Variable delivery hydraulic pump

In a variable delivery hydraulic pump including a housing having a suction and a discharge conduit, a variable control element movable between maximum and minimum displacement positions corresponding, respectively, to a maximum and a minimum fluid discharge amount, a pumping means associated with said variable control element and controlled thereby to vary the fluid discharge amount of said pumping means between said maximum and minimum amount, the improvement comprising:

• a biasing means for constantly urging said variable control element toward said maximum displacement position, a pilot plunger means and a control plunger means for adjusting said displacement position of said variable control element from said maximum to said minimum against the urging of said biasing means, said pilot plunger means having a rear end and being slidable through a limited stroke distance to thereby move said variable control element from said maximum displacement position to an intermediate displacement position between said maximum and minimum positions, to vary said fluid discharge amount from said maximum amount to a medium amount, said control plunger means having a rear end and being slidable through a stroke distance to move said variable control element from said intermediate displacement position to said minimum displacement position to vary said fluid discharge amount from said medium amount to said minimum amount,
• means for controlling the sliding movement of said pilot and control plunger means including first and second spool valves each disposed for axial movement in first and second spool chambers, respectively, each spool chamber having one end thereof in fluid communication with said discharge conduit of said pumping means, so that said spool valves will be exposed to the fluid discharge pressure of said pumping means, first and second push members disposed at the other ends of said first and second spool chambers for engaging the ends, respectively, of said first and second spool valves for urging said spool valves against the fluid discharge pressure from said discharge conduit, said first and second push members exerting different biasing forces on said respective spool valves so that said spool valves will each be caused to move against said respective push members in response to different fluid discharge pressures, a first control conduit for establishing fluid communication from a point intermediate the said ends of said first spool chamber, and thereby from said discharge conduit when said first spool valve is moved past said point toward its said other end, to the rear end of said pilot plunger means when said fluid discharge pressure exceeds a medium level to overcome the biasing force of said first push member, a second control conduit for establishing fluid communication from a point intermediate the said ends of said second spool chamber, and thereby from said discharge conduit when said second spool valve is moved past said point in said second spool chamber toward its said other end, to the rear end of said control plunger means when said fluid discharge pressure exceeds a maximum level to overcome the biasing force of said second push member, the biasing force of said first and second push members being such that said spool valves will be moved thereby to close off said first and second control conduits from said fluid discharge conduit when said fluid discharge pressure is lower than said medium level whereby said biasing means will move said variable control element to its said maximum level displacement position to cause said pumping means to discharge said maximum amount of fluid.

In a variable delivery hydraulic pump having a housing having a suction and a discharge conduit, a variable control element movable between maximum and minimum displacement positions corresponding, respectively, to a maximum and a minimum discharge amount, a pumping means associated with said variable control element and controlled thereby to vary the fluid discharge amount of said pumping means between said maximum and minimum amount, the improvement comprising:

• a biasing means for constantly urging said variable control element toward said maximum displacement position, a pilot plunger means and a control plunger means each having a rear end and each slidably disposed in said housing in operative engagement with said variable control element for adjusting said displacement position thereof from said maximum to said minimum against the urging of said biasing means, said pilot plunger means being slidable through a stroke distance that is limited to thereby move said variable control element from said maximum displacement position to an intermediate displacement position between said maximum and minimum positions to vary said fluid discharge amount from said maximum amount to a medium amount, said control plunger means being slidable through a stroke distance to move said variable control element from said intermediate displacement position to said minimum displacement position to vary said fluid discharge amount from said medium amount to said minimum amount,
• means for controlling the sliding movement of said pilot and control plunger means including first and second spool valves each disposed for axial movement in a first and a second spool chamber, respectively, said first spool chamber of said first spool valve having one end thereof in fluid communication with said discharge conduit of said pumping means so that said first spool valve will be exposed to the fluid discharge pressure of said pumping means, first and second push members disposed at the other ends of said first and second spool chambers for engaging the ends, respectively, of said first and second spool valves for urging said spool valves against fluid pressure from said discharge conduit, said first and second push members exerting different biasing forces on said respective spool valves so that said spool valves will each be caused to move against said respective push members in response to different fluid discharge pressures, a first control conduit for establishing fluid communication from a point intermediate the said ends of said first spool chamber, and thereby from said discharge conduit when said first spool valve is moved past said point towards its said other end, to the rear end of said pilot plunger means when said fluid discharge pressure exceeds a medium level to overcome the biasing force of said first push member, said first control conduit having a branch for passing fluid under pressure to said second spool valve, a second control conduit for establishing fluid communication from a point intermediate the said ends of said second spool chamber, and thereby from said discharge conduit through said first control conduit when said second spool valve is moved past said point in said second spool chamber toward its said other end, to the rear end of said control plunger means when said fluid discharge pressure exceeds a maximum level to overcome the biasing force of said second push member, the biasing force of said first and second push members being such that said spool valves will be moved thereby to close off said first and second control conduits from said fluid discharge conduit when said fluid discharge pressure is lower than said medium level whereby said biasing means will move said variable control element to its said maximum displacement position to cause said pumping means to discharge said maximum amount of fluid.