Hydraulic Symbols Tutorial

This is a good tutorial for people who would like to know more about hydraulics but have not been able to find a good resource. If you would like to know an excellent resource for information on hydraulics then purchase the Fluid Power Designer Lightning Reference Handbook by Paul Munroe Hydraulics, it is highly recommended.

Fluid Power Symbols

Hydraulic Tank (fluid reservoir)

All hydraulic systems must have some form of a reservoir to hold the fluid in the system. Most systems have vented tanks, however aircraft are one application where a closed tank is appropriate. The symbol shown here is a vented tank, a box with the line in the center would indicate a closed system. The line could also not go to the bottom of the tank, that would mean that the line stops above the fluid level in the tank and the fluid falls in. It is better to stop the line below the fluid level, otherwise the falling fluid may cause bubbles in the fluid.

Hydraulic Pump

A pump displaces fluid which creates flow. There are fixed displacement pumps and variable displacement pumps. The pump symbol is very similar to a hydraulic motor symbol, the difference is that the pump has the small triangle pointing out and a motor has the small triangle pointing in to the center. An angled arrow typically indicates that a device is variable, thus this is a variable volume pump. Fixed displacement pumps provide the same output volume with the same input RPM. Variable displacement pumps can change the output volume while maintaining the same input RPM. Hydraulic pumps are precision components and have very close tolerances, they must be treated with care.

Hydraulic Line

Hydraulic lines carry the fluid from the pump throughout the system. There are two basic types, rigid and flexible. Rigid lines are used to connect items that will not move in relation to each other. Manifolds connected with rigid lines are the most reliable transfer method. The dots at the end of the line show a connection point, if two lines cross and this dot isn't shown then the lines are not connected.

Hydraulic Hose (flexible line)

A flexible line is used to carry fluid to items that have a lot of vibration or movement in relation to each other. Some examples where flexible lines are used, the pump unit (vibration) or blades on a tractor, due to the movement.

Pressure Relief Valve

Hydraulic fluid is virtually non compressible, if the fluid can't go anywhere the pump will stall, and damage to the pump and motor can result. All hydraulic systems must have a pressure relief valve in line with the pump. The pressure relief will drain into the tank. The dashed line indicates a pilot line, this is a small line that only flows enough fluid to control other valves. The pressure of this pilot line acts against the spring on the other side of this valve. When the pilot pressure exceeds the spring force then the valve spool shifts over and opens the valve, this allows flow to the tank. This causes a drop in the pressure on the pump side, which also reduces the pilot pressure. When the pilot pressure is less than the spring force the spring closes the valve. The relief valve in the position described above will control the maximum pressure in the hydraulic system.

Directional Valves

A directional valve will control which device the fluid will flow to. These valves are the primary devices used to sequence the motion of equipment. There are many different types of directional control valves. The valve is generally specified by number of positions and number of ways (ports). The valve is made up of two parts, the body and the spool. When valves shift the spool is moved in relation to the body, this opens and closes passages that the fluid flows through. Remember that the valve actuator always pushes the spool, this will help you read the drawings. You read the operation of a valve in a circuit in the following manner. The box(s) with arrows in it show the flow of fluid when the valve is shifted. The box without arrows and/or away from the actuator shows the flow, if any, in the neutral position. This is also the box you use to count the number of ports the valve has.

Two(2) Position, Two(2) way

This valve has two positions (2 boxes) and 2 ways (ports); thus 2 position, 2 way. It is shown with a manual actuator (on the right) and has a spring return to neutral. This valve is called normally closed because both ports are blocked when in neutral. It could be used on a safety device like a safety gate, if the gate isn't closed, actuating the valve, then the flow will be stopped, preventing movement of the connected device.

Three(3) Position, Four(4) way

This valve has three positions (3 boxes) and 4 ways (ports); thus 3 position, 4 way. It is shown with a closed center, when the valve is neutral all ports are blocked. The small boxes on each end with diagonal lines through them, C1 and C2, are electrical coils, this is an electrically actuated valve. The port marked P is Pressure and the port marked T drains to tank. The ports marked A and B connect to an external device, like a cylinder. When C1 is energized the valve will shift, putting pressure to the B port and draining the A port to the tank. Likewise when C2 is energized the pressure port connects to the A port and the B port drains to the tank.

Three(3) Position, Four(4) Way

This valve has three positions (3 boxes) and 4 ways (ports); thus 3 position, 4 way. It also is electrically actuated. The jagged lines next to the coil indicates springs, when the coil is de-energized the opposite spring will force the spool back to the center position. This valve also drains to tank when in neutral, this is a standard valve on molding machines. They drain to tank when de-energized for safety.

Cylinder

A cylinder is one of the devices that creates movement. When pressure is applied to a port it causes that side of the cylinder to fill with fluid. If the fluid pressure and area of the cylinder are greater than the load that is attached then the load will move. Cylinders are generally specified by bore and stroke, they can also have options like cushions installed. Cushions slow down the cylinder at the end of the stroke to prevent slamming. If the pressure remains constant a larger diameter cylinder will provide more force because it has more surface area for the pressure to act on.

A Complete Circuit

Some Basic Hydraulic Formulas

If you use formulas occasionally a handy trick is to set up a spreadsheet that has the formulas built in, then all you need to do is enter the numbers.

