The Indicator Handbook 1000076023

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THE INDICATOR HANDBOOK.


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BY THE SAME AUTHOR.

THE SLIDE RULE:A PRACTICAL MANUAL FOR ENGINEERS.

Sixteenth Edition."Of all books on the slide rule, Mr Pickworth's is

the best. ... It is a book which every slide-rule usershould possess. " " The Engineer.

LOGARITHMS FOR BEGINNERS.Seventh Edition.

" An extremely useful and much-needed little work . . .giving a complete explanation of the theory and use oflogarithms, with numerous examples of a practical kind. "

"Mining Journal.

POWER COMPUTERFOR STEAM, GAS, AND OIL ENGINES.

"Accurate, expeditious, and thoroughly practical . . .We can confidently recommend it, and engineers willfind it a great boon in undertaking tests, etc. "-The Electrician.

C. N. PICKWORTH, WITHINGTGN, MANCHESTER.


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THE

INDICATOR HANDBOOK:A PRACTICAL MANUAL FOR ENGINEERS.

BY

XCHARLES Nf*PICKWORTH,///WHITWORTH SCHOLAR;

EDITOR OF "THE MECHANICAL WORLD";AUTHOR OF "THE SLIDE RULE," "LOGARITHMS FOR BEGINNERS." ETC.

COMPLETE EDITION

MANCHESTER:EMMOTT " Co.. LIMITED

65 KING STREET.

LONDON:EMMOTT A- Co., LIMITED. PITMAN A SONS, LIMITED.20 BEDFORD STREET, W.C. 1 AMEN CORNER, E.C. 4.


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635058

ALL RIGHTS RESERVED.


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PREFACE.

T)UBLISHED originally in two parts, dealing respectively with"The Indicator: Its construction and Application" and

" The Indicator : Its Analysis and Calculation," this work has metwith a considerable measure of success.

Both parts being out of print, I have taken the opportunity ofre-issuing the complete treatise in one volume, and trust it will enjoya continuance of the favour extended to the work in its originalform.

CHARLES N. PICKWOBTH.

WITHINGTON, MANCHESTER.March 1920.


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THE INDICATOR HANDBOOK.

PART I.

The Indicator: ItsConstruction and Application.

CHAPTER LPRELIMINARY CONSIDERATIONS.

THEndicator is an instrument primarily designed togive a graphic record of the manner in which thepressure of steam in an engine cylinder variesthroughout the stroke of the piston. The instrument is,in

fact, simply a "pressure recorder," and whether applied tothe cylinder of a steam, gas, oil or hot-air engine, or to anair or water pump, its sole function is to afford an indicationof the manner in which variations of pressure occur in thevessel to which it is attached. Considered more especiallyin connection with the steam engine, it will be seen that inorder to record the pressure variation during the stroke,means must be provided (1)to measure the pressure atevery instant throughout the stroke, and (2) to indicateclearly the exact position occupied by the piston at eachinstant of pressure measurement.

The pressure-measuring part of the apparatus consistsessentially of a small cylinder connected to the enginecylinder, in which a light piston is arranged to move freely.

9


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10 THE INDICATOR HANDBOOK.

By means of a piston rod passing through the upper coverof the cylinder, and suitable mechanism, any movement ofthe piston is transmitted to a pencil or marking point, thearrangement generally being such that the movement ofthe pencil is a magnified copy of that of the piston. Inthe upper part of the cylinder is a spiral spring, which isso arranged as to resist any movement of the piston ineither direction from its neutral position.

Upon the lower part of the cylinder being placed incommunication with the engine cylinder, the pressureexisting in the latter acts upon the under side of theindicator piston, while the upper side of this piston issubjectedo atmospheric pressure only. It is clear thatthe pressure which preponderates will cause a correspond-ng

movement of the piston to take place, the latter rising,if the steam pressure is the greater, until the increasingresistance of the spring due to its compression balancesthe greater pressure of the steam. On the other hand,the piston will fall if the atmospheric pressure is in excess,until the increased tension of the spring in elongating againproduces equilibrium. As the compression or extensionof a spiral spring is proportional to the pressure causing thechange in length, it follows that " knowing the pressurerequired to compress or extend the spring so as to cause thepencil to move through one inch vertically the pressureupon the piston at any instant will be indicated by thecorresponding height of the pencil above the neutralposition which it occupies when both sides of the pistonare exposed to atmospheric pressure only.

Thus far, only the pressure-measuring portion of theinstrument has been considered ; it remains to indicatehow the positions of the engine piston corresponding tothe successive positions of the pencil are recorded. Itwill be evident, in the first place, that if the pencil bearranged to move in contact with a suitably-mounted sheetof paper, the varying positions of the pencil will cause avertical line to be traced, the maximum height of whichwill represent the maximum pressure which has occurredin the cylinder during the stroke. But no indication ofthe manner in which the pressure fluctuateshroughoutthe stroke is thus afforded. If, however, the mounted


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PRELIMINAR Y CONSIDERA TIO\* " 1 1

sheet of paper is moved in a direction at right angles tothat in which the pencil moves " and in such a mannerthat the motion is a reduced copy of that of the enginepiston," the pencil will no longer trace and retrace avertical line, but will describe a curved outline representingthe resultant motion due to the simultaneous movements ofthe pencil and paper. On the other hand, when each sideof the indicator piston is subjectedo atmospheric pressureonly, the pencil will remain stationary in its neutralposition, while the movement of the paper only, will causea horizontal line to be traced, which, from the conditionsunder which it is drawn, is termed the atmotpheric line.From this it will appear that each point in the curvedfigure, or indicator diagram, as it is called, represents byits vertical height above the atmospheric line the pressureacting upon the engine piston at that particular point inits stroke.

In nearly all modern instruments the paper upon whichthe diagram is traced is mounted upon a cylindrical drumor barrel, capable of turning upon a fixed stud as an axis.Motion is given to this drum by a cord passing around agroove at its base and connected to the engine in such amanner as to give to the paper a movement exactly similarto that of the engine piston, but on a much reduced scale.Since the cord gives motion to the drum in one directiononly, a spring placed within the drum is employed to effectthe movement during the return stroke, and this maintainssufficient tension on the driving cord to keep the drumconstantly under its control.Assuming the indicator to be attached to one end of thecylinder of an engine which is working uniformly, and thatthe correct movement is given to the paper-carrying drum,the figure traced by the pencil supplies a record of thebehaviour of the steam while acting upon one side of thepiston during a complete double stroke of the engine. Thusin the hypothetical case assumed in Fig. 1, the vertical lineor ordinate 1 2 at the commencement of the strokeindicates a practically instantaneous attainment of theinitialpressure, the amount of which is represented by thevertical height above the atmospheric line A L. Then thehorizontal motion of the paper caused the line 2 3 to be


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traced, indicating that the pressure remained uniform untilthe latter point was reached. At 3 the steam supply wascut off,and the consequent fallof pressure, combined withthe continued movement of the paper, produced the curvedline 3 4. Release occurred at 4, the vertical line 4 5representing an instantaneous fallof pressure and markingthe termination of the forward stroke. The motion of thepaper now being reversed, the line 5 1 was drawn, repre-enting

by its height above the atmospheric line A L, theamount of pressure (supposedonstant in this case)whichopposed the movement of the engine piston during itsreturn stroke. At 1 the cycle of operations is again commenced by the fresh admission of steam to the cylinder.

FIO. i.

It will be shown, subsequently, that from the area of theindicator diagram, the power developed by the engine maybe ascertained, this application of the instrument consti-uting

one of its principal uses. Another and perhapsmore important function of the indicator is to show thedistribution of steam in the cylinder, by indicating themanner in which the admission, cut-off, expansion, release,and compression of the steam takes place, thereby acting asan invaluable guide to the correct adjustmentof the valvegear. The indicator diagram also shows whether the portsand passages for the admission and exhaust of the steamare of sufficient size and suitably arranged. Cylindercondensation and re-evaporation, leakage of steam past thepiston or valves, and (inconjunctionwith feed watermeasurements) the weight of the steam passing throughthe engine in a given time, are among other important


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PRELIMINARY CONSIDERA TlOXi,. 13

items of information which may be deduced by the intelli-entstudy of the indicator diagram.

Nor are the useful applications of the indicator restrictedto the engine cylinder. Applied to the steam chest, or tothe steam and exhaust pipes, the instrument is frequentlyfound of great value in determining the resistance offeredby the pipes and ports to the passage of the steam. Itsapplication to the receivers of compound engines should bementioned ; while in condensing engines, the application ofthe indicator to the condenser allows a direct comparison tobe made between the amount of back pressure shown bythe diagram and the pressure in the condenser. Again, byattaching the indicator to the air pump, the power requiredto operate this apparatus may be determined ; while muchuseful information may be obtained by applying theinstrument to air compressors, blowing engines, refrigeratingplant, etc.

