Publisher Summary Many machine systems require clutches that can pick up the drive to a machine that is already rotating or alternatively can release the drive when another driver takes over. A synchronizing self-shifting clutch has a pawl and ratchet mechanism to sense synchronism between the input and output shafts and to align the teeth, which are then shifted into mesh by the small torque applied through the pawls to helical splines. Conversely, reverse torque on the splines causes the teeth to disengage. Clutches can be mounted directly between two shafts, in a separate casing or in a gearbox. They can be fitted with an internal thrust bearing between the input and output sides so that the clutch is a fixed length and positions the output machine from the input or vice versa. Oil lubrication is required for the clutch teeth and pawls to prevent wear and corrosion. Lower power and speed clutches are usually self-contained units with an integral oil system. Higher powers and speeds require a force fed oil supply Such a bearing is usually only subjected to high thrust forces when the clutch is engaged and in this condition there is no relative rotation across the bearing so it has a high thrust capacity.