Hydrostatic transmission circuit

A hydraulic valve assembly for use in a hydrostatic transmission, said transmission including a variable displacement main pump, an interconnected charge pump, an interconnected hydraulic motor, and an interconnecting closed loop hydraulic circuit having low and high pressure leg portions, said hydraulic valve assembly controlling fluid transfer between a first, a second and a third line, within said hydraulic circuit, wherein two of said first, second and third lines define first and second pressure lines and are located at substantially similar longitudinal distances from the remaining one of said first, second and third lines, said remaining line being rotationally displaced relative to said first and second pressure lines, said valve assembly comprising in combination:

• a. a valve body defining a first port for connection to said remaining line, a second port for connection to one of said first and second pressure lines, and a third port for connection to the other of said first and second pressure lines, said valve body further including a spool bore in fluid communication with said first, second and third lines;
• b. a valve spool adapted for sealing reciprocation within said spool bore, having a first end portion, a second end portion, a connecting portion having a cross-sectional area smaller than the cross-sectional area of said first and second end portions, a first bypass orifice within said valve spool extending between said first end portion and said connecting portion, and a second bypass orifice within said valve spool extending between said second end portion and said connecting portion, said valve spool being movable from a neutral position, in which the valve spool is longitudinally centered within said spool bore and where the pressure forces in said first and second pressure lines are substantially similar, to a first position, occurring when the pressure forces in said first pressure line are greater than the pressure forces in said second pressure line, or to a second position, occurring when the pressure forces in said first pressure line are less than the pressure forces in said second pressure line, with said connecting portion being in fluid communication with at least a portion of said first port at each of said positions of said valve spool, wherein:
• while in said neutral valve spool position, said first bypass orifice is aligned with said first pressure line for fluid communication with said remaining line and said second bypass orifice is aligned with said second pressure line for communication with said remaining line;
• while in said first valve spool position, said first and second bypass orifices are at least substantially disabled and said connecting portion is in fluid communication with one of said first and second pressure lines;
• while in said second valve spool position, said first and second bypass orifices are at least substantially disabled and said connecting portion is in fluid communication with the other of said first and second pressure lines;
• c. dampers, located at both ends of said valve spool, for centering said valve spool, relative to said remaining line, in said neutral valve position;
• and
• d. wherein said first and second bypass orifices have a cross-sectional area sufficient to permit the equalization of the fluid pressure between said hydraulic circuit low and high pressure leg portions.

A method for increasing the efficiency of a hydrostatic transmission, in a neutral mode of operation, without impairing the performance in non-neutral modes of operation, wherein said hydrostatic transmission includes:

• a variable displacement main pump;
• a hydraulic motor;
• a closed loop hydraulic circuit operatively interconnecting said main pump and motor;
• a charge pump having an outlet line operatively interconnected only to said circuit;
• a valve block within said circuit, for controlling fluid transfer between a first, second and third line, within said hydraulic circuit, wherein two of said first, second and third lines define first and second pressure lines, said remaining line defining an outlet line;
• said valve block comprising a valve body defining a first port for connection to said remaining line, a second port for connection to one of said first and second pressure lines, and a third line for connection to the other of said first and second pressure lines, said valve body further including a spool bore in communication with said first, second and third lines;
• a valve spool adapted for sealing reciprocation within said spool bore, having a first end portion, a second end portion and a connecting portion having a cross-sectional area smaller than the cross-sectional areas of said first and second end portions;
• and dampers for centering said valve spool in a neutral mode of operation, said method comprising:
• a. including a first bypass orifice, within said valve spool, extending between said first end portion and said connecting portion;
• b. also including a second bypass orifice, within said valve spool, extending between said second end portion and said connecting portion;
• c. sizing the cross-sectional areas of said first and second bypass orifices to allow the passage of substantially the full flow of said charge pump, at a low restriction to flow, through said bypass orifices;
• d. keeping said connecting portion in fluid communication with said first port at all times;
• e. permitting substantially equal fluid flows from said second and third ports, via said first and second bypass orifices, respectively, to said first port, in said neutral mode of operation, when fluid forces acting on said first and second end portions are about equal;
• and
• f. shifting said valve spool from said neutral mode of operation to non-neutral modes of operation during which the fluid forces acting on said first and second end portions are unequal, to thereby at least substantially disable said fluid flows via said first and second bypass orifices while simultaneously permitting fluid flows from one of said pressure lines to said outlet port.

A hydraulic system comprising:

• (a) a variable displacement main pump;
• (b) a hydraulic motor;
• (c) a closed loop hydraulic circuit, including low and high pressure leg portions, operatively interconnecting said main pump and said motor;
• (d) a charge pump, within said circuit, having an outlet line to said circuit;
• and
• (e) a shuttle valve, said shuttle valve operable to reduce fluid temperature in said circuit by connecting a low pressure side of said circuit to a drain line, said shuttle valve also being configured to function as a primary relief valve for said charge pump.