How do single-acting cylinders work?

Single-acting cylinders are a fundamental component of hydraulic and pneumatic systems used in a wide range of industrial applications. Their simple yet effective design makes them widely used across many industries—from agriculture and construction to advanced production lines.

How single-acting cylinders work

Single-acting cylinders are characterized by a unique operating principle that distinguishes them from other types of cylinders used in power hydraulics and pneumatics. The fundamental principle of their operation is based on the use of a working medium (hydraulic fluid or compressed air) to perform a working stroke in only one direction.

In single-acting cylinders, the working fluid (hydraulic oil or compressed air) is supplied to only one chamber of the cylinder, causing the piston to move in a specific direction. The key feature distinguishing these cylinders from double-acting designs is the method used to return the piston. In single-acting cylinders, the piston’s return to its initial position is not achieved by supplying the working medium from the other side, but rather through other mechanisms.

Piston return mechanisms

There are three main mechanisms responsible for the return of the piston in single-acting cylinders:

1. Return spring – the most commonly used solution, in which the piston compresses the spring during its working stroke, and the potential energy stored in the spring is used to return the piston to its original position once the working pressure ceases.
2. Gravitational force—in certain applications, especially when the actuator is mounted vertically, gravity acts as the return mechanism. When the pressure is released, the weight of the lifted component causes the piston to return to its initial position.
3. External forces – in specialized applications, the piston’s return stroke may be driven by other external forces, such as a mechanical load or pressure from another device.

Duty cycle of a single-acting cylinder

The typical operating cycle of a single-acting cylinder can be divided into two main phases:

Phase 1: Operating
Cycle The working fluid (hydraulic oil or compressed air) is forced under pressure into the cylinder’s working chamber. The pressure of the medium acts on the piston surface, generating a force that causes it to move. In the case of a cylinder with a pushing function, the piston extends, and in the case of a cylinder with a pulling function, it retracts. During this movement, if the cylinder is equipped with a return spring, it is compressed.

Phase 2: Return
Stroke Once the working stroke is complete and the pressure is released (by draining the working fluid from the working chamber), the piston begins its return stroke. Depending on the actuator design, this movement is driven by a spring unwinding, the force of gravity, or other external forces. The working medium is forced out of the actuator chamber and returned to the reservoir (in hydraulic systems) or to the atmosphere (in pneumatic systems).

Construction of a single-acting cylinder

Although the design of a single-acting cylinder is simple, it consists of a number of precisely engineered components that work together to ensure the device operates effectively. A detailed analysis of each component is provided below.

Main structural components

1. Cylinder (body) – the outer casing of the actuator, typically made of high-quality steel. The cylinder must be sufficiently strong to withstand internal pressure and external loads.
2. Piston – a component that moves inside the cylinder and converts the pressure energy of the working fluid into linear motion. The piston is equipped with seals to ensure the system remains leak-free.
3. Piston rod – a rod connected to the piston that transmits the force generated by the piston to the outside of the cylinder. The piston rod is typically made of hardened steel to ensure adequate strength and resistance to bending.
4. Return spring – in actuators equipped with a spring mechanism, the spring is located inside the cylinder and is responsible for returning the piston to its initial position once the pressure is released.
5. Seals—key components that ensure the system’s leak-tightness. These include piston seals, rod seals, and seals at the joints between individual cylinder components. Modern cylinders utilize advanced sealing materials, such as polymers with high strength and abrasion resistance.
6. Connection port – an opening in the cylinder through which the working fluid is supplied. Single-acting cylinders have only one port, unlike double-acting cylinders, which have two ports.
7. A gland is a sealing element located where the piston rod exits the cylinder. It prevents the working fluid from leaking and protects the interior of the cylinder from external contaminants.
8. Foot – a mounting component located at the end of the cylinder, used to attach the actuator to the support structure.