The Four Ball Wear Test

The four ball test for wear as outline by ASTM D-4172 is used to determine the ability of a lubricant to prevent wear by operating the oil in conditions of sliding contact.  During this test three metal balls are clamped together in the fluid to be tested to form a base upon which the fourth ball will rotate under pressure.  The fourth ball has 40 kg of pressure applied to it and is rotated at a speed of 1200 rpm at a set temperature for a period of one hour.  Wear scars will form on all four metal balls, which can then be measured under a microscope to determine to determine the measurement in millimeters.  The smaller the size of the wear scar, the more effective the air compressor oil is at handling the the wear contact that is encountered in all parts of the compressor, but especially at the bearings of the compression element or rotors. 

A good way to compare synthetic oil and petroleum based products

The four ball wear test is a means to compare compressor oils, and is often seen as a good comparator, along with other standard testing to determine if non-OEM oil is suitable to replace the current manufacturers product offering.  It also is a good indicator of the advantages of synthetic oil over its petroleum counterpart.