Abstract: A hydraulic modulation valve for use in conjuction with a tractor power-takeoff shaft to prevent an instantaneous loading of the hydraulic clutch controlling the operation of the power-takeoff shaft is disclosed wherein the supply line leading from the valve spool to the hydraulic clutch is in flow communication with a modulation chamber. A yieldable spring-loaded piston is moved within the modulation chamber from an initial position to an operating position to allow the hydraulic pressure to increase from zero pressure to a working pressure for the hydraulic clutch. A bypass line bleeds hydraulic fluid from the inlet end of the modulation chamber to slow the rate at which hydraulic pressure is increased within the modulation chamber.

Abstract: A control valve system for controlling the capacity of a hydraulic clutch in a power transmission mechanism wherein the clutch torque established by the hydraulic clutch will be nearly equal to the engine torque for the engine with which the transmission is used, thereby resulting in a smooth clutch engagement as a torque transmitting path is established from the engine to a driven member.