Requirements for the system – numerous parameters
The choice of the sealing element for the piston rod surface is a crucial factor that determines the quality of a hydraulic cylinder and other products that work with fluids. Various, opposing parameters clash in this area. "Tightness and minimisation of friction are actually contradictory parameters," says Matthias Breisch, Head of Testing Department at Hänchen. "That's why, in the past, it used to be either 'tight or free-moving' in hydraulics." Tightness is nowadays a matter of course in quality products. Other important parameters include friction, wear, precision in positioning and repeatability, maximum and minimum speed, stick-slip behaviour, minimum and maximum pressure, suitability for particular fluids, such as Skydrol® or water-based fluids, and fluid contamination limit values. In addition to these, there are also different sealing systems: Classic seals with seal and lip seals, and non-contacting seals with hydrostatically supported piston rod guide or with the floating gap seal, which is, by the way, a Hänchen patent. In the case of classic seals, the various sealing materials play just as much a role as the geometry - in particular that of the sealing edge. However, even the nature of the counterface, such as that of the piston rod, and the friction and lubrication properties of different fluids are decisive factors. These include chromium-plated steel or light metals, but also modern materials with a special synthetic coating such as H-CFRP® – Hänchen's proprietary carbon composite material. The surface processing methods of these materials (e.g. honing or grinding) also have an influence on the sealing effect.