Patent GB-1400867-A (GB)
Title: Hydrostatic transmission systems
IPC: F15B11/16
Filed: 1972-10-03
Abstract: 1400867 Hydrostatic transmission control HYDROMATIK GmbH 3 Oct 1972 [22 Oct 1971] 45499/72 Heading F2W In a hydrostatic transmission for a tracked vehicle wherein separate transmissions 1, 2; 1<SP>1</SP>, 2<SP>1</SP> drive tracks on opposite sides of a vehicle and each transmission has variable displacement pump 1; 1<SP>1</SP> and motor 2; 2<SP>1</SP> units, the pump displacements are controlled by servos 4, 4<SP>1</SP> responsive to the pressures in control lines 13, 14; 13<SP>1</SP>, 14<SP>1</SP> between fixed upstream throttles 8, 9; 8<SP>1</SP>, 9<SP>1</SP> and manually variable downstream throttles 10, 11; 10<SP>1</SP>, 11<SP>1</SP>. Pressure liquid is supplied from an engine driven pump 6 to the control lines through a common control line 52 which is normally maintained at a predetermined pressure by a relief valve 20 and the pressure in the line 52 may be overridden by exhaust valves 25, 26 respectively responsive to engine speed and transmission working circuit pressure to effect proportional ratio changes in both transmissions. The motor displacements are controlled by a servo 33 which is biased by a spring 34 to maximum motor displacement and moved towards minimum motor displacement by pressure liquid supplied through a line 35 including a fixed throttle 36 from the pump 6. The pressure in the line 35 is adjusted by a manually variable throttle 37. The override exhaust valves 25, 26 act additionally to reduce the pressure in the motor control line 35. The manually variable throttles 10, 11; 10<SP>1</SP>, 11<SP>1</SP> may be adjusted to provide equal output speeds from both transmissions for forward or reverse drive or may be adjusted to provide differential speeds in the two transmissions for steering purposes. If the engine speed falls due to overload of the engine, the valve 25 opens to exhaust a line 21 which is connected through oneway valves 39 with the motor control line 35 and the common control line 52. The reduction in pressure in the line 21 initially reduces the pressure in the line 35 to increase motor displacement and thereafter reduces the pressure in the line 52 to cause proportional reductions in the displacements of both pumps 1, 1<SP>1</SP>. The engine speed at which the valve 25 opens is adjusted by the engine throttle control lever 24. If the pressure in the working circuit of either transmission exceeds a predetermined value the valve 26 opens to effect increased motor displacement and decreased pump displacement as described above. The motor displacement control elements may, as shown, be linked mechanically and operated by a single servo 33 or may be actuated by individual servos connected to a common motor control line. In Fig. 1 (not shown) the motor displacements are automatically controlled by servomotors (19, 19<SP>1</SP>) which are subjected to the highest pressure existing in the control lines (13, 14, 13<SP>1</SP>, 14<SP>1</SP>). The override control valves (25, 26) act solely to reduce the pressure in the common pump control line. In Fig. 4 (not shown) reversing valves (44, 44<SP>1</SP>) are provided in the control lines (13, 14; 13<SP>1</SP>, 14<SP>1</SP>) between the servo motors (4, 4<SP>1</SP>) and the manually variable downstream throttles (10, 11; 10<SP>1</SP>, 11<SP>1</SP>). The manually variable throttles are adjusted by a linkage controlled by a steering wheel. An additional manually adjustable relief valve (43) is provided in the override circuit and provides a speed control in that depression of a pedal (41) progressively loads the relief valve (43) and thus increases the pressure in the common control line (52) and the motor control line (35).