Patent GB-1404562-A (GB)
Title: Hydraulic systems
IPC: F15B11/05
Filed: 1973-07-02
Abstract: 1404562 Hydraulic control circuits BORG-WARNER CORP 2 July 1973 [13 July 1972] 31325/73 Heading F1P The circuit shown in Fig. 1 operates as follows. Valve 36 gives priority to the bucket-tilting function of ram 23 controlled by valve 14. If a spool 35 is as shown, no pressure is applied to a logic system 27 and, assuming that no signal pressure is applied to conduits 85, 88, 89, then no control signal pressure is delivered to control port 61, and any residual pressure in conduit 93 goes to reservoir 13g through a throttle 94. Thus, system pressure applied to chamber 57 moves an element 51 to the left against a spring 53 and opens a path from chamber 50 to chamber 46. The operation of valve 33 controlling the lifting ram 41 is thus not restricted by valve 36 when valve spool 35 is in the neutral position. When the valve 14 is slowly actuating the bucket ram 23 a maximum pressure drop exists from a chamber 31 to one or other of work ports 24a, 24b. The appropriate control port, 26a, 26b, receives a pressure equal to the work port pressure which is lower than system pressure by the amount of this maximum pressure drop. The system pressure in chamber 57 is able to overcome the combined force of the spring 53 and the signal pressure in chamber 62 to move the element 51 to the left so that valve 36 does not restrict flow to valve 34. When the spool 35 of valve 14 is fully actuated very little pressure drop occurs between chamber 31 and work port 24a or 24b. As a result the control signal pressure in chamber 62 is almost equal to the system pressure in chamber 57 so the element 51 is moved to the right by spring 53 thereby restricting flow from chamber 50 to chamber 48 and giving priority to valve 14. Valve 36 also functions as a load checker to prevent the load axis raised by ram 41 from descending if the pressure in conduit 37 drops below actuating pressure. To increase the operating threshold of valve 36, a pressure B is applied through conduit 91 to chamber 65 where it moves a plunger 72 to the right and pre-loads the spring 53. The pressure B in line 91 is selected by logic circuit 27 and is the highest pressure of those present in lines 85, 76, 77 as selected by check valves 81, 82, 83. By-pass valve 11 provides a variable system pressure and is controlled by a pressure A in line 93. Pressure A is the highest of pressures B and those present in conduits 88, 89. In Fig.2, (not shown), three-port shuttle valves are used in the logic circuits instead of check valves and the throttles 84, 94 are replaced by cut-away lands on spool 35. A variable displacement pump, Fig.3 (not shown), may replace the fixed displacement pump 10 and by-pass 11. Fig.5 shows a backhoe control circuit. When a limiting pressure is reached in either the bucket ram or the crowd ram 184 the higher of these pressures is fed to conduit 206 and an actuator 207. This moves valve 210 and applies pressure to ram 182 to raise the boom 181 thereby decreasing the digging load. Fig.6 (not shown), shows a modification of valve 210 which allows the boom to float when it is actuated as described above.