oving systems of indicating instruments must be supported to allow the desireddegree of rotation with a limited amount of

friction and eccentricity between fixed and moving parts. The most widely used means for supporting any type of instrument moving system,be it a moving coil, iron vane, or moving magnet,is that of a jewel and conical-pivot or cylindrical-shaft construction. The shaft or conical pivots areusually rigidly attached to the moving system androtate between two jewel bearings. Such con-struc-tion is also being increasingly applied in miniatur-ized devices of many sorts. This article willdescribe differences of application of the two mostcommon types of instrument jewel bearing systemsand will discuss the design considerations of each.

Types Of Bearing Systems: The two types of bearings to which this discussion will be limited arethe vee-jewel and the ringstone, Fig. 1.

Vee-jewels are used extensively in electric in-dicating instruments in which bearing frictiontorque must be kept to an absolute minimum inorder that instrument accuracy will always be within specified limits. For true indication of thequantity that is being measured, the restoring torque on the moving system must either equal the electrical torque or differ from it by a con-stant amount. Since friction is not a constant quantity, the bearing friction must be held to a

relatively low value for satisfactory instrument cal-ibration. Commercial instruments utilizing the vee-jewel are small panel, switchboard, hook-on,portable and laboratory types, and exposure me-ters, some of which are shown in Fig. 2. Fig. 3shows two typical moving systems supported byvee-jewels.

Vee-Jewel Design: If the weight of a moving system operating with the shaft vertical were supported by a theoretically sharp point, the con-tact area between pivot and jewel would be zero and therefore friction would be zero. An instru-ment such as this would be very short-lived, how-ever, because shocks and vibration which it wouldexperience in normal operation would soon flattenor distort the points. This would produce frictionmuch greater than would have occurred had thepivot points been slightly rounded originally. Foroptimum design of the jewel and pivot contour,then, the two major requirements of minimum friction and maximum resistance to deformationmust be balanced against each other.

In many applications of electric indicating in-struments using the vee-jewel system, the instru-ment can be operated in any position—with theshaft vertical, horizontal, or in any intermediateposition. It can be shown analytically that the maximum friction error exists when the shaft is horizontal. For this reason the remainder of the

DESIGNDATADesign Factors for Jewel Bearing Systems

By A. C. LawsonMeter and Instrument Dept., General Electric Co., West Lynn, Mass.Reprint from Machine Design -

discussion of vee-jewels will deal with horizontalfriction only. In this position, as the moving systemis actuated, friction causes the pivots to roll up thejewel wall until the wall becomes too steep to sup-port rolling, and slipping occurs. This action isillustrated in Fig. 4a. The maximum friction torqueresulting at this point is

T = µ W Rp cos . . . . . . . . . . . . . . . . . . . . . . . .(1)

whereRp = Pivot radius, millimeters

T = Maximum horizontal friction torque, gram-millimeters

W = Weight of moving systemµ = Coefficient of static friction between jewel

and pivotØ = Angle of pivot roll from neutral position to

point on jewel at which slipping occurs, degrees

A more accurate expression for friction torque ina horizontal axis system has been derived byNylander.1 It involves another important designconsideration—end-play of the moving system. Theequation is

T = µW Rp cos tan-1 (2)

wherePr = End play with the pivot contacting the

spherical surface of the jewel, millimetersRj = Jewel radius, millimeters.

Equation 2 is shown graphically in Fig. 5 which,for particular values of R21, W, and R1, showshow end-play affects friction torque. Two interest-ing observations can be made from this graph:

1. At end-plays of 0.001-inch and greater and acoefficient of friction of 0.10 or less, the frictiontorque is practically independent of jewel radius.

2. With the end-play reduced below 0.001-inch,an appreciable decrease in friction torque canbe attained even with a relatively large coefficient of friction.

However, Figs. 4b, 4c, and 6 illustrate that thenormal force at the contact area between jewel andpivot increases rapidly as end~play is decreasedbelow 0.001-inch. It is seen, then, that minimumend-play cannot be the only criterion in reducingfriction because high wedging forces accompanysmall end-play. If this wedging force is not keptwithin certain limits, serious pivot deformation canoccur resulting in increased friction. A series ofexperiments conducted by Stott2 showed that for anincluded pivot cone angle above 30 degrees themaximum allowable stress increases with coneangle. In addition, the rate of increase of bearingpressure with decrease in pivot radius depends uponthe weight of moving system, Fig. 7.

µ

cos sin-1 ( 1 )Pr

Pr Rµ

Equations 1 and 2 indicate that for a given bearing system the friction torque varies linearlywith moving system weight, provided, of course,the bearing surfaces are clean and polished so that does not vary with load. In most cases, therolling angle ø is small and Equation 1 reduces to

T = µ W Rp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .(3)

In the design of an instrument using a vee-jeweland pivot system, the coefficient of friction µ andmoving system weight W known, the maximum

allowable friction torque is usually known, (it de-pends primarily on the instrument sensitivity) andthe pivot radius can be calculated. The jewel radiusis made a maximum consistent with holding theeccentricity of the moving system within allowablelimits and the contact stress within the elastic lim-its of the materials used.

Shock-Resistant Jewel Mountings: In certaininstrument applications shock forces sometimesproduce stresses in the jewel and pivot whichwould exceed the maximum allowable stresses ifthe jewel were mounted rigidly. The most commonmethod for protecting the bearing system againstsuch forces is that of mounting the jewel on a heli-cal spring. Th the majority of applications, this con-struction is perfectly satisfactory. For conditions ofextreme shock and vibration, however, a recentdevelopment using sponge rubber has been provedsuperior because of its damping characteristics. Asan example, one small panel instrument design, tomeet an extremely stringent military specification,must withstand severe amplitudes and frequenciesof vibration, tumbling in a steel barrel, and nineshock blows ranging from 1200 to 2000 ft-lb. Theacceleration present when such shocks are appliedis in the order of 1000g.

