Servos are used in automatic transmissions to convert the hydraulic pressure acting on a piston to a mechanical force which is then applied to a brake band.
In the Borg Warner 35 transmission, two brake bands are used, a front band and a rear band, and a servo is required for the operation of each.
The rear servo is a pressure apply/spring release type and is engaged together with the rear clutch and its piston has a central stem which locates in one end of the rocker arm to transfer the apply force to the band.
A heavy lever type spiral spring returns the servo and the band to the Off position when the fluid is dumped.
The rear band holds the planet carrier stationary when it is engaged and since it is applied in drive ranges where low ratios are involved.
A high apply force is therefore required and this is provided by a combination of the large diameter piston and the leverage provided by the rocker arm.
The fluid supplied to the servo at line pressure is converted to an apply force by the piston and this acts on the longer end of the rocker arm.
The apply force is therefore multiplied by the leverage ratio to approximately five times its value and this force is applied to the "free" end of the band through a flat steel strut which connects the arm with the band.
The band contracts and wraps itself firmly around the planet carrier drum to lock the planet carrier to the casing.
An adjusting screw, threaded in the casing, provides the anchorage point and this means the band can be easily adjusted when necessary to compensate for wear.
