TIRE 1129813 - Caterpillar

The steering system pump is an automatic piston pump. The steering system pump senses pressure requirements and flow requirements. When the steering circuit is not being used, the pump is at low-pressure standby.The single highest pressure that is sensed flows to pressure compensator spool (13). This spool keeps the pump output at a level that is necessary to fulfill the requirements for the system flow and for the pressure.The actual system pressure will be greater than the highest work port pressure requirements unless the pump is at full stroke. The difference between the work port requirement and the higher main system pressure is called margin pressure.The pressure compensator valve also limits maximum system pressure. The pressure compensator valve prevents damage to the steering system components from excessive pressure.The pump is controlled by bias piston (4) and actuator piston (10). Bias spring (6) causes swashplate (2) to move. The swashplate then causes the pump to upstroke.Actuator piston (10) has a larger area than bias piston (4). Actuator piston (10) causes swashplate (2) to destroke the pump. Flow compensator spool (12) and/or pressure compensator spool (13) changes pump output by regulating the pump discharge pressure that is acting on actuator piston (10).Pressure and flow compensator valve (7) routes pump discharge pressure to actuator piston (10). Because actuator piston (10) is larger than bias piston (4), the oil pressure acting against piston (10) overcomes the force of bias spring (6). The oil pressure then causes the pump to destroke.Flow compensator spool (12) maintains pump outlet pressure at 2400 100 kPa (350 15 psi) above the work port pressure requirements.Pressure and flow compensator valve (7) also controls the maximum output of pump pressure. When work port pressure rises above 23400 700 kPa (3394 102 psi), pressure compensator spool (13) overrides flow compensator spool (12). This process causes the pump to destroke. This process occurs at approximately 690 kPa (100 psi) below the maximum pressure setting.Upstroking
Upstroking of the pump occurs because of increased flow demand. The increased flow demand is met by increased pump output.The increased flow demand causes a signal pressure in line (19). The signal pressure in line (19) combines with the force of spring (18) in cavity (20). The force of spring (18) causes pump pressure to be 2400 100 kPa (350 15 psi) greater than the signal pressure. However, the combination of signal pressure and of spring force is greater than the pump discharge pressure. This difference is called margin pressure.The pressure causes spool (13) to move down. As spool (13) moves down, the spool blocks the flow of supply oil to actuator piston (10). When the oil flow to actuator piston (10) is blocked, the oil in passage (22) drains to passage (23). The oil then flows past flow compensator spool (12). The oil then flows past pressure compensator spool (13) and through passage (16) into case drain (14).Supply oil flows through passage (15) to bias piston (4). The oil acts