In connection with pumping plant, the indicator furnishesinformation in regard to the resistance offered by pumpvalves, the utilityof air vessels in reducing pressure fluctua-ion

in pipe lines,etc. Applied to machinery operated byhydraulic or pneumatic power, many useful data are obtainedas to the power required for various operations. In factthere is scarcely any limitation to its useful applicationwherever fluid pressure variations are to be measured andrecorded.


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CHAPTER II.

THE CONSTRUCTION OF TEE INDICATOR" STEAMENGINE INDICA TORS.

INommon with many of the most important adjunctsfthe steam engine, the indicator was invented by JamesWatt. Few particulars of the earliestform of apparatusare available, doubtless owing in a great measure to the factthat Watt kept his valuable invention a secret for manyyears.

There is, however, reason for believing that in itsoriginal form the instrument consisted merely of a cylinderabout 6in. in length and lin. in diameter, which was fittedwith a piston. A rod attached to the latter,and passingthrough the upper end of the cylinder, was provided witha pointer ; while between the upper face of the piston andthe cylinder cover was placed a long spiral spring, the endsof which were attached to the piston and cover respectively.The upper side of the piston was open to the atmosphere,while the lower part of the cylinder communicated with theengine cylinder. With the low speeds usual at this period,some idea of the action of the steam could be obtained byclosely watching the movement of the pointer ; but thiscrude device was useless as a power-measuring and recordingapparatus, and it was not until Watt added the slidingpaper-carrying board " -representative of the motion of thepiston " that the principle of the present form of indicatorwas established. This sliding board was arranged to movein a direction at right angles to that of the piston, and wasactuated by a cord attached to a suitable point in somereciprocating part of the engine, the return movement beingeffected by a weight attached to another cord acting in theopposite direction to the driving cord.

Early Instruments. " Watt's indicator rendered very14


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CONSTRUCTION OF THE INDICATOR. 15

great service in the early days of steam engineering, whenlow speeds and low steam pressures were employed. Butwith increase of engine speed the necessity for modifyingthe apparatus soon became apparent. It was found thatthe reciprocating board which carried the paper, and thecords and counterweights used to actuate it,rendered theinstrument totally unsuitable for indicating any but veryslow-moving engines.With a view to remove this defect, McXaught introducedthe indicator bearing his name, and the improved instrumentenjoyedconsiderable favour for many years. In this indi-ator

a hollow vertical cylinder, capable of rotating aboutits axis, was substituted for the sliding board previouslyemployed, and the paper, wrapped around this cylinder, wassecured thereto by means of a clip. The driving cordpassed round a groove at the foot of the paper barrel, andwas secured to the latter. A guide pulley, by means ofwhich the driving cord could be led into the groove in thebarrel, was also added by McNaught.It will be readily understood that upon the cord beingpulled, the paper-carrying drum will rotate, and a diagramwill be traced, as previously described, provided means areemployed to ensure the prompt return movement of thedrum. This was conveniently effected by means of a flatcoiled spring contained within the base of the drum, which,by keeping the cord constantly in tension, formed a veryefficient substitute for the balance weight employed byWatt. The extent of the motion of the drum was limitedby means of stops to about three-fourths or seven-eighths ofa complete revolution.

The arrangement of steam cylinder, spring, etc., wassubstantially the same as that adopted by Watt, exceptthat the pencil was carried by a short arm secured to thepiston rod near the lower end of the sprint;,and projectingthrough a slot in the upper part of the cylinder.Slightly modified forms of the McXaught instrument weresubsequently made by Maudslay and others in this country,-and by Stillman in America, but the only one which cameat all into general use was the indicator made by Messrs.Hopkinson, of Huddersfield. In the original form of thisinstrument, the paper-carrying drum surrounded the lower


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1" THE INDICATOR HANDBOOK.

part of the steam cylinder, the axes of the two coinciding.As in the McNaught indicator, a short arm was fixed to thepiston rod at the lower end of the spring, and from thisdepended a long pencil rod, the lower end of which carriedthe pencil. In a later form of this instrument, the moreusual drum arrangement was reverted to, the pencil beingcarried by a horizontal arm mounted upon the top of thepiston rod, and capable of swivelling thereon in order to.adjustthe position of the pencil with regard to the paper.In order to satisfactorilyfulfilits purpose, the indicatormust produce a diagram sufficiently large to enable theaction of the steam to be shown distinctly, and experiencesoon led to the recognition of the fact that a diagram offrom 2 to 3in. in height was necessary to ensure successfulresults. It follows that in all direct-acting indicators "wherein the piston and pencil are rigidly connected " aspiral spring of considerable length must be, employed,which, upon the admission of high-pressure steam to theindicator cylinder, will be thrown into vibration, the extentof the resulting oscillations of the pencil being greatlyintensified by the momentum of the reciprocating parts.The effect produced upon the diagram by this vibratoryaction depends greatly upon the speed of the engine. Withslow-running engines the oscillations of the pencil mayalmost entirely cease before the paper has moved perceptibly ;but with higher speeds the oscillations distort the diagramvery considerably, the pencil tracing a series of undulatingcurves, which, being in no way due to the action of thesteam within the cylinder, seriously detracts from thevalue of the indicator diagram.

There are other disadvantages attending this arrangementof indicator. The compression of the spring is frequentlyaccompanied by a bending action which has the effect oftilting the piston and producing an undue amount offriction. Further, the extensive motion of the pistonoperates against a simultaneous indication being obtainedof the variations of pressure which are occurring within thecylinder.What appears to be the earliest attempt to obviate thesedisadvantages was made by Sir Daniel Uooch about 1840.In the form of instrument which he devised specially for


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use on locomotives, the piston moved in a horizontalcylinder rigidly fixed to a framing, while the springs werecurved flat "trips. similar in form to a carriage spring.An unequal-armed lever was used, the end of the longerarm carrying a pencil, while the end of the short arm wasconnected to the piston rod by means of a short link. Thepaper used in this instrument was in the form of a longstrip, being continuously unwound from one roller on toanother, and at a rate proportional to the speed of theengine The pencil moved through a considerable arc, andfrom the motion of the piper it will be clear that thediagram traced will not form a closed curve, but thatindications of successive strokes will be given in onecontinuous line. The atmospheric line was traced by asecond pencil fixed to the frame, and by means of a curvedscale the more usual form of diagram could be readilyconstructed. This indicator, which is said to have workedwell up to 400 strokes per minute, is worthy of notice asbeing the first instrument in which a lever was employedfor the purpose of amplifying the motion of the pencil.

Richard's Indicator. " It is not a littlesurprising that thelever movement used by Gooch was not generally adopted.No doubt the curvilinear movement of the pencil constitutedthe principal objectiono the arrangement, but it isremark-ble

that more than twenty years elapsed before this defectwas remedied. The introduction in 1862 of the instrumentdesigned by Mr. C. B. Richards, of Hartford, Conn., U.S.A.,marks a distinct epoch in the history of the indicator. Thisinstrument, representing the earliest form of the modernindicator, was in every respect a very great improvementupon any hitherto constructed, and at the present time itis stillextensively used for moderate speeds.Of the accompanying illustrations,Fig. 2 shows the instru-ent

partly in section, while Fig. 3 represents the indicatoras made by Messrs. Elliott Bros. From Fig. 2 it will beseen that a short spring D is employed, the upper end ofwhich is attached to the cylinder cover E, while the lowerend is secured to the piston B, the latter being arranged tomove with perfect freedom in the liner or bushing fixed inthe steam cylinder A A. The upper end of the piston rodis coupled by a short link to a point G in the upper bar


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H I of the parallel motion. An exactly similar bar M L,and a centre link I L, completes the parallel motion, thearrangement being such that the pencil K describes anapproximately straight line, the extent of the movementbeing four times that of the piston, since the length H G isone-fourth of H I. For the usual travel of the pencil, the

Fia. 2.

stroke of the piston is thus diminished considerably, and amuch shorter and stifferspring may be employed, resultingin a corresponding reduction of elastic vibration. The twofixed centres of the parallel motion are carried by suitablebrackets projectingrom, and forming part of, a cylindricalsleeve which fits upon the outside of the steam cylinder,and around which it can turn freely. The jointat the


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top of the piston rod is arranged to swivel, and this allowsthe .sleeve,together with the parallel motion and pencil, tobe rotated upon the cylinder, enabling the pencil to bebrought into contact with or withdrawn from the paper asdesired.

FIO. 3.

