Originally Posted By: Tom NJ
Film strength is defined as the load or shear rate necessary to break the lubricant film and push the contact into a boundary regime. It is usually measured by a method that progressively increases load until the film ruptures, such as the Falex test, and the results are reported in units of load. Polarity has a strong influence on film strength since the ionic bond between the negatively charged oil molecules and the positively charged metal surface, as well as the intermolecular bonds within the oil, require more load to break. Esters are well known for their high polarity and film strength.
Tom, I missed this yesterday. Thanks for the detailed and nuanced response.
My whole reason for bringing up HTHS was to use it as an analogue to film strength, which I often see done in technical contexts, such as those I quoted. I do understand the difference between the two, but HTHS values are much more commonly available and do often seem to be used in this way. The advantage of HTHS as a measure of viscosity seems to be that it gets around the effects of VIIs and other compounds that interfere with the normal correlation that would otherwise be very clear between viscosity and film strength. Would you agree?
In any case, my argument in this thread is simply that polarity can have a major effect on film strength and consequently on the ability of the oil to limit wear. You seem to have confirmed that in your first paragraph.
Another very general concept I often run into in technical writing on oil is that there is a very clear observed correlation between HTHS and wear. I have seen it written, as I am sure you have, that tests show that the reduction in wear with increasing HTHS implies that wear would be reduced to zero at an HTHS of around thirteen. I'm sure that's not true - wear would certainly reduce logarithmically and not linearly as it approaches zero: however, it does show the importance of true "newtonian equivalent" viscosity as measured in this test and its effects on limiting wear.
Of course, if one were to test every available oil and plot a graph showing wear vs HTHS, some oils would perform above the standard and some below, due to important differences in add packs, blending, and so on. However, the overall trend of the graph would show decreasing wear with increasing HTHS and a high level of correlation between the two. That is at least what I have concluded from the various technical bits and pieces that I have read on the subject over the years.
Finally, as I said before, I am personally not convinced of a superiority of Redline oil over others. However, I am impressed by the positive impact that Redline seems to have had on many people using it, such as in racing, and intrigued by the prospect that the high-polarity/high-HTHS/strong add-pack nature of the product might really make it something special. Obviously, every application is different - but are you aware of any real testing that has compared Redline or other high-POE oils to other Grp IV/V blends? I have not, and would be very interested in the results of any such experiment if it has been carried out.
Thanks again for your reply.
