Abstract: A U ring in an inwardly radially-open groove in a relatively stationary machine part for sealing a relatively axially-displaceable machine part extending therethrough has a resilient U ring body, a radially-outside statically-acting sealing lip for bearing on the bottom of the groove, and a radially-inside dynamically-acting sealing lip for bearing on the axially-displaceable machine part. The axial spacing of the statically-acting sealing lip from the adjacent end face of the groove is greater than that of the dynamically-acting sealing lip, whereby the dynamically-acting sealing lip tilts away from the axially-movable machine part when the latter forces the end of the U ring against the end face of the groove to relieve pressure along the axially-moving machine part.