Hydraulic Symbols

Fixed displacement hydraulic pump

Pump that delivers the same volume per revolution, drawn as a circle with a filled triangle pointing to the outlet and the drive shaft on the side. Use it for the power unit of simple circuits where flow only changes with shaft speed.

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Variable displacement hydraulic pump

Pump whose displacement can be adjusted, shown by the oblique arrow across the circle. It appears in circuits that match flow to demand, such as pressure-compensated or load-sensing systems.

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Fixed displacement hydraulic motor

Rotary actuator that turns fluid flow into shaft rotation; the filled triangle points inward at the inlet. Draw it wherever the circuit drives a winch, conveyor or wheel.

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Electric motor (prime mover)

Circle with the letter M and an output shaft, representing the electric motor that drives a pump. Place it next to the pump so the two shafts meet.

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Single-acting cylinder, spring return

Cylinder with one pressure port on the cap end and a spring in the rod chamber that retracts the piston. Used where force is needed in one direction only, such as clamping or lifting.

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Double-acting cylinder

Cylinder with a port at each end, so pressure both extends and retracts the rod. It is the standard linear actuator in hydraulic circuits.

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Double-rod cylinder

Double-acting cylinder with the rod passing through both end caps, giving equal effective areas on both sides of the piston. Common in steering and positioning drives that need the same speed and force in both directions.

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2/2 directional control valve

Two-port, two-position valve shown normally closed, with flow from P to A in the actuated position. Works as an on/off valve; add the actuator and return spring blocks beside the boxes.

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3/2 directional control valve

Three-port, two-position valve shown normally closed: at rest A is connected to tank and P is blocked, when actuated P feeds A. Typical for single-acting cylinders and pilot signals.

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4/2 directional control valve

Four-port, two-position valve that switches P and T between ports A and B, so a double-acting cylinder extends or retracts. Ports are drawn on the right-hand box, the rest position.

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4/3 directional control valve, closed centre

Three-position valve whose centre position blocks P, T, A and B, holding the actuator in place. Suited to circuits where several valves share one pump with an accumulator or a pressure-compensated pump.

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4/3 directional control valve, tandem centre

Three-position valve whose centre connects P to T and blocks A and B, so the pump unloads to tank while the actuator stays locked. A common choice with fixed displacement pumps.

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4/3 directional control valve, open centre

Three-position valve whose centre position connects all four ports, unloading the pump and leaving the actuator free to move. Used where the load may float when the valve is in neutral.

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Solenoid actuator

Rectangle with an oblique line for direct electrical operation by a single winding. Attach it to the outer side of the end box it shifts.

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Return spring

Zigzag spring that returns the valve to its rest position when the other actuator releases. Drawn for the right-hand side of the valve; mirror it for the left.

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Lever actuator

Manual operation by a hand lever, shown as the operating stem with a lever ending in a small circle. Used on mobile and manually operated valves.

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Push-button actuator

Manual operation by a push button, drawn as the operating stem capped with a rounded head. Usually combined with a return spring on the opposite side.

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Hydraulic pilot actuator

Direct pilot operation: a dashed pilot line with a filled triangle pointing at the valve shows that pilot pressure shifts the spool. Used when another valve or a pressure signal controls the main valve.

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Check valve

Ball pressed against a seat that lets flow pass from left to right and blocks the reverse direction. Draw it wherever the circuit must stop back-flow, for example at a pump outlet.

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Pilot-operated check valve

Check valve that a pilot signal, drawn as the dashed line at the seat, can open to allow reverse flow. It holds a cylinder load without leakage until the pilot line is pressurised.

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Throttle valve, adjustable

Adjustable restriction on the line, shown by two facing arcs crossed by an oblique arrow. It controls flow in both directions, so actuator speed depends on the load.

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One-way flow control valve

Adjustable throttle with a check valve in parallel: flow is restricted in one direction and passes freely through the check valve in the other. The usual way to set the speed of a cylinder in one stroke only.

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Pressure relief valve

Normally closed valve with an adjustable spring: inlet pressure, sensed through the dashed pilot line, opens the path to tank once it overcomes the spring. It sets the maximum pressure of the circuit.

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Pressure reducing valve

Normally open valve that throttles flow to keep the outlet pressure, sensed through the dashed pilot line, at the spring setting. Used to feed a branch that needs lower pressure than the main supply.

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Filter

Square set on its corner with the dashed filter element across the flow path. Place it in the suction, pressure or return line depending on what it protects.

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Gas-loaded accumulator

Vertical capsule divided by a separating element, storing fluid under pressure against a gas charge. Used for energy storage, pulsation damping and emergency supply.

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Reservoir (tank)

Open-top tank vented to atmosphere, with the line ending below the fluid level. Each return or drain line in a circuit ends at one of these symbols.

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Cooler

Square on its corner with an arrow pointing outward at both ends, meaning heat leaves the fluid; coolant lines are not shown. Typically placed in the return line before the tank.

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Pressure gauge

Measuring instrument circle with an oblique pointer arrow, connected to the line whose pressure it shows. Draw it at test points and after pressure control valves.

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Working line

Continuous line for pressure, return and suction connections between components. It is the main flow path of the circuit.

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Pilot or drain line

Dashed line for pilot control signals and for drain or leakage lines. Use it to tell control connections apart from the working lines.

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Quick coupling, connected, self-sealing

Coupled quick-release connector with a non-return valve in each half, so both sides seal when disconnected. Used on hoses to attachments and on test points that are connected and disconnected often.

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