What is a hydraulic cylinder?

Hydraulic cylinders are straight-line devices for converting hydraulic power into work power (mechanical energy). Today, a cylinder is an indispensable device in the hydraulic circuit. It is the link between the hydraulic circuit and the working machine.

In contrast to rotary drives (hydraulic motors), the hydraulic cylinder performs translatory (straight-line) movements. In doing so, it transfers the forces to the working machine.

Cylinders have a wide range of applications and can be found in a wide variety of machines and vehicles.
The maximum transmittable cylinder force “F” depends on the maximum permissible operating pressure “p” and the effective area “A”. Under ideal conditions, i.e. neglecting frictional influences, the following applies:

F = p ⋅ A in kN

Where F is the force in newtons, p is the pressure in bar, and A is the cylinder area (piston or ring area). Basic information on the calculation of hydraulic cylinders can be found in our articles.

Advantages of hydraulic cylinders

When using hydraulic cylinders, the following advantages result for linear movements in working machines:

• A direct drive with cylinders is simple in design and therefore easy for the designer to arrange.
• Depending on the design, a hydraulic cylinder can apply either only compressive forces or also tensile and compressive forces
• By dimensioning hydraulic cylinders, it is possible to realize drives with large power at small installation dimensions.
• Converting hydraulic energy into linear motion gives better efficiency than converting it into rotational motion.
• With a hydraulic cylinder, both forces and speeds can be kept constant over the entire stroke.

Structure of a hydraulic cylinder and areas of application

Hydraulic cylinders essentially consist of a cylinder tube, a piston rod and a piston. The piston rod moves along the cylinder when fluid pressure is applied to the piston.

In addition to industrial applications, cylinders are also used in the automotive industry, aircraft industry, agriculture as well as mechanical engineering, construction and mining.

Types of hydraulic cylinders

Hydrocylinders can be divided into two groups according to their mode of action:

Single acting cylinders

Single-acting hydraulic cylinders have only one effective area, they can deliver their force in only one direction. The piston can be returned by a spring, the piston’s own weight or by external force.

Double acting cylinders

These have two oppositely acting surfaces, of the same or different size.Unlike single-acting cylinders, they have two line ports “A” and “B”. By applying pressure via port “A” or “B”, the cylinder piston can transmit tensile or compressive forces in both stroke directions. Double-acting hydraulic cylinders can be found in a wide range of fluid power applications and are further subdivided into differential cylinders and synchronous cylinders.

End-position cushioning in a hydraulic cylinder

Cylinder pistons can reach high speeds (> 1 m/s) or have large masses to move. This can mean that the unbraked stop of the piston in the end position leads to damage to the cylinder and the machine. Hydraulic cylinders with end-position cushioning are usually used for these applications. This damping is normally installed in both end positions, i.e. on both rod and piston sides. The illustration shows a cylinder with end-position cushioning on the piston side. The damping is adjustable via a throttle valve.

6 Lock valve

7 Cylinder base

A Connection piston side

B Rod side connection

Hydraulic cylinder sealing

As a rule, hydraulic cylinders are sealed by elastic seals. These should have the following characteristics:

• Suitable for the hydraulic fluids used
• High durability
• Designed for high travel speeds
• Low friction

The frictional forces on the piston and piston rod are so high that a pressure of several bar is required to set the cylinder in motion.

For cylinders with two piston rods (synchronous cylinders), the friction is higher than for a differential cylinder with one piston rod. Without load, a minimum pressure of 10 bar is recommended for hydraulic cylinders with one piston rod to ensure safe retraction and extension.

Hydraulic cylinder manufacturer

There are very many manufacturers of hydraulic cylinders. The table below shows only a small selection of manufacturers known to us. If you would like to be added to the list, please contact us.

All contributions to hydraulic cylinders