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Understanding Hydraulic Cylinders: Part 1 - Single & Double Acting

Hydraulic cylinders or “cylinder jacks” are used commonly in a variety of lifting and pushing applications in the load and house moving industry.

For expert users cylinder jacks are very easy to configure and use. For people that have never used these jacks, configuring a system and accurately using the system seems extremely complicated. The reason is that between the 3 main manufacturers of cylinders in the US, over 10,000 cylinders, pumps and fittings are available. A novice user does not know how all these components work, what job they do and how to configure them.

Our article here aims to explain the basics to understanding hydraulic cylinders.

The basic set up of a hydraulic cylinder entails a hydraulic hose that connects the cylinder to a hydraulic pump. The jack cannot be operated without the basic set up of these 3 components:

• The Pump
• The Hose
• The Cylinder

The pump takes hydraulic fluid (oil) and pushes it into the connected hydraulic hose. The oil flows through the hose into the connected hydraulic cylinder. Inside the cylinder, the oil then pushes and raises the piston out of the cylinder.

This basic set up is called “Single Acting” which has a “single” hydraulic hose that pumps the oil into the bottom of the cylinder.

In a “Double Acting” system, the cylinder and the pump have two connections for two hydraulic hoses. One hose runs from the pump into the bottom of the cylinder and one hose runs from the pump into the top of the cylinder (Both hoses connect to the side of the cylinder – top and bottom).

In this set up, oil is pumped into the bottom of the cylinder to extend the piston just like with a “Single Acting” system. However, the user can reverse the flow of oil by now pumping the oil into the second hose which pumps oil into the top of the cylinder which then pushes the piston back down. The hose at the bottom of the cylinder now transports the oil that is inside the cylinder underneath the piston back to the pump.

A “Single Acting” cylinder and a “Single Acting” pump are outfitted with one connection nipple each for one hose.

A “Double Acting” cylinder and a “Double Acting” pump are outfitted with one connection nipple for each of the two hoses. The double acting pump also has a lever to change the oil flow from one hose to the second hose which pushes the piston up or down respectively.

With lower capacity cylinders you can push the extended piston back down and force the oil to flow back to the pump by simply pushing down on the piston with your foot. A single acting system is consequently an adequate set-up.

However, with larger capacity cylinders the piston will retract extremely slowly or will not retract at all with just the pressure of your foot. Consequently the user must rely on either the weight of the load to press the piston back down or the user needs to have a double acting system that can force the piston to retract.

A double acting system is more expensive than a single acting system. Also there are more components to connect and if you are operating multiple cylinder jacks at the same time too many hoses become user unfriendly. Most users stay with a single acting set-up if at all possible.