Wear abrasion on O-rings is characterized by flattening of the inner side (illustration), the rest of the circular cross section remaining unaffected. This type of wear abrasion occurs only on O-rings whose seal face is forced to perform a correction or adjustment function. If a seal face's misalignment and/or stationary seat's axial run-out caused by shaft displacement exceeds the elastic formability of the O-ring, the O-ring makes an axial movement. With each revolution the O-ring pushes a distance of a few hundredths/tenths of a millimetre across the metal surface, resulting in wear abrasion of the O-ring as well as shaft wear. Fine-grained, hard solids in the pumped medium increase abrasion and wear. The problem is rectified by eliminating the run-out error and/or using a bellows mechanical seal.