Patent GB-823848-A (GB)
Title: Improvements in or relating to pressure control arrangements
IPC: F15B21/045
Filed: 1955-12-05
Abstract: 823,848. Change-speed control. ZAHNRADFABRIK FRIEDRICHSHAFEN A.G. Dec. 5, 1955 [Dec. 3, 1954; Feb. 26, 1955], No. 34868/55. Class 80 (2). [Also in Group XXIX] A pressure control relief valve, particularly for providing a slowly increasing engaging pressure and a rapid disengagement in hydraulically operated gear shift clutches such as K 1  . . . K 4 , Fig. 5, comprises a valve piston 60 controlling an outlet passage 67 to a sump 59, a control piston 71, and an interposed spring 66, the pressure oil supplied by a gear pump 63 communicating with the control piston 71 through a delaying thermostatic throttle opening indicated at 73, and communicating directly with the piston 60. When a control S 1  is operated to engage a clutch K 1  a valve O is closed and oil pressure in pipe 79 moves piston 60 to open partially the passage 67 until the oil pressure on the piston equals the spring pressure. Simultaneously the opening 73 allows a gradually increasing oil pressure to act on piston 71. Since piston 71 is larger than piston 60, the former is slowly moved until it abuts a shoulder 72, the spring 66 being compressed so that the oil pressure in pipe 79, and consequently the clutch pressure, slowly reaches a maximum. The thermostatic throttle indicated at 73 is constructed, as shown later in Fig. 9, to operate in only one direction of flow so that when the control S 1  is actuated to disengage the clutch K 1  and opens the valve O to vent the pipe 79 then the spring 66 moves both pistons 60, 71 rapidly to the opposite ends of the housing 65. In modifications employing a thermostatic throttle opening acting in both directions of flow so that although a rapid clutch disengagement is still obtained the return of the control piston is delayed, one or other of the two pistons may have an additional spring acting thereon and the two pistons may be of the same diameter. The pressure-time graphs obtained may be straight or curved. In such a relief valve having a thermostatic throttle opening indicated at 12, Fig. 12, spring-separated pistons 30, 31 and an additional spring 33, an auxiliary chamber 201 is connected to the chamber in front of the control piston 31 and has a piston 203 loaded by a weak spring 204. When a clutch is being engaged the pressure in pipe 27 is maintained at or near its low value determined by the initial opening of passage 28 until the oil passing throttle 12 has fully depressed piston 203 (for which a stop may be provided), whereafter the control piston 31 is slowly depressed and the pressure in pipe 27 rises to a maximum as in the Fig. 5 arrangement. The extended low-pressure period provided by chamber 201 allows time for synchronism to be approximately established in the clutch before the pressure is increased to its maximum. In another modification of the Fig. 5 arrangement the pipe 69 is replaced by a pipe passing coaxially through the two pistons. The coaxial pipe may contain adjustable one-way thermostatic throttle means, as shown in Fig. 9, wherein spring-separated pistons 122, 123 in a housing 47 slide on a tube 113 containing a cone 111 forming, with an adjustable screw sleeve 118, an adjustable throttle. The cone 111 has oil flow cavities and is on a shaft 110 having a guide cylinder 112 engaging a shoulder in the tube 113. When the relief valve is vented the oil displaced by the returning control piston 123 shifts the shaft 110 to open the throttle so that the throttle is operative in only one direction of flow. The shaft 110 and tube 113 are of materials having different coefficients of expansion to maintain a constant flow past the throttle as the oil viscosity changes with temperature changes. In a modification the shaft 110 is replaced by a hollow shaft screwed into the surrounding tube so that it is not slidable, i.e. the throttle operates in both directions of the oil flow. In the relief valve shown in Fig. 6 and comprising spring-separated valve and control pistons 80 and 81, the thermostatic throttle indicated at 88 is mounted in an antechamber 94 having a spring-loaded piston 86 and containing oil which does not mix with the gear oil, the antechamber preventing impurities from reaching the throttle opening. The oilfilled chamber between the pistons 80, 81 communicates with a displacement chamber 90 so that oil leaking past the piston 81 on depression of piston 86 is sucked back when the piston 86 returns. The valve may be surrounded by a jacket connected to the sump by a single pipe. In a modification an antechamber is formed by two resilient bellows which may be filled with a fluid having a low change of viscosity. In Fig. 10, a relief valve piston 131 controlling an outlet 139 slides on a control piston 132 loaded by a spring 135, and a spring 133 between the piston 131 and a guide ring 134 on the piston 132 holds the two pistons together. Oil entering at 138 immediately moves piston 131 against spring 133 to open partially the outlet 139 and thus determine the initial oil pressure. Simultaneously the oil entering at 138 slowly moves the control piston 132 until it abuts a stop 136, the escape of oil from the closed chamber 143 into a vented chamber 147 being delayed by an adjustable thermostatic throttle 144, similar to that in Fig. 10. As in the previous constructions of relief valve, the slow movement of the control piston 132 compresses the spring 133 so that the oil pressure rises slowly to a maximum.