The diameter of the piston in this indicator is 0'7978in.,giving an area of O5sq. in The extreme travel ofthe pencil is 3in. The piston, of a specially hard metal,is an easy fitin the cylinder, in order to ensure a movementfree from friction : while, to prevent an accumulation .of2


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20 THE INDICA TOR HANDBOOK.

steam above the piston, large escape holes are provided inthe cylinder cap, as shown in Fig. 2.At the lower part of the instrument a coupling U isprovided in order to connect the indicator with the stop-ockT. The lower extremity of the stem of the instrumentis conical in form, and fitsinto a correspondingly coned seatin the stop-cock, and as the thread upon the indicator andthat upon the stop-cock are of different pitches, the rotationof the coupling draws the cone firmly into its seat, forminga very effectual and conveniently-made joint. The lowerend of the stop-cock is screwed with a f-in.hitworththread.

The diameter of the paper-carrying drum 0 is 2in.,giving a maximum length of diagram of 5in. ; a lengthof 4^in.will, however, generally be found sufficient. Inthe place of the usual form of pencil, a pointed brass wireis employed to draw the diagram upon specially preparedmetallic paper. A split clip P Q is provided, by means ofwhich the paper is readily attached to the paper drum.The grooved pulley xipon which the cord is coiled, and theguide pulleys, are cleai'lyshown in Fig. 3.

The " detent " device shown applied to the paper drum inFig. 3 offers a convenient means for instantly stopping orstarting the motion of the paper drum without disconnect-ng

the driving cord. It consists of a small pawl mountedon a short standard, and so arranged that it may be thrownin or out of gear with the teeth of a ratchet segment on thebase of the paper drum, this being conveniently effected bythe short spring partly encircling the cylinder and providedwith a small knob as shown.

In order to adapt the indicator to the various pressuresnow used, springs are made of strengths ranging from 10 to2501b. per square inch. When it is required to change thespring, the small coupling is unscrewed from the end of thepiston rod, and the cover from the cylinder. The springmay then be unscrewed from the piston and the cover, thenew spring inserted, and the parts replaced.

The Thompson Indicator. " In the Richards instrument agreatly improved action was obtained by diminishing themotion of the piston, and consequently reducing the lengthand vibration of the spring. But with a stillfurther increase


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CONSTRUCTION OF THE INDICATOR, 21

in engine speeds, the momentum of the parallelmotionexercised a disturbing influence upon the diagram, and afurther reduction of the weight of the mechanism wasfound to be necessary.To meet this requirement, Mr. J. W. Thompson, of Salem,Ohio, U.S.A., introduced in 1875 the improved instrumentwhich bears his name, and of which Figs. 4 and 5 areillustrationsof the form made by the American SteamGauge Company, the original makers of this instrument.

FIG. 4. FIG. 5.

It willbe observed that the parallelmotion differsessentiallyfrom that used in the Richards instrument, the single leverbeing reverted to and the pencil caused to move in astraight lineby a modified arrangement of the Scott-Russellparallelmotion.From the illustrationsit willbe seen that the fulcrum ofthe pencil arm, being placed at the upper end of a swinginglink, is free to move laterally. A short and light radiusbar has one end attached to a point in the pencil arm andthe other to a fixed standard, as shown, the length andpoint of attachment being so arranged that the curved


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22 THE INDICA TOR HANDBOOK.

path which the pencil would have described if the fulcrumhad been fixed, is counteracted by the effect of the shortradius rod, and the pencil describes an approximately straightline. The link connecting the piston rod and pencil arm isfitted at the lower end with a modified form of ball-and-socket joint,which provides for the lateral movementrequired by the parallel motion, as well as for the rotativemovement which occurs when the pencil arm is moved to orfrom the paper drum. Means are provided for taking upany lost motion in the jointsresulting from wear or othercauses.

This construction of parallel motion reduces the weightof the moving parts to a very considerable extent, thedownward pull at the pencil due to this weight in theThompson indicator being less than one-third of that in theRichards instrument ; but the reduction of disturbingeffect is stillgreater, since the pencil arm, not having tocarry other links, may be made very much lighter, and thisresults in a very material reduction of the weight of thoseparts which move at a high velocity, rendering the instru-ent

available for speeds up to 400 revolutions per minute.The links, pencil bar, etc., have been reduced in weight tothe lowest limit consistent with the requirements of every-ay

use ; while a considerable improvement has beeneffected by placing the drum spring close to the base of thedrum, thereby lessening the weight and reducing the spindlefriction.

During recent years indicators with external springs havecome largely into favour, and nearly all the leading makersnow supply instruments of this type. As will be shownlater,the strength of a spring isinfluenced by its temperature.Hence, when a spring calibrated in air under normal con-ditions

is used in an indicator cylinder, it is necessary tomake an allowance for the effectof the increase in tempera-ure

to which it is subjected.The more satisfactory methodadopted by many makers is to calibrate the spring at thetemperature at which it is to be used. It will be obvious,however, that while this temperature may reasonably betaken at about 212" Fah. for the ordinary conditions ofsteam practice, it may be considerably higher when theinstrument is used with superheated steam or on a gas or


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oil engine. But by so locating the spring that it is un-affectby variations of temperature, any uncertainty ofthis kind is avoided altogether. The spring can be moreaccurately calibrated under normal conditions, and morereadilv tested either by dead weight or steam pressure if

FIG. 6.

necessary at any time. .Minor, but not insignificant,advan-agesare found in the greater comfort with which the springscan be handled, and in most instruments, the greater facilitywith which they can be changed.These conditions are approximately fulfilledin the instru-


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ment shown in Fig. 6, which represents a very recent formof Thompson indicator made by the American Steam GaugeCompany. As will be seen, the spring is exposed and iskept cool by the air circulating around it. In other respectsthe instrument is of standard design.In the Thompson indicator, as made by Messrs. Schaffer

Fio. 7.

and Budenberg, Ltd., of Whitworth Street, Manchester,and Magdeberg-Buckau, the parallel motion is modified soas to avoid the hollow piston rod used in the original form,a shorter connecting rod being employed. The indicatorshown in Fig. 7 is "suitable for pressures up to 2501b. persquare inch and for speeds up to 400 revolutions per minute,while diagrams 3m. high and 5in. long may be obtained if


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desired. For speeds up to 600 revolutions the makerssupply a smaller pattern, giving diagrams Ifin.high and3|in.ong."Fig. 8 shows a form of Thompson indicator by the same

FIO. 8.

makers and known as the " 1911 " pattern. The piston, of0-5sq. in. area, is of steel,and the instrument is availablefor pressures up to 3501b. per sq. in. The cylinder lineris surrounded by steam and can be readily removed for


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cleaning. The springs are double-wound and made with a

Fio. 10.

ball which forms a ball jointbetween the piston and pistonrod. The movement of the piston is multiplied six times


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at the pencil. The drum is ordinarily made l|in.diameter by 2"in.high, for speeds up to 600 revolu-ions.

By using an alternative cylinder and piston of"sq. in. area, the instrument can be used to 7001b.per sq. in.In another form of the same instrument, the cylindercasing is cut away (Fig.9) so that the spring is exposed.With this arrangement the spring is cooled by the circu-ating

air, while accumulation of pressure above the pistonis impossible.

In the Thompson indicator with inside spring, shown inFig. 10, the spring is attached to a bridge connected withthe cylinder cover, and the spring is extended instead ofbeing compressed. The springs are tested and adjustedin a cool state, and hence difficulties due to differenttemperatures of the springs are entirely avoided. Accumu-ation

of steam pressure above the piston due to leakagepast the piston is prevented by the large openings providedin the cylinder casing above the piston. The indicator ismade with a piston of |sq.in. area and is available in foursizes, for 400, 500, 700, and 1000 revolutions per minuterespectively.

A very convenient instrument is the " Willner :' outsidespring indicator, shown in Fig. 11. In this instrument thespring is screwed on to a boss on the cylinder cover, andthe Thompsoa pencil movement is inverted, the pencil armbeing forked at the suspended end so as to clear the spring.To remove the spring it is only necessary to unscrew themilled nut at the top of the piston rod, when, by tilting itover to the left,the spring can be unscrewed from the bosson the cylinder cover. If the cover is now unscrewed, thepiston and piston rod can be withdrawn. The cylindercover is insulated by a vulcanised fibre disc, and it is foundthat the spring remains quite cool even during prolongedtests. The pencil mechanism is very strong and rigid : but,owing to the very compact design, the weight of the movingparts is exceptionally small for an outside spring indicator.All these instruments are provided with a stop motion forregulating the pressure of the pencil upon the paper.Threaded into a projectionrom the rotating sleeve whichcarries the pencil motion is a screw, the point of which


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comes into contact with a small fixed post or stop when thepencil approaches the paper. By regulating the position ofthis screw, a very minute adjustmentof the pencil pressuremay be readily effected. The screw is provided with aninsulated handle, which is also of service in rotating the

FIG. 11.

pencil mechanism to and from the paper when using theinstrument.

