, when a bearing is ordered, the series, type, and size are specified.

Figure 9 shows a range of bearings to a common bore at A and to a common outside diameter at B. A selection can therefore be made for a given shaft size or for a given housing diameter, and the series selected will depend on the load which is applied to the bearing.

9. SHAFT AND HOUSING FITS

If a ball or roller bearing is to function satisfactorily, both the fit between the inner ring and the shaft and the fit between the outer ring and the housing must be suitable for the application. The desired fits can be obtained by selecting the proper tolerances for the shaft diameter and the housing bore.

Seals for Grease Lubrication

In order that ball or roller bearings may operate properly, they must be protected against loss of lubricant and entrance of dirt and dust on the bearing surfaces.

In its simplest and least space-requiring form, this is accomplished in some types of bearings by the use of a thin steel shield on one or both sides of the bearing, fastened in a groove in the outer ring and reaching almost to the inner rings as illustrated in Fig. 10. All other types of bearings require a seal between the bearing housing and the shaft; the types and designs of which are shown in Fig. 11.

Seals for Oil Lubrication

With oil lubrication, the sealing devices have the double function of protecting the bearing against contamination and retaining the lubricant in the housing. Protection is obtained by means of friction seals or fingers, as when grease lubrication is used. The essential feature for retaining the oil is a groove in the rotating shaft, or a rotating ring or collar from whose edges the oil is thrown by centrifugal force. The oil-groove seal shown in Fig. 12A retains the oil effectively but should be used only in dry and dust-free places where there is little danger of contamination. Figure 12B shows examples of labyrinth seals, which retain the oil and protect against contamination.

10. BEARING SYMBOLS

Simplified Representation

The simplified representation (general symbol) of rolling bearings (see Fig. 13) should be used in all types of technical drawings, wherever it is not necessary to show the exact form or size of the rolling bearings or details of their inner design. Where it is desirable to show the functional principle of the set of rolling elements, symbols for the appropriate type of rolling element and raceway surface are added. See Fig. 13C.

Pictorial Representation

Pictorial representation of bearings, as shown in Fig. 14A, is used chiefly in catalogs and magazines. It is not recommended for production drawings produced using traditional pencil/board because of the extra drafting time required. However, using AutoCAD already drawn symbol for bearings could be inserted.

Schematic Representation

Designers and engineers frequently use schematic layouts in their initial design layout. The schematic diagrams of bearing types and their application are shown in Figs. 14C and 15.

BALL BEARINGSROLLER BEARINGSTHRUST BEARINGSNEEDLE BEARINGS

RADIAL DEEP-GROOVEANGULAR CONTACTRADIL DOUBLE ROWSELF ALIGNING DOUBLE ROWCYLINDRICALSPHERICAL SELF ALIGNINGTAPEREDBALLROLEERRADIALAXIAL

BIBLIOGRAPHY[1] http://en.wikipedia.org/wiki/Bearing_(mechanical)[2] http://www.msm.cam.ac.uk/phase-trans/2010/types/index.html[3] http://www.msm.cam.ac.uk/phase-trans/2011/Bearings/index.html[4] http://www.bearing-king.co.uk/how-to-measure-a-bearing.php[5] http://www.ntn.co.jp/english/products/pdf/ball/pdf/Bearing_en_all.pdfFig. 2 Journal or sleeve bearings.

Fig. 3 Thrust bearings.

Fig. 4 Supplementary lubrication for oil impregnated bearings.

Fig. 5 Antifriction-bearing nomenclature. (SKF Company)

Fig. 6 Types of bearing loads.

Fig. 7 Ball bearings. (SKF Company)

(D) NEEDLE

Fig. 8 Roller bearings.

Fig.9 Standard bearings sizes.

Fig. 11 Housing seals for grease lubrication.

Fig. 10 Bearing seals for grease lubrication.

Fig. 12 Housing seals for oil lubrication.

Fig. 13 Simplified representation of ball and roller bearings.

Fig. 14 Representation of bearings on drawings.

Fig. 15 schematic representation of bearings.

Kragujevac, january 2014

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