Patent GB-1027416-A (GB)
Title: Hydrostatic power transmission
Inventor: NORTH JOHN DUDLEY
IPC: F15B1/02
Filed: 1964-03-02
Abstract: 1,027,416. Hydraulic transmissions. BOULTON PAUL AIRCRAFT Ltd. Feb. 10, 1965 [March 2, 1964], No. 8712/64. Heading F2C. [Also in Division F1] In an hydrostatic transmission comprising a positive displacement pump connected by lines 11, 12 with a positive displacement motor, a low pressure accumulator 13 is connected to the transmission pump inlet line 12 to accommodate variations in the total liquid volume in the transmission, a low-pressure gear pump 18 feeds transmission pump leakage in a leakage collecting line 17 to the line 12 through a non- return valve 21, and a spring-loaded by-pass valve 22 arranged between the accumulator and the leakage collecting line maintains a substantially constant pressure in the latter. The transmission pump or motor may be continuously rotatable with the volume variation per revolution fixed or variable, partly rotatable, or linearly movable as in a piston and cylinder unit. The transmission pump may be a swashplate type with a rotating swash-plate and/or cylinder barrel. As shown, the transmission pump 1 comprises axial pistons 3 housed in a rotating barrel 2 and having slippers urged into engagement with a swash-plate 4 by pressure in the associated cylinders. The motor may be a simple doubleacting piston or a multi-cylinder rotary motor. The line 17 collects from a casing 16 which surrounds the cylinder barrel and has a seal effective on a pump driving shaft 5. The pipe 17 connection with the casing may be such as to ensure that some liquid remains in the casing for lubrication purposes or, as shown, may extend from the lowermost part of the casing; alternatively, means may throttle flow into the gear pump if the level in the casing falls below a predetermined height. Total liquid volume variation accommodated by the accumulator may result from temperature or pressure variation, and may occur when the motor is a simple double-acting ram, in which case the capacity of the accumulator must exceed the maximum volumetric variation of the ram. The accumulator shown comprises a floating piston 15 acted on by a quantity of inert gas forming a spring medium. Low pressure from the accumulator rests on the pistons 3 which are connected to the pump inlet port 8. The gear pump 18 drawing from the casing 16 and delivering to the accumulator maintains the casing at a lower pressure than that at the inlet port so that each piston in connection with the inlet port is urged into contact with the swashplate. The valve 22 responds to pressure in the casing 16 such that on excessive pressure in the casing it closes against a spring 25, by-pass flow through the valve is prevented, and the gear pump is fully effective to pump from the casing; and on insufficient pressure the valve opens so that liquid is drawn from the casing at a reduced rate. The pressure in the casing is chosen so that sufficient liquid remains for lubrication. The pumps 1 and 18 are driven by the same prime mover, 6. Leakage from the motor may also be collected, as by traps, and fed to the line 17. Motor speed may be varied by direct manual or servo adjustment of the transmission pump swash-plate angle. Motor reversal may be affected by a reversing valve in the lines 11, 12, or by reverse inclination of the main pump swash-plate in which case the accumulator, the delivery of the gear pump 18 and the connection of the valve 22 are connected through a shuttle valve selecting the low pressure one of the pipes 11, 12. The transmission pump may have a fixed delivery and be provided with a by-pass relief valve. A valve may short-circuit the transmission pump when no motor movement is required.