Fluid operated clutches have a substantially cylindrical apply chamber which is filled with pressurized fluid when the clutch is to be engaged. When the clutch is disengaged, the pressure of the fluid is reduced. All of the fluid is not, however, exhausted and continued rotation of the clutch housing can result in a centrifugal pressure being generated in the clutch apply chamber. This can lead to "drift on" of the clutch when the centrifugal pressure overcomes the force in the apply piston return springs.
There have been many approaches to solving this problem. The most common of these solutions involves the use of a ball dump valve which exhausts the clutch apply chamber of fluid when the rotating speed is above a design speed and the clutch is in a disengaged state. This can result in air ingestion into the apply chamber if the clutch apply passage is fully evacuated.
Another solution is to increase the release force of the return springs to a level above the maximum centrifugal pressure expected. This will increase the apply pressure needed to engage the clutch thereby increasing the energy requirements of the hydraulic control system and decreasing the overall efficiency of the transmission.
A third solution is to provide a balance chamber opposite the apply chamber of the clutch. This design option has become useful when two or more clutches are nested in radial alignment. However, this option requires that the balance chamber to have a continuous supply of hydraulic fluid, separate from the clutch apply fluid to insure the operation thereof. To satisfy this requirement an additional fluid passage must be incorporated in the shaft and/or the support sleeve for the clutch. While each of the above solutions will prevent "drift on" of the clutch, they do present some drawbacks and design complications.