GB1352028A - Fluid power transmissions - Google Patents

Abstract

1352028 Hydrostatic transmission of power CLARKE CHAPMAN-JOHN THOMPSON Ltd 1 Aug 1972 [4 May 1971] 12927/71 Heading F2D In a hydrostatic transmission in which a variable displacement axial piston pump 4 supplies fluid to a motor 6, the pump displacement can be controlled so as to maintain the load (i.e. the pressure in the main line 52 to the motor) constant. As shown the pump 4 and three auxiliary pumps, 8a, 8b and 8c, are driven by an electric motor 2. A follow-up servo 10, supplied with pressure via a composite valve unit 14 (which also provides pressure relief for the main hydraulic circuit) from the main hydraulic circuit, controls pump 4 displacement and is acted upon by a lever 16 which is pivoted with a rod 18 when this rod is pivoted about its longitudinal axis by one of levers 20, 22 or 24. The rod 18 is shown rotated by 90 degrees in relation to servo 10 in order to clearly show its construction. Each lever 20, 22, or 24 is pivoted by a cam 30, 32 or 34, each cam being moveable by pressure acting on pistons (38, 70 and 78 respectively). In the case of piston 30 an adjustable pressure acts (along with a spring 44) in a chamber 42 at one end of the piston, this coming from auxiliary pump 8b via either a hand controlled valve 50 or a valve 52 controlled remotely by a potentiometer 54 controlling fluid supply to the ends of a piston 68 which determines the position of a cam 60 which acts on valve 52. The pressure in the main line of the hydraulic circuit acts via lines 46, 76 on the opposite side of the piston 38 so that the cam 30 is moved, and so the lever 20 moved and, via rod 18 and lever 16, the pump output adjusted, which again causes the cam to be moved, and so the pressure in line 52 can be maintained constant. Pipe 76 is of small bore to reduce the effect of cyclic pressure variations in the pump output. The lever 22 is controlled in similar fashion to the lever 30, except that a spring 74 alone acts on one side of piston 70 and that the cam 32 is of a special shape so that it can limit the maximum power output of the transmission (Fig. 3, not shown). The lever 24 is controlled through a piston 78 whose position is determined by a manually operated potentiometer 82, controlling pressures on either side of the piston via an electrical follow-up servo system. The lever 16 is connected to the rod 18 through a spring 26 so that it can be moved by a member 28 to change the transmission to neutral (pump output zero) irrespective of the positions of the other levers 20, 22, 24. This acts as a safety cut-out and can operate (as shown) on failure of pressure in the hydraulic circuit, or electrical failure. The auxiliary pump 8c pumps fluid through a cooling device (not shown). The transmission shown, which is only suitable for drive in one direction, can be used to control a mechanical grab on a crane, wherein a constant load to the grab is required irrespective of any hoisting movement. In a further embodiment two levers such as 20 and two levers such as 22 are provided, one for each direction of rotation so that the transmission can operate in reverse. The motor 6 shown is fixed stroke but it can be variable stroke, so that its stroke can be reduced on a fall of pump output to help maintain a constant power output. This control can be via a further cam and piston arrangement. In addition it is possible for the motor to be a ram, instead of rotary, and the maximum power lever 22 may be omitted, in both embodiments. The cam 32 of the unit for limiting maximum power can additionally be shaped to limit the maximum speed of the motor when this is being driven in reverse by the pump in a zero load condition (in the embodiment of Fig. 1).