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How do I decipher a hydraulic circuit diagram? PART 1 SuperUser Account
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Ever tried to decipher a hydraulic circuit diagram and make sense of its symbols? Our blog will help you to understand the meanings and functions of the common ISO1219 hydraulic symbols. As there are so many possible combinations of system parts and functions possible, we look at the base component circuits that make up a typical hydraulic power unit.

Hydraulic Pump


A hydraulic pump typically comes in either a fixed or variable form with the variable version allowing adjustments in flow rate and outlet pressure. They both have the same aim though and that is to pump oil from the hydraulic reservoir back into the system.

There are typically 4 main types of pumps used in hydraulic systems:

• Vane pump – Vane pumps are reliable compact units that, in comparison to their size, produce the equivalent hydraulic power of larger units. They come in single and double stage forms and often require less maintenance in general due to their low failure rates.
• Gear pump – Available in either external or planetary/internal variants, gear pumps normally consist of two or more gears which link with a closely fitting housing. External gear pumps are usually found in construction or industrial based machinery as they have high tolerances and excellent durability whereas internal pumps are commonly found in high pressure machining tools such as forging applications and moulding presses.
• Piston pump – These pumps closely mimic the workings of a typical air cylinder type pump commonly seen inflating bicycle tyres or similar. The pistons in the pump work to discharge oil at a very high pressure through the pumps outlet port. Available in either radial or axial forms they are commonly found in heavy industry applications such as marine shipbuilding equipment.

• Hand pump – These pumps are handheld mechanical devices which pump high pressure fluid in one direction




Filters are an essential part of any system as they help filter our particulate from fluids and, in turn, they keep components in good shape and the system running effectively. They come in different sizes, with some in cases and functions and can be placed almost anywhere on a hydraulic unit if they are of the pressure filter type. Another popular type of filter is the return filter when filter oil is deposited back into the reservoir.

All in all, filters help to maximize the service life of a system provided they are changed at the specified manufacturer recommended intervals.

Pressure relief valve


The role of a pressure relief valve is to transfer fluids from areas of high pressures to those of low pressures. This most likely involves the tank.

The dotted line on the diagram indicates the hydraulic pressure which acts as a pilot that’s function is to actuate the relief valve, in turn, moving the arrow along the box. When the pressure from the pilot creates a force equal to the spring load, the valve will open and release flow.

Join us again soon for the second part of this blog which looks at the rest of the valves range as well as hydraulic cylinders and tanks.

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