HEAD A.-CYL. 8B4464 - Caterpillar

Low Pressure Standby
Illustration 4 g01141206
Low pressure standby (1) Load sensing spool (2) Horsepower control spool (3) Main control valve (4) Main relief valve (5) Power shift pressure (6) Lever (7) Actuator piston (8) Maximum angle piston (9) Signal limiter valve (10) Load sensing signal (11) Minimum angle piston (12) Margin pressure spring (13) Slow return check valveWhen all controls are in the NEUTRAL position, the main pump is in the low pressure standby condition. No load sensing signal (10) is directed from main control valve (3) to the pump control. As the pump system pressure increases, the load sensing spool (1) is moved to the right against the spring force. Since no load sensing signal pressure (10) is present, pump system pressure increases to only the margin pressure setting.As load sensing spool (1) moves to the right, pump system pressure is directed around load sensing spool (1), through slow return check valve (13), around horsepower control spool (2) and to minimum angle piston (11). Minimum angle piston (11) is larger than maximum angle piston (8). The pressure in minimum angle piston (11) moves the swashplate toward the minimum angle in order to decrease flow.As the pump destrokes, flow decreases and load sensing spool (1) reaches a balance point or a metering position, which causes the pump system pressure to be directed to minimum angle piston (11). The pump swashplate reaches a balance point for standby flow.Upstroke by Load Sensing Spool
Illustration 5 g01141207
Upstroke by load sensing spool (1) Load sensing spool (2) Horsepower control spool (3) Main control valve (4) Main relief valve (5) Power shift pressure (6) Lever (7) Actuator piston (8) Maximum angle piston (9) Signal limiter valve (10) Load sensing signal (11) Minimum angle piston (12) Margin pressure spring (13) Slow return check valveThe pump is upstroked and the main pump flow increases under the following conditions:
The flow demand is increased when the pump flow is being controlled by load sensing spool (1) and the flow demand is not on the horsepower curve.
Power shift pressure (5) is increased when the pump flow is being controlled by horsepower control spool (2) .
The system pressure is decreased when the pump flow is being controlled by horsepower control spool (2) .The pump is upstroked in Illustration 5 and flow increases due to a change in the position of load sensing spool (1) .The force that is working on the left end of load sensing spool (1) and the force that is working on the right end of load sensing spool (1) are no longer balanced when the following conditions occur:
A hydraulic circuit is activated from the low pressure standby condition.
Additional circuits are operated.
A single circuit is shifted and the circuit requires more flow.On the right end