Page 482 - Cam Design Handbook

THB14 9/19/03 7:58 PM Page 470

470 CAM DESIGN HANDBOOK

14.13 UNIDIRECTIONAL CAM CLUTCHES

Freewheeling or overrunning clutches are employed when torque must be transmitted in
one direction only or where the driven member is to be permitted to “overrun” the driver.
They can be considered simple and inexpensive substitutes for a differential drive and also
be used in multiple-speed drives where they automatically disengage the lower-speed unit
when the higher-speed unit is engaged. When one of the members of the overrunning
clutch is fixed, the other member can rotate in only one direction and the clutch becomes
“backstopping.”
The most common designs utilize the cam wedging principle. Figure 14.16a shows a
commercially available “sprag” clutch, which has an inner and outer race like a ball
bearing. It utilizes hardened, specially shaped wedge cam bodies called sprags to lock or
free the movement. A similar result is obtained with balls or rollers in wedge-shaped cham-
bers (having designed wedge angles of inclination) in one of the races called a roller clutch.
Figure 14.16b is an application of the clutch principle in the rear wheel of a bicycle when
the wheel speed exceeds that of the drive. The construction consists of a shell-shaped
outer member and an inner member with wedge-shaped pockets, or flat cam profiles,
around its periphery; the coupling between the two members is provided by the locking
action of the rollers. Generally, instantaneous wedging action at any position without back-
lash or slip is required. The wedge-shaped pocket may have the disadvantage of high man-
ufacturing costs.
For proper locking action, the condition for self-locking, a £ 2f, must be satisfied; see
Fig. 14.16c. Here a = the angle between tangents to the cam contour and to the roller
-1
surface at the point of contact. And f = tan m (m = coefficient of friction). To check the
strength of the roller the crushing for

Outer race Sprag
Energizing
spring

(b) Roller clutch. (c) Forces.

Inner race Spring retainer
(a) Sprag clutch.
FIGURE 14.16. Unidirectional cam clutches.