Patent GB-1452023-A (GB)
Title: Hydrostatic transmission anti-stall system
IPC: B62D11/18
Filed: 1974-05-02
Abstract: 1452023 Hydrostatic transmission control EATON CORP 2 May 1974 [3 May 1973 13 March 1974] 19253/74 Heading F2W In a hydrostatic transmission for a tracked vehicle wherein tracks (22, 24) on opposite sides of the vehicle are drien by a pair of hydrostatic transmissions (14, 16) from a common engine (20) which also drives a pump (62) supplying pressure liquid through an antistall valve 64, speed control and reversing valve 42 and steering valves (46, 48) to pump and motor displacement adjusting follow-up servomotors (50, 52, 56, 58), the anti-stall valve 64 reduces the pressure supplied to the servomotors and thus transmission output speed on engine overload and a rate control element 70 retards operation of the anti-stall valve 64 and thus reduces the rate of change of transmission output speed in response to rapid changes of engine load. The pump (62) supplies pressure liquid at a pressure proportional to engine speed directly through a line 106 to a chamber 102 on one side of a piston 110 and through a fixed orifice 98 to a chamber 96 on the other side of the piston 110 which is additionally biassed by a spring 114 that is variably loaded by a linkage 90 connected to the engine throttle. During normal operation the pressure in the chamber 102 balances the opposing pressure in the chamber 99 and the spring force to hold the piston 110 and an integral valve spool 120 in the position shown. On engine overload the reduced pressure from the pump (62) causes an immediate reduction in the pressure in chamber 102 whilst the pressure in chamber 96 falls slowly due to the fixed orifice 98. The valve spool 120 thus moves left to reduce the pressure in a line 74 acting on a second valve spool 150 of the anti-stall valve The valve spool 150 moves up to throttle the pressure liquid supplied from the pump (62) through line 164 and speed control and reversing valve 40 to the servomotors (50, 52, 56, 58) for reduced output speed. The pressure reduction in the line 74 is transmitted through a line 182 to a chamber 190 of the rate control element 70 to move a piston 206 thereof left against the bias of a spring 212. When the pressure in the line 74 falls slowly due to a slow increase in engine load the piston 206 moves slowly left and the accompanying increase in volume of a chamber 194 of the rate control element 70 is compensated for by a flow of liquid through a fixed orifice 186 and the chamber 96 to the chamber 194, but when the pressure in the line 74 falls rapidly due to a rapid increase in engine load the flow through the orifice 186 is insufficient to compensate for the rapidly increasing volume of the chamber 194 whereupon the pressure in the chamber 96 is reduced to slow movement of the valve spool 120 and thus reduces the rate of change of transmission ratio. When the engine load is reduced the increased pressure in line 106 moves the valve spool 120 right to increase the pressure in the line 7 and thus reduce the throttling action of the valve 150 to restore the pressure supplied to the servomotors. A rapid rise of pressure in the line 74 due to a rapid reduction of engine load causes the piston 206 to move rapidly right and thus increase the pressure in the chamber 96 to retard movement of the valve spool 120 and thus the rate of increase of transmission output speed. The construction of the speed control and reversing valve 42, the steering control valve (46, 48) and the follow-up servomotors (50, 52, 56, 58) may be identical with those described in Specification 1435932. In Fig. 3 (not shown) oppositely directed relief valves (254, 256) are connected in parallel with the fixed orifice 186 to render the rate control element 70 ineffective to retard operation of the anti-stall valve 64 upon the occurence of a large rapid variation in the loading of the engine.