Control valve for variable displacement pump or motor

A control valve for use with a hydraulic pump or motor having a swashplate movable from a neutral position to an actuated position to vary the displacement of said pump or motor in response to the porting of fluid from a relatively high pressure fluid source to one servo attached to the swashplate and the porting of fluid from another servo attached to the swashplate to a relatively low pressure fluid source, the control valve comprising:

• a. a housing defining an axially extending spool bore;
• b. a control spool located within said spool bore and having a pair of axially spaced lands engaging said spool bore, a reduced center portion intermediate said lands and spaced from said spool bore, and a pair of reduced end portions axially outwardly from said lands and spaced from said spool bore, each of said lands having a given axial width defined by an axially outer edge adjacent said respective reduced end portion and an axially inner edge adjacent said reduced center portion;
• c. means to center said control spool relative to a given position within said spool bore;
• d. linkage connected to said control spool and to said swashplate to move said control spool in response to movement of said swashplate;
• e. a movable control handle;
• f. linkage connected to said control spool and to said control handle to move said control spool in response to movement of said control handle;
• g. a controlled pressure port in said housing adapted for fluid communication with said relatively high pressure fluid source and in fluid communication with said spool bore intermediate said lands when said control spool is centered;
• h. means fluidly communicating said portion of said spool bore encircling said reduced end portions of said control spool with said relatively low pressure fluid source;
• i. a first servo port in said housing adapted for fluid communication with one of said servos and in fluid communication with said spool bore at a position axially spaced in one direction from said controlled pressure port, said one servo port being partially covered by one of said lands when said control spool is centered;
• j. a second servo port in said housing adapted for fluid communication with the outer of said servos and in fluid communication with said spool bore at a position axially spaced in the other direction from said controlled pressure port, said other servo port being partially covered by said other and when said control spool is centered;
• and
• k. means providing fluid communication between said controlled pressure port and each of said servo ports, said fluid communication being provided via an orifice which has an area that is no greater than one-fourth of the area of the respective servo port which is not covered by the respective land when said control spool is centered.