Hydraulic symbology 102: understanding basic fluid power schematics
I covered basic constituent lines, shapes and their respective symbols. Due to space constraints, I left out some of the major components represented by symbols in hydraulics, so here I continue with Hydraulic Symbology As it turns out, cylinders use rectangles in three forms. The basic differential cylinder is a wide rectangle partially bisected by a line being the rod, itself attached at one end to the piston.
Although not always necessary, the ports are appointed by stubby lines. This first example is a differential cylinder, meaning its single-rod construction has a differential of both area and volume on either side of the piston.
It will extend with more force than it retracts but will retract with more velocity than it extends. A ram, by definition, is single acting, so the rod side port is omitted. Replacing the second but adding a third rectangle to the mix is the choice of head and cap cushions. In the symbol shown, the diagonal arrow of adjustability is included to call out some type of needle valve to control cushion rate in the head and cap.
Hydraulic symbols are functional representations in their simplest forms, so get used to reading them with that in mind. After rectangles come ovals, which are often confused with an ellipse. The oval is oblong with two straight lines, where ellipses are stretched circles. In fluid power symbology, an oval represents an accumulator, or energy storage vessel. Most accumulators are energized with inert gas, such as nitrogen, and the symbol shows a partition separating the top and bottom of the oval.
A spring on the other side of oil in a piston accumulator compresses to store energy as air would, although a spring cannot compress as infinitely as gas. The jagged spring symbol is the exact one used in a directional or check valve. Turning the spring into a square signifies we now have a weighted accumulator, which is my favourite type. A weighted accumulator is the only one able to supply continuous and stable pressure as it depletes itself, contrary to the limited and varied pressure and flow available from the other two.
The small circle is next in the hierarchy and portrays auxiliary components important to complete a hydraulic system. The check valve uses a small circle stuffed into a wedge to represent very accurately a chrome ball nested into a conical seat. Any flow from the top port will do nothing but force the ball harder into the seat, absolutely restricting flow. Next up are two common ways to identify pressure in a system; the pressure gauge and pressure indicator. The pressure gauge is logical, showing a needle within a circle, making it not hard to imagine the bottom-mount unit most common in our industry.Understanding schematic symbols will help to better understand at a glance the application, control, direction, amount of flow for actuated valves, cylinders and rotary actuators.
Two way and three way valves with a mechanical return spring that automatically return the valve back. Actuators provide clamping, movement, rotation, turn and position from fluid power, pneumatic and hydraulic circuits.
Powered by pneumatic air or hydraulic lines. Rotary actuators provide rotational force to actuate ball, butterfly, gate and globe valves. These can be quarter turn, half turn, full turn and multi turn actuators powered by pneumatic or hydraulic lines.
Process equipment details and drawings relate to the type of schematic drawing provided. Typically a process flow schematic drawing will provide basic levels of technical information to highlight basic essential flow control and flow paths. An engineering line diagram of P and ID will show more detailed and precise information. Solenoid Valve Symbols. Basic Symbols.hydraulic and pneumatic part 1
Square or rectangular block specify a valve position. Two square or rectangular blocks specify the valve has two possible positions Three square or rectangular blocks specify the valve has three possible positions Arrows specify the direction of flow, in this case in one direction, if a line has an arrow at each end then the flow is bi-directional or flow in both directions.
Symbol for closed port. Symbol for two port valve with both ports closed. Symbol for spring or rest position of valve. The spring pushes from the side that is drawn on and will return the valve back to its failsafe position. Symbol for pressure inlet port of valve. Symbol for 2 port valve with two positions, one position valve open with flow in both directions or valve closed with both ports blocked.
Symbol for 3 port valve with 2 positions. Basic Valve Symbols.
Mechanical valve symbols. Common Valve Symbols.Directional air control valves are the building blocks of pneumatic control. Pneumatic circuit symbols representing these valves provide detailed information about the valve they represent. Symbols show the methods of actuation, the number of positions, the flow paths and the number of ports.
Here is a brief breakdown of how to read a symbol. Most valve symbols have three parts see Figure 2A below. The Actuators are the mechanisms which cause the valve to shift from one position to another. The Position and Flow Boxes indicate how the valve functions. Every valve has at least two positions and each position has one or more flow paths, thus every valve symbol has at least two Flow Boxes to describe those paths.
