Overrunning Clutch

An overrunning clutch is used in automatic transmissions to lock a planetary gearset to the transmission case so that it can act as a reactionary member. The overrunning clutch for the planetary gears is similar to the one in a torque converter stator or an electric starting motor drive gear. A planetary gearset overrunning clutch consists of an inner race, a set of springs, rollers, and an outer race. Operation of the overrunning clutch is very simple to understand. When driven in one direction, rollers lock between ramps on the inner and outer race, allowing both races to turn. This action can be used to stop movement of the planetary member, for example. When turned in the other direction, rollers walk off the ramps, and the races are free to turn independently.

Brake Band The brake band is used to lock a planetary gearset element to the transmission case

so that the element can act as a reactionary member. The brake band is made up of the following elements:

• Band—The brake band is a circular piece of spring steel that is rectangular in cross section. Its inside circumference is lined with a friction material. The brake band has bosses on each end so that it can be held and compressed.

• Drum—The drum fits inside the band and attaches to the planetary gear-set element, and is to be locked by the band. Its outer surface is machined smoothly to interact with the friction surface of the brake band. When the open ends of the band are pulled together, the rotation of the drum stops.

• Anchor—The anchor firmly attaches one end of the brake band to the transmission case. A provision for adjusting the clearance between the band and the drum is usually provided on the anchor.

• Servo—The servo uses hydraulic pressure to squeeze the band around the drum. The servo piston is acted on by hydraulic pressure from the valve body that is fed through an internal passage through the case. The servo piston has a seal around it to prevent leakage of hydraulic pressure, and is spring loaded to allow quick release of the band. Some servos use hydraulic pressure on both sides of their pistons so that they use hydraulic pressure for both the release and the application of the band.

The operation of the brake band is as follows (Figure 10-23):

Released—When the brake band is released, there is no hydraulic pressure applied to the servo, and the drum is free to rotate within the band.

Applied—When the brake band is applied, hydraulic pressure is applied to the servo that in turn tightens the band around the drum. The result is that the drum is locked in a stationary position, causing an output change from the planetary gearset.

In the applied circuit of a clutch or band, an accumulator is used to cushion initial application. It temporarily absorbs some of the hydraulic pressure to cause slower movement of the applied piston.