U-ring seals of polyurethane materials
Polyurethane materials are based on either ether (polyetherurethane) or ester (polyesterurethane) and are very wear resistant, although many varieties exist. It is therefore very important to make sure to choose a seal made of a material with appropriate properties for the demands of the application in question and a material which does not vary from one manufacturing batch to another. Polyetherurethane has better low temperature properties than polyesterurethane and is more resistant to environmentally adapted hydraulic oils. Other thermal and physical properties of polyetherurethane are now also equivalent to those of polyesterurethane. Most rod seals manufactured of polyurethane, normally with a hardness value of 92 to 95° IRH, are designed as U-ring seals in order to provide optimal sealing properties.
High contact force is concentrated to the sealing edge, which can be made very sharp if manufactured of polyurethane. This results in a high specific surface pressure on a small contact surface, which provides optimal sealing ability at low-pressure conditions through effectively wiping off the lubrication film from the rod surface (fig 1).
The axial width of the seal section is somewhat increased at the assembly since its radial section dimension decreases. In order to enable assembly into closed housing grooves, the axial width of the seal housing groove therefore always must be 0,5 to 1 mm larger than the axial width of the seal section (depending on seal dimension). Furthermore, a certain free space is needed to let the pressure media work all over the seal section, also after expansion of the seal material due to increased temperature. This results in the fact that the seal can move axially just as much as allowed by the space of the housing groove at varying operating conditions.
In order to achieve a good static sealing function with rod seals of polyurethane a careful treatment of the seal housing groove surface is needed. In our installation instructions, we normally suggest generally established surface roughness values. These are, however, not good enough in most demanding applications. We then suggest a surface roughness value of 0,8 µm Ra.
Type SIL of polyurethane is our all-round rod seal (fig 2). This seal is designed with an asymmetrical cross section with a short and strong dynamic seal lip in order to provide good sealing performance also at zero-pressure conditions. The outer seal lip is slightly longer and slimmer than the inner one in order to effectively seal statically at radial and axial movements at both low and high temperatures.
Type TIL of polyurethane is designed with short and strong seal lips providing a good contact force towards the surface of the seal housing groove. Type TIL is more compact than type SIL and is especially suitable for small radial seal section dimensions, i.e. 4 to 6 mm, where this type retains a good sealing performance at low- or zero-pressure conditions.
SIL and TIL are designed with a secondary sealing edge, the main task of which is to reduce the contact surface towards the rod at both low temperatures (when the seal diameter is decreasing) and high temperatures (when the seal material is getting softer). Additionally, their ability to retain the contact force and sealing ability against the seal housing groove surface is improved.
We nowadays suggest both type SIL and type TIL to be produced of polyetherurethane.