Definition of Oil Shear

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Jan 9, 2005
Sarasota, Florida
People often use the word SHEAR to mean a decrease in viscosity of a motor oil. We all know that mineral based motor oils initially thin a little with use. This is in part from the dilution with gasoline and also the consumption of viscosity index improvers. It is less of a problem today than in the past but still occurs. However, the definition of SHEAR is not a permanent thinning that occurs with use but a temporary, functional thinning that occurs in the actual bearing. It is an instantaneous thinning. When the oil leaves the fast moving, hot bearing the oil reverts back to it normal viscosity. Often we hear of oils “shearing” to a lesser grade. This is as if a 30 wt. oil becomes a 20 wt oil. This is not the case. It is an inappropriate use of the term. An oil SHEARS to a thinner oil because it has less viscosity than the temperature alone would account for. At 150 C an oil may have a viscosity of 4. But under high RPM, pressure and temperature it is found to have a viscosity of 3. After that oil comes out of that higher load/RPM/pressure situation it becomes the same oil as before entering that bearing condition. It goes back to it’s regular viscosity of 4. This is my understanding of the use of the word SHEAR in automotive motor oil applications. aehaas
So then, what DO you call the activity by which a 30 wt thins to a 20 wt or thickens to a 40 wt during use? Thanks.
When an oil shears permanently from a higher viscosity to a lower one, VII polymers are actually cut (sheared), so "shear" is the correct term. When oil "shears" temporarily under stress, as you described it, nothing really "shears", so shearing is a misnomer. So I guess "shear" is an improper term, as you defined it, but I don't know what else to call it.
Answers: 1. The temperature for an HTHS measurement is done at 150C or 302 F, which is the average peak temperature likely to be encountered in a bearing. 2. The oil is mechanically sheared at a rate of 1 x10^6 shearing operations/second. 3. Minimum Oil Film Thickness measurements (MOFT) of operating engines did not correleate well with actual wear in service. A method was devised by which the oil temperature would be elevated to worst case and sheared to determine the optimum viscosity which better correlated with wear. Three Exxon Researchers found that a minimum HTHS of about 2.8 mPA.s was the MINIMUM HTHS viscosity needed for normal wear, with the higher the HTHS being better for minimum wear. IN general, the higher the viscosity, the greater the HTHS. For example, in a fleet of taxicabs using a GM 4.3L V6 engine, if the HTHS was 2.35, the startup film was 0.097um and 2.56 um at running; if the HTHS was 2.98, cP, the Startup oil film thickness was 1.231 um while the running film thickness was 3.22 um. In Dynomometer wear tests using four GM 3.8L engines, the wear mass of a connecting rod bearing was as follows: HTHS 2.1 mass loss (gm.) - 190 HTHS 3.2 mass loss (gm.) - 28 For "mains" bearings: HTHS 2.1 mass loss (gm.) - 150 HTHS 3.2 mass loss (gm.) - 40 A jump in HTHS by about +1.5 results in approximately 1/5 the wear. Now this relationship is not linear and flattens as one nears a 40+ weight oil. I should also mention that this test showed little differences in wear between a high quality 5W20 and a 10W30 for oils of close HTHS. For example, The average wear of one of the 3.8L V6's showed a total wear of the Connecting Rod bearings as 48.4 grams for the 5W20 verses 44.3 grams for the 10W30. For a 10W40 oil, the wear was 39 grams!!! [Mola's comment: I think this test verifies my earlier comments that most daily driver engines can use any oil from a high quality 5W20 to a 15W40 fleet oil.] Summary: It was found that HTHS correlates better with wear values found in actual oil analysis and actual tear-down measurements than does measuring the oil film thickness in situ. -------------------------------------------------------------------------------- I too think that viscosity at any given temp. while a factor is not the most important! I think that when compareing wear HTHS and MOFT are the most important things we consider wear rates with different viscositys. Redlines 5W20 has an HTHS of 3.3 while M1 0W30 has an HTHS of either 2.9 or 3.0. SO I would expect the Redline 5W20 to perform better so long as presure adn flow were within spec. If a vechile's oilpump can not maintain adequate flow and presure then the thin oil even with the higher HTHS is not going to do you any good! I belive that TooSlick's poor results with 5W20 were do to flow and presure issues in a system not designed for 5W20. DC had to re-clearance their G-Rotor oil pump on the 4.7 and 3.7 from it's origanal spec. whena at the last minute they decided to make 5W30 the prefered grade instead of 10W30. The rest of the engine remained the same. [Cheers!]
Seems to me it's definition is based on it's context. You could both "cut" (shear) the VII molecules or apply a force in a particular way to deform something. ------------------------------ shear (shîr) v. sheared, sheared, or shorn (shôrn, shrn) shear·ing, shears v. tr. To remove (fleece or hair) by cutting or clipping. To remove the hair or fleece from. To cut with or as if with shears: shearing a hedge. To divest or deprive as if by cutting: The prisoners were shorn of their dignity. v. intr. To use a cutting tool such as shears. To move or proceed by or as if by cutting: shear through the wheat. Physics. To become deformed by forces tending to produce a shearing strain. n. A pair of scissors. Often used in the plural. Any of various implements or machines that cut with a scissorlike action. Often used in the plural. The act, process, or result of shearing. Something cut off by shearing. The act, process, or fact of shearing. Used to indicate a sheep's age: a two-shear ram. also sheers (shîrz) (used with a sing. or pl. verb) An apparatus used to lift heavy weights, consisting of two or more spars joined at the top and spread at the base, the tackle being suspended from the top. Physics. An applied force or system of forces that tends to produce a shearing strain. Also called shearing stress, shear stress. A shearing strain. A deformation of an object in which parallel planes remain parallel but are shifted in a direction parallel to themselves; "the shear changed the quadrilateral into a parallelogram."
In J300 May 2004 the HTHS definitions based on ASTM D 4683 and D 5481 measure the EFFECTIVE viscosity (a momentary viscosity less than the rated viscosity because of high-shear, high temperature conditions occurring within certain journal bearings and between the rings and cylinder walls). aehaas
So then, what DO you call the activity by which a 30 wt thins to a 20 wt or thickens to a 40 wt during use? [Big Grin]
Hi, pscholte - please teacher I know, I know (hand in air!!) thinning = sheer bad luck! thickening = duh bad luck! Am I right teacher, am I right.............? The Elves tried to tell me something too..... Regards Doug [Cheers!]
Originally posted by Doug Hillary: Hi, pscholte - please teacher I know, I know (hand in air!!) thinning = sheer bad luck! thickening = duh bad luck! Am I right teacher, am I right.............? The Elves tried to tell me something too..... Regards Doug [Cheers!]"creative" answers, Doug. [Big Grin]
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