ELBOW 0013816 - Caterpillar

Illustration 4 g00915529
(1) Load sensing signal (3) Margin spring (4) Flow compensator spool (11) Pump outlet (12) Bias spring (13) Swashplate (14) Actuator pistonWhen an implement hydraulic circuit requires flow, the pressure from pump outlet (11) is reduced. The reduced pump output pressure causes the force of margin spring (3) and load sensing signal (1) to be greater than pump outlet pressure (11). This force pushes flow compensator spool (4) downward.The spool moves to the bottom which blocks the flow of oil to actuator piston (14). Oil that is in the chamber for actuator piston (14) is vented to the case drain across flow compensator spool (4). This allows bias spring (12) to move swashplate (13) to a greater angle.The pump now produces more flow. This condition is known as "upstroking".The following conditions can result in upstroking the pump:
If an implement hydraulic circuit is initially activated from low pressure standby, the load sensing signal increases the pump output flow.
If a main control spool in a hydraulic control valve is in a given position, the pump will slightly upstroke in order to compensate for system leakage.
The hydraulic pump will upstroke when the demand increases from changing the position of the main control spool in a hydraulic control valve.
If another implement hydraulic circuit is engaged, there is a need for increased pump flow.
If the demand on the implement hydraulic system remains constant or the demand increases, the hydraulic pump will upstroke when the engine speed decreases.Note: The load sensing signal pressure does not need to increase in order to upstroke the hydraulic pump.For example, if one implement hydraulic circuit is activated at an operating pressure of 13800 kPa (2000 psi), the system pressure is 15500 kPa (2247). The pressure of 15500 kPa (2247 psi) is a combination of the margin pressure and the pressure of the signal oil.If another implement hydraulic circuit is activated at an initial operating pressure of 6900 kPa (1000 psi), the maximum pressure of the load sensing signal will still be 13800 kPa (2000 psi). The pressure of the supply oil will decrease momentarily as more flow is now demanded by the additional circuit.The pressure of load sensing signal (1) plus the force of margin spring (3) is now higher than the output pressure of the pump at the bottom end of the spool. Flow compensator spool (4) is pushed to the bottom. This allows oil that is behind actuator piston (14) to be vented to the case drain. The angle of swashplate (13) now increases and the hydraulic pump provides more flow.Constant Flow
Illustration 5 g00915531
(1) Load sensing signal (3) Margin spring (4) Flow compensator spool (11) Pump outlet (13) Swashplate (14) Actuator pistonWhen a constant flow of oil is demanded by an implement hydraulic circuit, the supply oil pressure from pump outlet (11) will increase at flow compensator spool (4). Force from margin spring (3) and pressure from load sensing signal (1) will act on the top end of flow compensator spool