Check out our Interactive Pneumatic Circuit Symbols here. Flow direction is indicated by the arrows in each box. These arrows represent the flow paths the valve provides when it is in each position. In the example above, when the lever is NOT being activated, the spring return actuator right side is controlling the valve, and the box adjacent to the spring shows the flow path. When the lever IS actuated, the box next to the lever shows the flow path of the valve.
A valve can only be in one position at a given time. With this 3-position valve, the center flow box shows the flow path when neither actuator is active and the springs are holding the valve in the center position. The number of ports is shown by the number of end points in a given box. Count only the ports in one flow box per symbol For example there are three boxes in the Figure 2B valve symbol showing each of the three different positions possible for the valve.
In Figure 2Cthere are a total of 5 ports. Sometimes a port usually an exhaust port goes directly to atmosphere and there is no mechanical means for attachment of silencers, flow control valves, or any other accessories. Port labels are typically shown on a single flow box per symbol. Different manufacturers label valve ports with different letters, but the labels at right are fairly standard.
In most situations the number of ports and ways are the same for a given valve, but take a look at Figure 2C above. It has five ports, but it is considered a 4-way valve because two of the ports share the same exhaust function. This is a holdover from hydraulics — where the two exhaust paths are joined internally to the valveso that only one return port is required, and only one return line is required to get the hydraulic oil back to the storage tank for re-use.
The symbols on the next page detail many of the ports, ways, and positions of common pneumatic valves.The top symbol shows a solenoid that pushes in the direction of the valve element shown by the dotted lines. This is by far the most common type of solenoid actuation. The bottom symbol shows an electrical control mechanism with two coils that will both push and pull the valve spool. Caution must always be shown when analysing a circuit to this detail.
It is likely the designer has shown the symbol drawn incorrectly but you should always double check especially if it does not match the part code or components supplied. There are generally a wide range of valve actuation types available and they are usually not all shown in the manufacturers standard datasheets.
Directional Valve Symbols
The bottom symbol shows an electrical solenoid that is continuously controlled. The actuation is towards the valve element. The black triangle shows a two stage, pilot operated valve. The direction the triangle is pointing represents the direction of fluid flow e.
The top symbol shows a continuously controlled, bi-directional, electrical solenoid, with mechanical spool position feedback. Typical of a traditional servo valve. Electrical solenoids. The middle solenoid symbol shows a direction of actuation away from the valve element. Mechanical operation. These symbols show mechanically operated valves.
The top symbol shows a manual lever operation The second symbol shows a roller activate valves. The third symbol shows a manual push button The bottom symbol shows a foot pedal activation. Valve control options. The middle symbol shows a turning mechanism with 4 fixed detent positions. Two-stage valves. The middle symbol shows a two-stage valve with an external pilot drain line. The bottom symbol shows an electric stepping motor driving the valve.
Last Updated The basic idea behind any hydraulic system is very simple: Force that is applied at one point is transmitted to another point using an incompressible fluid. The fluid is almost always an oil of some sort. The force is almost always multiplied in the process. The picture below shows the simplest possible hydraulic system:. A Simple hydraulic system consisting of two pistons and an oil-filled pipe connecting them.
Click on the red arrow to see the animation. In this drawing, two pistons red fit into two glass cylinders filled with oil light blue and connected to one another with an oil-filled pipe. If you apply a downward force to one piston the left one in this drawingthen the force is transmitted to the second piston through the oil in the pipe. Since oil is incompressible, the efficiency is very good -- almost all of the applied force appears at the second piston. The great thing about hydraulic systems is that the pipe connecting the two cylinders can be any length and shape, allowing it to snake through all sorts of things separating the two pistons.
The pipe can also fork, so that one master cylinder can drive more than one slave cylinder if desired. The neat thing about hydraulic systems is that it is very easy to add force multiplication or division to the system. If you have read How a Block and Tackle Works or How Gears Workthen you know that trading force for distance is very common in mechanical systems.
In a hydraulic system, all you do is change the size of one piston and cylinder relative to the other, as shown here:. Hydraulic multiplication. The piston on the right has a surface area nine times greater than the piston on the left. When force is applied to the left piston, it will move nine units for every one unit that the right piston moves, and the force is multiplied by nine on the right-hand piston.
