Single-acting cylinder

A single-acting cylinder is the hydraulic linear actuator with the lowest level of complexity. As it requires an external return force, its use is only possible to a limited extent.

Design and function of a single-acting cylinder

A single-acting cylinder consists of an internal piston, an external cylinder barrel, seals, and only one valve. This is installed on the side opposite the extending piston. The supply and return flow of the hydraulic fluid is controlled by a directional control valve.

During operation of the single-acting cylinder, the piston only extends. It maintains its position until the directional control valve is shifted and an external force returns the piston. To a certain extent, the return force can be reduced by a vacuum in the hydraulic line. However, hydraulic systems fundamentally operate under pressure or overpressure, so if self-return is required, the double-acting cylinder is the standard solution.

Applications of the single-acting cylinder

Single-acting cylinders are a cost-effective solution for applications in which a high load is moved vertically by the linear actuator. They are often used in very small, manual applications. In addition, there are special applications in which the single-acting cylinder performs additional tasks. Typical examples of this linear actuator include:

• Hydraulically vertically opening flaps
• Lifting platforms
• Car jacks
• Transmission jacks
• Engine hoists
• Plunger cylinders in hybrid systems

When operating a single-acting cylinder, it is important that the load is not removed before it returns. Otherwise, retracting the linear actuator requires manual force or the labor-intensive application of an external load to the piston. With permanently installed hydraulic cylinders on flaps or lifting platforms, this is ensured. In manual applications such as car jacks, transmission jacks, or engine hoists, this is often forgotten and results in additional work.

Special case: hybrid cylinder

In hydropneumatic systems, plunger cylinders can make a significant contribution to reducing energy consumption. The piston of a plunger cylinder consists of a solid rod with a constant cross-section. During retraction, part of the hydraulic force is transferred into a gas-charged piston accumulator. The pressure of the preloaded gas is then available again as energy for extending the cylinder.

A typical example of the use of a single-acting cylinder

A typical example of the use of these single-acting cylinders can be found in demolition excavators. The operating principle of these machines is as follows:

1. Lifting the unloaded arm.
2. Picking up a load at a high lift level (demolishing a section of wall).
3. Lowering the arm: The weight of the picked-up load retracts
   the plunger cylinder.
4. Redirecting the hydraulic pressure from the plunger cylinder into the gas piston accumulator
5. Setting down the load (unloading the arm).
6. Raising the unloaded arm again with support from the gas pressure in the piston accumulator.

This process can reduce the fuel costs of a demolition excavator by 15–25%.