An experimental study into the mechanisms of false brinelling contact damage

Abstract

False brinelling is a type of surface damage that occurs in rolling bearings when the contacts of bearing rings and rolling elements are subjected to small amplitude oscillatory motion. It manifests itself as deep craters with relatively polished surfaces that are evenly spaced around the rings. False brinelling can be considered as a specific example of a more general damage mode of fretting that occurs in many engineering components other than rolling bearings. False brinelling can be found in applications where nominally stationary bearings are subjected to external sources of vibration such as in vehicles transported by sea or rail, in spare machinery located adjacent to a working machine and in the pitch bearings of wind turbines. False brinelling is increasingly becoming of major practical importance because it is an important failure mode in hybrid vehicle components, where one power train is often stationary while the other is operating and hence acting as a vibration source. There is currently limited understanding of the influence of relevant contact conditions on false brinelling hindering our ability to predict and prevent the onset of this type of damage. This work investigates the influence of lubricant properties and contact conditions, including the amplitude and frequency of oscillations and applied load, on the onset and progression of false brinelling damage. Experiments were conducted with AISI 52100 steel specimens in a ball-on-flat configuration under small amplitude reciprocating conditions. Crucially, this set-up was adapted to allow optical access to the contact and direct observation of the damage onset and progression as well as development of fluorescence techniques. Additional experiments were conducted with the same lubricants on a full bearing rig which employs a thrust ball bearing subjected to small amplitude oscillatory motion and the observed trends compared to those from the ball-on-disc set up. It is shown that the false brinelling damage initiates locally at discrete micro-contacts and then spreads relatively quickly with continued rubbing to cover the whole contact area. Stroke lengths shorter than the Hertz contact width were shown to be most damaging but the damage amount diminished rapidly for stroke lengths longer than this. Lower oscillation frequencies and lower loads generally produced less damage. Oils with low viscosity significantly reduced the amount of damage and oils in general were more effective at reducing damage than greases using the same base oil. Surface roughness can trap lubricant to supply the contact. A consistent trend emerged across all tests that strongly indicates that contact conditions and lubricant properties that promote the ability of lubricant to effectively resupply the contact during its oscillatory motion are able to mitigate against false brinelling. Reducing adhesion by means of a coating or a reacted tribolayer can also reduce the damage. The insights presented in this thesis can help in devising mitigating measures against false brinelling, establishing new design guidelines and informing the development of models to predict its onset.