Wear-In and Boundary lubrication – Even the most apparently looking smooth surface of a bearing is actually quite rough, showing high and low peaks when seen under the microscope. These surfaces actually contact each other in a rotating journal bearing during the “wear-in” period, causing the asperities to either bend flat or brake off to create “wear-in” debris.
The resulting worn surfaces react with anti-wear additives e.g. ZDDP or Extreme Pressure sulphur based compounds, to form a toughened surface called the boundary lubrication layer (coloured brown below). If the lubrication film (coloured yellow above) should fail to separate the surface, boundary lubrication is the last line of defence to prevent metal to metal contact and severe wear.
The wear-in process, to create boundary lubrication, occurs very early in the life of the machine. In the example graph below, the level of wear is high in the early stages of the machine life, which rapidly decreases as the wear-in process completes, to leave relatively smooth mating surfaces – allowing full fluid film lubrication to form between the two mating components (see below). There is naturally a high correlation between wear and the likelihood of machine failure, so there is a high risk of failure at the start of any equipment life. This may prove to be manufacturing faults or poor installation practices etc.; once these problems have been resolved, the normal wear-in process can begin and these wear problems decrease. Then the machine enters the steady state or random region; a period where problems are likely to occur only as a result of random problems, such as dirt contamination, insufficient lubrication or lack of maintenance etc.
No oil Sample means no early failures detected or corrective actions in place (blue). Early failures detected and corrected by oil sampling (red)
The above wear curve graph, often called a bath tub curve, shows the pattern of wear in a newly installed machine or engine. The rate of wear begins relatively high until the shearmix layer is formed, when the wear rate becomes relatively low and steady, unless a random event creates a problem, which will most likely lead to an expensive failure. Otherwise, the graph shows the machine components (e.g. bearings) will start to wear out due to their age and normal use. Oil Analysis is one of the best techniques available to detect the random events likely to cause a failure, such as contamination, poor quality maintenance or wrong lubricant etc. Indeed regular Oil analysis programmes are frequently reported as helping extend the expected life of a machine, by simply detecting these random problems before abnormal wear occurs.
Full fluid film – This is the lubrication type that springs to mind when describing lubrication; where two contacting surfaces are separated by a full film of lubricant. Typically after any asperities (surface imperfections shown as peaks on the pictures below) have already broken off and reacted with additives during the normal wear-in process to produce the essential smooth and toughened boundary lubrication layer.
Note – within full fluid film lubrication systems there are subcategories: Hydrodynamic, Hydrostatic or Elastohydrodynamic. (See later)
Mixed film – this is in-between Full-fluid film and Boundary lubrication where there is some fluid film separating the majority of the asperities, but where there is also some boundary lubrication at certain points. These situations tend to be temporary scenarios and eventually become either boundary or full fluid film.
Hydrodynamic lubrication is where a rotating bearing produces the centrifugal forces leading to a wedge of oil collecting just ahead of the load zone, enabling the journal to ride the oil wedge around the bearing and separating the two surfaces of the bearing. When the bearing comes to rest and oil is squeezed or drains out allowing the two surfaces to touch again. Then when the bearing starts to move again the lubrication temporarily relies only on the boundary lubrication layer. However, when starting up and before getting to operational speed sliding wear can and does occur. It is at this stage most bearing wear occurs.
Hydrostatic Lubrication – Under extreme loads, boundary lubrication alone is insufficient to protect the bearing surfaces. So before the shaft starts to rotate a film of oil is pumped into the bearing to lift and separate the mating surfaces. Thereafter as the shaft rotates Hydrodynamic lubrication comes into play.
Elastohydrodynamic Lubrication – In systems such as roller bearings the point of contact is <1 micron between the two opposing curved surfaces. In this exceptionally small area – the load can be between 200,000 psi and 500,000 psi – causing the oil in this small area to become solid. The bearing element surface deforms around this oil similar to how a tyre deforms to the shape of the road. This constant deformation of the surface, which springs back into its original shape as the load moves to the next section of the bearing during normal rotation, relies of the bearing material being “elastic”. Eventually this repeated bending action will cause the material to work-harden, become brittle and fail through fatigue wear. Simultaneously, dirt contamination within this local small area will only exasperate the problem, the dirt itself promoting work hardening of the bearing metal too and both situations will eventually lead to fatigue wear and component failure.