Click the red arrow to see the animation. To determine the multiplication factorstart by looking at the size of the pistons. Assume that the piston on the left is 2 inches in diameter 1-inch radiuswhile the piston on the right is 6 inches in diameter 3-inch radius.
The area of the left piston is therefore 3. The piston on the right is 9 times larger than the piston on the left.
What that means is that any force applied to the left-hand piston will appear 9 times greater on the right-hand piston. So if you apply a pound downward force to the left piston, a pound upward force will appear on the right.
The only catch is that you will have to depress the left piston 9 inches to raise the right piston 1 inch. The brakes in your car are a good example of a basic piston-driven hydraulic system.Fluid power diagrams and schematics require an independent review because they use a unique set of symbols and conventions.
Different symbology is used when dealing with systems that operate with fluid power. Fluid power includes either gas such as air or hydraulic such as water or oil motive media. Some of the symbols used in fluid power systems are the same or similar to those already discussed, but many are entirely different. In the broad area of fluid power, two categories of pump symbols are used, depending on the motive media being used i.
The basic symbol for the pump is a circle containing one or more arrow heads indicating the direction s of flow with the points of the arrows in contact with the circle. Hydraulic pumps are shown by solid arrow heads.
Pneumatic compressors are represented by hollow arrow heads. Figure 19 provides common symbols used for pumps hydraulic and compressors pneumatic in fluid power diagrams. Reservoirs provide a location for storage of the motive media hydraulic fluid or compressed gas. Although the symbols used to represent reservoirs vary widely, certain conventions are used to indicate how a reservoir handles the fluid. Pneumatic reservoirs are usually simple tanks and their symbology is usually some variation of the cylinder shown in Figure Hydraulic reservoirs can be much more complex in terms of how the fluid is admitted to and removed from the tank.
To convey this information, symbology conventions have been developed. These symbols are in Figure An actuator in a fluid power system is any device that converts the hydraulic or pneumatic pressure into mechanical work. Actuators are classified as linear actuators and rotary actuators. Linear actuators have some form of piston device. Figure 21 illustrates several types of linear actuators and their drawing symbols.
Rotary actuators are generally called motors and may be fixed or variable. Several of the more common rotary symbols are shown in Figure Note the similarity between rotary motor symbols in Figure 22 and the pump symbols shown in Figure The difference between them is that the point of the arrow touches the circle in a pump and the tail of the arrow touches the circle in a motor. The sole purpose of piping in a fluid power system is to transport the working media, at pressure, from one point to another.
The symbols for the various lines and termination points are shown in Figure Valves are the most complicated symbols in fluid power systems.The valves shown are all two way, two position directional valves.
This means they have two flow pipe connections and can switch into two different positions. The top symbol shows a manually activated valve that is pushed over against a spring. The double arrow in the left box shows that the flow may pass in both directions. Valves should always be drawn in the de-activated position e. This top valve, therefore, shows a 'Normally Closed' NC valve e.
When activated the left-hand box would be connected to the pipes and flow could pass through the valve. The bottom symbol shows a hydraulically operated valve, but in this case, flow can only pass in one direction because the spring chamber is connected to the low pressure, return line connection. When using valves that switch pressure lines, without a direct, low pressure, return line connection it is important to make sure that the valves solenoid and spring chambers are rated for the maximum pressure they may see.
Not all valves will operate or work safely without a low-pressure drain signal. Learn more about directional valves. The top symbol shows a two-position valve that is switched by a mechanical roller. An example might be that as the cylinder extends, it contacts the roller, which switches the valve and this stops the cylinder stops moving. The middle valve also has two positions but it is operated by a solenoid and includes a detented hand emergency button. The ball on seat symbols inside the valve also indicates that this is a low leakage poppet style valve rather than a traditional spool style valve.
The bottom symbol shows a three-way three position valve that is hydraulically operated. This valve also has two end springs to return the spool to the centre position when no pilot signal is available.
Note how the hydraulic pilot is shown as a solid triangle although if this was a pneumatic pilot it would be shown as a clear triangle. Both valves shown are four-way two position valves.
The 'two position' means that it has two switched positions i. The bottom valve is solenoid operated with two detent positions to hold the valve in its last switched position. All symbols show four-way three position valves.