The Tabor Indicator. " With a view to still furtherreducing the weight of the pencil mechanism, Mr. HarrisTabor, of New York, introduced in 1879 another form ofindicator, in which the straight-line movement of the pencilis obtained in a manner which constitutes a radical de-artur

from the systems previously employed. Fig. 12 is


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CONSTRUCTION OF THE INDICATOR 29

a sectional illustration of the instrument on a scale of two-

FlQ. 12.

thirds fullsize. P H is the pencil lever,J H itsswingingfulcrum, and E F the connecting rod. At a suitable point


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in the pencil lever a pin is fixed,upon which a small rollerR is arranged to turn freely, while in a vertical platesecured to the cylinder cover a curved slot S is formed, inwhich the roller R is arranged to travel, the curvature ofthe slot and the position of the roller being arranged so asto cause the pencil P to describe a straight line. Theouter curve of the slot is approximately that of a circlehaving a radius of lin.

From the construction it will be seen that the links H Jand F E -always remain parallel to each other ; that astraight line may be drawn through the points J, E and Pin every position ; and also that if an imaginary link,parallel to F H, be supposed to connect the point E with thelink H J, the arrangement would form an exact pantograph.The upper part of the steel piston rod is hollow, havingan external diameter of ^in. The lower part is solid,reduced in diameter, and provided at the end with a ball,forming part of a ball-and-socket joint,y means of whichthe piston and rod are connected. The socket is an inde-endent

piece, which fitsinto a square hole in the piston,and is secured by means of a central threaded stem andthumb-nut C. Shallow grooves, cut upon the outside of thepiston, tend to keep it steam-tight by the so-called " waterpacking." The steam cylinder proper is separate from theouter casing, the space between the two preventing excessiveheating of the exterior of the instrument.

The duplex type of spring used in this indicator (Fig.13)consists of two spiral coils of wire, secured at the ends tosuitable winged nuts. The springs are mounted so thatthe points of attachment of the two coils are upon oppositesides of the nut. By this means the side strain induced bythe bending of one coil of the spring is exactly counteractedby that of the other, and the piston is kept in a centralposition in the cylinder. To enable the pencil to be broughtinto contact with the paper drum, the cylinder cover isfitted with a swivel plate upon which the pencil mechanismis mounted, the attachment being made by means of thesmall standard J on the one side,and by the slotted verticalplate on the other. From Fig. 14 it will be seen that theslotted plate referred to is backed by another plate of thesame size, but without a slot. This serves to receive the


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spring Q, which is of the watch-spring type, has one endsecured to the circular box in which it is contained, whilethe other is fixed to the long sleeve of the carriage. Thestop B, which limits the travel of the paper drum, is shownto the right of the spring box. In order to adjustthetension of the drum spring, the thumb-nut N is loosened,when the carriage may be raised clear of the stop, and thetension adjustedy winding or unwinding as desired.The form of guide pulley used is simple and ingenious.The pulley G is contained within a vertical disc or sheave,this disc being held by a circular frame arranged toswivel upon a pivot, which may be locked in any positionby means of a nut as shown. By this device the cord maybe readily guided in any direction required.The piston of the Tabor indicator isof the usual diameter,0'7978in., corresponding to an area of 0-5sq. in. Thestroke of the paper drum is 5'5in., its diameter 2'063in.,and its height 4in. The extreme range of motion of thepencil is 3'25in., and as the pencil mechanism multipliesthe piston motion fivefold,the extreme travel of the pistonis 0'65in. For very high speeds an instrument is suppliedwith a paper drum 1'Sin. in diameter, 2'8in. high, andwith a stroke of 4in.

Fig. 14 shows a Tabor indicator with the spring outside.The thrust of the spring is taken by a yoke attached tocolumns fixed in the cylinder cover as shown.The Darke Indicator. " The method adopted by Mr. E. T.Darke for producing the rectilinear movement of the pencilis shown very clearly in Fig. 15, which gives a general viewof the instrument as made by Messrs. Elliott Bros. Inthis case a single light steel pencil lever is pivoted at oneend upon fixed steel centres as shown. For "ome distancefrom the fulcrum the lever pencil is circular in section, andis fitted with a small steel sleeve, which can slide easilyupon the lever, but without lost motion. The sliding sleeveswivels in the fork at the upper end of the piston rod, andis therefore enabled to accommodate itself to the varyingangular positions of the pencil lever.

In order to render the lever somewhat elastic the greaterpart of it is reduced in thickness, the requisite strengthbeing provided by increasing the depth. Upon the outer


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end of the lever is a sliding carriage, which is constrainedto move in a straight line by sliding in a slot formed in avertical guideplate, and carries the pencil in the form of ametal pin. As in the Richards instrument, the piston

FIG. is.

movement ismultiplied four times by the pencil mechanism.The marking point is kept in contact with the paper by theelasticity of the lever, and both the guideplate and thestandard carrying the fulcrum centres are mounted upona socket piece, which may be rotated by means of the


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the piston,and in which a diminution of weight is of thegreatest importance.The spring is connected to the piston through a ball-and-socket joint,hich allows the spring freedom of motion,while the connection of the piston and rod is effectedin thefollowing manner :" A boss on the piston is screwed

internally to receive the lower end of the hollow piston rod10, while diametrically across this boss a slot is cut, extend-ngdown to the body cf the piston,and of a width sufficientto allow the straight part of the spring to enter freely.Another boss on the underside of the piston is fittedwitha set-screw 9, the upper face of which is formed with a

3


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V-shaped recess, providing the. lower seating for the ballon the spring. The upper seating is formed in the end ofthe piston rod, which is firstscrewed into the slotted boss,and all slackness taken up by adjustinghe set-screw 9.The upper part of the tubular piston rod receives therod 11, which is threaded for the greater part of its length.By screwing this rod in or out of the tube 10, the totallength of the piston rod, and consequently the position ofthe atmospheric line on the paper, may be altered as foundconvenient.

Another distinguishing feature of the Crosby indicator isthe special form of drum spring employed. In place of thevolute spring usually adopted, a short spiral spring 31 is

used, the arrangement being clearly shown inFig. 16. The lower end of the spring isattached to the base of the paper drum, andthe upper end to a collar which fits on thesquared top of the spindle 28. This collarmay be raised above the squared part of thespindle, and the tension of the spring readilyadjustedto suit the various engine speeds byturning to the right to increase or to the leftto decrease the tension. In the standardinstrument the paper drum 24 is l|in.indiameter, but for low-speed work the makers

Fro. 17. fika 2-in. drum if desired.Crosby External- Spring Indicator. " Thisinstrument, known as the Crosby " New " indicator, andshown in Figs. 18 and 19, presents several novel features.The spring, which is mounted on a strong cross bracket, hasthe usual Crosby ball connection with a long hollow rod, thelatter being slotted to admit the pencil mechanism, as shownclearly in Fig. 19. The piston, attached to the lower end ofthis rod, is Isq. in. in area, and takes the form of the centralzone of a sphere. This gives only a line contact with thecylinder, and as the rod passes freely through the cylindercover and iipper guide, the piston is able to accommodateitselfto any slight eccentricity in the movement of the spring.But this action is without effect upon the pencil mechanism,as the latter is independently connected to the piston by theinner rod shown in Fig. 18. This rod, which has a ball-and-


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socket connection with the piston, slides through a sleeveattached to the base carrying the pencil mechanism, and thusensures the movement transmitted be'ng perfectly axial.The ease with which the spring can be removed and replacedis a good feature.

The makers are introducing an indicator similar to theinstrument justdescribed, but with a piston of 0'5sq. in.

Fro. 18.

area. This is known as the Crosbv "New" Indicator,No. 2.

