What are single-acting hydraulic cylinders?
In single-acting hydraulic cylinders, only one piston side is pressurized with pressure fluid. For this reason, the force and movement is generated hydraulically only in one direction.
The piston is returned to its initial position either by its own weight, an external load (external load) or by built-in springs. This type of cylinder is usually designed as a plunger or plunger cylinder. In them, the effective piston area is formed by the cross-section of the piston rod, which is sealed in the rod guide.
This eliminates the need for sealing against the cylinder barrel. Thus, no requirements are placed on its surface and diameter accuracy. Single-acting cylinders thus have a high degree of efficiency because there is only one sealing point. It also has few wear parts and is inexpensive.
Single-acting hydraulic cylinders – designs
Plunger or plunger cylinder
This type of cylinder transmits only compressive forces, as mentioned above. Plunger cylinders can be designed with or without internal stops and guide pistons, depending on the application. The magnitude of the compressive force is calculated for all designs by multiplying the piston rod area by the operating pressure.
Plunger cylinders are used wherever a clear direction of force enables safe return to the initial position. Examples of applications here are lower piston presses, scissor lift tables, lifting devices, etc.
The piston extends when the effective piston area is pressurized with the operating pressure via port “A”. The cylinder is retracted by the piston’s own weight or by an external force.
Hydraulic cylinder single-acting with spring return
Cylinders with return springs are used when there is no external force to return the piston rod. These springs can be either inside the cylinder or attached to the cylinder as separate components.
Since they can apply limited travel and forces, they are mainly used in smaller cylinders. They are often used in fixture technology as clamping cylinders or as assembly tools for repairs.
To extend the piston rod, pressure is applied to the effective piston surface via port “A”. The return spring ensures the return movement of the piston rod. To retract the piston rod, pressure is applied to the effective ring surface via port “B”. The return spring provides the extension movement.
Single-acting hydraulic cylinders with rapid traverse
Rapid traverse cylinders are widely used in press construction in particular. They work differently than conventional cylinders. When the full force is not required, only part of the effective piston area, called the rapid traverse piston, is pressurized. Only later, via pressure valves or limit switches, is the entire effective piston area switched on by the control system using a hydraulic pump.
This type of cylinder has the following advantages:
- High rapid traverse rate due to low volume
- High pressing force due to large effective piston area
Drive rapid traverse = connection A1
Full pressing force = connection A2
Single acting telescopic cylinders
When the pistons are pressurized via port “A”, they move out one after the other. The pressure depends on the load and the effective area. Consequently, the piston with the largest effective area moves out first.
At constant pressure and flow rate, the extension movement starts with the highest force and low speed and ends with the lowest force and higher speed.
The required lifting capacity must be designed for the smallest effective piston area. For single-acting telescopic cylinders, the sequence of the retraction movement is reversed by external load. This means that the piston with the smallest area moves to the end position first.