If contact seals must seal against moving surfaces, friction is produced that causes wear. The friction force necessary to overcome this friction reduces at the same time the movement force (e.g. piston force, solenoid force on valves).
The friction is essentially influenced by the seal shape, material, pressure (Figure D 17), surface and lubrication conditions in the sealing gap. There are also numerous other effects that are mostly difficult to quantify.
In addition a differentiation must be made between stiction at the start of a movement and dynamic friction that, given adequate lubrication, is in the mixed friction area at low velocity and can then change to hydrodynamic friction (Figure D 18).
Friction force measurements are difficult to reproduce and even more difficult to transfer to different conditions such that quantitative information on the magnitude of the friction is rarely available.
Figure D 17: Friction of various seals at four pressures
Figure D 18: Change in friction as a function of the velocity