Patent GB-978842-A (GB)
Title: A method of and means for controlling the motor of a power machine in a hydraulic power transmission feeding a plurality of machines
Inventor: BURRETT THOMAS MARK
IPC: F16H61/46
Filed: 1962-07-18
Abstract: 978,842. Hydraulic transmissions. R. C. PEARSON. June 28, 1963 [July 18, 1962], No. 27641/62. Headings F2C and F2D. An hydraulic transmission comprises a pump or pumps supplying a plurality of hydraulic motors, at least one of which is an individually controlled variable displacement motor having its displacement control means actuated by a jack the piston of which is subjected on one side to pump delivery pressure and on the other side to motor supply pressure, such that an increase in the motor supply pressure relative to the pump pressure increases the motor displacement, and a decrease of motor supply pressure causes a decrease of displacement. As shown in Fig. 1 two variable displacement pumps 1, 2 in parallel, which may be driven from the same or separate prime movers, supply liquid through a pressure conduit 4 to motors 9, 109 and may supply other hydraulic apparatus as indicated by the arrow A. The motor 9 is supplied through a conduit 22 incorporating a manually adjustable throttle valve 12 the pressure on either side of which is applied through lines incorporating restrictions 34, 35 to either side of a differential piston 25 of a jack 13 controlling the displacement of the motor 9. The jack 13 thus ensures a constant pressure drop across the throttle valve 12, and for any setting of this valve the motor delivers constant power. The motor 109 is controlled in a similar way by a manually adjustable throttle valve 112, a reversing valve 166 being inserted in the circuit for reversing the direction of rotation of. the motor. In certain conditions, for example due to the sudden loss of load of a motor driving a winch if a wire breaks, motor overspeed would result, and in order to prevent this a positive displacement pump 150 is driven by the motor 109 and supplies liquid from a reservoir 6 to a conduit 161 leading to a chamber 162 of the throttle valve 112 and leading through a restriction 155 back to the reservoir 6. In the event of overspeed of the motor the spool of valve 112 is urged upwardly by the pressure in chamber 162, compressing a spring 163 between the spool and its manual control lever so that fluid supply to the motor is restricted. The requirements of the motor drive may be such that from time to time the load acts on the motor in the same direction as its drive, for example, such as occurs when a heavy load is being lifted by a crane hoist, and the motor reversing valve 166 is operated to lower the load. In this event additional control means for controlling the motor speed are required, as now the closing of the control valve 112 would not prevent the motor 109 from being rotated by the load and complete closure of the valve 112 might cause cavitation. The fluid return conduit 170 to the reservoir is thus provided with a pressure-maintaining valve 171 producing a back pressure such that the largest load likely to be experienced in the motor over-run direction will be balanced by the torque of the motor 109, acting as a pump, so that overrun will not occur. To avoid cavitation in the motor supply conduit 122 it is connected to the reservoir 6 through a non- return valve 175. Under normal operating conditions it is undesirable to have the high back pressure produced by the valve 171, and it is therefore by-passed under normal conditions by a valve 173 which has a spool 180 subject at one end to the motor supply pressure applied to a chamber 186, and at the other end has a chamber 185 connected to the motor exhaust line 170 upstream of the valve 171. Under normal conditions the spool 180 is maintained in its right-hand position by the motor supply pressure, and exhaust from the motor passes through conduit 170, a conduit 177, through the valve 173 and by a conduit 179 to the reservoir 6. If the motor tends to overrun the supply pressure falls and the valve spool 180 is moved to the left by a spring to close the by-pass so that liquid returning from the motor passes through the back pressure valve 171. Fig. 2 shows an embodiment in which the throttle valve, instead of being actuated directly by a manual lever, is actuated through an hydraulic control system, and the valve serves the dual function of a throttle and reversing valve. Liquid is supplied to a motor 224 by a pump 201 through a valve 206 which is centred by springs to a neutral position in which the motor supply is cut off, and can be shifted to either side of this position to give throttled flow to the motor in the desired direction by application of pressure to end chambers 213, 214 by means of a manual control lever 260 which selectively actuates throttle valves 250, 251 in parallel conduits 252, 253 between the pump supply conduit 205 and a reservoir 203. The conduits 252, 253 are connected through fixed restrictors 254, 255 to the end chambers 214, 213 of the valve 206. Actuation of lever 260 to restrict flow through valve 250 or 251 increases the pressure in the associated line 252 or 253 and hence in the valve chambers 214 or 213 to shift the spool of valve 206 in the desired direction. The motor displacement is controlled by a jack 227 subjected on one side to the pressure in the pump delivery conduit 205 and on the other side to motor supply pressure which is connected to the jack through a shuttle valve 237. The motor 224 drives a unidirectional-flow type pump 240 delivering liquid to a conduit 243 connected to a valve 245 which is subjected to the pressures in conduits 252, 253 so that if conduit 252 or 253 is at the higher pressure liquid from the conduit 243 is supplied through a line 265 or 264 to the conduits leading to the end chambers of the valve 206. If, for example, the conduit 252 is at the higher pressure, the pump 240 supplies through conduit 243 and the valve 245 to the conduit 265 and returns through the restrictor 255 and the variable restrictor 251 to the reservoir. The faster the speed of the motor 224, the greater the delivery of the pump 240 and the greater the pressure in the conduit 265 and hence in the end chamber 213 opposing the pressure in the chamber 214 and tending to move the valve to its closed position with consequent reduction of output torque of the motor 224, until the speed of the motor corresponds with the setting of the operator's control lever 260. With this arrangement all the motors may be controlled through conveniently grouped control levers at some remote station.