Friction of seals to be overcome at the start of a movement (Figure H 1). It is dependent on both the form of the seal (preload) and the seal material and on the service life of the seal: during longer periods of inactivity, the oil film under the seal is squeezed away allowing dry friction to predominate (Figure H 2).
Stiction of seals is particularly evident during stick-slip movements and during oscillating movements where the stroke remains within the height of the seal. No new lubricating film can develop during this kind of movement, which means the whole process is affected by dry friction.
Figure H 1: Stribeck curve
Figure H 2: Stiction depending on the period of inactivity a) polyurethane, b) NBR, c) fabric