Patent GB-1302741-A (GB)
IPC: F15B11/00
Filed: 1970-01-02
Abstract: 1302741 Fluid-pressure servomotor systems CATERPILLAR TRACTOR CO 2 Jan 1970 [7 April 1969] 212/70 Heading G3P [Also in Division F2] A hydraulic circuit wherein a low-pressure system is employed for controlling a high-pressure fluid work output system includes a selectively actuable control valve 71, Figs.1 and 2, connected to a source 61 of low-pressure fluid, a variable displacement pump 37 having a high-pressure variable flow output and a control servo 79 for adjusting the output, conduit means 139, 183 connecting the control valve to the servo to adjust the pump output in response to the position of the control valve, a directional valve 231 having its output connected to the work output system and its inlet 53 connected to the outlet 47 of the pump, and fluid actuated means 137 between the control valve and the directional valve to control the direction of the work output. In the preferred embodiment, the circuit is applied to a bucket loader having "lift" and "tilt" cylinders 261, 375 each controlled by a separate control valve 71, 73 through a pair of fluid actuated valves, such as 137, 297, and a directional control valve 231. The low-pressure source provides a maximum pressure of 625 p.s.i. and the control valve regulates the output pressure in a line 99 so that within the range 0-300 p.s.i. control of the valves 137, 297 and 231 is effected while pressures above 300 p.s.i. adjust the servo valve 75 through a fluid-actuated priority valve 141. The pump 37 provides a high-pressure output of 6000 p.s.i. and receives its supply through a line 33, a filter 31 and a cooler 29 communicating with a pump 21. A relief valve 59 returns the surplus pump charging flow to tank. When valve 71 is manually actuated to "raise" position, fluid is directed through a line 99 and a shuttle valve 133 to a priority valve 141 and through a line 135 to a "kick-out" valve (not shown) on the "lift" cylinder and to a fluid-actuated valve 137. Actuation of the latter causes shuttle valves 227 and 233 to connect lines 149, 229 and 235 to close a connection 55, 57 to tank and allow pressure to build up in a line 53 in response to increased delivery from pump 37 and causes a line 243 to be connected to tank to free a spring-loaded valve 253, Fig.14, in the directional control valve 231. Further movement of the control valve 71 causes the priority valve 141 to supply actuating fluid to the servocontrol valve 75 which adjusts the servo cylinder 79 until the pump 37 is producing the required output and the valve 75 is again closed by a moving seat follow-up. Increased pressure in the line 53 now opens the valve 253 against load reaction pressure from the cylinder 261 which expels fluid through a valve 295 to tank, the valve 295 having been prepared for opening by the application of pressure from the line 229 to a piston 285 to reduce the load on a pilot poppet valve 291. Should the cylinder 261 attempt to run faster than the delivery from the pump 37 permits, the valve 295 closes to restrict the flow of fluid to tank 23. When the cylinder 261 reaches the end of its stroke a cam opens a spool in the "kick-out" valve to cause the valve 71 to be restored by fluid-pressure to its neutral position. A similar sequence of events follows opening of the "tilt" control valve 73. During a lowering operation, a line 331 directs fluid to a second pilot valve 297 to pressurize lines 339, 343 and 235, closing the connection 55 to tank and causing the valves 265 and 267 in the directional control valve 231 to function as inlet and outlet valves, respectively, for the cylinder 261. When the control valve 71 is set to "float" position both pilot valves 275, 291 are relieved of pressure from the pistons 285 and 349. If the "tilt" control valve is actuated when the "lift" control valve is already calling for maximum pressure, the valve 141 interrupts the 'lift" signal until the displacement of the pump 37 has been increased to supply both circuits adequately. In a modified arrangement, a directional control valve having inlet  poppet valves 254, 266, Fig.16, and exhaust valves 268, 296 released by fluid actuated spools 350 and 286 are used. Relief valves, such as that shown at 276, protect the circuit, which also supplies steering and brake systems, against excess pressure. Specifications 1302742, 1302743 and 1302744 are referred to.