False brinelling
From Wikipedia, the free encyclopedia
False brinelling is a type of bearing damage caused by fretting, with or without corrosion,[1] that causes imprints that look similar to brinelling, but are caused by a different mechanism. False brinelling may occur in bearings which act under small oscillations[2] or vibrations.[3]
The basic cause of false brinelling is that the design of the bearing does not have a method for redistribution of lubricant without large rotational movement of all bearing surfaces in the raceway.[4] Lubricant is pushed out of a loaded region during small oscillatory movements and vibration where the bearings surfaces repeatedly do not move very far.[5] Without lubricant, wear is increased when the small oscillatory movements occur again. It is possible for the resulting wear debris to oxidize and form an abrasive compound which further accelerates wear.
In normal operation, a rolling-element bearing has the rollers and races separated by a thin layer of lubricant such as grease or oil.[6][7][8] Although these lubricants normally appear liquid (not solid), under high pressure they act as solids and keep the bearing and race from touching.[9][10]
If the lubricant is removed, the bearings and races can touch directly. While bearings and races appear smooth to the eye, they are microscopically rough. Thus, high points of each surface can touch, but "valleys" do not. The bearing load is thus spread over much less area, increasing the contact stress[11] and causing pieces of each surface to break off or to become pressure-welded and then break off when the bearing rolls on.
The broken-off pieces are also called wear debris. Wear debris is bad because it is relatively large compared to the surrounding surface finish and thus creates more regions of high contact stress. Worse, the steel in ordinary bearings can oxidize (rust),[12] producing a more abrasive compound which accelerates wear.
Simulation of false brinelling
The simulation of false brinelling is possible with the help of the finite element method. For the simulation, the relative displacements (slip) between rolling element and raceway as well as the pressure in the rolling contact are determined. For comparison between simulation and experiments, the friction work density is used, which is the product of friction coefficient, slip, and local pressure. The simulation results can be used to determine critical application parameters or to explain the damage mechanisms.[13]
Physical simulation of the false brinelling mechanism has been standardized since the 1980s in the Fafnir bearing test instrument, where two sets of thrust ball bearings are compressed with a fixed load, and the bearings are oscillated by an eccentric arm under standardized conditions. This culminated in the ASTM D4170 standard.[14][15] Although an old method, this is still the leading quality control method for greases that need to avoid the false brinelling damage.
Test bearings for ASTM D4170 False Brinelling fretting wear test
False Brinelling Fretting wear tester
