A clutch pressure control valve for a hydraulically actuated friction clutch comprising: a source of hydraulic pressure; a valve bore communicating with the source of hydraulic pressure and having an outlet port from which the friction clutch is supplied; valve means movable within the valve bore for opening and closing communication between the outlet port and the hydraulic pressure source;Balance Valves a reaction piston movable within the valve bore; means for producing a force on the reaction piston whose magnitude varies non-linearly with the displacement of said reaction piston; means for producing a speed dependent pressure on the reaction piston that opposes the effect on the reaction piston of the non-linear force producing means; and means for producing a force on the valve means whose magnitude varies proportionally with changes in the distance between the valve means and the reaction piston.Reducing Valves The pressure control of the invention includes a valve spool disposed within a valve bore, which spool has two control collars and is acted upon at one end by a linear compression spring and a speed dependent pressure signal, the spool serving to regulate the clutch pressure by regulating the free cross sectioFORGED STEEL VALVESn of an outlet port disposed between its control collars. The present invention seeks to provide an improved clutch pressure control valve for a hydraulically actuated starting friction clutch, in which an improved variation of the clutch pressure with speed can be achieved. According to the present invention, the control collars of the valve spool have two different diameters and the spool is disposed within a stepped bore. The end of a linear compression spring remote from the valve spool abuts a reaction piston, which is disposed in the valve bore and which is acted upon on one side by the speed dependent pressure signal (Pitot) and on the other side by a second, non-linear compression spring, which is compressed between the reaction piston and a shoulder in the valve bore. The movement of the reaction piston acted upon by the speed dependent pressure, which varies as the square of the speed, is opposed by a non-linear compression spring, which imposes a non-linearity on the displacement of the reaction piston as pressure varies. The movement of the piston is not, therefore, proportional to the square of the speed as is the pressure acting upon the piston. The movement of the piston compresses the linear spring and exerts an additional force on the valve spool, proportional to the displacement of the reaction piston due to the speed dependent pressure. Therefore, by designing the non-linear compression spring in the form of a progressive spring, the effect of the pressure signal, which increases as the square of the speed, can be reduced. Preferably, the force of the non-linear compression spring is calibrated in such a manner that at idling speeds the reaction piston remains in a rest position, whereby only the force of the linear compression spring acts on the clutch- pressure valve. The resulting clutch pressure, which leads to a slow crawling of the vehicle during idling, can then be kept constant despite variations in idling speeds. At a specific partial load speed, the reaction piston may move to a second end position against the force of the non-linear compression spring to limit the clutch pressure. The non-linear compression spring may be constructed in the form of a conical spring, a spring with varying pitch between the coils, or a spring with varying wire gauge, or it may comprise several separate springs of differing stiffness. In this way, the speed dependence of the clutch pressure may be varied within very wide limits.

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