Patent GB-1122798-A (GB)
Title: Hydrostatic drive for vehicles
IPC: B60K17/10
Filed: 1965-10-21
Abstract: 1,122,798. Hydrostatic transmissions; hydrodynamic and hydraulic brakes. ROTENBURGER METALLWERKE G.m.b.H. 21 Oct., 1965 [21 Oct., 1964], No. 44543/65. Headings F2D, F2E and F2F. [Also in Division B7] A hydrostatic transmission for a tractor is downshifted by depression of the service brake pedal 24, and the service brakes are then applied by the differential fluid pressure in the working circuit of the transmission. The swashplate control 12 of an engine-driven pump 13, feeding a fixed-capacity motor 18, which drives the rear and/or front wheels of the tractor, is adjusted from zero to maximum, in opposition to a zeroizing spring 14, by fluid-pressure 5 supplied to a cylinder 6 by an engine-driven control pump 1, the output and input lines 5, 10 of which are subject to short-circuit by three by-pass valves 7, 8, 9 and a permanently open adjustable by-pass restrictor 16. The restrictor 16 acts in combination with a performance pedal 3, replacing the usual vehicle accelerator pedal, and acting on a swash-plate 4 controlling the capacity of the control pump 1, whereby control pressure 5, and hence the servo-control 11 of the main pump swash-plate 13, are rendered proportional to depression of the performance pedal 3, which latter thus directly controls vehicle speed through transmission ratio. Release of the performance pedal 3 causes the main swash-plate 13 to downshift slowly towards zero, for engine-braking, at a rate determined by the setting of the by-pass, restrictor 16. Depression of the service brake pedal 24 opens the by-pass valve 8 which provides unrestricted short-circuit of the control pump 1, so that the main pump swash-plate 13 is rapidly downshifted by the spring 14. The pressure subsequently developed in the return line 17b of the main motor 18 acts through lines 26, 28 on a piston 29 acting on the master-cylinder 32 of the service brakes in opposition to pressure 17a, 49 in the supply line 17a of the main motor. The effective pressure on the brake piston 29, and the stage at which it begins to operate the service brake, are determined by an adjustable relief-valve 34 connecting the idle side 30 of the piston to the drive pressure side 17a of the main pump 18. Brake-line pressure 36 is fed to a piston on the valve 8 to provide " feel " of brake-pressure on the pedal 24. A direct mechanical strut 40 to the master cylinder is also provided. A forward-reverse selector-lever 43, operable only in the release position of the performance pedal 3, reverses the input and output lines of the control pump 1 and connects the main motor return line 17b to a reservoir through a valve 27, thus preventing power brake operation in reverse. If power braking is required in both directions, the valve 27 actuated by the reverse lever 43 interchanges the pipes 49 and 26. The additional ratio-shift control by-pass valves 9, 7 act as follows. The by-pass valve 9 is an end-position valve which short-circuits the control-pump 1 when the swash-plate 13 of the main pump 18 reaches maximum capacity position to prevent further increase in capacity and hence vehicle speed. The by-pass valve 7 is a safety-valve which short-circuits the control-pump 1 when main working pressure 17a exceeds a value determined by a loading spring 53 variably compressed by a cam 23a on the rod of the main servo-piston 11.