213 TRACK-TYPE EXCAVATOR Main Relief Valve Caterpillar

Main Relief Valve
1.1. General
2.1. Function And Operation
3.1. Pressure Adjustment

Fig. WH 106

(1) Support rings

(2) O-ring

(3) Piston

(4) Bushing

(5) Pressure spring

(6) Valve set

(7) Cone

(8) Pressure spring

(9) O-ring

(10) O-ring

(11) Nut

(12) Regulating shim

(13) 6.2 x 12 x 0.5 screw

(14) Cap


(P) Pressure-oil channel

(T) Return channel to tank


In a hydraulic system, if the resistance counteracting the actuator rises to an extent which is both undesired and detrimental to the system, the weakest point of that system would be destroyed. To avoid this, the system is provided with an artificial weak point, in other words, a kind of "predetermined break point".

The purpose of this is to allow the increasing pressure in the entire pressure line system to act on a hydraulic valve, which is initially closed, by means of an adjustable spring.

The increase in pressure lifts the sealing element (ball, cone or piston) and discharges the excess fluid, which is delivered by the pump but not consumed by the actuator to the tank, converting its hydraulic energy into heat.

This valve is known as a pressure relief valve.

Function And Operation

The pressure in the system acts on the piston (3). At the same time, pressure reaches the cone (7) through the small orifices in the piston (3) and the valve seat (6). If pressure in the system exceeds that of the spring (8), the cone (7) is lifted and allows some fluid to drain off into the return channel. The pressure of the fluid now flowing through the small orifice in the piston (3) is not sufficient to compensate the pressure behind the piston. Thus, a pressure differential is established between the spaces in front of and behind the piston. This difference now causes the pressure in front of the piston to shift the piston (3), allowing the main body of fluid to drain off into the tank.

Pressure Adjustment

------ WARNING! ------

Make reference to WARNING on first page of the TESTING AND ADJUSTING Section.


Adjustment is by loosening the nut (11) and turning the regulating screw (12) CW or CCW. After adjustment, the screw should be sealed again.