GB1269141A - Improvements relating to hydraulic transmissions - Google Patents

Abstract

1,269,141. Hydrostatic transmission. INTERNATIONAL HARVESTER CO. 16 May, 1969 [23 May, 1968], No. 25090/69. Heading F2D. In a vehicle hydrostatic transmission having by-pass valve means preventing excessive pressures during overrunning to prevent overspeeding of the pump and engine, an anti-coast valve 139 is operative in forward only to prevent complete by-pass of fluid during overrunning to maintain engine braking effective. The transmission comprises a pump and a motor of the variable-stroke swash-plate type having by-pass relief valves 19, 19A between their pressure and return lines. The valves have spring-loaded spools 38, 38a subject to the pressure in a line 18a, 18 acting via a line 20, 20a and connected to a respective transmission line. Passages having restricted orifices 48, 47 and 48a, 47a are provided in the valves and lead via lines 46, 46a to chambers between relief valves 82, 93 and 82a, 93a which are connected by the anti-coast valve 139 and a line 50 to a manually-adjusted spring- loaded pilot relief valve 52. Valves 82a and 93a are selectably unseated, according to the direction in which the transmission motor is to be driven, by a slidable cam 117 acting through push rods 107, 106a, and positioned by an F-N-R lever 118. In neutral, push rod 107 is raised by land 135 on cam 117 to open valve 93a and, through push rod 106a, to open valve 82a. In forward gear, the rod 107 abuts reduced diameter portion 133 and is raised sufficiently to open valve 93a but not valve 82a. In reverse gear, the rod 107 abuts portion 134 and both valves remain closed. Under normal drive in forward gear, line 46 carries high pressure acting between valves 82, 93 to open valve 82 and force spool 140 of anti-coast valve 139 down so that the high pressure may be regulated by the relief valve 52 which relieves excessively high pressure or allows the transmission to be brought into neutral by depression of pedal 76. On overrun, high pressure in line 46a acts on the spool 140 via the open valve 93a and a passage 111, to raise the spool and close line 50 preventing complete by-pass. The pressure also acts to open valve 93 thus reducing the pressure acting on the rear of spool 38a which is thereby shifted to interconnect the lines 18, 18A to reduce the excess pressure. In reverse, during normal operation, high pressure in line 46a opens valve 82a and forces spool 140 down to open line 50. Similarly on overrun, high pressure in line 46 opens valve 82 and holds spool 140 down. High pressure is thus regulated by only the pilot relief valve 52 during reverse. Depression of pedal 76 will again neutralize the transmission, and may also be used to provide smooth power take-ups. A shuttle valve (24), Fig. 1 (not shown), is opened by high pressure to direct low make-up pressure from an engine-driven pump (12) for cooling.