In order to interpret hydraulic circuit diagrams, it's important to know what the basic symbols are and what they mean. This post is meant for beginners, but can also serve as a good review and reference for those more experienced with reading hydraulic schematics.
Here are a few other Shop Talk Blog posts you might find extremely useful:
- The Ultimate Glossary of Terms About Hydraulic Motors
- Heavy Equipment 101: Classifying Hydraulic Motors by Their Function
- Basics of Hydraulics Part 1: Introduction to Pressure
Hydraulic Circuit Diagrams
The purpose of a hydraulic circuit diagram is to show how different components are interacting within a system. The circuit diagrams usually differ in appearance significantly from the physical hydraulic setup because circuit diagrams are intended to be a more abstract representation.
Hydraulic circuit diagrams are very similar to electrical circuit diagrams. Electrical diagrams have lines that represents electrical connectors between components. In a hydraulic circuit, the lines represent connectors that allow fluid to flow between components. Electrical circuits symbols representing electrical components (resistors, power sources, transistors, etc.) and systems. For a hydraulic circuit, the symbols represent components (valves, pumps, motors) and systems. And both use industry standard symbols for components, which for hydraulic diagrams would include things such as valves, pumps, motors, and tanks.
A hydraulic reservoir is used to hold the hydraulic fluid needed by the system. These reservoirs, or tanks as they are often called, can either be above or below the fluid level, and either pressurized or non-pressurized.
Common Hydraulic Valves
An on-off valve is very simple in its purpose: it either allows flow through or it does not.
A check valve has quite a few different names, including one-way valve, retention valve, non-return valve, foot valve, and reflux valve. It's operation is quite simply: is allows fluid to flow through in one direction only. These valves have two ports: one for the fluid to flow in, and the the other for the fluid to flow out.
Flow control valves automatically adjust flow rates based on factors such as temperature and pressure. They can range from very simple components to extremely complex electromechanical devices.
There are also pressure relief and pressure reducing valves.
Here are some additional types of flow valves you may see.
Pressure, temperature, and flow rate are key characteristics of hydraulic fluid as it proceeds through a system. Venturi meters are a specific instrument used to measure the flow rate of a fluid.
We usually think of ports as the point of connection that makes pumps, hydraulic motors, etc. a functioning part of a hydraulic system. When you see the simple for a port in a hydraulic circuit, it represents the beginning or end of a passage of fluid flow. Multiple ports are combined into a manifold.
Here are some more complex port arrangements.
Motors and Pumps
There are a variety of symbols for hydraulic pumps and hydraulic motors, and that makes sense because there are several different types. Based on the symbols present, you can tell at a glance is the pump/motor is fixed-displacement or variable displacement, as well as whether they are unidirectional or bidirectional.
Referring to the hydraulic circuit diagram for a piece of equipment can be extremely helpful when troubleshooting a hydraulic system. While this is certainly not an exhaustive list of hydraulic symbols, it does provide good foundation to begin studying hydraulic circuit diagrams.