Crosby Indicator with Continuous Drum. " Fig. 20 showsa Crosby " New " indicator fitted with a continuous drum fortaking a series of diagrams, and of particular service forindicating winding engines, rolling mill engines, etc. Theouter shell of the drum has an opening cut in it aboutl^in.wide, and centred within this space is a pin uponwhich a roll of paper is mounted. Within the shell, and


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38

concentric with it, is a smaller cylinder connected to themain drum. The roll of paper (2in.ide and 7ft. or 12ft.in length)is placed on the pin within the opening, passedround the outer shell of the drum, and in through theopening to the inner cylinder to which it is attached. Thepaper is advanced by pawls and a ratcheted collar on thetop of the drum, an adjustmentbeing provided by meansof which it is possible to vary the number of diagrams,taken on each foot of paper, from 6 to 100. When the

FIG. 20.

complete roll of paper has been wound on the inner cylinder,the latter may be removed through the top of the drumafter disengaging and removing the knurled knob shown.A pencil mounted in a slotted bracket near the opening inthe drum continues the atmospheric line throughout thelength of the paper, the position of the line having beenfirstdetermined by pulling the drum round by hand, andthe stationary pencil adjusted

o continue it. A detent isprovided on the top of the drum, allowing the indicator tobe used as an ordinary instrument.The Casartelli Indicator. " This strongly made and


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convenient instrument, made by Messrs. J. Casartelli "k Son,Market Street, Manchester, is shown in Fig. 21. It has apencil motion of the Crosby type, made of hardened steeland giving a sixfold multiplication of the piston motion.The piston and rod are of hardened steel and ground ; a

FIG. 21.

ball jointat the base of the rod prevents binding of thepiston in the cylinder and ensures a free movement. Thecylinder liner is isolated from the body of the instrument,and the latter has an opening formed on one side, as shownin the illustration. This prevents overheating and enablesthe piston to be lubricated while in use. For convenience


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cylinders are connected with the two ends of the enginecylinder, and the drum actuated in the usual manner. Themovement of the latter causes the spindle E to rise, and bymeans of the cam F and ratchet wheel G, the cam wheel His slowly rotated. When projectingoints on the rim of Hreach the pawl I, the latter, by means of the connectinglink K, carries the two pencil points L to the paper, and asthe paper drum is reciprocating and the pencil arms risingand falling at each stroke, one pencil gives a diagram from

FIG. 26.

the top end of the cylinder and the other a diagram fromthe bottom end, both being taken simultaneously and onthe same length of paper. Immediately the diagrams aretaken the pencil points are released, and the same mechanismnow causes the length of paper occupied by the diagramsjusttaken to be drawn forward into the drum, leaving afresh portion of the same paper ready for the next set ofdiagrams. This goes on automatically until the paper isexhausted. The cam H, which determines the intervals at


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which the diagrams are taken, can be replaced by others,

Ki--.._'

so that cards can be taken, automatically, at intervals of25, 50, or 100 revolutions.


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A revolution and time recorder is also included in thisinstrument. The spindle of the wheel G carries a paperdrum M over which a strip of paper is led after passingunder the pencil P. This latter receives a slight movementat each stroke of the large paper cylinder, producing theserrated record as shown. As the pointer is arranged tomiss at every tenth stroke, an interval appears on therecord which enables the result to be read in multiples of10. To register the time, an electrical connection with aclock is arranged to actuate the second pencil S, therecord, which may be in intervals of 30 seconds, beingshown on the other edge of the paper strip as seen on theleft of M.

The Simplex Indicator. " This outside-spring indicatorexhibits a departure from usual practice in the form of thespring, which is " tongs-shaped." As shown in Fig. 27, themethod of application is both simple and convenient. Thespring can be readily inserted and is quite out of theinfluence of heat and steam. The makers (Messrs.ElliottBros.)also claim that this form of spring lends itself toaccuracy of calibration to any range.

By removing the milled nut at the top of the instrument,the piston,piston rod, and pencil motion can be entirelyremoved, as shown separately in Fig. 28. From this viewit will be seen that a simple pantagraph pencil motion isemployed. A minor, but much appreciated, -feature, isthe provision of the insulated handle shown, by which theinstrument can be held in comfort while hot. Thisinstrument is made in two sizes,of which the larger givesdiagrams up to Sin. high and 5in. long ; while the smallsize gives diagrams up to Hin. high and Sin. long.


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CHAPTER III.

INDICATORS FOR GAS AND OIL EXGIXES, ETC.

ALTHOUGHell adapted to meet all ordinary require-entsof modern steam practice, it is found that thestandard instruments, when used on gas and oil

engines, do not by any means give equally satisfactoryresults,the sudden shock which the indicator piston receivesat the instant of explosion, and which is transmitted to thelevers of the pencil movement, not infrequently resulting inthese being strained and distorted to a very serious extent.This has led to the introduction of special instruments forindicating internal-combustion motors, in which the linksand levers of the pencil motion are of more substantial con-structio

and with these satisfactory results are obtained.With an ordinary indicator the high pressures met with

in gas engine work would necessitate the use of muchstronger springs than are usual in steam engine practice,and to obviate this gas engine indicators are usuallyfurnished with a piston of one-half the standard area,that is. 0-25sq. in. in place of 0'5sq. in. For very highpressures, pistons of 0'125sq. in. in area, or even smaller,are available.

It has now become customary for makers to supplycombination indicators, the cylinder having two bores toreceive pistons of 0-5 and 0'25sq. in. respectively. In thisway. while the instrument is available for steam enginework, it can be readily converted into a gas engineindicator, by attaching the smaller piston. The samesprings are used in both cases, but it will be understoodthat, when the small " gas engine " cylinder and piston areused, the strength of the spring is,in effect,doubled, and,consequently, if the pressure is read on the diagram by thescale corresponding to the normal strength of the spring,this reading must also be doubled.

47


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48 THE INDICATOR HANDBOOK,

The Casartelli Gas Engine Indicator. " This indicator,shown in Fig. 29, has a strong pencil motion giving amovement to the pencil of four times that of the piston.

FIG. 29.

The piston is fitted with a ball-jointonnection, giving afree moving piston. The cylinder cover is fitted with flatspring cushion plates, to which a steel sleeve is attached,passing down inside the cover. When the piston reaches its


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Iy LIC A TOES FOR GAS AND OIL ENGIXES 49

upper limit of movement a shoulder on the piston rodstrikes the lower end of the sleeve, so that the shock due to

FIG. 30.

any excessive pressure is transmitted to the cushion plates,which absorb it and prevent injuryto the instrument.


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The piston usiially furnished is O5sq. in. in area, but half-size pistons can be supplied and also combination sets.Atkinson's Spring Relief Gear. " Fig. 30 shows anindicator fitted with Atkinson's Patent Spring ReliefAttachment, designed to relieve the instrument of theexcessive momentary shocks met with in indicating gas

FIG. 31.

engines. The swinging fulcrum rod is not connecteddirectly to the indicator but is carried in a stirrup held incontact with a bracket by a strong spiral spring. Thetension of the spring can be regulated by the milled nutsshown. In addition to relieving the pencil movement ofshock, the attachment lessens the vibratory oscillations,which in gas engine diagrams are sometimes considerable.


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INDICATORS FOR GAS AND OIL ENGINES. 51

The Thompson Gas Engine Indicator. " This instrument,made by Messrs. Schafter "feBudenberg, Limited, is shownin Fig. 31. It is fitted with Atkinson's spring relief gearand also with the spring cushion plates, as previouslydescribed. The piston has an area of 0'25sq. in., but an

FIG 32.

additional piston of 0'5sq. in. area can be fitted to enablethe instrument to be used for ordinary steam engine work.The Tabor Gas Engine Indicator. " As shown in Fig. 32,this instrument is a combined steam and gas engineindicator, the small piston (0'25sq.n.)operating in theupper part of the cock tube.The Crosby Gas Engine Indicator. " The standard gas


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engine indicator made by the Crosby Steam Gage andValve Company is similar in external appearance to thesteam engine indicator (Fig.16) except that the pencillever is of a stronger section, combining lightness with stiff-ess,

and a stronger connecting link is provided. The

piston is 0"25sq. in. in area.The Crosby combined gas andsteam engine indicator (Fig.33) has a cylinder of extralength, bored to two diameters,for two pistons of 0*5 and0-25sq. in. area respectively.

An external-spring indicatorof the type shown in Figs.18 and 19 is also supplied forgas engine work ; the piston in

this case is 0'5sq. in. in aiva, but the springs are ratedproportionally so that the scale of the spring gives thescale of the diagram, as in steam engine practice.

Dobbie-Mclnnes Gas Engine Indicators. " For indicat-nggas and other explosive engines, Messrs Dobbie,Mclnnes, Limited, supply an external-spring indicator similar

FIG. 33.


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INDICATORS FOB GAS AND OIL ENGINES. 53

to that shown in Fig. 23, but with a piston 0'3989in. indiameter, giving an area of 0'125sq. in. It is suitable forspeeds up to 800 revolutions per minute, and gives diagrams3 1in. long by Hin. high. The instrument is also madewith a magazine drum accommodating a continuous roll ofpaper. For very high speeds a "half-size" instrument ismade. This gives diagrams up to 2in. long and lin. high,and is suitable for speeds up to 2000 revolutions perminute.

In the same makers' combined gas and steam engineindicator, a spare cylinder is supplied, which can bescrewed into the indicator cylinder. In this instrumentthe steam cylinder is 0'25sq. in. in area, and the supple-entary

piston, 0'125sq. in. With the steam cylinder inuse the springs are read at their rated strengths, these beingdoubled when the gas engine piston is in use.

