The hydraulic schematic appears intimidating initially, with its letter, arrow and square symbols rather than real life images, but it’s a language all hydraulics speak and it saves time. The more you learn about the pattern, the more you’ll be able to figure out what is going on in the system. The diagram will read like a story that makes sense of flow from point A to point B.
Each diagram has four letters: P, T, A and B. Pressure (P) powers it; tank (T) is where it returns. Work ports A and B makes the machines go. To figure out how the machine works, learn where the fluid flows. The main working pressure runs in a solid line. A drain or pilot signal is represented by a dashed line. These can be wired wrong if they are confused causing circuits to fail. Learn to differentiate between two.
How to Read Hydraulic Diagrams
Motors and Pumps is similar looking except for the direction indicated by a triangle. Outward facing triangle with a solid fill indicates it’s a pump that pushes fluid around the system. An inward facing triangle with a hollow fill indicates the motor draw flow to turn it. Arrow going thru the circle indicates a variable displacement unit. The variable unit can adjusts to cool the system, while the fixed unit will continue pushing no matter what causing the system to overheat. Two triangles is bidirectional. This is common on mobile units. This allow reverse flow.
Rectangles represent cylinder. Rectangles come in different sizes based off how many ports there are. One port means that the rectangle is a single-acting cylinder; it goes out under hydraulic pressure but then relies on gravity or some other method like a spring to pull back into place. Having ports on both ends means it’s a double-acting cylinder, which means that it can be pushed out and pulled back in using hydraulic power. That means you can control force and speed in both directions. Double-acting cylinders typically goes with four-port valves and that combination lets you precisely control retraction and extension. You’re in control of the load instead of hoping for it to retract.
The system’s logic lives in its valves, which are notated by the number of positions and number of ports. A position is represented by each square in the symbol; therefore, three squares denote a 3-position valve capable of being switched into 3 states. The central position is important. An open center links pressure and tank to conserve energy whereas a closed center closes off all ports to retain a load. Selecting the incorrect center position either wastes power or allows movement without control. A middle ground is found with tandem centers that unload the pump yet lock the actuator, a deciding point for separating amateur from professional design.
Hydraulic systems operate at thousands of pounds per square inch and safety is vital. If there’s even a small leak, it will inject hydraulic fluid into your skin, a medical emergency. To protect the system, use a relief valve a bit higher than working pressure, but don’t go past the rating for hoses. Don’t forget that the pressure doesn’t know or care who’s in control. It follows no other rules. Follow the solid lines beginning from the reservoir and you’ll start reading the system moddern.
