In brief
A proportional valve adjusts pressure, flow or direction continuously and in proportion to an electrical setpoint, usually by means of a proportional solenoid.
Contents
What is Proportional valve?
Whereas a switching valve knows only open or closed, a proportional valve can take up any intermediate position. This allows speeds, forces and pressures in hydraulics and pneumatics to be finely metered, for example when a press starts smoothly, for positioning in materials handling or on injection moulding machines with varying injection profiles.
There are proportional directional valves, proportional pressure valves and proportional flow valves. A proportional solenoid generates a force corresponding to the coil current; amplifier electronics convert the setpoint, for example 0–10 V or 4–20 mA, into this current. These electronics are either external or mounted on the valve as on-board electronics. Valves with a position transducer and closed-loop position control achieve higher accuracy and lower hysteresis. They should be distinguished from servo valves, which are even more dynamic and precise but also more sensitive to dirt.
Failures are caused mainly by contaminated oil that jams spools, by defective coils or connectors and by faults in the amplifier electronics. As proportional valves often work in closed control loops, a replacement affects the behaviour of the entire axis. After installation, adjustment of the zero point, gain and ramps is therefore usually required.
In practice: maintenance and procurement
For a replacement, not only the nominal size, function and nominal flow but also the setpoint signal, supply voltage, connector pin assignment and electronics variant must match. Amplifier electronics parameters should be backed up before the replacement.
Synonyms and abbreviations: proportional directional valve · proportional pressure valve · continuous valve · proportional solenoid valve
Frequently asked questions
What is the difference between a proportional valve and a servo valve?
Both are continuous valves that adjust infinitely. Proportional valves are usually operated directly or pilot-operated via proportional solenoids and are sufficient for many positioning and pressure tasks. Servo valves work with very high dynamics and accuracy, often via nozzle-flapper systems, and place higher demands on oil cleanliness. In practice, the boundary is fluid.
What does on-board electronics mean for proportional valves?
In valves with on-board electronics, the amplifier is integrated directly on the valve. The valve receives only the supply voltage and setpoint, and the built-in electronics handle the control of the solenoid. Valves without integrated electronics require a separate amplifier card in the control cabinet. For a replacement, it must be clear which variant is installed.
What information is needed to replace a proportional valve?
Required are the valve function, nominal size and mounting pattern, nominal flow or pressure stage, direct or pilot operation, setpoint signal, supply voltage, connector type and electronics variant. The complete type designation from the nameplate is particularly important, because small characters in the designation encode significant differences between versions.
Why does a proportional valve respond inaccurately or jerkily?
Common causes are contaminated oil that makes the valve spool stiff, an incorrectly set amplifier with unsuitable ramps or characteristics, interference in the setpoint signal or a damaged coil. Air in the system can also impair behaviour. Before replacing the valve, it is worth checking oil cleanliness and the signal curve.