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 (fig. 4-33):

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 of 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. By pulling the open ends of the band 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

Figure 4-33. - Brake band.

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 (fig. 4-34):

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 (fig. 4-35). It temporarily absorbs some of the hydraulic pressure to cause slower movement of the applied piston.

HYDRAULIC SYSTEM

The hydraulic system of an automatic transmission serves four basic purposes:

1. Actuates clutches and brake bands by hydraulic pressure from the hydraulic slave circuits.