It is self-evident that bearings need to be lubricated. What is often not so well understood is that for optimum results the lubricant application needs to be controlled:
- The right of amount of lubricant – less is more
- The correct lubricant
- Clean lubricant
Separation of the rolling elements of a ball or roller bearing depends on hydrodynamic lubrication – the maintenance of a continuous presence of a very thin – micron sized – elastohydrodynamic film of lubricant between rolling elements and raceways for correct lubrication. Under high magnification even the smoothest surface will appear rough, but the lubricant nevertheless must be able to maintain separation if the bearing is to achieve its full service life. The coefficient of friction [μ] in a rolling element bearing in hydrodynamic lubrication conditions is in the range: μ ≈ 0.001 – 0.01.
If this is not achieved and maintained, then mixed friction results. If there is insufficient lubricant, or if the lubricant has lost its lubricating properties, an oil film with sufficient load-carrying capacity cannot form. The result is partial lubrication with metal-to-metal contact – mixed friction – between the rolling elements and raceways, leading to adhesive wear. Mixed friction: μ ≈0.01 – 0.2.
Maintenance of a separating oil film is critical for friction and wear reduction. As can be seen from the above figures, in approximate terms the hydrodynamic friction coefficient in a rolling element bearing is 10x – 20x less than the mixed friction coefficient and 25x – 100x less than the boundary friction coefficient. Maintenance of the separating oil film depends on the selection of a correct lubricant with the necessary properties for the application and correct lubricant application to ensure the necessary replenishment of lubricant at the right rate to maintain the film separating the bearing elements.
Discoloured (blue/brown) raceways and rolling elements is a sign of lubrication failure and from this wear rates will increase, which in turn causes excessive heat and will lead to catastrophic failure.
Adhesive wear occurs as a result of lubrication failure, under mixed-friction or boundary-friction conditions. Microscopic projections on the sliding interfaces between components weld together on contact and are then torn apart by the sliding forces, resulting in minute cavities on one surface and a build-up on the other. This damage has a compounding effect, as once it starts the lubrication problems increase due to the problems created by the surface damage, which then leads to an increase in the rate of surface damage. Adhesive wear includes scoring, galling, seizing and scuffing.
Skid smearing occurs when rolling elements slide, rather than rotate, as they pass from the unloaded zone to the loaded zone, especially in large bearings, a situation worsened by old or too stiff grease. Smearing can occur on the roller surfaces and in the raceways of spherical roller bearings with roller rotation being retarded in the unloaded zone, the lower rotational speed then resulting in sliding and smearing as the rolling elements are accelerated in the loaded zone.
GreaseMax lubricators provide the 3 vital elements for correct lubrication:
- The right amount of lubricant – less is more. Rolling element bearings in most applications operate at their optimum temperature when the minimum amount of lubricant required is used.
The quantity of lubricant required also depends on the other functions it must perform, such as in slow moving and/or heavily loaded bearings, corrosion protection, sealing and purging however if this is a requirement then ideally provision to lubricate the seals separately to the bearing should be made.
- The right lubricant – consideration needs to be given to bearing type, speed, load, vibration, temperature and conditions.
- Clean lubricant