In Reply to: Re: The Hydraulic Oil Story posted by raVen on December 30, 2013 at 01:29:23:
"Thought the debate was all about UTF being too thin to work as a hydraulic fluid in summer months in warm climates in tractors with worn out hydraulic pumps"
No - that was a given from the beginning and is well documented by 36Coupe's repeated references to his personal situation.
"At 100C there is small difference in viscosity between 80W90, 90, UTF & AW46 and the numbers are useless, except for grading purposes."
Then why do we go to so much time and expense to grade them? While the difference is numerically small it DOES make a measurable performance difference. More so with engine oils where the main and rod bearing lubrication regime is designed to be almost totally hydrodynamic and the oil film thickness is critical. At engine operating temperatures hydrodynamic oil film thickness is very sensitive to even tiny (1-2 cSt) differences in viscosity. Insufficient oil film results in increased frequency of boundary mode operation and greatly accelerated bearing wear.
Gear lubes are somewhat different because the parts operate primarily in boundary conditions with very little if any hydrodynamic effect and EP type chemical anti-wear agents play a much larger role than viscosity. That is why you don't see a lot of SAE 140 gear lubes in use anymore - with modern EP chemistry you don't need the molasses effect of a super heavy oil to get the needed performance. That means less drag and higher efficiency in the mechanism - especially in colder conditions. But - all other things equal - an SAE 90 gear lube will still out perform an SAE 80 gear lube in demanding boundary lubrication tests like the Falex four ball test (ASTM D2266 and D4172). It may not be a dramatic difference but it is still a measurable difference.
TOH
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