Speed control of hydraulic and pneumatic actuators

In some applications, there are times when it is necessary to vary the speed of an actuator. One method of controlling an actuator’s speed is by using a variable-volume pump. This works well for a circuit with a single actuator or in multi-actuator circuits where only one actuator moves at a time. However, most circuits that need actuator-speed control have multiple actuators and some of them operate simultaneously. For most circuits, a variable orifice called a needle valve or flow control is common. Fixed orifices may be used in some cases.

Non-compensated flow control valves

Figure 13-1 shows non-compensated flow devices in symbol and cutaway form. At the top are non-compensated fixed-orifice in-line flow controls for tamper-proof applications. These can be purchased as in-line valves or they could be a drilled plug or insert located in a pipe fitting or valve port.

Flow through standard orifices is affected by viscosity changes in the fluid, while flow through knife-edge (or sharp-edge) orifices changes very little when fluid viscosity changes from thin to thick. A knife-edge orifice is the style used on most valves that are designated as temperature compensated. (A classic example of a non-compensated fixed orifice with a bypass check is the orificed check valve shown in Figure 10-2.)

Pressure-compensated flow control valves

The pressure-compensated flow control cutaway view and symbols depicted in Figure 13-2 are the component used with actuators that must move at a constant rate. A non-compensated flow control passes more or less fluid as pressure raises and lowers. This is because more fluid can pass through a certain size orifice when pressure drop across the orifice increases.

The needle valve section of a pressure-compensated flow control is the same as any flow control. The difference is the addition of a compensator spool that can move to restrict Inlet flow at the compensating orifice. The compensator spool is held open by a 100- to 150-psi bias spring that sets pressure drop across the knife-edge orifice.

Flow from the inlet goes through the compensating orifice, past the compensator spool, and out through the knife-edge orifice. A drilled passage ports Inlet fluid to the right end of the compensator spool, which forces the spool to the left when pressure tries to go above 100 to 150 psi at gauge PG01. After pressure reaches or goes above 100 to 150 psi, the compensator spool moves to the left and restricts flow to the knife-edge orifice flow control. Pressure at gauge PG01 never goes above 100 to 150 psi (plus any backpressure at the outlet). Pressure at the outlet is ported to the bias-spring chamber and increases the spring force. The compensator spool assures that pressure drop across the knife-edge orifice flow control stays at a constant 100 to 150 psi. With a constant pressure drop, flow stays the same regardless of inlet or outlet fluctuations.

Pressure-compensated flow controls are four to eight times more expensive than standard controls so they should only be applied to actuators that must move consistently.

The no-jump option is an adjusting screw that holds the compensator spool within a few tenths of an inch of its operating position. This is an especially important option when the valve is oversize for the present flow setting. A compensator spool without a stroke limiter may close and open violently until it stabilizes and sets pressure drop for the orifice. During this time the actuator also moves erratically.

The two symbols represent the American National Standards Institute (ANSI) and the International Standards Organization (ISO) way of indicating that the valve is pressure compensated. The arrow indicating pressure compensation is easier to distinguish in the ANSI symbol -- especially when the schematic drawing has been reduced to fit into a machine's documentation book.