In gear lubricant applications, fluid film thickness is a critical parameter for ensuring adequate gear protection. Modern gear lubricants are specially formulated to protect gears from wear and fatigue under a wide variety of conditions. However, if the fluid film breaks down, the lubricant is subjected to extremes of pressure and temperature that will diminish its ability to protect gear surfaces. This can result in reduced equipment life.
Increased emphasis on fuel economy, extended fluid life, and other performance parameters, have resulted in increased use of wide-span multigrade gear lubricants, which may contain relatively high levels of polymer compared to the more traditional SAE 80W-90 and SAE 85W-140 grades. SAE 75W-90 and SAE 80W-140 grade gear lubricants are common examples. If not formulated properly, these “polymer-rich” formulations have the potential to shear significantly and reduce the fluid film provided by the lubricant, thus increasing the potential for premature failure. This is of particular concern considering that the shear stresses encountered in geared equipment can be far greater than those in other lubricant applications.
With no industry shear stability specification currently in place, there is a danger that improperly formulated gear lubricants could experience significant shear in the field resulting in increased wear and reduced equipment life.