Patent GB-974464-A (GB)
Title: Transmission control system employing a differential check valve
IPC: F16D67/00
Filed: 1963-06-05
Abstract: 974,464. Change-speed control. CATERPILLAR TRACTOR CO. June 5, 1963 [Feb. 25, 1963], No. 22305/63. Heading F2D. A control system for a transmission especially for heavy duty vehicles such as earth moving equipment, which require more than a single clutch or brake to be engaged in order to engage a desired gear ratio, provides fluid pressure for the engagement of a pair of clutches or brakes at the same time and at the same rate of pressure increase until one reaches its maximum pressure after which time the other continues to receive increased pressure. A pump 11 supplies liquid at a pressure determined by a valve 15 to a conduit 13a leading to a pressure control valve group 14 having output conduits 17, 21 leading to selector valves 18, 22 controlling servo devices 19, 23. On initially starting the engine liquid from the conduit 13a passes to a chamber 61 and thence through passages 62, 63, 64 in the spool 36 of a sequence valve 31 to a chamber 66 and thence to a drain line 67. Before a ratio can be engaged, a control shaft 27 must be set to a netural position in which a valve 16 is actuated to supply liquid through a conduit 26 to depress the spool 41 of a safety valve 24 against spring bias to cut off the passages 64 from the chamber 66 so that the pressure in chamber 61 rises, valve spool 36 is depressed against spring bias, and liquid from chamber 61 passes through a passageway 71, and an annular orifice 72 formed between the spool 46 of a modulating valve 32 and the valve housing to a chamber 73. The chamber 73 communicates with the line 17 leading to the selector valve 18 and also communicates with an internal chamber 74 in the valve spool 46 and reacts against a slug 75 to urge the spool upwardly against a biasing spring 48. With the control shaft 27 in neutral the valve 18 blocks communication between conduit 17 and a conduit 29 leading to the conduit 21 and the control valve 22. On moving the control shaft 27 to give a ratio, conduits 17, 29 are interconnected and the pressure in chamber 73 drops causing a corresponding drop in the chamber 74 so that the spool 46 is urged downwardly to give the maximum area of orifice 72. Conduit 29 communicates round the spool 51 of a valve 33 with a passage 54 communicating with the conduit 21, and the passage 54 also communicates by way of restricted passages in the spool of a check valve 34 with a chamber 87 of a load piston 49 which determines the loading of the spring 48 which biases the modulating valve spool 46. The spool 46 is thus moved downwardly to increase the size of the orifice 72 to gradually increase the pressure in the lines 17, 21. When the desired maximum pressure to be applied to the selected servo device 23 is obtained in the passage 54, the pressure in an internal compartment of the valve spool 51 and reacting on a slug will be sufficient to raise the spool to cut off the passage 54 from the conduit 29, after which the differential pressure in the passage 54 and chamber 87 due to the restriction in the check valve spool 34 will gradually disappear so that the load piston 49 will continue to move downwardly to increase the pressure in conduit 17 and the selected device 19 still further. The pressure in chamber 87 is applied through passages 91 to a chamber 92 above the check valve spool 34 to assist a spring 58 to move the valve downwardly when a gear change is made so that chamber 87 is opened to a drain line 88 and the modulating valve spool 46 receives its lightest biasing.