The basic principle.

A solenoid turns an electrical command into mechanical force. In a hydraulic valve, this can move a closing element directly or control a small pilot passage that influences a larger stage.

The coil is only one part of the assembly. A command can be present while a mechanical or hydraulic condition prevents the expected result. Conversely, not every change in fluid behaviour is caused by the coil.

A pilot-controlled lowering path.

A worked-through example

The controller energises a coil. A pilot passage changes pressure acting on a main element, which changes the larger flow path. This is a conceptual example, not a description of every lowering valve.

Direct action and pilot action, inside the valve.

Both examples below are normally closed two-port valves. “Normally closed” means closed in the specified unenergised rest condition. Their internal shapes are deliberately simplified to expose the difference between direct mechanical action and hydraulic assistance.

1. Direct acting: at rest

1. Direct acting: at restIn this normally-closed example, the spring holds the plunger’s seal against the main seat. The inlet and outlet are separated.123INOUT

In this normally-closed example, the spring holds the plunger’s seal against the main seat. The inlet and outlet are separated.

2. Direct acting: energised

2. Direct acting: energisedThe coil lifts the plunger and its attached seal away from the main seat. Fluid can pass directly through the opening.123INOUT

The coil lifts the plunger and its attached seal away from the main seat. Fluid can pass directly through the opening.

3. Pilot operated: at rest

3. Pilot operated: at restThe pilot drain is closed. The restricted feed admits inlet fluid to the control chamber. Pressure on the larger upper area, assisted by the spring, holds the main piston on its seat in this design.124567INOUTMain seat

The pilot drain is closed. The restricted feed admits inlet fluid to the control chamber. Pressure on the larger upper area, assisted by the spring, holds the main piston on its seat in this design.

4. Pilot operated: energised

4. Pilot operated: energisedThe small plunger opens the pilot drain. Fluid leaves the control chamber faster than the restricted feed replaces it. Chamber pressure falls; the resulting force balance lifts the main piston and opens the main flow path.124567INOUTMain seat

The small plunger opens the pilot drain. Fluid leaves the control chamber faster than the restricted feed replaces it. Chamber pressure falls; the resulting force balance lifts the main piston and opens the main flow path.

  1. Coil and return spring. The magnetic field provides the actuating force; the spring biases the mechanism towards its rest position.
  2. Moving plunger. Direct action moves the main seal itself. Pilot action moves a much smaller seal in the control passage.
  3. Main seat in the direct-acting example. The plunger’s seal sits here to interrupt flow. The coil must overcome the forces opposing its movement.
  4. Main piston in the pilot-operated example. This larger element carries the main seal. The small solenoid does not lift it directly in the arrangement shown.
  5. Restricted feed. Connects the inlet to the upper control chamber. Its limited flow lets the open pilot drain reduce chamber pressure.
  6. Control chamber. Pressure acting over an area creates force. Changing the pressure above the piston changes its balance with the inlet pressure beneath it and the spring force.
  7. Pilot drain. Connects the control chamber to the outlet when the small pilot seal lifts. Its flow is much smaller than the main valve flow.
Original conceptual cutaways; not a manufacturer’s valve or a universal lift-lowering circuit. Thick solid arrows show main flow; fine dash-dot arrows show the smaller pilot flow. Dashed grey passages have no through-flow in the rest state, but can remain pressurised. Black arrows at the main piston show the direction of the opening tendency or closing action; they are not force magnitudes. The drawings use an external feed passage for clarity; real valves may route it through the piston.

The pressure condition matters. The uncoupled pilot-operated example needs a suitable inlet-to-outlet pressure difference to move correctly. That is a requirement of this design, not a rule for every solenoid valve: mechanically coupled or externally piloted designs can work differently. The manufacturer specifies the pressure range and any minimum differential pressure.

A lift valve block can use the same pilot principle to control a lowering, bypass or other hydraulic function. Which coil changes which path must come from that block’s own circuit description; “energised” does not universally mean “main valve open”.

Connect it to the lift.

Normally open and normally closed describe specified rest conditions. Energising a valve does not universally mean opening it. The circuit diagram and manufacturer’s description establish what each coil and passage does.

A closer look

Proportional valves control force or opening over a range using appropriate electronics. They should not be modelled as ordinary on/off valves operating at arbitrary supply voltages.

Check your understanding

If the coil energises, is that proof that the main flow path has changed correctly?

Reveal the explanation

No. The actuator, pilot stage and main hydraulic element must all perform their intended functions.

Further reading.

Optional reading to explore this lesson in more detail.

  • Blain: service manual (PDF)

    Technical example: pages 4–5 identify solenoids A–D and their roles in the EV100 hydraulic circuit.

  • Bürkert: direct-acting valves

    Explains how the coil moves the armature and seal directly, including the opposing spring and fluid forces.

  • Bürkert: servo-assisted piston valves

    Follow the restricted feed, upper chamber and pilot outlet. The text distinguishes designs requiring differential pressure from mechanically coupled variants.

  • Blain: EV / KV service manual (PDF)

    Pages 4–5 show how several solenoid-controlled functions are integrated into a real lift valve block. The generic cutaways above are not drawings of an EV100.