6 ____________________________________________________________________ Warner Electric/Formsprag (800-927-3262)

Principles of Operation

Sprag DesignThis sprag-type design overrunning clutchgenerally consists of an inner race, anouter race, a set of sprags, a sprag retain-er, energizing springs, and bearings.

The wedging of the sprags between theraces transmits power from one race tothe other. The sprags have a greater diag-onal dimension across one set of cornersthan across the other (see Figure 1). Thewedging action occurs when the relativerotation of the inner and outer races tendsto force the sprag to a more upright posi-tion where the cross-section is greater.

Ramp & Roller DesignRamp & roller clutches consist essentiallyof an outer race with a cylindrical insidediameter, an inner race with ramps, and aset of rollers which are individually springloaded to provide constant contactbetween the rollers and both races. Thisarrangement assures instant action at allrunning speeds and guarantees immediatedriving capability whenever one of the tworaces rotates with respect to the other inthe drive direction.

ApplicationClutches of this type can be used in alltypes of overrunning, backstopping andindexing applications.

When used as an overrunning clutchthe ramp & roller clutch should be mount-ed in such a way that the outer race is theoverrunning member. This is especiallyimportant with higher overrunning speeds.In applications where the inner race over-runs, the centrifugal force on the rollersresults in lower overrunning speed limits.

When used as a backstop a ramp &roller clutch with rotating inner race isespecially suited at lower speeds. If theRPM is higher than the RPM recommend-ed in the tables, we suggest using a spragtype clutch.

When used as an indexing clutch theouter race should always be the oscillatingmember and the inner race should be thedriven member. Otherwise, the inertia ofthe rollers and springs will lead to inaccu-

OUTER RACE

OVER

RUN

INNER RACE

DRIVE

DRIVE

DRIVEOUTER RACE DRIVE

INNER RACE DRIVE

OUTER-RACE OVER-RUN

INNER-R

ACE OVER-RUN

Figure 3Gripping angle increases as load increases andannular space increases.

Figure 1Detail of sprag. Dimension A is greater thandimension B.

Gripping AngleWedging action depends upon the wedg-ing, or gripping angle of the spragsbetween the races. The fundamental con-cept of sprag clutches requires that thecoefficient of friction of the sprag, withrespect to the inner race at the instanttorque is applied in the drive direction,must be greater than the tangent of the

gripping angle, GA. If the condition is notsatisfied, wedging will not occur.

The gripping angle is determined by theconstruction of Figure 2, where points A and B are the points of contact of thesprag with the inner and outer races,respectively.

Sprags are designed to have a low initialgripping angle to insure positive initialengagement. As torque increases, thesprags produce radial forces which causerace deflections, which make the spragsroll to new positions. Sprags are usuallydesigned to have an increasing grippingangle as they roll from overrunning posi-tion to maximum load-carrying position. A higher gripping angle reduces the radialload imposed by the sprag, thus permit-ting higher torques to be transmitted with-in the limits of race stretch and brinelling.

Sprag

Energizing Spring

Sprag Retainer

Inner Race

Outer Race

AB

Figure 2Geometry of sprag, showing gripping angle GA.

Higher Load

Lower Load

A

A1

B

GA

B1

GA1

racies, especially at high index frequen-cies. The use of thinner lubricating oil and stronger springs will provide higherindexing accuracy and can be suppliedupon request.

Sprag

GrippingAngle (GA)

Inner Race

Outer Race

A

B

GA

Free action

In the Formsprag free action retainer all sprags are permitted to have free andindependent action. During overrunningthis allows each sprag independently toadapt itself to any variations in annularspace caused by runout or by foreignmatter which may inadvertently get insidethe clutch. Since each sprag operatesindependently, it cannot transfer theeffects of variations from one sprag to thenext. With all sprags in uniform engage-ment at all times, the load is evenly dis-tributed. The free action principle alsodistributes wear evenly for a minimum of wear on all components.

Formsprag and Stieber manufacture a widevariety of sprag sizes and shapes to meet themarket requirements.

Warner Electric/Formsprag (800-927-3262) ____________________________________________________________________ 7

Principles of Operation

The C/T Sprag PrincipleCentrifugal Throwout, or C/T, retainer assemblies are designed for high speedinner or outer race overrunning, and lowerspeed drive conditions. C/T sprags areavailable in a variety of models. ModelFSO sizes 300 through 700 are availablewith PCE or C/T sprag option with outerrace overrunning.

Outer Race C/TIn the outer race centrifugally disengagingsprag design, the mass of the sprag islocated so that when the outer race isoverrunning, the centrifugal force of thesprags overcomes the force of the ener-gizing spring causing the sprags tocompletely lift off of the inner race.

Inner Race C/TModel RSBI & RIZ sizes 20 through 240are available with the inner race C/Tfeature.

In the inner race centrifugally disengagingsprag design, the mass of the sprag islocated so that when the inner race isoverrunning the centrifugal force of thesprags overcomes the force of the ener-gizing spring causing the sprags to com-pletely pull away from the outer race.

The point at which the sprags lift off of theinner or outer race is listed as the lift-offspeed. The maximum drive speed isalways less than the lift-off speed toinsure positive sprag energizing.

The primary advantage of the centrifugalthrowout sprag retainer is that when thesprags lift off the inner or outer race thereis no rubbing contact in the clutch.Therefore, the life of the clutch is deter-mined by the life of the bearings.

Note: In centrifugal throwout designs, the sprags lift off the inner or outer raceduring overrunning. Therefore, for driveconditions, C/T designs require that thedriving speed be lower than the lift-offspeed.

Lift Off

Sprag Energizing

Torque transmission

Lift-off

The sprags are energized by springs thatact upon each end of each sprag.Formsprag has developed several differ-ent types of energizing springs, such ascontracting springs, expanding springsand a torsional type spring. In each over-running clutch the type of energizingspring used will reflect Formsprags broadexperience in the design and applicationof overrunning clutches in the choice of amethod of energizing best suited for theparticular design of clutch.

In all cases, whether the spring is anexpanding spring, a contracting spring, or a torsional spring, the spring designenergizes each sprag individually withouttransfer of motion or effect from one sprag to the next.

PCE spragPCE sprags are designed to overcome the effects of severe torsional and linearvibrations as well as high transient torqueoverloads. It is aFormsprag exclusive.This design, whichprovides built in pro-tection from otherwisedamaging overloads,is now standard inmodel sizes 300through 700.

Formchromeextends sprag lifeUltra-hard Formchrome sprags provideextra-long life, maximum wear resistanceand lower maintenance costs. Formchromesprags exclusive with Formsprag are made by diffusing chromium into thesurface of hardened high carbon alloy steelto form a chromium-carbide alloy.

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