Double-acting hydraulic cylinder

In contrast to the single-acting cylinder, the double-acting cylinder has two fluid ports. By applying pressure to the opposing piston surfaces on both sides, highly controlled extension and retraction movements are achieved.

Since the hydraulic pump delivers the fluid flow in only one direction, a directional control valve is essential for controlling double-acting cylinders. The valve controls start, stop and the fluid flow to the respective ports, thereby controlling extension and retraction.

In this cylinder type, the piston rod and piston are rigidly connected. Due to the different effective pressure areas inside, efficiency and movement speed differ between extension and retraction. While the full area of the piston head can be used as the pressure area at the bottom end, only an annular area around the piston rod is available on the rod side.

Movement speed and the resulting force depend on the area the fluid acts on and are inversely proportional to the direction of movement. During extension, a greater force can be applied due to the larger area, but this movement is slower. By contrast, retraction is significantly faster but with less force, as the effective area of the fluid is smaller.

Calculations carried out in advance when designing a hydraulic cylinder may differ from the actual force due to friction and possible pressure losses. However, thanks to high-quality seals, the calculated piston speed in practice also corresponds to the calculated values.

Applications for double-acting hydraulic cylinders

Because this cylinder type can be moved in a controlled manner in both directions, it is preferred for construction machinery, conveying technology and other mobile work machines and power arms. Due to its versatility, this cylinder type is by far the most widely used cylinder in hydraulics.

Differences between double-acting hydraulic cylinders

This large cylinder group can be divided into further subcategories. These include synchronous cylinders, differential cylinders, ISO standard cylinders and tandem cylinders.

Differential cylinders and tandem cylinders

Differential cylinders are a synonym for the most common standard design, which represents the basic principles of a double-acting cylinder.

As a special design, tandem cylinders are used wherever immense forces must be applied in a confined space and with short strokes. This is made possible by coupling several internal pistons. These can be controlled separately within the cylinder housing and are arranged in series. The cylinders connected in series then act together on a single piston rod. By using multiple pistons, their partial forces add up to a powerful total force. Despite a comparatively compact design, very high forces can still be achieved with this cylinder type.

Synchronous cylinder

As another special design of the double-acting cylinder, the synchronous cylinder eliminates the drawbacks of different speeds and force output during retraction and extension through its special design. This cylinder type achieves the same speed and force during extension and retraction.

This improvement of the main drawback of the basic design is made possible by fitting a piston rod on both sides. As a result, the inflowing and outflowing fluids occupy approximately the same volume. If there is not enough space in the housing for a second piston rod due to the cylinder’s application, it is also possible to achieve an approximately equal area ratio for both directions of movement by means of internal bores, throttle valves or special piston rod designs.

ISO standard cylinders

In industry, the many variants of ISO standard cylinders are used in particular. They have internationally standardized lengths and designs and must withstand specified pressures.

The advantage of ISO cylinders is that customers receive exactly the same installation dimensions for the models, despite different manufacturers. When changing the manufacturer, their equipment therefore does not need to be adapted to different designs.

Industry prefers ISO cylinders for machines that require frequent but straightforward replacement of individual components. Since not only the cylinder itself is standardized to ISO/DIN, but the parts are as well, replacing defective individual parts and carrying out repairs is not only efficient and fast, but also economical. Even special parts that give the cylinder additional properties are standardized. This makes it possible to equip a standard model with standardized additional seals that, for example, enable the cylinder to function reliably even under adverse temperature conditions.