Single-acting hydraulic cylinders

In single-acting cylinders, actively controlled force is generated in only one direction through pressure changes and fluid flow. Cylinders of this type have only one fluid connection, whereas double-acting hydraulic cylinders have two. While it can happen that these hydraulic cylinders have an active return stroke, the controllable power conversion is generally only effected by the active forward stroke.

Functional principle of single-acting hydraulic cylinders

The piston moves actively only in the extension direction. The return to the starting position via a return stroke is then effected either by a weight force acting on the outer piston head or by an internally mounted spring mechanism. The piston rod is pushed outward by the fluid connection at the rear end and the build-up of pressure on the inner piston head.

Only in the case of upright cylinders is the return to the starting position effected by weight force. A well-known example of the use of this type is car lifts. The car only needs to be actively pushed upwards; sliding back down then occurs solely through the weight force that the car and the lift itself exert on the cylinder.

For cylinders in other positions, internal spring mechanisms are preferred to guide the piston back into the housing. However, internal springs have weaknesses. The spring material is subject to constant stress and can fatigue and break over long periods.

To prevent pistons in this type of cylinder from escaping the housing, the piston rods have an internal stop so that they can only extend until the stop makes contact with the seal.

Differences in single-acting hydraulic cylinders

Single-acting cylinders are divided into two subcategories. The two categories are telescopic cylinders and plunger cylinders.

Plunger cylinders

Plunger cylinders are the basic form of a single-acting cylinder. A common feature of all types in this cylinder group is that the sealing and sliding surfaces must be extremely smooth to prevent a reduction in the converted force due to friction.

The simple design and universal application possibilities make this cylinder a generalist. It generally corresponds to the previously described design of a single-acting cylinder. Due to their simple construction, they are cost-effective to manufacture and experience very little wear, as they exhibit low friction losses during retraction. The piston rod can emerge from the housing through the build-up of fluid pressure until the internal stop reaches the housing’s exit seal. Retraction is realized by weight or spring force.

Telescopic cylinders

As a special form, the telescopic cylinder allows for the generation of enormous stroke lengths despite the small dimensions and volumes of the cylinder housing. It consists of several nested pistons, which allows very long stroke dimensions to be achieved despite the compact design.

Telescopic cylinders are installed in many systems as both single-acting and double-acting. The individual telescopic piston elements can be of different types, meaning single-acting and double-acting stages can exist simultaneously in one cylinder.

During manufacturing, the shortest partial piston is decisive for the final force assessment of the cylinder, as it must ultimately be able to keep up with the other partial pistons during power transmission as the weakest element.

Due to the sequential extension of the partial pistons, impact forces occur that cause faster signs of wear. Nevertheless, these impact forces are quite desirable in certain areas of application, such as extending a tipping bed, as they facilitate the unloading of bulk material.

In systems that should avoid these impact forces as much as possible, synchronous telescopic cylinders are installed, whose partial pistons are all the same length to enable constant force over the duration of operation.