Hydraulics - Seafish

8
Pressure Control Valves
Positive displacement pumps can generate very high pressures.
The pressure in the system will increase if the actuator stalls due
to an excessive load, or if the actuator (a hydraulic cylinder) is to
hold a load in place.
Excess pressure is reduced by a relief valve. Every hydraulic
system is fitted with at least one relief valve on the pump outlet
line and this valve discharges back into the tank.
A simple relief valve is shown in Figure 7.
Locknut
Pressure
Adjusting
Screw
Spring
Reservoir
Pressure
Line
Figure 7.
The spring holds the ball on its seat until the pressure in the line
lifts it off and allows oil to return to the tank. When the pressure
drops the ball is re-seated. The lifting pressure can be altered by
turning the pressure adjusting screw above the spring.
A direct operated relief valve as shown in Figure 7 is only suitable
for systems with small flow variations. For systems with large
flow variations, relief valves are operated by a pilot valve as
shown in Figure 8.
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