GB1437424A - Hydrostatic transmission - Google Patents

Abstract

1437424 Hydrostatic transmission control KOEHRING CO 26 July 1973 [13 Sept 1972] 35598/73 Heading F2W In a hydrostatic transmission for control of the swing of the boom of, e.g., a crane, a fluid motor 12 is supplied with fluid under pressure from a variable displacement pump 11, the swash plate 13 of which is adjusted by pressure fluid supplied to servos 14 and 15 under the control of a valve assembly 23 and a mechanical feedback linkage between the assembly 23 and the swash plate 13. A control lever 33 adjusts an air pilot valve 22 to supply a pneumatic signal to a converter 24, which in turn supplies oil under pressure from a pump 25 to either a pilot port 30 or 31 (the other being vented) in assembly 23. Assembly 23 comprises a valve 36 having a spool 37 and a valve 40 having a spool 41 in the illustrated embodiment, charge pressure for pump displacement adjustment being supplied to a port 60 and ports 61 and 62 being connected to drain. On pilot pressure being supplied to port 31 valve spool 37, centred by springs 55, is moved to the left (the position shown) to put port 60 in communication with a port 63 and pressure is also applied to the right hand side of spool 41 (chamber 72) to move this to the left against the force of a spring 88 which acts between an abutment 97 on a pivoted member 90 linked to the spool 41, and a fixed abutment 98. This movement of spool 41 brings port 63 into communication with a port 76 from which fluid passes to servo 14 to adjust the pump swashplate 13. A link 104, pivoted at 105 to the swashplate 13 and a further link 100 interconnects swashplate 13 and spool 41. The movement of the swashplate 13 tensions spring 88 further and the spool 41 is returned to the central position shown when the force of this spring equals the pressure in chamber 72, so that the adjustment of the swashplate depends on the pressure supplied to the pilot port 31. The load on the motor is reflected in the pressure in the servo which holds the pump adjusted, 14, if pilot pressure is supplied to port 31, which pressure is transmitted through a groove 75 in a land 73 of spool 41 and an orifice 65 to a chamber 57 of valve spool 37, which pressure opposes that from port 31, in chamber 54, and so the spool 37 is moved to the right if this load is too great. This movement connects port 63 to drain and with spool 41 to the left of centre the servo 14 is connected to drain until the load is reduced. On return of the control lever 33 towards neutral the pressure at port 31 is reduced, spools 41 and 37 move to the right and the pressures in servos 14 and 15 reverse, thereby braking the motor 12. In an alternative embodiment (Fig. 7, not shown) the groove 75 and orifice 65 are replaced by a bore in the spool 37. With the lever 33 in its neutral position the motor 12 can rotate freely since neither servo 14 or 15 will be pressurized and so the motor 12 can move the swashplate of the pump by means of the fluid flow when the motor rotates. The system can work in reverse due to its symmetry.