In the released position, the plates must retain a set clearance. This allows them to separate from one another and avoid excessive or premature wear.
Premature wear can also be caused by partial application of a clutch at high speed. This can occur if fluid is trapped in the clutch cylinder after release.
Trapped fluid accumulates at the outer portion of the cylinder. At high speed and under the action of centrifugal force, this can produce enough pressure to effect partial application.
This can be prevented by a pressure relief valve in the piston.
It is normally a flat-reed or ball-check type, which is easily closed by the apply fluid.
On release, the valve is unseated by the rotation of the assembly, letting fluid escape.
One-way clutches are used in the gear train to allow rotary motion in one direction only.
They simplify the hydraulic and mechanical control of the members.
The hydraulic control system requires a continuous supply of fluid under pressure, normally provided by a fixed displacement pump. It is driven by the torque converter when the engine is running.
The pump driving member is engaged by tangs on the converter spout, or machined flats, or by slots.
Pump output always exceeds normal operating demands so pressures are controlled by regulating valves. Excess fluid returns to the pump inlet or is dumped into the oil pan, where it can reach the pump intake strainer.
Crescent-type pumps are commonly used in automatic transmissions.
