The usual way of judging seal materials for hydraulic applications is to read the material specifications and try to compare the specifications for various materials. This unfortunately leads to several pitfalls. Most specifications and test results can seldom be compared with one another if they have not been carried out in the same way and under the same conditions.
Many test results reproduce values based on test samples produced with standardized forms and sizes. Some properties can then show better values than could be expected from tests with seals from serial production. In particular, this is the case with seals with small cross sections, when e.g. the compression set shows high values. To avoid drawing the wrong conclusions, it is essential that comparisons are based on correct assumptions.
Because of costs and time pressure, test results often show values which have been recorded following short test periods, e.g. 24 or 72 hours. These provide only a very limited amount of information about the properties of the material. Experience has shown that only results from long test periods, normally 1 000 hours, provide reliable bases for comparisons.
The properties which initially should be studied are those most important for the sealing function. We consider the following properties to be the most important:
- compression set,
- elasticity at low temperature, e.g. TR 10-value (retraction test),
- change of hardness in oil, and
- change of volume in oil.
It is naturally not just the material's properties that determine the function of the seal. The design of the seal is just as important, i.e. how the properties of the material can be optimally used for the task which the seal in question has to carry out.