Linear Coefficient of Friction

Linear Coefficient of Friction

In this installment of Diamond Claw® Notes, we discuss what we call the Linear Coefficient of Friction (L-COF) and the reasoning behind our use of the term to describe test results on Diamond Claw® Friction Shims. 

The L-COF is simply the highest load at which the test behaves in a linear fashion with no surface-to-surface slip.  This is shown graphically in Figure-1.

Figure 1: Typical COF Test Result

Some sources will use an offset value, similar to the 0.2% offset strain used for yield strength.  The main reason we don’t do this is that a friction shim that has started to slip has already caused damage to the surface.  As shown in Figure 2, if the test surfaces slip, the diamonds will plow through the surface of the mating material leaving furrows behind them.   

Figure 2: ~600x Image Showing Post Test Slip Damage

The concern is that any load reversal might push the diamonds back along that furrow at a reduced load setting the stage for increased damage on subsequent peak tensile loads.  

Conclusion: Given that most applications have load reversals of some amount, it simply makes sense to be conservative and report the L-COF.