Jewel Bearings SolveLight Load ProblemsThese miniature bearings are unsurpassed for lowbreakaway friction,long life, and resistance to harshenvironments.

PAUL BAILLIOSales ManagerBird PrecisionWaltham, MA

Jewel bearings earned their reputa-tion as reliable low-friction devicesby keeping watches and timepiecesworking accurately, some for over acentury. In low-load and low-speedapplications, they remain un-equalled as maintenance-free bear-ings in precision analog movementssuch as altimeters and guidance sys-tems. Furthermore, advances incrystal growing and manufacturingnow make the bearings cost-effec-tive alternatives to conventionalbearings in selected applications.Ruby, sapphire, and hard glass makethe best jewel bearings be-causethey are hard and take a near-perfect polish, down to 1 or 2µin. While diamond holds promiseas the ultimate jewel bearing be-cause of its extreme hardness andlow friction, its high cost makes itunacceptable.Jewel bearings perform best when

loads are less than 500 g and whenconstant speeds are low or momen-tary at higher rates. Within thisrealm, the bearings compete withminiature ball bearings, flex-uralpivots, taut-band suspensions, and avariety of low friction materialssuch as Teflon. the most com

mon jewel-bearing designs are ringsor journals, cups, Vs, balls, rollers,and endstones.Other features the bearings offerare:

� Low friction coeffecient. Forsteel on sapphire. Cƒ is 0.1 to 0.15.Breakaway torque is so low that ajewel bearing recently was used in amechanical torque meter sensitive

Clear sapphire, red rubies, and hard glass make ideal miniature bearings forlow-torque, low-speed and light-load applications such as compasses and sensitive analog meters. The same machining techniques that produce thebearings provide precision orifices for use in flame-off torches, miniaturecheck valves and air bearings.

enough to measure the thrust pro-duced by a fruit fly.

� High hardness. Sapphire hasa compressive strength of 300,00

psi (2,000 knoop).� High temperature stability.

Melting point is about 2,050˚ C.� Low relative cost. Typically,

a jewel bearing costs 20% as muchas a ball bearing and 10% as muchas a flexural pivot.

� Tight concentricities, to 0.0002 in. in most cases.

� Small sizes for compact de-signs. Shafts can be as small as0.006-in, diameter, but more typi-cally range from 0.02 to 0.125-in.diameter.

Ring jewelsThese are journal or annular bear-ings commonly found in instru-ments such as watches, dial indica-tors, and gimbal suspensions.Because of their manufacturingmethod, ring jewels offer a feature

not available in conventional bear-ings. The hole is contoured like anhourglass or olive shaped, ratherthan a straight cylindrical bore. Theadvantage is reduced contact area,hence, lower friction between shaftand bearing.The hourglass profile also allows adegree of radial freedom in the bear-ing. If the shaft becomes mis-aligned, the hole shape preventsbinding. Also, it allows closely fit-ting the shaft to the bearing; diame-

The simplest jewel bearing is a journal-like ring. The hole may be cylindrical,but rounding the sides to an hourglassshape allows for shaft misalignmentwithout binding. A rounded surface onthe bearing face, called a bombé, en-ables the bearing to handle thrust loadsas well.

Greater thrust capacity results frommounting an endstone below or behind asingle-cup, olive-hole ring jewel. Theendstone has a radiused surface, like abombé, but without a center hole. Thisarrangement is not suitable for shockloading. Also, the fixed assembly mustbe held in place with a retaining ring orother device.

For shock loads, an endstone can bespring loaded below a bombé or ringjewel. The stepped shaft and bombecarry thrust loads. The threaded assem-bly is another method for securing thejewel bearing.

MOUNTING JEWEL BEARINGS

tral clearances of 0.0002 in. or better are possible.

