Patent ID: 6837047

Claim:
A hydraulic system for use with a hydrostatic transmission comprising in combination: a variable displacement main pump; a hydraulic motor; a hydraulic circuit operatively interconnecting said main pump and motor; a charge pump, within said circuit, having an outlet line; and a valve block, within said circuit, having an inlet line, in fluid communication with said charge pump outlet line, and having a first and a second outlet line in fluid communication with said hydraulic circuit, said valve block comprising: a valve body having a first port connected with said inlet line, a second port connected with said first outlet line, a third port connected with said second outlet line, and a spool bore in fluid communication with said inlet, first outlet, and second outlet lines; a valve spool, adapted for sealing movement within said spool bore, having a first end portion, a second end portion, and a connecting portion having a cross sectional area smaller than that of the first and second end portions, said valve spool having at least one orifice in each of said first and second end portions in communication with said connecting portion, with said connecting portion being in fluid communication with at least a portion of said first port at all times, said valve spool being longitudinally movable, via fluid pressure, within said spool bore from a neutral position to one of a first and second position, wherein the fluid pressure forces, acting on the first and second end portions, are approximately equal in said valve spool neutral position, the fluid pressure forces acting on said first end portion being greater than the fluid pressure forces acting on said second end portion in the first position, and the fluid pressure forces acting on said first end portion being less than the fluid pressure forces acting on said second end portion in the second position, said at least one orifice in each of said first and second end portions having fluid flow therethrough when said valve spool is in said neutral position and having substantially no fluid flow therethrough when said valve spool is in one of said first and second positions; a first check valve in physical contact with said valve spool first end portion, having a fully open position when said valve spool connecting portion is in fluid communication with both said inlet line and said first outlet line thus permitting fluid transfer from said inlet line to said first outlet line when said valve spool is in the second position, and having a closed position when said valve spool is in the first position; and a second check valve in physical contact with said valve spool second end portion, having a fully open position when said valve spool connecting portion is in fluid communication with both said inlet line and said second outlet line thus permitting fluid transfer from said inlet line to said second outlet line when said valve spool is in the first position, and having a closed position when said valve spool is in the second position.