The author is to be commended for his attempt to cope analytically with the complex problem of friction. A few comments regarding the present paper are in place. The present simplified friction model is essentially based on the 1949 Mindlin approach (see Ref. 1). According to this concept, it is assumed that the contact area of an elastic spherical asperity is given by the frictionless Hertz solution (see Eq. (2)) and that slip of such an asperity will ensue when the average shear stress at its contact area reaches a critical value (see Eq. (6)). These simplifying assumptions may lead to some unrealistic situations where the local equivalent von Mises stress in the contact interface can exceed the yield strength of the sphere material. Additional shortcomings of the Mindlin concept is that an increase of the contact area due to...

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