A guide to common hydraulic symbols

A hydraulic circuit represents all the hydraulic components in a system. This includes the arrangement of the components and the behavior of the system as a whole in a universally accepted symbolic manner. In this article we will discuss the most common hydraulic symbols as represented in ISO 1219-1:2012. Armed with knowledge of how basic hydraulic components are represented in the hydraulic circuit; one can understand a wide range of different hydraulic symbols, representing components performing similar tasks with minor modifications.

The most commonly used hydraulic symbols are as follows:

Hydraulic Reservoir

A hydraulic reservoir stores hydraulic fluid. This is a must-have component in any hydraulic system. All hydraulic reservoirs are open to the atmosphere except in the case of those used in aircraft and submarines.

Hydraulic Pump and Motor

A hydraulic pump converts electrical and/or mechanical energy into hydraulic energy. The lower end (suction side) of a pump is connected to the hydraulic reservoir, the upper end is connected to the remaining circuit. The dark upper triangle in these hydraulic symbols indicates fluid going out of the system and hence represents a pump.
In the case of the hydraulic motor, the dark triangle is inverted indicating that the fluid is entering into the system. A hydraulic motor converts hydraulic energy into mechanical energy.
System output is represented by an arrow at 450 – this can be adjusted, In other words, that the pump/motor can deal with variable flow rate per shaft rotation. Most industrial applications use electric motors as prime movers to rotate hydraulic pumps. The electric motor is represented by the letter M inside of a circle. The curved arrow represents the direction of shaft rotation.



Hydraulic Cylinders

Hydraulic cylinders can be categorized as single acting cylinders and double acting cylinders.

Single acting cylinders can perform operations in only one direction and return to the initial position by spring.
Double acting cylinders can actuate in either direction depending upon the position of the direction control valve.

Controlling valves

Pressure Relief Valve

A pressure relief valve is a NC (normally closed) type safety valve which operates when system pressure increases above a maximum working pressure. The normally closed position is indicated by the arrow away from the center line. The dashed line indicates that the system pressure acts against spring force for valve actuation.

Direction Control Valve

A direction control valve is a vital component in a hydraulic system. It controls the actuator’s position and direction by controlling the fluid flow into the actuator. Therefore direction control valves can be designated by number of ports and number of positions and are selected based on the application.

The way to decipher a direction control valve symbol is as follows:

  • Every square box indicates one valve position.
  • The central position is a neutral position and various neutral positions are available depending upon the application. All ports closed will increase the system pressure to the maximum – actuating the pressure relief valve. Whereas all ports connected in the neutral position will relieve the system by diverting fluid from the pump to the tank directly.
  • DCV can be distinguished depending upon the type of actuation. Hand levers, mechanical systems or solenoids are used to change the valve’s position. A spring is used to return to a neutral position.

Flow control valve

The flow control valve is used to control the flow rate as well as the speed of the actuator. The position of flow control valve will lead to varied system behavior – an arrow representing the adjustable flow control.

There are a number of ways to control the flow:

  1. Meter-in: – controlling the flow rate at the inlet side of the actuator
  2. Meter-out: – control of the flow rate at the outlet side of the actuator.
  • Bleed-off: – delivering partial pump output to the tank

Check Valve

Check valve allows the fluid to pass only in one direction and restricts flow in the opposite direction.

Note: Arrow is not part of the symbol. It represents the direction in which fluid can flow

Hydraulic symbols representing methods of valve actuation

Push Button
Pull / Push Lever
Servo controlled

Hydraulic symbols for indicators

A pressure indictaor is used to measure hydraulic pressure at any one point. Hence it is generally connected between the hydraulic pump and direction control valve
Temperature indicator is used to measure the fluid temperature in the system.
Flow indicator indicates the flow rate.

Based on the above information, can you understand the hydraulic symbols and circuit below?

First of all you can see the electric motor driving the fixed delivery hydraulic pump in the above circuit.  A safe pressure level is maintained using the pressure relief valve which is connected after the pump.

4/3 Direction control valve is being actuated by a solenoid control with all the ports are closed during the neutral position. In the figure, the DCV is in its 1st position and hence pressurized liquid will flow towards the right side of actuator. The left side of the actuator is connected to a reservoir meaning the actuator will move towards the left side.