All bands are externally contracting types and have a thin layer of plain or grooved friction material bonded onto a spring steel or cast steel backing.
One end of the band rests against a stop or strut in the transmission case. The other end accommodates a push rod or linkage from a hydraulically operated servo, which contracts the band onto the drum.
The simplest servo consists of a cylinder containing a piston, a piston return spring, and a pushrod.
Fluid leakage past the piston is normally prevented by neoprene type seals.
Fluid under pressure is directed onto the piston in the closed cylinder, moving it to apply the band.
When this fluid is "dumped" or exhausted from the supply line, the spring returns the piston. And the flexible band springs back to its released position.
In a pressure release type servo, the band is applied in the normal way by fluid pressure acting on the piston head.
However this fluid is not dumped to accomplish band release.
Release is obtained by supplying fluid at the same pressure to the opposite side of the piston.
As soon as pressure is equal on both sides of the piston, the piston is moved to the release position by the force of the return spring, acting on the release side.
The band’s speed of application, or release, is controlled by the rate at which fluid on the release side enters or leaves the servo.
When admitting release fluid is unrestricted, the band is released quickly.
When dumping it is unrestricted, a snap-band application occurs. This is because fluid under pressure is already acting on the servo piston.
Pressure-release servos can also have pistons with unequal areas.
In this case the release side has a larger surface area than the apply side.
The band is brought into operation by directing fluid onto the reduced area that surrounds the hollow portion, accommodating the return spring.
When fluid at the same pressure is directed to the release side, the differential area results in a higher release force. The band can be released even while apply fluid continues to exert force on the piston
The fluid directed to the release side can be directed at the same time to a clutch piston, to bring the next component into operation.
