The service life of a bearing is typically limited by raceway damage, which, in turn, is commonly caused by incorrect lubrication, contamination, or markings that result from poor handling or storage.
Bearings often fail to reach their potential service life as a result of contamination; a dent created by an over-rolled hard particle will disturb the load distribution along the roller/raceway contact surface, causing stress to be locally increased, which accelerates fatigue. Surface distress is typically associated with poor lubrication conditions, where high local friction and stresses are generated.
Recent research has underlined the importance of appropriate lubrication to maximise bearing performance and lifespan. Effective lubrication is crucial to the efficient operation of machinery, but the need for re-lubrication at regular intervals is often overlooked. Even if lubricants are applied at specified intervals it is common for either too little or too much lubricant to be used.
Engineers generally show a tendency to over lubricate because, understandably, they are keen to prevent bearings from running dry. However, it is a less widely understood fact that over-lubrication can be equally problematic in that it can increase friction, raise temperature and promote the migration of grease into parts where it may cause damage, such as electric motors. Also, while engineers are aware that lubrication is regularly required, they do not always apply the correct type and this can have a significant effect on bearing lifespan, as shown by the new research into surface damage.
The tell-tale signs of surface distress are generally recognised as an increasingly dull surface appearance and, under the microscope, tiny micro spalls, micro cracks or micro pits. SKF has developed detailed models, based on the interaction between surface micro-crack generation and mild loss of material, in order to investigate the critical parameters and predict surface damage in far greater detail.
Indentation marks in rolling bearing raceways, combined with scratches and surface pitting, may at first seem inconsequential. Their interaction with lubricants can, however, have an important and detrimental impact on bearing life. For example, indentations can create fluctuations in local film thicknesses and pressure ripples, leading to stress concentrations, which in turn cause surface fissures to occur. These problems can be exacerbated if contaminating particles in the lubricant become trapped within the indentations, as this increases the shear stress in the immediate vicinity.
G.E. Morales-Espejel, a senior scientist at SKF’s Engineering and Research Centre at Nieuwegein, The Netherlands, has spelled out the importance of lubrication in no uncertain terms:
“Surface distress is greatly influenced by the lubrication regime and notably enhanced by boundary and/or mixed-lubrication regimes,” Morales-Espejel says. “Consequently dry or boundary friction plays a very important role. In a full-film lubrication regime, the friction force is introduced by shearing the lubricating film by means of sliding.
“The shear stress (and thus the friction force) depends on the rheology of the lubricant. However, whenever asperity tips are in contact, dry friction (or boundary lubrication friction) is regarded, approximately, as a Coulomb type of friction, which has zero value at pure rolling and a nearly constant value as soon as sliding starts.
“Higher sliding does not necessarily mean higher friction. In a rough boundary or mixed lubricated contact, the ‘dry’ spots will not have traction forces on the surface, unless there is some sliding, regardless of how little, as long as it is different from zero.”
To understand the causes of surface distress in rolling bearings and prevent damage, there are a number of issues to consider. The first is that surface roughness and lubrication quality are extremely important influencing factors as to whether surface distress will occur or not.
Also, mixed and boundary lubrication both result in areas of ‘dry’ contact, where increased surface tractions, stress concentrations and micro stress cycles apply, all of which individually can facilitate fatigue. Surprisingly, we also discover in tests that increased sliding does not typically increase the risk of surface distress, and that mild wear can actually be a positive force by removing the surface before surface distress can develop. To reduce the negative effects of surface distress, a number of steps can be taken:
- Good (full film) lubrication appropriate to the application, (for example providing higher lubricant viscosity, higher running speed, lower temperature, etc.) should be the first approach wherever possible.
- Where boundary lubrication cannot be avoided, boundary friction should be reduced as much as possible through the use of lubricant additives or other methods such as low-friction coatings, etc.
- Reducing stress raisers by eliminating contamination and preventing damage caused during mounting etc will minimise the risk of surface distress.
- Running in will help achieve equal levels of roughness in the different contacting elements.
- Even a controlled mild surface removal of material (wear) induced through the introduction of additives to the lubricant can reduce the surface distress level.
- Select bearings that have a surface topography balance proven to avoid surface distress risk.
For those who want to gain a deeper insight into the causes of surface distress, this research explains much, such as why a failure in high sliding conditions can be the result of an indentation that is distant. However, for all concerned, the research also underlines the importance of cleanliness, careful and efficient bearing mounting and appropriate lubrication in reducing the risk of surface damage.
To return to where we began, it is fair to say that engineers are aware that lubrication is regularly required, but they do not always apply the correct type. Whether this is a cost or time-saving exercise, it is, in both cases, a false economy; investments in components such as high quality precision bearings are typically made to reduce failures and keep machinery running smoothly, so it is counter-productive to then maintain these parts with the wrong type of lubricant. However, the correct type, frequency and quantity of lubricant will extend bearing life and guard against premature wear and failure.
Phil Burge is with SKF in the UK