The True Value of Hydraulic Circuit Diagrams

Brendan Casey

I am regularly involved in troubleshooting problems with hydraulic equipment. In these situations, there are two steps I always complete before reaching for my test gear.

The first is to conduct a visual inspection of the hydraulic system, checking all the obvious warning signs that could cause the problem in question (never overlook the obvious). The second is to ask for a circuit diagram for the hydraulic system.

Figure 1. Graphical Circuit Diagram

There are four types of hydraulic-circuit diagrams: block, cutaway, pictorial and graphical.

Block Diagrams show the components of a circuit as blocks joined by lines, which indicate connections and/or interactions.

Cutaway Diagrams show the internal construction of the components and flow paths. Because these diagrams typically use colors, shades or patterns in the lines and passages, they are very effective at illustrating different flow and pressure conditions. This makes them ideal for training purposes.

Pictorial Diagrams show a circuit’s piping arrangement. The components are seen externally and are usually in a close reproduction of their actual shapes in scaled sizes. This aids in component recognition and identification.

Graphical Diagrams are the shorthand system of the fluid power industry (Figure 1). They comprise simple, geometric symbols, drawn to American National Standards Institute (ANSI) or International Organization for Standardization (ISO) standards, that represent the components, their controls and connections. Graphical diagrams are preferred for design and troubleshooting purposes.

A graphical circuit diagram is a road map of the hydraulic system, and to a technician skilled in reading and interpreting them, it is a valuable aid in identifying possible causes of a problem. This can save time and money in a troubleshooting situation.

If a circuit diagram is not available, the technician must trace the hydraulic circuit and identify its components in order to isolate possible causes of the problem. This can be a time-consuming process, depending on the complexity of the system.

Worse still, if the circuit contains a valve manifold, the manifold may have to be removed and dismantled - just to establish what it’s supposed to do. Reason being, if the function of a component within a system is not known, it can be difficult to discount it as a possible cause of the problem. Circuit diagrams eliminate the need to reverse-engineer the hydraulic system.

Where are All the Hydraulic Circuit Diagrams?

As most hydraulic technicians know, it’s likely that a circuit diagram will not be available for the machine they’ve been called in to troubleshoot. This is unlikely to bother the technician because it is the machine owner who usually pays for its absence through prolonged service calls and increased downtime.

Where do all the hydraulic schematic diagrams go? They get lost or misplaced, they don’t get t ransferred to the new owner when a used machine is traded, and in some cases, they may not be issued to the machine owner at all. Why? Because generally speaking, hydraulic equipment owners don’t place a lot of value on them.

If you’re responsible for the maintenance of hydraulic equipment and you don’t have circuit diagrams for your existing machines, try to obtain them - before you need them.

In the case of a mass-produced machine, the hydraulic circuit diagram should be available from the equipment manufacturer on request. If the machine is custom-built, it may be necessary to have the circuit drawn from scratch.

A fluid power engineer or your preferred hydraulics supplier could provide this service. When obtaining circuit diagrams, request both electronic (dwg format) and hard copy.

Having an electronic copy makes it easy to update the drawing if the circuit is modified at any time. And always ensure that you are issued circuit diagrams for any additional hydraulic machines you acquire. The nominal cost involved could save you a lot of money in the long run.

Read more on hydraulic system best practices:

The Seven Most Common Hydraulic Equipment Mistakes

How Do You Know if You're Using the Right Hydraulic Oil?

10 Hydraulic Reliability Checks You Probably Aren't Making

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About the Author

Brendan Casey has more than 20 years experience in the maintenance, repair and overhaul of mobile and industrial equipment. For more information on reducing the operating cost and increasing the...