The Mathot Explosion and Pressure Recorder. " Thisapparatus, shown in Fig. 34, attached to a Dobbie-Mclnnes_ras engine indicator, gives a graphic record showing thenumber of explosions, the initialpressure of each, the orderof their succession, together with the irregularity or other-ise

of the variations, the number of revolutions and theproportion of "misses," the rate of compression, etc. Thisrecord is in point of fact a continuous succession of indicatordiagrams, so greatly foreshortened that the distance fromone xipward movement of the pencil to the next is onlyabout TVn- The apparatus is of service in determining thebest proportion of air and gas or oil, while the effects ofvarying the compression, speed, point of ignition,dimensionsof inlet and exhaust valves, etc., are also made evident.

In Fig. 34 the recording drum A, 3|in.in diameter,carries a band of paper and is rotated by clockwork withinthe drum, B being the winding key. The rotation of thedrum can be stopped at any time by pressing the knob D.In this recorder the drum makes a complete revolution intwo minutes and is restricted to speeds under 300 revolutionsper minute.In the instrument shown in Fig. 35 the band of paper,8ft. 6in. long, is contained in a cylinder A, from which it isled over the drum B, and re-wound on the cylinder C. Gearwheels at D can be adjustedto regulate the rate of feed of


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the paper, which can be varied from Sin. to about 50ft.per minute. The winding key is at N. As will be seen,the pencil arm passes between the cylinder of therecorder and the paper drum of the ordinary indicator,enabling continuous and ordinary diagrams to be taken asrequired.

FIQ. 31. FIG 35.

SPECIAL FORMS OP INDICATORSIn addition to the standard types of indicators already

described, there are a few special instruments which call forconsideration.

The Hopkinson Flashlight Indicator. " In the variousoptical indicators, which have been introduced from timeto time, a thin diaphragm of steel usually takes the placeof the piston and spring of the standard instrument. Theslight movements of the diaphragm caused by the variations


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INDICATORS FOE GAS AND OIL ENGINES. 55

of pressure are communicated to a pivoted mirror upon whicha beam of light is allowed to fall. The reflectedbeam thusrecords the pressure fluctuations, and, as the mirror alsoreceives motion in a direction at right angles to the firstand proportional to the movement of the engine piston, theline of light traces out an indicator diagram.

In the " Hopkinson ;I indicator,shown in Fig. 36, a pistonand spring are used, the latter taking the form of a steel strippassing over the top of the cylinder as shown in theillustration, the ends being fixed to the upper frame ofthe indicator by screws. The piston moves in the bore ofthe indicator body and is provided at the top with a hook


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embracing the spring, but in such a manner as to allow thepiston to move laterally in order to prevent binding action.Three pistons are used, the areas being in the ratios of 1, 2,and 4 ; and, as there are t\vo springs, the strengths of whichare in the ratio of 1 to 5, a wide range of sensitivenessis obtained. The smaller pistons fit inside liners insertedin the main bore.

The mirror is clamped to a steel spindle, the ends ofwhich are pivoted in small holes in the light spring supportsshown. The movement of the spring is communicated tothe mirror by a flat vertical spring attached to an arm onthe mirror spindle, this spring, while sufficiently rigid totransmit the motion without buckling, being flexibleenoughto allow for the slight angular motion of the arm.

The upper frame of the indicator is bored to fitover thebody of the instrument ; it is held up to the cylinder coverby the coil spring shown, a ball race being interposed soas to allow the frame to turn easily around the axis of theinstrument. A light lever clamped to the frame receivesmotion from a reducing gear, and in this way the frame isgiven a slight angular oscillation around the body of theinstrument, this corresponding to the drum movement ofthe ordinary indicator.The resulting movement of the line of light (wh'ch inhigh-speed engines forms a continuous figure)an beinspected through a telescope arrangement provided witha graduated screen, or photographed for future reference.As will be seen, the instrument has the advantage that thespring can be calibrated accurately, and when in use it isnot affected by temperature variations ; further, the effectsof the inertia of the piston, pencil movement and paperdrum are enormously reduced. It is therefore particularlyadapted for indicating internal combustion engines, high-peed

steam engines, etc. Messrs. Dobbie, Mclnnes, Limited,are the makers.Other Optical Indicators. Among other optical indicatorsof recent introduction, mention may be made of theHospitallier-Carpentier Manograph and the " Clerk " opticalindicator. In the latter a piston and spring of the ordinarytype are employed. A chain, replacing the piston rod,gives a slight rocking motion to the mirror spindle, the


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INDICATORS FOR GAS AXD OIL EXGIXES. 57

latter being under spring control to keep the chain taut.The line of light is received on a sheet of sensitised paperheld in a frame sliding in a direction at right angles to thepiston movement and actuated by a reducing gear.Other Types of Engine Indicators. " Another method bywhich inventors have sought to overcome the distortion dueto the momentum of the moving parts of piston indicatorsis by restricting the vertical movement within small limits,and gradually changing the position of these limits. Inthis way the indicator card is made up of a series ofdiagrams of small vertical height. The Wayne indicator,introduced by Messrs. Elliott Bros, (who have now discon-inued

its manufacture),ave diagrams of this kind. InRipper's Mean-pressure Indicator, made by Messrs. Schafferand Budenberg, two pressure gauges record the meanforward and back pressures acting in the cylinder towhich the instrument is attached,- the difference of thesereadings (subjecto a small percentage correction)iving avery close approximation to the mean effective pressure asordinarily determined from the area of the indicatordiagram.

Indicators for Ammonia Compression Machines. " Owingto the corrosive action of ammonia upon brass andbronze alloys, the instruments used in connection withammonia refrigerating machinery are invariably made ofiron or steel. For this service, steel indicators of theordinary type are furnished by the Crosby Steam Gageand Valve Company and other makers. As leakage pastthe piston is objectionablehen dealing with gaseousammonia, a pistonless indicator, such as that made byDreyer, Rosenkranz and Droop, of Hanover, possessesadvantages for this work. In this instrument a corrugatedsteel disc about 2in. in diameter is used hi place of theusual piston, and as the movement of the centre of thisplate is only slight (about^in.),the connecting link,which communicates this movement to the pencil lever, isattached to the latter at such a point that a multiplyingmovement of about twenty times is given to the pencil.

Indicators forHydraulic Work. " Another form of piston-lessindicator introduced several years ago is that known asKenyon's Indicator. Although only indifferentlysuccessful


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when used for indicating steam engines, it has found a use-fulpresent-day application in connection with such high

pressures as are met with in hydraulic work. In thisinstrument, of which several forms are made by Messrs.Schaffer and Budenberg, a bent steel tube of ellipticalsection forms the pressure-measuring element, the move-ment

of the free end of this tube being transmitted to apencil movement of the Richards or Thompson type, and arecord being obtained of the variation of pressure in theapparatus to which it is attached. Pressures as high as10 tons per square inch are thus readily dealt with.Indicators for Very High Pressures.- For excessively highpressures, indicators having exceptionally small pistons areoccasionally used. In a special instrument made by theCrosby Steam Gage Company for testing the action ofpneumatic gun carriages, and for hydraulic work, etc., theusual piston has an extension in the form of a plunger0'18in. in diameter, corresponding to an area of 0-025sq.in., or -^ of the area of the ordinary piston.The pencil motion is of a similarly heavy pattern tothat used in the same makers' gas engine indicator, and inaddition, the bracket of the instrument carries a verticalpost against which the pencil arm bears lightly, whenneeded to prevent the pencil point being thrown awayfrom the paper by very sudden shocks.A bye-pass is formed in the"metal at the side of the smallcylinder, by which the pressure, when not too high for thecapacity of the spring, can be admitted to the larger piston.When the small piston is to be used, this bye-pass istightly closed, the pressures being read off on the ordinaryscale, and the results multiplied by 20.


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CHAPTER IV.

ERRORS OF THE INDICATOR.

