Copyright Goodheart-Willcox Co., Inc.
430 Hydraulic Systems for Mobile Equipment
• Pressure supplied to the DCV spool.
• 1000 psi + 40 psi = 1040 psi
• Pressure drop across the DCV spool (equals the value of the
PC spring).
• 1040 psi – 1000 psi = 40 psi
The addition of the shuttle valve and the pressure compensator valve pro-
vides precise fl ow to the actuator. As the hydraulic system’s flow fl and pressure fl
changes, the pressure compensator valve will automatically compensate and
adjust the flow so that the actuator remains at the same speed. fl
There is a limit to pressure compensation. If the engine-driven pump is
slowed and is no longer delivering minimum fl ow as requested by the oper- fl
ator, the pressure compensator valve cannot magically magnify oil flow that is fl
not there.
The shuttle valve and the pressure compensator valve also help alleviate
an additional problem. As oil fl ows through a spool valve, oil velocities create fl
forces that affect the position of the spool valve. See Figure 18-11.
The addition of the compensator valve results in a smaller pressure drop
across the DCV spool, which minimizes the negative effect of flow forces. If fl
the DCV spool is manually operated, the PC valve will help by lowering the
amount of effort the operator must use to actuate the spool valve and the effort
required to maintain the position of the spool valve.
Figure 18-11. Oil flowing through a DCV spool creates forces, causing a resistance when attempting to open and close
the spool valve. High-pressure pump flow is directed to extend the cylinder as the spool is opened. The shuttle valve
and PC valve work in unison to keep the pressure drop across the DCV spool relatively low, 60 psi. Decreasing the size
of the orifice between the pump input oil and cylinder extend oil ports by closing the spool increases these flow forces.
High-pressure
relief (3000 psi)
1500 psi
to extend
Unloading
valve (300 psi)
Shuttle valve
Pressure
compensator
valve (60 psi)