Figures 10-68 through 10-70 diagram how to control the output of a fixed-volume pump with a 3-port flow control. A 3-port flow-control circuit operates the same as a bleed-off flow control. The main difference is that a 3-port flow control does not need an external relief. The flow control’s integral relief valve protects the pump and makes the flow control pressure compensated.

fig 67

Fig. 10-67. Meter-in flow-control circuit for variable-volume pump supplying hydraulic motor.

A 3-port flow control used with a running away load requires a counterbalance valve (as was shown in Figure 10-15). Normal use is with a single-actuator circuit because it controls maximum pump pressure and flow, and affects all actuators in a circuit.

Figure 10-68 shows a typical 3-port flow-control circuit at rest with the pump running. Flow from the pump is 10 gpm. The needle valve is set for 5 gpm. The bias spring on the main relief valve is about 25 to 70 psi (according to the manufacturer). In the at rest condition, 5 gpm flows to tank through the directional valve and 5 gpm flows to tank through the main relief valve (at a pressure drop of approximately 70 psi). The normally closed pilot operator on the relief valve stays closed until system pressure reaches its setting.

fig 68

Fig. 10-68: Three-port flow-control circuit at rest.

When the cylinder starts to move, as in Figure 10-69, its resistance will increase pressure at the outlet of the flow control. Pressure build-up sensed through the fixed orifice acts on the main relief valve with the 70-psi spring. Any resistance from the actuator -- plus the 70 psi of the bias spring -- will increase pressure at the pump. The constant 70-psi pressure drop will maintain an accurate and constant flow to the cylinder while it moves. This 3-port flow control generates very little heat when the pressure requirement is low. It works the same as a bleed-off flow control except it has a built in relief valve.

fig 69

Fig. 10-69: Three-port flow-control circuit with cylinder extending.

When the cylinder reaches the load, as in Figure 10-70, pressure increases. As pressure at the cylinder rises, the main relief valve senses it and maintains a 70-psi differential right up to the setting of the pilot operator. When cylinder pressure reaches 1000 psi, all pump flow will go to tank across the main relief valve. The pilot-operator section of the 3-port flow control sets maximum system pressure.

fig 70

Fig. 10-70: Three-port flow-control circuit with cylinder stalled.

The 3-port flow control is similar to the priority flow divider that will be explained in Chapter 11. The main difference between these two valves is that tank flow always goes directly to tank on a 3-port flow control. Any backpressure in the tank line will add to the maximum system pressure. A priority flow divider can use the excess flow to operate other valves and actuators. Relief valves in the lines leaving the priority flow divider protect the circuits from overpressure.