Patent GB-1534719-A (GB)
Title: Vehicle including an auxiliary hydrostatic drive arrangement
IPC: F16H61/452
Filed: 1975-10-15
Abstract: 1534719 Hydrostatic transmission control DEERE & CO 15 Oct 1975 [23 Oct 1974] 42165/75 Heading F2W In an auxiliary hydrostatic transmission driving the front steerable wheels of a tractor which has its rear wheels driven through a mechanical transmission wherein a variable displacement pump 22 supplies pressure liquid through a reversing valve 28 and series-parallel valve 38 to fixed displacement wheel motors 20L, 20R, a manually actuated switch 64 selects series or parallel operation of the motors and switches 120, 124, 150, 130, 132, 142, which are closed by magnets on range and ratio shift levers 154, 152, override parallel motor operation and impose series motor operation when the range selector lever 152 selects high forward range in the mechanical transmission and cuts out the hydrostatic transmission when the range and ratio selector levers 152, 154 respectively select high ratio and high range. In addition pressure responsive switches 250, 252 are respectively closed to override parallel motor operation and impose series operation and to cut out the hydrostatic transmission when the working circuit pressure falls below first and second predetermined values. With the switch 64 in its lower (series motor operation) position closure of the low forward range switch 120 or reverse range switch 150 and one of the ratio switches 130, 132, 142 renders the forward transistor circuit 80, 82 112 or reverse transistor circuit 84, 86, 114 conductive to energize forward or reverse solenoids 98, 102 which move the reversing value 28 to its forward or reverse drive position. The forward solenoid 98 is maintained energized when the high forward range switch 124 and one of the two lower ratio switches 130, 132 are closed but is de-energized when the high range switch 124 and high ratio switch 142 are closed whereupon springs move the reversing valve 28 to its central neutral position. With the switch 64 in its upper (parallel motor operation) position the reversing valve is actuated as above described. In addition a series-parallel transistor circuit 166, 168, 180 is rendered conductive when the low forward range switch 120 as reverse range switch 150 and one of the ratio switches 130, 132, 142 are closed whereupon a solenoid 172 shifts the series-parallel valve 38 to its parallel operation position. In high forward range the solenoid 172 is de-energized and a spring moves the valve 38 to its series operation position. When the working circuit pressure falls below a first predetermined pressure the switch 252 closes and transistor circuit 212, 220 renders the transistor 180 non-conductive to impose series motor operation. If the working circuit pressure falls below a second predetermined value lower than the first predetermined value the switch 250 closes and a transistor circuit 210, 218 renders the transistors 112, 114 non-conductive whereupon the reversing valve 28 moves to neutral. The pump 22 additionally supplies pressure liquid through a line 32 to power steering, braking and lifting mechanisms not described. The motors 20L, 20R are connected to wheels 14 through hydraulically engaged clutches 21.