What’s the difference between single acting and double acting piston seals?

Piston seals or piston rings are used in hydraulic cylinders for fluid sealing. They are designed to ensure that pressurized fluid does not leak across the cylinder head as the system pressure pushes the piston down the cylinder bore.

There are two main types of hydraulic cylinder piston seals—single-acting and double-acting. Single-acting piston seals are unidirectional and can seal dynamic pressure from just one side. A car jack is an example of a simple single-acting cylinder. The jack handle is pumped, thus putting building pressure on the single-direction seal, which allows the jack to lift the car. When the job is complete, the jack is let down by release of the pressure. The weight of the car (e.g. gravity) returns the rod back into the cylinder, and the car returns to the ground. Because the sealing process occurs in only one direction in this application, a single-acting piston seal is all that is required.

A double-acting cylinder is bidirectional and can seal dynamic pressure from both sides. An excavator is one example. When digging a trench, the excavator’s arm is first pushed in one direction by extending the rod. Then it is pulled down in another direction by retracting the rod. Both directions are under pressure.

Choosing a piston seal is determined by the method in which the cylinder operates, and many other factors. For reference, piston seals are manufactured in materials such as high-performance polyurethane, rubber/fabric composite, PTFE or polyester elastomer. There must also be consideration for the amount of time and work that is being done by the cylinder and in what direction and at what pressure.

For example, a situation in which a cylinder has high-bias (i.e. one side of seal operates at a much higher pressure that the other) requires a much different approach to design compared to an application where pressures are somewhat equal from side-to-side. One must also consider the kind of equipment or machinery running and the operating environment. For example, an elevator’s operation requires a much smoother low-friction solution versus what one may use inside an excavator. And finally, fluid medium considerations must be understood to select the most appropriate materials for the design of the seal as it is critical to overall seal performance in the application.

Contributed by Ryan Webster, Director of Engineering, Hallite Seals International