Slide ring seals of PTFE
PTFE has properties which surpass all other known seal materials when it comes to low friction and chemical and thermal resistance. Compared with polyurethane and nitrile rubber materials used in sealing elements, PTFE is hard and non-elastic. In order to take advantage of the excellent properties of PTFE, sealing units made of PTFE must be combined with an energizing element of rubber (fig 1), or metal. The housing dimensions of type GS are in accordance with ISO 7425-2, which the chamfer on the seals’ outside diameter indicates (fig 2).
In addition to this, the slide ring seal must have a well-planned design and the energizing element must provide the necessary initial sealing force and guarantee the static sealing function in the seal housing groove.
To obtain optimum service life, PTFE materials are used with different types of fillers which improve the resistance to deformation and wear, and reduce the risk of extrusion into the clearance between the rod and the cylinder head. Our PTFE material for the widest range of applications contains bronze and molybdenum disulphide and has the designation MS-292.
An O-ring is most frequently used as the energizing element for the slide ring. This is mainly due to the fact that it is easy to obtain O-rings in the desired size and material. It is important to choose an energizing element consisting of material with the best possible properties, since the function of the slide ring is totally dependent on the properties of the rubber material. An O-ring with bad compression set values reduces the service life of the entire seal.
It is a great advantage from a functional point of view if the slide ring along its entire width can be pressed evenly against the rod surface. We therefore suggest whenever possible that the slide ring be combined with a square ring as an energizing element instead of an O-ring. It is also important that the surface of the seal housing groove is uniformly even, otherwise there is a risk of static leakage.