461 Overrun clutch with single diameter rollers

A roller clutch is comprised of an inner and outer ring member and a series of cylindrical rollers spaced between them (see Fig. 4.13). Incorporated between the inner and outer members is a cage which positions the rollers and guides so that they roll up and down their ramps simultaneously. One of the members has a cylindrical surface concentric with its axis, this is usually made the outer member. The other member (inner one) has a separate wedge ramp formed for each roller to react against. The shape of these wedge ramps may be flat or curved depending upon design. In operation each roller provides a line contact with both the outer internal cylindrical track and the external wedge ramp track of the inner member.

When the input wedge member is rotated clockwise and the output cylindrical member is prevented from rotating or rotates anticlockwise in the opposite direction, the rollers revolve and climb up the wedge ramps, and thereby squeeze themselves between the inner and outer member tracks. Eventually the elastic compressive and frictional forces created by the rollers against these tracks prevents further roller rotation. Torque can now transfer from the input inner member to the outer ring member by way of these jammed (locked) rollers.

If the output outer member tries to rotate in the same direction but faster than the inner member, the rollers will tend to rotate and roll down their ramps, thereby releasing (unlocking) the outer member from that of the input drive.

Type Clutch
Fig. 4.13 Overrun freewheel single diameter roller type clutch

4.6.2 Overrun clutch with triple diameter rollers (Fig. 4.14)

This is a modification of the single roller clutch in which the output outer member forms an internal cylindrical ring, whereas the input inner member has three identical external inclined plane profiles (see Fig. 4.14). Situated between the inner and outer tracks are groups of three different sized rollers. An anchor block and energizing shoe is arranged, between each group of rollers; the blocks are screwed to the inner member while the shoes (with the assistance of the springs) push the rollers together and against their converging contact tracks. The inclined plane profile required to match the different diameter rollers provides a variable wedge angle for each size of roller. It is claimed that the take-up load of each roller will be progressive and spread more evenly than would be the case if all the rollers were of the same diameter.

When the input inner ring takes up the drive, the rollers revolve until they are wedged between the inclined plane on the inner ring and the cylindrical internal track of the outer member. Consequently the compressive load and the frictional force thus created between the rollers and tracks locks solid the inner and outer members enabling them to transmit torque.

If conditions change and the outer member overruns the inner member, the rollers will be compelled to revolve in the opposite direction to when the drive was established towards the diverging end of the tracks. It thus releases the outer member and creates the freewheel phase.

4.6.3 Sprag overrun clutch (Fig. 4.15) A very reliable, compact and large torque-carrying capacity overrun clutch is the sprag type clutch. This dispenses with the wedge ramps or inclined plane formed on the inner member which is essential with roller type clutches (see Fig. 4.15). The sprag clutch consists of a pair of inner and outer ring members which have cylindrical external and internal track surfaces respectively. Interlinking the input and output members are circular rows of short struts known as sprags. Both ends of the sprags are semicircular with their radius of curvature being offset to each other so that the sprags appear lopsided. In addition a tapered waste is formed in their mid-region. Double cages are incorporated between inner and outer members. These cages have rectangular slots formed to equally space and locate the sprags around the inner and outer tracks. During clutch engagement there will be a slight shift between relative positions of the two cages as the springs tilt, but the spacing will be

Fig. 4.14 Overrun freewheel triple diameter roller type clutch

accurately kept. This ensures that each sprag equally contributes its share of wedge action under all operating conditions. In between the cages is a ribbon type spring which twists the sprags into light contact with their respective track when the clutch is in the overrun position.

When the inner ring member is rotated clockwise and the outer ring member is held stationary or is rotated anticlockwise, the spring tension lightly presses the sprags against their track. This makes the inner and outer members move in opposite directions. The sprags are thus forced to tilt anticlockwise, consequently wedging their inclined planes hard against the tracks and thereby locking the two drive and driven members together.

As conditions change from drive to overrun and the outer member rotates faster than the inner one, the sprags will rotate clockwise and so release the outer member: a freewheel condition is therefore established.

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