Plunger cylinders: Maximum efficiency for linear actuators

At their core, hydraulic cylinders consist of a pressure cylinder and a piston. The piston rod generally has a significantly smaller diameter than the force-bearing piston. In a plunger cylinder, the piston rod is almost as large as the cylinder’s internal diameter and does not require an additional welded piston. Due to their design, plunger cylinders are exclusively single-acting. Compared with double-acting cylinders, plunger cylinders do offer advantages:

• Simple design
• More cost-effective to manufacture
• More efficient

The working piston of the plunger cylinder consists of a thick piston rod that is interrupted only by two seals. Plunger cylinders are therefore quite robust, but can be manufactured more cost-effectively than double-acting cylinders. However, these cylinders also have disadvantages:

• Limited functionality
• More complex retraction

A plunger cylinder is single-acting. To retract it, it must be acted upon by an additional external force, or alternatively the hydraulics can be switched from pressure to suction to return the plunger cylinder to its initial position. Along its force stroke, however, the plunger cylinder offers higher efficiency than other cylinders. This is reflected in greater force output or a smaller effective area. Due to its design and operating principle, the plunger cylinder is also referred to as a “ram cylinder”.

Applications of plunger cylinders

Plunger cylinders are particularly useful where a counterforce is available virtually free of charge. A preferred installation orientation for these single-acting hydraulic cylinders is vertically upward. Since the plunger cylinder itself is extremely robust, at least the entire weight force of its piston is available as a counterforce.

Hydraulic vehicle lifts are usually equipped with plunger cylinders. The vehicle is raised by hydraulic force, but lowers again under its own weight. The example of the vehicle lift already makes it clear that the plunger cylinder is used only rarely for cyclical, productivity-focused applications. It is simply too slow for most applications.

Plunger cylinders in hybrid systems

However, there is one exception in which the plunger cylinder can be used with maximum efficiency: in hydraulic-pneumatic hybrid systems. If the plunger cylinder acts on a preloaded pneumatic pressure system, the counterpressure of the compressed gas can also be used to retract the pressure cylinder. This design is now more common in hydraulically driven work machines. Large material-handling machines from an Upper Bavarian manufacturer of industrial vehicles achieve fuel savings of up to 30% during operation with this functional principle.

However, hybrid systems in which hydraulic and pneumatic components act on the same force system are very demanding in terms of design and maintenance. They are particularly hazardous and may only be handled by trained specialists. The greatest danger when working on hybrid systems is that even when the hydraulic pressure has been relieved, the gases in the piston accumulators may still be under high pressure.

Emergency solution for plunger-cylinder retraction systems

Another way to retract a plunger cylinder is to connect a second ram cylinder in parallel. When one cylinder extends, the other retracts. This simple but very space-intensive solution is therefore used only rarely. If no technically viable solution can be found, springs or electromagnets are also used to retract plunger cylinders. However, both solutions are prone to faults, and electromagnets also require significant energy.

Overall, using a plunger cylinder involves extensive planning and calculation.