Pump Outlet Flow

The outlet flow of a pump in GPM is:Flow (GPM) = RPM * Pump Displacement(Cu.in./Rev) ____________________________________ 231

Area of a Cylinder

Both the force and speed of a cylinder are dependent on knowing the area of the cylinder. Remember that the area on the rod end of a cylinder is different than that of the non rod end. You must subtract the area of the rod itself from the overall area of the cylinder. The same formula can be used to determine the area of the rod. As you will see when pressure is applied to the rod end of the cylinder (pump pressure and volume constant), it will move faster and have less force.Area (Sq.In.) = Pi * Diameter^2 _________________ 4

Area (rod end) = Area - area of the rod itself

Force of a cylinder

A cylinder usually has two forces, the force when applying pressure to side with the rod, and the side without the rod. An exception can be a double ended cylinder, it has a rod end sticking out of each cylinder end. This can also be applied to pneumatics.Force = Pressure (PSI) * Net Area (Sq. In.)

Speed of a cylinder

Note - Inaccuracies have been discovered in the following equation and are being evaluated. The speed of a cylinder is dependent on the flow rate to the cylinder and the area of the cylinder. This formula assumes no loss of fluid over a relief valve. Velocity (Ft./Sec.) = 231 * Flow Rate (GPM) ___________________________ 12 * 60 * Net Area (Sq.In.)

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HYDRAULIC & PNEUMATIC CONTROL

  FLUID POWER GRAPHIC SYMBOLS

The standard icons to graphically represent fluid power components are defined in the Australian standard AS

1101.1-1993 Graphic symbols for general engineering - Hydraulic and pneumatic systems. The following are

some of the commonly used components:

The valve designation is as follows: p/n DCV = Directional Control Valve with n settings

and p ports. It can be Normally Open (NO) or Normally Close (CO).

The port numbers on DCVs have the following standard designation:

Port Number

Designation

1Pressure port (coming from the pump)

3Exhaust port (back to the tank)

2, 4 Output ports 

VARIABLE FLOW VALVES The great of majority of the valves used in fluid power applications are discrete valves as represented by the

above valve symbols. The opening area for these valves are constant and can be either open or closed. In variable flow valves, the flow area is a function of

the spool position. By adjusting the spool position, one can adjust the flow rate.

The flow rate obviously depends on the flow area as well as the pressure difference across the valve. Only the flow area can be controlled by the spool position.

Therefore, if precise control of the flow rate is essential, then a control loop has to be implemented

that modulates the spool position to keep the flow rate constant. Such valves are called the servovalves.

Valve selection The choice of the valve is an important consideration in

any fluid control application. This choice is a usually compromise between the control requirements and the

system cost. In difficult environments (eg mining or construction industry), reliability also is a factor.

Best control performance is usually offered by servo valves due to their good linearity and high bandwidth

characteristics. However, these valves have a very low tolerance against contaminants in the hydraulic oil.

The standard of cleanliness required by a servovalve may be difficult to achieve and maintain in "dirty"

environments. In some instances, capacity may also be a problem. For large machines, it simply is not possible

to find a servovalve with the required flow capacity. Regular directional control valves can be made

extremely robust and are available at the desired size and they should be the first option for jobs where

control requirements are not too demanding. Solenoid-driven proportional directional valves (SDPDV) provide a reasonable trade-off. SDPDVs are relatively new additions to control world. They were introduced

in the 1980s and they have become very popular since then due to their simplicity and cost effectiveness. Their severe non-linearity (in comparison with the

servovalves), however, may offer a significant challenge for the control designer.

EXAMPLES OF FLUID POWER CONTROL We will now provide some examples of how the fluid power components are connected and controlled to perform simple tasks. The examples all use simple

DCVs. The treatment of proportional or servo-valves are beyond the scope of this course. Control of a single-actuating ram

Example: Draw the hydraulic circuit and the electrical diagram for a hydraulic ram. The piston is to be

extended when a manual switch is closed. The piston should return back when the switch is released. Use a

single-acting cylinder and a 3/2 valve. Answer :

Example: A hydraulic press is controlled by two manual switches placed 1 m apart (switches S1 and S2) and a

third switch S3 representing the status of the protective cover. The switch S3 is automatically closed

when the protective cover is in place.The press is to be activated whenever

(a) both S1 and S2 are ON; or(b) either of the S1 or S2 and S3 are ON.

Once the press is activated, it will stay down until a normally closed lift button switch (S4) is released and

breaks the circuit.Design a hydraulic circuit and the electrical logic to

drive this hydraulic press.Answer:

There is a deliberate mistake in the PLC diagram. See if you can spot it.

Notes:1. The symbol S1 appears twice on the PLC diagram. It refers to signals from the same switch on the hydraulic

circuit.2. ditto for S2.

3. K is the relay that engages the latch switch. The latch switch provides the signal for the solenoid relay X

even if S1, S2, and S3 are broken.4. S4 is the STOP button. When it is broken, K is to

deenergise; the latch switch opened; the solenoid relay

deenergised and, under the spring action, the valve returns to its normal position, causing the ram to

retract. Is this happening in the above diagram? If not, how can you correct it?Regenerative Circuit

 

No motion

Extension at normal speed (piston moves to right)

Rapid retraction (piston moving to left)

Centre position: The piston does not move. Right position: Retraction.  The piston speed is given

by Vpist = Qpump /(Apist - Arod)

where Vpist     Piston Speed, m/s

Qpump  Pump Flow rate, m3/s Apist    Piston area, m2 Arod     Rod area, m2

Left Position: Rapid Extension Apist Vpist = Qpump + (Apist - Arod) Vpist        ==> Vpist =

Qpump/Arod A small rod area leads to very rapid extension.

OIL HEATING In a hydraulic system, the power is transmitted by pushing the working fluid (usually oil) through the circuit. During this process, some of the power is spent in heating the oil. For example, when oil is pushed through a valve, no external work is done but the pump still has to exert effort to push the oil through the pressure differential over the valve. You can think of this as work done on the oil and all of it is converted into heat:

The work is done on the oil is given by

This is converted into heat and the resultant temperature increase for the oil volume can be calculated by