GB1310351A - Hydrostatic transmission systems - Google Patents

Abstract

1310351 Hydrostatic transmission control JOSEPH LUCAS (INDUSTRIES) Ltd 1 July 1970 [3 April 1969] 17534/69 Heading F2D In a vehicle hydrostatic transmission cornprising a pump 10 and a reversible motor 12, the inclination of the pump swashplate 11 is varied by a servo 22 controlled by a valve 28 positioned by plungers 37, 38 acting on a lever 36 and subjected to pump delivery pressure and an engine speed signal respectively, the pivot pin 39 of the lever being adjustably positioned by a cam 40 fast with the servo piston-rod 26 to provide a mechanical feedback to the valve, and a manual override being provided, Fig. 2 (not shown), to hold a low ratio, but being itself automatically overriden on overrun when the pump input pressure, or engine speed, exceeds a predetermined value. The plunger 38 has a differential area subjected to fluid pressure from opposite sides of a restriction 20 in a line from an engine-driven pump 15 which, together with an output-driven pump 16, provides the operating pressure for the transmission. A relief valve 9 limits the pressure drop across the restriction 20 and is adjusted by the accelerator pedal. The manual override allows the transmission to be held in either of two low positions and comprises a bifurcated lever (52), Fig. 2 (not shown), pivoted at one end on the piston-rod 26 and at its other end on a control rod (49) so as to abut a spring sleeve (54) on the valve stem (35) when in either of its operative positions and hence limit upshift of the transmission. The rod (49) is controlled by a pair of pistons (43), (44), individually controlled by a pair of switchoperated solenoid valves (H 1 ), (H 2 ) to move the rod (49) axially to position lever (52) for abutment by spring sleeve (54). A pressure switch P is included in the valve switch circuit and is operated to override the manual low hold by high pressure in line 7 connected to the pump inlet. The high pressure in line 7 arises due to excessive engine braking on overrun, or due to the closing of valve 8 in line 7 by high flow through the valve from the engine-drive pump 15 during excessive engine speeds. Again, Fig. 3 (not shown), the valve spool (128) is surrounded by a valve sleeve (129) which follows up due to a pivoted lever (136) connected to the servo (122). The spool (128) is subjected at one end to a plunger (137) and at the other to the differential piston (138) and a spring (140) between the spool and a cup (141) fast with the piston (138). A sleeve (144) is moved axially by a servo-actuated rack and pinion (146), (147) to hold a low ratio, and the servo is relieved when the pump input pressure exceeds a predetermined value. The input pressure is also directed to a plunger (149) which acts directly on piston (138) to upshift the transmission when the hold is released on overrun.