HAVINGescribed in some detail the various forms ofmodern indicators, we shall now consider the variouserrors inherent in the instruments and incurred in

their operation.Piston Friction. " A considerable amount of error is in

some cases traceable to excessive friction of the indicatorpiston, this being often caused by grit carried into thecylinder from the pipes and other connections. Frequentlythis condition of affairs may be detected by placing the

FIG. 37.

finger lightly upon the upper end of the piston rod. Theremedy in this case is obvious. Occasionally a newinstrument will be found to exhibit an unusual want offreedom, in which case, after it has been thoroughly cleanedand lubricated, itmay be connected to the engine cylinderand allowed to run for some time, the pencil mechanismhaving first been disconnected. An abnormal amount ofpiston friction frequently exhibits itself in the diagram bya series of step-like serrations in the expansion line asshown in Fig. 37, the horizontal portions of the steps

59


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indicating a sticking of the piston at each of those positionsin its descent.When a strong spring is used, the friction caused by a

tightly-fittingclean piston may not reveal itselfin this way.If, however, the resulting diagram is compared with onetaken under precisely similar conditions, but with a freely-moving piston, it will be found that the various events inthe stroke are shown as occurring unduly late, owing to thesluggish response of the piston to the variations of steampressure. A vertical admission line with the freely-movingpiston may appear so distinctlysloping in the other diagramas to suggest insufficiencyof lead, while the initialpressureattained will be less and the back pressure greater in thelatter case than in the former. With a fairly early cut-off,however, the area of the two diagrams may not greatlydiffer,as the first-mentioned losses are compensated by thedelayed formation of the expansion line ; but the diagramtaken with the tight-fittingpiston may be very misleadingas an indication of the action of the valve gear.To determine the amount of the piston friction, theinstrument is firstallowed to make a few working strokes,after which the pencil lever is pressed down by hand, so asto slightly extend the spring, and then allowed to return torest, the indicator being rapped with a small wood stick.When at rest, a horizontal line is drawn on the diagram.The pencil lever is next raised slightly, agaia allowed tocome to rest, and another line drawn as before. Theinterval between these two lines is a measure of the sum ofthe total frictional resistances in both directions, and,assuming the pencil mechanism to be in order, almost thewhole of the error so measured is attributable to pistonfriction. In some tests of one of the best forms of modernindicators,the mean amount of double friction with varioussprings was found to be as follows :

Scale of spring (lb.)...20 36 40 60Double friction(lb.)...-6201... 0-6342. ..0-6763...1-008It is scarcely necessary to remark that careful attention

to the lubrication of the piston will conduce to smoothrunning, and prevent any tendency to stick or bind. Cleancylinder oil, castor oil,or va.lvoline will be found a much


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ERRORS OF THE INDICATOR. 61

better piston lubricant than the thin oil used for the jointsof the pencil movement. The amount of lubricant requiredwill depend upon the character of the work in hand, frequentattention being necessary when indicating gas, oil,or petrolengines.

^j'i-inys. The chief error met with in this importantdetail is a want of regularity in compression under uniformincrements of pressure, any inaccuracy in this respect beingmagnified in the diagram four, five,or six times, accordingto the type of pencil movement used. When itisconsideredthat in the Crosby instrument, for example, an error of0-0028in. in the compression of a 601b. spring will producean error of lib. in the diagram, the importance of accuratesprings will be readily admitted.From a number of tests of springs which have been madefrom time to time it would appear that the character andextent of this error varies considerably in instruments bydifferent makers. The general tendency, however, is forthe spring to give a continual gain in the pressure indicated,owing to the spring becoming gradually weaker as it iscompressed. The amount of this error in new springs bythe best makers does not usually exceed 3 per cent, whentested under steam, but after having been some time in usethe error may be materially increased, especially if emerycloth has been generously used to remove rust from thecoils. It is undoubtedly very desirable to periodically testthe springs, and for exact work this involves the use of astandard mercury column. If an accurate steam gauge isavailable, an approximate test may be made by connectinga short horizontal length of piping to the boiler or mainsteampipe, as most convenient, fitting a regulating valveat the supply end of this pipe, while the outlet end is closedby a small cock, by which water of condensation may beallowed to escape. Two tee-pieces are inserted in the lengthof piping, in one of which the indicator is mounted, whileto the other the steam gauge is attached by means of theusual syphon. Steam is first allowed to blow freely throughthe pipe to expel water and to thoroughly warm up theinstruments. Then by means of the two cocks the pressureis regulated until some definiteamount " lOlb., for example" is indicated by the gauge. The paper drum of the


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indicator is then rotated and a line drawn. This process isrepeated at various intervals of pressure (whichfor springsup to 401b. may be of 51b. each, and for those above, lOlb.),a similar series of lines being drawn as the piston descendsunder successive reductions of pressure. The regulatingvalve is then closed, the small cock opened fully, and theatmospheric line drawn. The difference between any pair

of corresponding lines is, asalready pointed out, a measureof the friction of the piston,etc., at that point, and linesdrawn midway between eachpair will give a scale whichmay be regarded as the scaleof the spring tested.It will be seen that thismethod of calibrating thespring covers any errors inthe area of the piston or inthe pencil movement. It doesnot, therefore, give an abso-utely

true spring scale, butrather a true scale of the com-bination,

which is of far morepractical utility. If,however,the spring isused in another in-trume

the scalewillof courserequire to be redetermined.A standard gauge not beingalways available, various de-ices

for testing indicatorsprings by weights have beenemployed. One method usedby Bollinckx is shown in Fig.

38. In this case the instrument is invei'ted,and is heldin an exactly vertical position in the manner indicated,D being a bracket fixed to a wall or other convenientsupport. A light frame carries a rod A, the point of whichbears upon the centre of the piston, as shown, while to thelower end of the frame weights may be attached, thesebeing so selected that each represents a definite increase of

FIG. 38.


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pressure per square inch upon the piston. It will be seenthat in this method of testing, errors in the pencil move-ent

will be covered, but any inaccuracy in the piston areawill not be taken into account. The piston diameter (whenhot) must therefore be ascertained, and the scale of thespring corrected accordingly.The apparatus shown is quite suitable for outside-springindicators. For enclosed-spring instruments it would notbe difficultto arrange a method of jacketinghe indicatorcylinder with steam, the instrument being heated up tothe working temperature (taken as 212" R), since thestrength of springs at this temperature is from 2| to 3 percent, less than at 60" F.

Pencil Movement. " This may be briefly examined withrespect to (a)Parallelism of the motion as influencing theaccuracy of the vertical movement of the pencil ; (b)Uniformity of ratio of pencil and piston motion ; (c)Amount and distribution of the weight ; and (d)Frictionand effectof wear.Commencing with the Richards pencil movement, it maybe said that the greatest deviation of the pencil from thestraight line occurs when the centre or pencil link is vertical.When this occurs in the lower part of the stroke, the pencilis drawn slightly too far from the line of motion of thepiston rod, the deviation being similar in amount, but in theopposite direction, when it occurs in the upper part of thestroke. Assuming the path of the pencil to be divided intofour equal parts, the two centre portions would satisfy everyrequirement of practical exactness ; beyond this, in eitherdirection, the error increases slightly until the maximum isreached, as already indicated, after which the deviationbecomes less, vanishing at about the extreme limits ofmovement. As a result of this, the ratio of the movementof the piston and pencil is not maintained exactly, the scalebeing slightly more open in the centre than in the outerportions of the pencil travel.The investigation of the effect of the weight of the pencilmovement resolves itself into the determination of theresulting pressure upon the upper end of the piston rod.In the Richards instrument, the effect of the two mainlevers of the parallel motion will be found by multiplying


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their weight by the distance of their centres of gravity fromthe fulcrums, and dividing this by the distance between thepoint at which the piston rod is connected and the nearestfulcrum. The effect of the pencil link is, however, muchmore serious, since this is to be multiplied directly by four,the ratio of the pencil motion to that of the piston in thisinstrument. In one indicator the weight of the two mainlevers, referred to the piston rod, was found to be 0'02421b.,while the weight of the centre link, similarly resolved, was0-05481b.

In order that the pencil movement may be compactlyarranged, the small link connecting the piston rod tothe parallel motion is made as short as convenientlypossible. This implies a fair amount of movement of thelink, and the consequent friction and wear at its two endsrequire to be well provided for, since any lost motion herewould produce a fourfold effect upon the movement of thepencil. The pressure of the pencil on the paper also tendsto increase the friction somewhat.

The Thompson pencil mechanism gives a close approxi-ationto perfect rectilinear motion throughout the greater

part of its movement : while the use of a comparatively longlink to connect the piston and pencil lever ensures themotion of the pencil bearing practically a constant ratio tothat of the piston.

It will be observed that although the original 'Thompsonmovement has the same number of jointss the Richards,three of them have a very small angular movement, andthe friction and wear are therefore correspondingly less.Moreover, the arrangement is very "stable," owing to thecomparatively long rods employed.

