Patent GB-1325958-A (GB)
Title: Hydrostatic drive for vehicles
IPC: B60K17/10
Filed: 1971-04-19
Abstract: 1325958 Hydrostatic transmission ROBERT BOSCH GmbH 19 April 1971 [14 April 1970] 20388/71 Heading F2D A vehicle hydrostatic transmission, Fig. 1, comprises a reversible and adjustable pump 2 feeding a reversible and adjustable motor 6, e.g. of the axial piston type, the motor displacement being adjusted, during normal operation, by a power booster and pressure regulator 9, Fig. 2, in response to the circuit pressure, but being held non-adjustably during engine braking so that the high pressure in the return line has no adjusting effect on the motor displacement. As shown in Fig. 2, the booster comprises a differential piston 21, 24 in a bore 22, the larger diameter piston part 24 defining two pressure chambers 26, 27 at its opposite ends. The chambers are interconnectible by a passage 43 controlled by a spool valve 31. In forward, a valve 20 in a line 11 connected to chamber 27 is moved to its closed position I, line 4 is the pump delivery line and with increasing pressure communicated to chamber 26 to act on spool 31 urging it against a spring 36 to connect the  high pressure to chamber 27 to move the piston 21, 24 to the left, the motor displacement is increased until the pressure falls sufficiently for spring 36 to return spool 31 and connect chamber 27 to exhaust through line 39. Similarly falling pressure in line 4 is insufficient to open valve 31 but acts on the differential piston 21, 24 to move it to the right to decrease the motor displacement and increase vehicle speed and the pressure. On engine braking occurring, the high pressure in line 5 is cut off from chamber 27 by valve 20 so that the motor displacement is not adjustable. In reverse, valve 20 is moved to its position II so that high pressure in line 5 is transmitted directly to chamber 27 and the differential piston 21, 24 is moved and held in its left hand position with the motor displacement at a maximum. On engine braking occurring, the high pressure in line 4 is connected through line 10 and chamber 26 to open valve 31 and act in chamber 27 to hold the motor displacement at a maximum without adjustment. Again, Fig. 3, (not shown), a three position valve (45) is included in line 10, selectively connecting the lines 4 or 5 to line 10, or blocking line 10. Line 11 is connected only to line 4, and forward control is identical to that of the Fig. 1 embodiment. Reverse control is effected with valve 20 open and valve (45) connecting line 5 to line 10. Valve (45) blocks line 10 during slow travel, e.g. tractor moving over cultivated land. Again, the larger diameter part of the differential piston carries a seal (52), Fig. 4 (not shown), preventing inadvertent movement of the piston by reaction of the motor pistons during engine braking. A cut-off valve (53), Fig. 5 (not shown), may be included in passage (61) in the differential piston similarly to prevent inadvertent movement. In Fig. 6 (not shown), the line 11 is omitted, the line 10 including a one-way valve (65<SP>1</SP>), and the feed pump 3 having a weather relief valve (69) to hydraulically lock the motor displacement member in position during engine braking.