The basic design approach was essentially that ofprotecting the most vulnerable part of the instru-ment, the moving system. This was accomplishedby cushioning each jewel with a small piece ofsponge rubber.

The internal portion of the jewel screw is sodesigned that shock forces applied in any direc-tion will move the jewel away from the pivot, Fig. 8. The jewel moves due to its own inertiarather than from armature acceleration force

directed through the pivot. When the shock force isalong the axis of the pivot, the jewel moves as aresult of the blow. The pivot is restrained to a trav-el of 0.010-inch by a shoulder on the pivot support.The action of the soft sponge rubber is to damp thejewel motion severely, thereby preventing it fromrebounding onto the pivot. Shock forces appliedperpendicular to the axis tend to tip the jewels,allowing the pivot to strike the jewel on the periph-ery of its cone rather than the tip. The armature isrestrained to a travel of 0.010-inch in this directionalso.

Ringstone and Endstone Bearing Systems: Amoving system using ringstone jewel bearings hashigher friction torque than a comparable vee-jewelsystem because of the larger contact surface be-tween pivot and jewel Some types of electric in-struments can tolerate higher friction torque, how-ever. Ringstones can therefore be used to advan-tage in producing much more sturdy bearing sys-tems. For the same weight of moving system, theone supported by ringstones has a much lower bear-ing pressure than the vee-jewel.

Ringstones are used in preference to vee-jewelsunder the following conditions:

1. When bearing friction is negligible comparedwith the active electrical torque or with other friction or error-producing torques. An example is a recording instrument in which the moving system drives a pen across a chart. Here, the friction of pen on paper far exceeds the friction present in the bear-

ings.2. When the instrument in normal operation is

subjected to continuous vibration, therebyreducing bearing friction torque. Aircraftinstruments mounted on an airplane panel arean example of this application.

3. When the mechanical construction requires thatthe shaft extend through the bearing.

4. When side-play of the moving system must beextremely small.

There are various types and combinations of ring-stone systems, as illustrated in Fig. 9. In instru-ments such as recorders where the shaft is alwaysvertical, Fig. 9a, a ringstone-endstone combinationis used for the bottom bearing, while the top bear-ing can be either a ringstone or vee-jewel. Fig. 9bshows horizontal shaft ringstone bearing systems.Certain applications require lower friction than canbe obtained by the straight-hole ring-jewel. Theolive-hole stone and the "bombe" type are refine-ments which give lower friction by virtue of thesmaller pivot to jewel contact area. The bombe typeis used in very special applications where minimumthrust as well as journal-bearing characteristics arerequired.

It is possible to hold the side-play of a movingsystem employing ringstones to a lower value thancan be obtained with vee-jewels. Fig. 10 showsdimensions of a typical ring jewel. Note that the tolerances are given in ten-thousandths of an inch.The shaft can, of course, be made to the same

Jewel Materials: Jewels of various materials, suchas glass, sapphire, synthetic spinel, titanium oxide,beryllium copper, brass, etc., have been tried andtested. Depending upon the moving system weight,it has been determined that borosilicate glass andsapphire afford the optimum in initial friction,resistance to impact, and long life.3

In recent years, glass has become a substitute forsapphire in some vee-jewel applications, partic-ularly small panel instruments. As a result of anextensive development program during the waryears, when the accessibility of sapphire jewelsfrom Switzerland was uncertain, glass was sub-sti-tuted for sapphire in vee-jewel applications wherethe total moving system weight is less than 0.75-gram.

The 0.75-gram value for glass vee-jewels hasbeen empirically derived for rigidly mounted jewelsand results from an exhaustive series of compar-ative tests on impact and shock resistance of glassversus sapphire. Of course, this moving systemweight limitation can be exceeded if the jewel isshock-mounted.

Pivot Materials: The most common pivot mate-rial for use with vee-jewels is high carbon tool steelin the form of ground and polished drill rod. Afterhardening to a minimum Knoop hardness of 690,the pivot tip radius is generally put on by tumbling.Since in ordinary usage the pivot may be subjectedto very high tensile, compressive, and impactforces, the microstructure of the hardened pivot is

closely controlled.Some vee-jewel applications require nonmagnet-

ic pivots to reduce the errors produced by magnetichysteresis. In such cases monel metal, stainlesssteel, or tantalum shanks with osmium tips areused, the osmium giving a hardness comparable tothe high carbon steel. Pivots of 92 1/2 percent tan-

talum, 71/2 per cent tungsten are also used for non-magnetic qualities.

Shafts for use in conjunction with ring jewels arecobalt-tungsten, monel metal, stainless steel, beryl-lium copper, tantalum, or high carbon steel.

For all pivot applications the material must behighly polished, and free of surface scratches orflaws for minimum friction and longest life.

Summary: Most electric indicating instrumentsin use today have a jewel-pivot bearing system. Inapplications where minimum friction consistentwith safe loading is the prime factor, vee-jewels andconical pivots are used. When extremely low mov-ing system friction torque can be sacrificed in favorof higher strength or mechanical constructionadvantage, ringstones are used.

REFERENCES

1. A. L. Nylander— “Instrument Bearing Friction”, General ElectricReview, July, 1946.

2. V. Stott— “Pivots and Jewels in Instruments and Meters,” Col-lected Researches of National Physical Laboratories, Vol. 24,Paper No. 1, 1931.

3. P. K. McCune and J. H. Goss—“A New Jewel for IndicatingInstrument,” AIEE Trans., Vol. 61. 1942.