Patent GB-1285543-A (GB)
Title: Control system for a hydrostatic transmission
IPC: F02B1/04
Filed: 1970-02-04
Abstract: 1285543 Hydrostatic transmission control ROBERT BOSCH GmbH 4 Feb 1970 [4 Feb 1969] 5245/70 Heading F2D The control for an hydrostatic transmission comprising a variable delivery reversible pump 11 and a variable delivery motor 12 includes a servo 22 for adjusting the pump in response to the pressure difference across a variable throttle 63 in the feed line 43 from an engine-driven make-up pump 21, the throttle 63 being positioned by the accelerator pedal 64, and the pump 21, throttle 63 and servo 22 acting as an engine speed governor maintaining the engine speed constant for a given position of the pedal 64. Should the engine speed increase without moving the accelerator pedal, the consequent increase in flow from the pump 21 causes an increase in the pressure difference across the throttle 63. The pressure difference is applied via lines 53 and 78 to opposite sides of a control valve 60 to close the valve, blocking a discharge line 61 connected to the discharge line 81 of a manuallypositioned valve 79. Pump pressure in a line 43<SP>1</SP> is delivered to the valve 79 and passes into a line 85 or 87 for forward and reverse and interconnected by a throttle 8711, to act on opposite ends of a relief valve 86 which opens when the pressure difference exceeds a predetermined amount, viz when exhaust line 81 is relieved. By closing valve 60 and hence line 81 then, valve 86 also is closed allowing the pump servo 22 to be connected to the high transmission pressure in a line 105 via a valve 89 acted on at opposite ends by make up pump pressure 90 and the pressure in a line 88 connected to the relief valve 86. The servo upshifts the transmission to increase the load on the engine and hence reduce its speed until the pressure difference allows the valve 60 to again open and connect the servo 22 to exhaust. The servo 22 includes pistons 23, 24 in one cylinder 25 and an opposing pair of pistons 26, 27 in a second cylinder 28, the pistons acting to position a movable cam ring of the radial piston pump. The pistons 24 and 27 act to provide stops to define neutral positions for the pump in forward and reverse, or to float freely, depending on the position of a reversing valve 95 mechanically coupled to the manual reverse valve 79.  Brake valve 70 is operated by brake pressure and closes an overflow line 69 from a valve 68 opened by the accelerator, and relieves the line 61 to reduce the volume of pump 11 which is acting as a motor, to increase engine braking. If this is not sufficient, further depression of the brake pedal 112 brings into action the normal brakes. To prevent progressive increase in engine braking whilst holding the pedal in one position, the high transmission pressure is used to provide a return force on the pedal via a pressure reducer 108. Over-revving of the engine due to engine braking results in the closure of a safety valve 57 to block line 61 and hence upshift the transmission. The valve 57 is opened by a relief valve 54 which opens to limit the pressure difference across the throttle 63. Safety devices.-Make up pump fluid in line 45 acts to load a high pressure relief valve 19 so that a pressure drop due to loss of oil or failure of the feed pump 21 is initially relieved by the valve 19 and then, on further pressure drop, the low pressure in line 90 acts to open a valve 92 to connect the line 94 to piston 27 to exhaust, to lock the pump in neutral. The transmission is similarly moved to neutral by the valve 92 on overheating sensed by a thermostat 40 which opens a valve 41 to relieve the line 90.