Lube-Tech
a lubricant is measured in a Mini Traction Machine [7] as a function of the mean speed between the ball and disk. It should be noted that results of such experiments are typically plotted on a logarithmic scale, which gives the impression that boundary friction is much more important that it actually is. Figure 4 shows results plotted on both a logarithmic scale and on a linear scale.
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.152 page 3
As mentioned in the Introduction, the amount of mixed/boundary lubrication also depends critically on whether the components are brand new or have been allowed to “run-in”. Figure 5 shows the substantially greater amount of measured mixed/boundary friction of a new piston ring, compared to a “run-in” ring [8]. It has also been reported that the friction power loss of a new piston ring pack, compared to a “run-in” piston ring pack, is approximately 10-15% higher [9].
Figure 5: Measured friction force of new piston assembly compared to “run-in” piston assembly. Data replotted from reference [8].
Models for Mixed/Boundary Lubrication If W is the load applied to a lubricated contact, and it is assumed that a fraction of the load, WA, is carried by rough asperities, and the remaining fraction, WF, is carried by the fluid, then it is reasonable to assume that:
[2]
If the friction coefficients for the asperities and the fluid are fA and fF, and the overall friction coefficient is f, then the following equations result:
[3]
Figure 4: Typical friction measurements from laboratory tribological test equipment (in this case a Mini Traction Machine [7]). The graphs show the same data, although the upper graph uses a logarithmic horizontal scale, whereas the lower graph uses a linear horizontal scale.
If we write X = WA/W, then the above equation simplifies to become:
[4]
LUBE MAGAZINE NO.181 JUNE 2024
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