Cylinder tube

Seamless cylinder tubes generally connect the hydraulic components within a hydraulic system. This type of line design is characterized by low ovality, high straightness, and tight dimensional tolerances.

In addition to these properties, cylinder tubes typically exhibit low roughness values as well as favorable sliding properties on the inner surface and are resistant to corrosion. Both welded and seamless tubes can be used for this purpose. Seamless cylinder tubes can be either cold-finished or hot-finished. Cylinder tubes are used not only in hydraulics but also in pneumatics and as working cylinders.

Comparison of cold-finished and hot-finished cylinder tubes

Although the transitions between cold-finished and hot-finished cylinder tubes are fluid, standards, application regulations, use, and processing are bindingly regulated. The German Steel Construction Association (Deutscher Stahlbau-Verband) has defined production-related differences between the two product groups. Due to the different temperature ranges during the production process, cold-formed cylinder tubes have an inhomogeneous hardness distribution as well as a higher residual stress distribution than comparable hot-formed cylinder tubes. Cold-finished cylinder tubes, for example, suffer from severe limitations in weldability. For certain welding processes, it is necessary to normalize the material. Such restrictions do not exist for hot-finished cylinder tubes.

Furthermore, cold-finished and hot-finished cylinder tubes differ in their static properties. Hot-finished cylinder tubes possess higher static values and are thus able to absorb significantly higher loads with the same dimensions and wall thicknesses than comparable cold-finished cylinder tubes. This means that hot-finished cylinder tubes must be used if the structural analysis is based on the values of hot-finished tubes.

The cylinder tube as the main component of a hydraulic cylinder

The rigid element of a hydraulic cylinder, which guides the piston rod or the piston tube, is referred to as the cylinder tube. The hydraulic lines and seals are attached to this tube in order to move the piston rod with the help of the hydraulic medium used. It should also be noted that the cylinder tube is simultaneously the outer skin of the cylinder.

The surface and material properties mentioned in the previous sections are influenced by specific processes during the production of the tubes. Examples include honing or roller burnishing processes, in which the inner walls are optimized with regard to the flow properties of the medium. Roller burnishing has clear advantages in terms of surface quality, costs, and process time. Process reliability in roller burnishing, also known as smooth rolling or deep rolling, is significantly higher than in honing, which is particularly suitable for large components in smaller quantities.

Typical areas of application for conventional hydraulic cylinders include excavators, cranes, all types of agricultural machinery, tail lifts, and guides for automatic machines. To provide an idea regarding the dimensioning of cylinder tubes, a data sheet for an example tube is provided below.

Data sheet for a suitable seamless cylinder tube with a precision-finished functional surface:

• Outer diameter: 25–280 mm for cold-finished tubes, up to 660 mm for hot-finished tubes
• Inner diameter: 20–250 mm for cold-finished tubes
• Material used: E355 / St 52 (Unalloyed base steel, standard steel for use in mechanical engineering or for general technical purposes)
• Minimum tensile strength: 570 N/mm²
• Minimum yield strength: 470 N/mm²
• Minimum elongation at break: 15%
• Optional delivery condition “bright drawn and stress-relieved” or “as rolled”
• Inner surface: roller burnished or honed
• Maximum permissible roughness: 0.3 µm for cold-finished cylinder tubes, 0.5 µm for hot-finished cylinder tubes
• The length of such cylinder tubes can be chosen as desired, as they are cut directly from the semi-finished material.