Parker Hannifin Corporation is introducing the H1 and H1A Series of over-center piston pumps, its next-generation piston pumps for light duty hydrostatic transmissions, designed to operate more efficiently in closed-loop wheel drive systems. Compared with today's conventional light duty piston pumps, the new pumps provide increased volumetric efficiency and smoother control of transmissions. The technology used in the new H1 and H1A pumps is embodied in the HydroSmooth neutral valve. The neutral valve provides extremely fast response and precise metering. The neutral valve is integrated into the H1 and H1A pumps either between the make-up check valves or on the hot oil shuttle valve of the closed loop. Combining the new innovative valve with make-up check valves or the hot oil shuttle valves can significantly reduce the cost and size of the hydraulic system. The fast, ultra-compact neutral valve enables a simpler, more cost-effective design. This paper presents an invention relating to the neutral valve assembly and design theory of increasing the width of the dead band of a hydrostatic transmission in a neutral mode of operation without impairing the performance of the hydrostatic transmission in operating modes. The new technology brings new innovation to widely used light duty hydrostatic piston pumps. Pumps with the proposed innovative neutral valve could maintain high volumetric efficiency even at very high operating pressure. Thus, the new pumps are ideal for many applications including the propulsion of vehicles, such as zero-turn-radius mowers.
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