HYDROSTATISCHE ANTRIEBSVORRICHTUNG

1480031 Hydrostatic transmission control DEERE & CO 13 Jan 1975 [14 Jan 1974] 1392/75 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 38 to fixed displacement axial or radial piston motors 20L, 20R, the motor casings are normally connected to a reservoir 24 through a valve 72 which, when the reversing valve 38 is in a neutral setting, connects the motor casings to the output ports of the motors to pressurize the casings when the motors are driven by the vehicle wheels and thus create a small pressure differential across the motor pistons to thereby reduce damage between the pistons and their associated cam or swash plates by reducing the period of contact therebetween. The pump 22 supplies pressure liquid through a line 36 and priority valve 52, which opens when the pressure upstream thereof exceeds a predetermined value, to the reversing valve 38. The valve 38 is spring biased to its neutral position and is moved to its forward or reverse position by pressure liquid supplied from the high pressure line 36 through solenoid operated pilot valves 88, 90 and lines 100, 110. The pressure in the lines 100, 110 is additionally supplied to opposite ends of the valve 72 which, when the reversing valve 38 is moved to its forward or reverse position, moves to a position in which it connects lines 68, 70 from the motor casings to a reservoir line 86. The motors 20L, 20R may be connected in parallel or in series by a valve 58 controlled by a solenoid operated pilot valve 92. When the pilot valves 88, 90 are both deenergized the valves 38, 72 are moved by spring to the positions shown. If under these conditions the vehicle wheels drive the motors 20L, 20R the pressure output therefrom in line 54 or 60 passes through a one-way valve 78 and the valve 72 to pressurize the motor casings. The pump 22 additionally supplies pressure liquid through a line 48 to power steering and braking mechanisms not described. The return line 40 to the reservoir contains a cooler 42 and is maintained at a predetermined low pressure by a throttle 46. In Fig. 3 (not shown) the valves 38 and 72 are combined as a single unit.