Lube-Tech Introduction
It is well known that real engineering surfaces, at the micron scale, are rough (Figure 1). The roughness of surfaces depends on the type of surface finish used. Typical engineering steels have a root mean square surface roughness of between 0.5 to 1 µm, which is similar to the minimum oil film thickness that separates lubricated components [1]. The fact that surfaces are rough has some major implications for lubrication. Firstly, for a given load, engineers will usually use the simple geometrical area of a sample to work out the average pressure. So, for a sample of size 10 mm x 10 mm, an applied load of 100 N would give an average pressure of 106
N/m2 (or about 10
atmospheres). However, for rough surfaces, the real contact area is much smaller than the geometrical area of the contact and so the actual pressure on each individual asperity is much higher – if the real contact area is just 1% of the geometrical area, the actual pressures will be 100 times greater. For brand new components, when they are first loaded, these high pressures will result in substantial plastic deformation of the highest asperities, and the process of “running-in” [2-4], over a few tens or hundreds of hours usually results in a smoother surface, and the change in surface shape of the component that can occur can also result in thicker oil films. It should be said that there are two possibilities for highly loaded rough surfaces, either they “run-in” and subsequently have a long life (many thousands of hours), or they fail quickly during the “running-in” process.
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.152 page 1
The Economic and environmental impact of mixed and boundary lubrication
R.I. Taylor, Visiting Professor, Jost Institute for Tribotechnology, University of Central Lancashire
Once “running-in” has finished, there can still be metal-to-metal contact, during stop-starts for example, or at times when there is low speed and/ or high load operation. However, any metal-to-metal contact that occurs after “running-in” generally involves elastic deformation of the asperities, rather than plastic deformation.
Figure 1: Schematic showing rough surface lubrication – whether or not the rough asperities touch each other depends on the value of the minimum oil film thickness, hmin
Much useful information can be captured using the schematic curve of Figure 2, which shows how the friction coefficient typically varies with the λ ratio, defined as [5]:
(dotted lines show the centre line average of each surface).
Where hmin (m) is the minimum oil film thickness separating the moving, lubricated surfaces, with σ1
and σ2 roughnesses of each surface. LUBE MAGAZINE NO.181 JUNE 2024 31
(m) being the root mean square surface
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