A Traction (Friction) Curve Is Not a Flow Curve

Round 1

Reviewer 1 Report

Comments presented in the added file

Comments for author File: Comments.pdf

Author Response

Thank you.

Reviewer 2 Report

Bair discusses the assumptions that the lubricant is Newtonian in nature within the EHL contact and that the so-called Stribeck curve is representative of the rheological properties of the lubricant. Using a few shear-thinning lubricant examples from the literature and Bair's own earlier measurements, the author states that the traction curve is not the same as a rheological flow curve. I generally agree with this statement except that it seems obvious that rheological data obtained from bulk viscometers will not match with EHL tribology. In steady state EHL friction measurements, typically something like an MTM machine is used, where the normal load is controlled but not the film thickness. In proper rheological measurements conducted with a parallel plate or cone-and-plate accessory on a rheometer, the gap is controlled but the load is allowed to fluctuate. So, in reality, the traction curve *may very well* represent the flow curve but only if the film thickness and shear rate is normalized by the rate- or pressure-dependence of the lubricant viscosity. In the examples shown in Figs. 1-6, although the curves and onset of shear thinning are at clearly different shear stresses, I am not quite convinced of the title of the manuscript since the experimental conditions for the viscometer and friction measurements were not explicitly discussed. These figures are not really providing an apples-to-apples comparison, and the author, should at the very least, understand this point and moderate the claims in this manuscript.

 

Minor point: The figures are of low resolution in the pdf, and Fig. 6 is especially sloppy - what are the solid lines?

Author Response

Reviewer 2

Bair discusses the assumptions that the lubricant is Newtonian in nature within the EHL contact and that the so-called Stribeck curve is representative of the rheological properties of the lubricant. Using a few shear-thinning lubricant examples from the literature and Bair's own earlier measurements, the author states that the traction curve is not the same as a rheological flow curve. I generally agree with this statement except that it seems obvious that rheological data obtained from bulk viscometers will not match with EHL tribology. In steady state EHL friction measurements, typically something like an MTM machine is used, where the normal load is controlled but not the film thickness. In proper rheological measurements conducted with a parallel plate or cone-and-plate accessory on a rheometer, the gap is controlled but the load is allowed to fluctuate. So, in reality, the traction curve *may very well* represent the flow curve but only if the film thickness and shear rate is normalized by the rate- or pressure-dependence of the lubricant viscosity. In the examples shown in Figs. 1-6, although the curves and onset of shear thinning are at clearly different shear stresses, I am not quite convinced of the title of the manuscript since the experimental conditions for the viscometer and friction measurements were not explicitly discussed. These figures are not really providing an apples-to-apples comparison, and the author, should at the very least, understand this point and moderate the claims in this manuscript.

Minor point: The figures are of low resolution in the pdf, and Fig. 6 is especially sloppy - what are the solid lines?

My paper does not address the Stribeck curve.  The Stribeck curve is a record of friction in a sliding contact with increasing sliding speed such that the friction covers the boundary, mixed and hydrodynamic regimes. A traction curve is a record of friction from a full film rolling contact in the piezoviscous-elastic regime as sliding speed is increased. 

I have much experience with viscometers and rheometers; however, I am not aware of one in which “the load is allowed to fluctuate,” unless it is desired to measure the primary normal stress difference, which is not the case here. 

Superposition requires that the onset of shear thinning be at a critical shear stress which is roughly independent of conditions.

“These figures are not really providing an apples-to-apples comparison”.  That is the point exactly.  A traction curve should not be compared with constitutive behavior.  They are different.

The figures are TIFF files of 1200 DPI resolution-not low resolution

The solid lines in Figure 6 were identified in the text.  I have included this description in the figure caption.

Reviewer 3 Report

Dear author,

The manuscript "A Traction (Friction) Curve is not a Flow Curve" is a very interesting work in the field of EHD lubrication. Your demonstrations are clearly and your finally  conclusion that " Decades of wasted effort demonstrate that the extraction of constitutive behavior from a traction curve must end" is very interesting for future researches in the field of EHL.

Congratulations!

We propose to be accepted for publication in present form.

Author Response

Thank you.