Surface distress and the role of lubrication
09 June 2013
Most engineers are well aware that lubrication plays a significant role in maintaining the performance and lifespan of bearings.
However, recent research has revealed more details of the precise activity that takes place on the bearing surface, illustrating just why regular and appropriately specified lubrication is so important. Phil Burge explains.
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.
Maintenance personnel 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 can cause damage, such as electrical motors.
Also, while maintenance personnel 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.
The effect of lubrication is one aspect of bearing operation that has been examined in recent research into 'surface distress. This phenomenon is typically associated with poor lubrication conditions, where high local friction and stresses are generated.
The tell-tale signs of surface distress are generally recognised as an increasingly dull surface appearance and, under the microscope appear as tiny micro spalls, micro cracks or micro pits.
SKF is one company that has developed detailed surface distress 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 than ever before.
Guillermo Morales-Espejel, a senior scientist at SKF's Engineering and Research Centre in 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,” he 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.”
More recent research has been carried out by Dr Morales-Espejel and his colleagues at Nieuwegein into damage mechanisms of indentations in raceways of rolling bearings. This has explored in detail the interactions between scratches and indentations, the lubricant present and levels of sliding to give an insight into the way the damage process takes place. A notable feature of the research was the comparison of results between low and high sliding conditions.
At low values of sliding present in the Hertzian contact of ball and roller bearings, the maximum over-rolling pressure is always in the trailing region of the dent. Wear dominates the leading edge of the dent. Within this slip range, the over-rolling dent pressure does not significantly increase with the increase of the amount of slip. The surface distress risk of the dent is therefore mainly controlled by the number of over-rolling cycles.
At the centre of rolling bearing raceway contacts, where low sliding conditions are typical, the research revealed that when there is a good lubricating film, indentations are likely to fail first on the trailing edge of the dent, since the pressures are higher and not from metal-to-metal contact in the leading edge of the dent.
However, if the lubrication is poor, wear, and possibly surface distress, can be expected at the leading edge of the dent. This contrasts with results from rolling-sliding contacts with high sliding conditions such as gears, where the outcome is different.
According to Dr Morales-Espejel and colleague, Dr Gabelli, the failure of the contact will depend on how the pressure waves produced by the steady state elastic deformation of the surface disturbance and the disturbance at the inlet of the contact overlap and combine in the many cycles of fatigue.
The failure could appear quite distant from the indentation because the two wave components separate from each other over time, as both pressure and film thickness ripples, they add. The film thickness ripple generated at the inlet of the contact will already have left the contact while the pressure ripple is still inside.
For those who want to gain a deeper insight into the causes of surface distress, this research explains why a failure in high sliding conditions can be the result of an indentation that is distant. However, the research also underlines the importance of cleanliness, careful and efficient bearing mounting, and appropriate lubrication in reducing the risk of surface damage.
Firstly, avoid mounting damage by ensuring that mounting recommendations are carefully followed, as incorrectly mounted bearings are a common cause of indentations on raceways. Lubricant should be kept as clean as possible, and that means clean grease, clean oil, undamaged seals and good particle filtration for oil.
And, as the research has made clear, a good lubricating film with the right viscosity must be specified for the operating conditions of the bearing.
To return to where we began, it is fair to say that engineers are aware that good lubrication is 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