For actuators, or motors, under the action of negative forces (e.g. pulling forces during extension) it is necessary to provide a restriction in the flow outlet in order to create a resisting, or back, pressure.

Load control valves of the counterbalance type are shown in Figure 1. That in (a) is a line mounted valve where the valve opens when the force from the pilot

Figure 1. Counter Balance Load Control Valve  

pressure, P , exceeds that of the spring. The operation of the valve is independent of the inlet pressure P2 whereas for the cartridge valve in (b) this is not the case as the valve opening is controlled by both of the pressures. The ratio of the areas exposed to the pressures P, and P2 can be selected this usually being in the range 3:1 and 10:1.

A typical operating characteristic of valve type (b) is shown in Figure 8 for the cracking pressure and for two flows. The flow through the valve depends on its opening and the pressure P2 so as P2 reduces, the valve opening has to be increased which requires P, to have a higher value over that for the valve just cracking open. This effect can be seen from Figure 2.

The maximum value of P2 which is set by pre-loading the spring with a screw adjustment, is chosen to provide a maximum safe pressure for the actuator and occurs when Pt is zero. For the actuator the variation of P2 with Pj depends on its area ratio which has a minimum value of unity for an equal area cylinder.

For a given force on the actuator the pressure relationship for extension of the actuator is shown in Figure 2 superimposed onto the valve characteristics, the point of intersection providing the operating condition of the system. Standard cylinders normally have a maximum area ratio of around 2 but some applications use actuators having ratios considerably higher than this. Clearly this has an influence on the operating pressures when under the control of a counterbalance valve.

Figure 3. Actuator Controlled by Counter Balance Valve