The Tabor pencil movement is essentially a modificationof the Thompson arrangement, a point in the pencil leverbeing guided by a curved slot in place of being attached toa radius rod. In this way the effective weight of themechanism is still further reduced, while a more exactrectilinear movement of the pencil is secured. Thecurvature given to the slot is such that the ratio ofmovement of piston and pencil is constant. In this casethe number of jointss reduced to four, but there is to beadded the friction of the roller in the slot,which, however,


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tests have shown to be very small. It will be noted that asit is impossible for the small roller to be simultaneously incontact with both sides of the slot, the pencil lever musthave a small amount of lateral play, but in this well-made instrument this is reduced to the smallest possibleamount. It will be noted, moreover, that in this instancethe effect upon the position of the pencil is not increased inamount by the magnifying action of the mechanism.The Darke pencil movement (Fig.15)gives a motion whichis theoretically perfect, but although fewer actual jointsreemployed than in any other movement, the accuracy maybe seriously impaired by wear of the sliding socket, orsleeve at the top of the piston rod, of the small carriagecarrying the pencil, and of the latter in the vertical slotguide. Any lateral wear of the piston rod in the plane ofthe motion will also affect the accuracy. The makershave, however, provided against these sources of error bygood workmanship and the provision of large bearingsurfaces. Obviously everything depends upon the correctnessof the position of the vertical guide plate, and this shouldbe tested from time to time, as explained in Chapter IX.During the upward stroke the friction of this pencilmovement is somewhat greater in the lower and upper partsof its range, reaching a minimum in the centre of its travel ;in the downward movement the friction is less marked.Careful lubrication of the sliding surfaces will, however,reduce this effectvery considerably.The Crosby pencil movement involves the u"e of six joints,and of the four links employed the radius rod marked 15 inFig. 16 moves through a fairly large angle. The mechanismgives very good results as regards both the rectilinearmovement of the pencil and the constancy of multiplicationof motion. The multiplying ratio in this instrument issix, and therefore any lost motion at the jointsmarked 12,18, and 19, Fig. 16, would affect the accuracy of the diagramto a greater extent than in other instruments. The makershave duly recognised this, however, and have providedmeans for accurately adjustingthe mechanism, while,further, the workmanship bestowed upon this instrument isof a very high class. The mechanism as a whole is verylight,but it will be noted that it is not independent of the


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piston rod, and if disconnected therefrom, it ceases to forma " stable linkage."The Dobbie-Mclnnes pencil movement is in effect aThompson movement, in which the piston rod is connectedto the radius rod instead of acting directly upon the pencil.lever as in the older instrument. Both the radius rod andconnecting link are somewhat shorter than in the Thompsoninstrument, but in general the remarks made in connectionwith the latterapply.Inertia of Piston and Pencil Movement. " In consideringthe effect of the inertia of the piston, piston rod, and theattached mechanism, it becomes necessary to determine foreach separate part its "equivalent mass'3 at the piston.The weight of the piston and piston rod, together with thatof the collar or fittingat the lower end of the spring, arethe only parts which are taken into the computation withtheir weights simply, although the connecting link maygenerally be also so included without material error. Theequivalent mass of the spring (withoutthe end fitting)sone-third of its actual weight.Space will not allow of a full investigation of this point,but it may be remarked that in a Richards instrument itwas found that while the weight of the piston, piston rod,and connecting rod, together with the equivalent mass ofa 201b. spring, was 0'10531b., the equivalent mass of thepencil mechanism was 0'22531b., of which latter amountO'lfiGlb.was due to the pencil link alone.The effectof the inertia of the moving parts in producingoscillationof the pencil has been very fully investigated byProf. Osborne Reynolds.* He found that the resultingerror, so far as the area of the diagram is concerned,depends upon the magnitude of the oscillations in somemeasure, but to a greater extent upon the smallness of thenumber which appears in each revolution. Their magni-ude

is affected by the degree of suddenness of the steamadmission, and will therefore be different for differentvalvegears and engine speeds. The general effect of the impactof the steam upon the piston approaches that of a suddenlyimposed load, and tends to force the piston through twice

* Proceedings Inst. Civil Engineers, vol. 83.


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the distance that it ought to rise. A similar but much lessmarked action takes place at any sudden change ofpressure. The oscillations thus induced are, however,modified and gradually destroyed by the friction ofthe piston, together with that of the pencil and attachedmechanism.It frequently results that undulations of considerablemagnitude do not affect the area of the diagram to anyvery great extent ; but the distortion of the figure thusproduced quite destroys its value as an indication of theaction of the steam. The minimum number of oscillationsin a complete revolution, consistent with clearness, appearsfrom Prof. Reynolds' experiments to be about thirty, andwhen the number approached fifteen,a fairdiagram couldonly be obtained by exerting considerable pressure on thepencil. It is needless to remark that such a method ofreducing the distortion of the diagram is quite inadmissibleif the results are to have any pretension

to accuracy.It is obvious that a reduction of the inertia effect is to be

obtained by diminishing the weight of the moving parts.This has been the constant endeavour of the makers ofmodern indicators, and further progress in this directionappears to be limited to the use of aluminium for thepiston, piston rod, etc. It will be understood that it isalways possible to diminish undulations by employing astronger spring, but this entails a consequent reduction inthe height of the diagram, which is often objectionable.It is true that this may in turn be remedied by increasingthe multiplying leverage of the pencil movement, but forthe reasons already cited this plan is not to be recommended.On the whole, the most successful method of obtaining adiagram free from undulations at very high speeds is to usesome form of optical indicator.

Pt-wil Friction. " The general effectof the frictionresult-ngfrom the pressure of the pencil on the paper, and thatof the mechanism in sustaining it,is to increase the area ofthe diagram, since by always acting in opposition to themotion of the pencil it causes the steam and expansion linesto be drawn above, and the back pressure and compressionlines below, their true positions. The extent of the errorthus incurred depends in some degree upon the condition

5


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of the surface of the paper and the kind of pencil used, butchiefly upon the pressure brought to bear upon the latter.This pressure should never exceed that necessary to draw afine working line, since, in some instruments, even thisproduces an error of 0'51b. in each direction. From theabove considerations it will be apparent that the extent ofthe error from pencil friction willdepend upon the cut-offof steam and other conditions. Thus, in the case of alocomotive notched up to mid-gear, it appears possible forthis error to amount to 5 per cent. ; while on slow-movingengines having a late cut-off and little compression, theeffect will be very much less marked.

Errors in the Drum Motion. " The chief causes of errorsin the motion of the paper-carrying drum are (a)the inertiaof the drum, (b)the imperfect action of the spring, and (c)the friction of the drum. The general effect of thesedisturbing influences is to vary the tension of the drivingcord, causing it to stretch unequally, and hence producingan irregular displacement of the diagram in the directionof its length. With a positive drum-driving arrangementthese errors would be eliminated, but the practical difficultyof adopting such a method of actuating tne drum appearsto prevent its use under any but exceptional circumstances.Reducing gears, in which only a very small amount of cordis used, will be found described in Chapter VII., and withthese the inaccuracies due to the drum action are reducedto a very small amount indeed.

It will be seen that the conditions governing the oscilla-ionsof the drum are very similar to those influencing the

motion of the balance-wheel of a watch. Every drum,together with its attached spring, has its natural period ofoscillation, which becomes longer as (1) the moment ofinertia of the drum or (2) the length of the spring isincreased. When the drum and spring are so arrangedthat the period of oscillationof the drum coincides with theperiod of movement of the engine crosshead, the tension onthe driving cord, and consequently the amount of stretch,will remain uniform. On the other hand, if these twoperiods differ materially, the cord tension will be corre-spondingl

affected. With speeds so low that the inertia ofthe drum is negligible,it is readily seen that the length of


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the diagram will be diminished by an amount d //,d beingthe difference between the tensions of the drum spring at theextremities of the stroke in pounds, and y the stretch ofthe driving cord in inches per pound of tension. As thespeed increases, the inertia of the drum gradually reducesthis shortening of the diagram, until at the critical speedthe one effect exactly neutralises the other. With stillhigher speeds the inertia effect preponderates, and the dia-rambecomes elongated. This action is modified by theaction of the spring, and it may be shown that " since theeffortof the drum spring is usually directly proportional tothe angle through which the drum moves, and also sincethe accelerating force vanishes at mid-travel " if thedifference of the drum-spring tensions at the ends of thestroke is twice that necessary to overcome the inertia of thedrum, the cord will remain uniformly stretched.All modern indicators are provided with means for ad-usti

the drum-spring tension, and this should be variedto suit the particular speed in each case. Since, however,increase of tension results in the ultimate stress on the cordbeing increased, only sufficient tension should be used aewill ensure the cord being kept taut.The general effect produced by the combined influencesof the imperfect spring action and the inertia of the drumis to change the location of portions of the diagram, thedirection and amount of such disturbance depending uponthe consideration outlined in the foregoing; but with asuitable drum-spring tension the effect upon the meaneffective pressuie is very slight. Some indication of thetension required may be obtained by comparing the lengthsof diagrams taken at low and full speeds. It may be noted,however, that when the driving cord or wire iskept taut bymeans of an auxiliary spring (describedlater),he dmm-spring tension nwy be correspondingly decreased, leavingthe turn of the tensions about equal to the drum-springtension which would have been required ordinarily.The error caused by friction of the drum is one whichoften affectslow-speed diagrams appreciably. The extentof the error depends upon the lubrication of the drumspindle and the elasticityof the cord, and may therefore bereduced to a negligible amount by car

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