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Module 2- Apply and Hold Devices (Page 3 of 6)

Brake Bands

A brake band is a very simple device. It is a metal band with friction material glued to the inside of the band. Pictured below is a heavy duty split-style brake band.

The only purpose of a transmission band is to hold a component of the planetary drive train stationary. They simply wrap tightly around the outside of a transmission component that is attached to a planetary gear component. See diagram below.

One end of the brake band is attatched to the transmission case. Pictured below is a transmission band that is attatched to the transmission case by means of an adjustable screw.

The other end of the transmission brake band is attatched to an operating rod. See above diagram. Fluid pressure enters the servo and pushes the operating rod which applies the band at the appropriate time.

The servo (pictured above) activates the push rod that applies the brake band. The servo is in turn controlled by the valve body (pictured below). You'll recall from the previous theory lesson that the purpose of the valve body is to control the shift timing, gear changes and shift feel.

The valve body is the brains of the automatic transmission. See diagram below. It decides what to do, which gear to shift, how hard to shift into a gear, how long to stay in that gear before shifting to the next one.

Example of the Operation of a Brake Band
You will recall from Module 1, that reverse reduction gear is obtained any time the planet carrier is held stationary and the sun gear is the input (driven by the engine). What holds the planet carrier stationary?

So, when the driver selects reverse gear, the automatic transmission valve body applies fluid pressure to the band servo which applies the band. The band wraps around the housing that is connected the planet carrier. Therefore the planet carrier is held stationary and we have reverse gear.
Module 2 (Page 3 of 6)