BOOK 2, CHAPTER 10: Flow control circuits

Flow Control Circuits

To control the speed of an actuator, most designers use flow controls. Air circuits normally need controlled flow because the plant air compressor is greatly oversized for almost any given circuit. Hydraulic circuits usually have a dedicated power source sized to meet the cycle time so flow restrictors are unnecessary.

Flow controls always generate some heat in hydraulic circuits, so consider some other method of controlling actuator speed where possible. The circuit examples in this chapter explain the types of flow-control systems and how to apply them.

Figures 10-1 and 10-2 show symbols for fixed orifices, rudimentary components that will control flow. A fixed orifice can be a simple restriction in a line or a factory-preset control with pressure compensation and a bypass. Their low cost and the fact that they are tamper-proof are two main reasons for using fixed orifices.

Fig. 10-1: Fixed orifice.

Fig. 10-2: Pressure- and temperature-compensated fixed orifice.

Use the needle valve shown in Figure 10-3 when control of fluid flow in both directions is necessary. Add the check valve arrangement shown in Figure 10-4 when a needle valve needs pressure compensation in both directions. These check valves, sometime referred to as bridge rectifiers, force fluid to flow through the needle valve in the same direction regardless of actuator movement. (Remember, pressure compensation only works in one direction of flow.)

Fig. 10-3: Needle valve.

Fig. 10-4: Pressure- and temperature-compensated needle valve.

When talking about flow-control hardware, some manufacturers use different terminology. Normally the term flow control refers to an adjustable needle valve with an integral bypass, as pictured in Figure 10-5. This type of flow control meters flow in one direction and allows free flow in the opposite direction. However, some companies identify the flow control in Figure 10-5 as a throttle valve. These companies say a flow control must have a bypass and be pressure-compensated as shown in Figure 10-6.

Fig. 10-5: Flow control with bypass (or throttle valve).

When a hydraulic actuator needs accurate speed control, use a pressure-compensated flow control. System pressure fluctuations or load changes will cause actuator velocity to change. Regardless of the cause of the pressure differences, flow across the orifice will change unless the flow control is pressure compensated. Only use a pressure-compensated valve when very accurate speed control is needed because its cost is as much as six times that of a non-compensated valve.

Fig. 10-6: Pressure- and temperature-compensated flow control.