TM 5-3810-306-20
Relief Valve
Relief valves are used to protect a component, a circuit, or a system from overpressurization. Most of the relief valves are
located in the directional control valves, while others are part of a component.
Relief valves are checked and adjusted by causing a given circuit to reach its prescribed pressure limit (stall). At this point
the relief valve opens, returning oil to the reservoir. Hydraulic motor circuits may be stalled by preventing rotation of the
motor shaft prior to actuating the control valve. Cylinder circuits may be stalled by extending or retracting a cylinder to its
limit of travel.
By placing a pressure gauge in the proper line or port, a pressure reading will indicate the point at which the relief valve
opens. The needle on the meter face will climb until it reaches the relief valve setting. At that point the needle will stop
climbing and fluctuate, indicating the relief valve is open and returning oil to the reservoir.
Correct relief valve adjustment is mandatory if any hydraulic circuit is to function properly. Settings must be within
tolerances. Therefore, adjustment should be performed only by qualified technicians using the correct equipment, after
the need for adjustment has established.
The hydraulic system utilizes four valve banks. Each valve bank has one main relief valve limiting maximum operating
pressure of the component(s) in that circuit. In addition, circuit relief valves further limit operating pressures as required by
circuit design.
Directional Control Valves
The directional control valves are located within the valve compartment behind the operator’s cab. Access to the
directional control valves is gained by lifting the access cover over the valve compartment and locking in place.
The directional control valves are four-way, three-position valves with either an open or closed spool. Whether a valve has
an open or closed spool is determined by whether, with the spool in the neutral position, the work ports are open to the
reservoir return passage. If the work ports are open to the reservoir return passage, the valve is classified as an open
spool type: if they are not, the valve is a closed spool type. Additionally, the valve spool is spring loaded to the neutral
position.
The open spool directional control valve is constructed with a through passage to allow flow to pass to the next valve(s) in
the bank and on the reservoir, when the valve spool is in the neutral position. By positioning the valve spool to a work
position, the through passage is blocked and flow is diverted to the dead end parallel passage. This causes flow to be
directed to the component’s open supply work port. Return flow is routed from the return work port to the reservoir return
passage by the opposite end of the valve spool. This flow pattern is applicable in either direction the valve may be
positioned. If it is necessary to open more than one
1-36