V and cup bearingsThese bearings are most often madeof hard glass and sapphire, whichhave among the lowest frictioncoefficients of all miniature bear-ings, about 0.12 for steel-on-sap-phire. Cup and V-bearings are usedin sensitive magnetic meter movements, compasses, encoders, turbine flow devices, and gal-

vanometers.The low-friction coefficient is due largely to the moving shaft’snear point contact on a V-bearing.In contrast, other miniature bear-ings use line contact. Also, glass

and sapphire deform less underload than competing materials. This property keeps friction andtorque nearly constant over the lifeof the bearing. Wear is usuallyfound on the pivot or shaft ratherthan the jewel. The amount of wearusually depends on load, speed, and shaft material.Glass V-bearings have slightlylower friction because their sur-faces are fire polished. Sur-prisingly, some compass manufac-turers add a small drop of lubricantto the glass V-bearing to damp compass card oscillations, whichotherwise would continue for longperiods. A fire-polished surfaceminimizes the asperities, hence, thelubricant provides viscous damp-

ing to the shaft. This is not a prob-lem with sapphire V-bearings,which have microscopic, drag-pro-ducing asperities on their lappedsurfaces.The effects of friction producingasperities are especially evidentwhen a V-bearing is used horizon-tally. In this position, the bearinghas higher load bearing capacity.However, the mating conical shafttries to roll up the surface of the V-

Other shock mounting techniques in-clude a silicone cushion behind theend-stone or a spring loaded ball. Thespring is used for light shock loads, andthe silicone for higher shock loading,up to 1,000g.

Mounting a ball on a shaft allowhigher thrust loads than just a round-ed shaft end. The silicone cushionsshock loads, and the straight-holering jewel carries loads normal to theaxis.

SEEINGCLEARLYTHROUGHSAPPHIREOptical-quality sapphire has five timesthe resistance of carbide to scratchesand chemicals. Because it is next to diamond in hardness (9 Mobs vs. 10for diamond or 1,800 to 2,200 Knoop)sapphire lenses work well on barcodereaders, watch crystals, and otherdevices with moving lenses. Highstrength and a moderate refractiveindex also make sapphire a goodselection for high-pressure windowsand view ports.Sapphire is unaffected by acids even at1,0000C. It is virtually inert to all re-agents and alkalis at room tempera-ture, including hydrofluoric acid.Furthermore, sapphire exhibits goodinfrared transmission across a widespectrum of wavelengths, and is usedin infrared detectors and filters.The mineral’s high-temperature con-ductivity, strength, and stability makeit ideal for high-energy laser applica-tions. Sapphire pinholes and aper-tures typically outlast those made ofmetal. The crystal’s availability andlow cost make it an excellent materialfor high volume use. For example, a 1-in. diameter watch crystal costsfrom $10 to $12.

bearing radius, and the shaft endoperates on the side of the conerather than at the point of the V.Like the ring jewel, the V-jewel pro-vides more radial freedom thancompeting designs.

When a V-jewel is used horizon-tally, a 0.003 to 0.005-in, clearancemust be maintained between bear-ing and shaft. To make the adjust-ment, fine-pitched screws aremounted behind the bearings. Inthis arrangement, turning a 100-100(100 threads/in.) screw once, forexample, produces linear travel of0.0 13 in. After gently tightening,backing the screw off one-quarterturn easily produces the clearance.Threaded screws that work best foradjusting end-play are the 2-80,100-100, and 4-90 sizes.

Sizing the radiiLoad-bearing capabilities and

friction are governed by V-bearingor cup size, and cone radius. Gener-ally, the V-radius should be threetimes the radius of the pivotingmember. This allows room for thepivot radius to wear in and conformto the V-radius without binding.

Best performance results whenthe conical V-angle is 85˚ and theconical pivot angle is 55˚. Typi-cally, glass V-bearings with mini-mum radius points tolerate loads to200 mg. Larger points handle high-er loads.

As shaft radius decreases in abearing, friction and load carryingcapacity also decrease. The smallcontact radii produce large contactstresses.

High hardness can be a detrimentfor a jewel bearing because they canfracture under high impact loads orhigh vibration. The bearings are notrecommended for shock applica-tions unless they are properlymounted with a spring or siliconepad to absorb damaging vibration.Some examples are shown in thebox Mounting jewel bearings.

Pivot material also deserves at-tention. Nickel chrome and 420stainless-steel alloys are goodchoices for pivot and shafts. Gener-ally, hardness in excess of R~ 50and surface finish of 2 to 4 µin isrecommended. Nickel-chromealloys of-ten are preferred becausethey are corrosion resistant, non-magnetic, and have a hardness overR~ 65.