GB1412830A - Torque limiting means for variable displacement pumps - Google Patents

Abstract

1412830 Fluid-pressure servomotor systems R J IFIELD 4 Oct 1973 [23 Oct 1972] 46385/73 Heading G3P [Also in Division Fl ] The driving torque of a variable displacement piston pump having a rotor (1), Fig. 1 (not shown) supported by hydrostatically balanced slipper bearings 14, 15 is limited by means which regulate the pump displacement in response to excessive hydrostatic bearing pressure. The pump includes a servomotor 12 Fig. 3 whose piston has a rack engaging corresponding teeth on an adjustable swash-plate (10) against which the axial pistons (2) carried by the rotor (1) bear. The high and low-pressure pump ports A, B are inter-connected downstream of non- return valves 22, 23 so as to supply the highest pressure via line G to the centre gallery of a manually operated reversing valve 26. The latter directs fluid via servovalve 30 to one or other ports E, F of servomotor 12 for forward or reverse operation respectively. Spool 31 of servovalve 30 is normally held in the position shown by pressure from lowpressure pump 17 supplied to a chamber 33 below an enlarged piston 32 on the spool. Chamber 33 is also connected to a pilot valve device 38 having plungers 39, 40 engaging closures 41, 42 urged against their seatings by adjustable springs 43, 44 to close a drain line H leading to reservoir 18. Plunger 39 is subject to the main pump output pressure in line G so that if this pressure becomes excessive closure 41 unseats to vent chamber 33 to reservoir 18, causing spool 31 to move downwardly under the action of spring 34. Thus communication between manual valve 26 and servomotor 12 is cutoff and ports E, F of the latter are connected to pump ports B, A respectively causing the servomotor piston to move downwards and reduce pump displacement. The highest of the pump pressures at ports A, B is also communicated via restrictors 24, 25 and connections C, D to the rotor slipper bearings 14, 15. The highest pressure at the latter, which is proportional to the driving torque on the rotor (1), is supplied via non-return valve 45 or 46 and line J to act on pilot plunger 40. Thus excessive pressure indicative of excessive torque causes unseating of closure 42 to vent chamber 33 with consequent reduction in pump displacement as previously described. To reduce instability a device comprising a piston 47 having a roller 48 bearing against cam 49 on the servomotor piston transmits to the spring chamber of pilot valve 38 via line L a biasing pressure which is a function of the rate of change of pump displacement. The adjusting screw for loading spring 44 of the torque limiting closure 42 may be replaced by a push-rod operated from a cam-shaft coupled to the controls of the prime mover driving the main pump so that the driving torque of the latter is a function of the setting of the controls. In a further modification Fig. 6 (not shown) the torque limiting closure 42 is biased to the closed position by the differential pressure across a variable orifice (62) and acting on a piston (63) connected to the closure. Since the orifice (62) is subject to the output pressure of low-pressure pump 17, which is driven by the prime mover, the driving torque of the main pump increases as a function of prime mover speed. In another modification, Fig. 4 the functions of reversing and servovalves 26, 30 are combined in a single valve comprising a ported sleeve 55 rotatably mounted in housing 56 and connected to lever 54 for manual setting of the desired pump displacement in forward or reverse modes. A rotary valve element 57 inside sleeve 55 has a gear 58 driven by teeth 59 on the swash-plate (10) or on the rack of servomotor 12 to provide a moving seat follow-up. The chamber 60 at one end of valve element 57 is subject via restriction 61 to the pressure within the main pump housing, i.e. the output pressure of low-pressure pump 17, which normally urges element 57 to the right as shown. The chamber 60 can be vented under the control of pilot valve device 38, as previously described, to cause element 57 to move leftwards to reduce main pump displacement in response to excessive output pressure or driving torque.