A gear oil and a motor oil are both 8.5 cSt at 100C. The motor oil is graded SAE 20, but the gear oil is graded SAE 80. Am I correct to understand that the discrepency is due to the grade applied to the gear oil being figured when the gear oil is at 150F (65.5C) and thus shows a much higer cSt, say 100 or so at that lower temperature? I assume that came about because gear oil does not run as hot as engine oil.
SAE Grade: Gear Oil vs. Motor Oil
- Thread starter TallPaul
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The way I understand it - it's to keep people from getting cornfused - just a different numbering system.
Well look what I found! Scroll to the first chart. So What I supposed above is not the case. They actually show in this chart that the viscosity is measured at 100C. Somewhere I read that it was measured at 150F, but I can't find that reference.
I don't know about it being done to prevent confusion. Seems to create confusion. Probably more likely just that the two scales developed independently and from older systems and thus were not related.
I don't know about it being done to prevent confusion. Seems to create confusion. Probably more likely just that the two scales developed independently and from older systems and thus were not related.
Off course the 100°C vis is measured at 100°C.
The reason it shouldn’t create confusion is that a 15 cSt@100°C gear oil is called a 75W-90 so people don’t confuse it with a 15W-40 motor which may be 15 cSt@100°C - the ranges don’t overlap perfectly but the SAE wanted to use different viscosity description numbers for gear oil and motor oil so people don’t walk into an oil retailer and buy 20W-50 gear oil and put it in the engine, etc……
The reason it shouldn’t create confusion is that a 15 cSt@100°C gear oil is called a 75W-90 so people don’t confuse it with a 15W-40 motor which may be 15 cSt@100°C - the ranges don’t overlap perfectly but the SAE wanted to use different viscosity description numbers for gear oil and motor oil so people don’t walk into an oil retailer and buy 20W-50 gear oil and put it in the engine, etc……
Oh I see. Now it makes more sense. Ha ha, my boss wanted to top up his hydraulic clutch master cylinder, walked into a parts store and came out with gear lube. He proceeded to pour some in the master cylinder. Ouch!!!!! Fortunately he was so inept that he didn't realize the rubber diaphram from inside the cap remained in teh master cylinder, so he only filled the rubber diaphram cup with gear oil and his braking system was okay. His mechanic figured it out really quick.
Gear lube in the brakes?? Never heard that one before... That takes skills...
Major Skills
let see if I can explain it to you. oil is checked at 100c. that means oil goes though the viscometer (called a tube) at a certain speed(in your case 8.5 cst)@ 100c.
now, look at this chart visc chart , at 100c cst the motor oil is sae 20wt sae motoroil but if it is a gear oil at 100c cst it is a 80wt gear oil.
now that means the base oil if a motor oil or a gear lube goes though the tube at a speed of 8.5cst(which the technician has a second chart that coverts to a cst chart)
since both run at the same time they are the same except that... motor oil and gear oil have different additives and are different between the two. so just because they move the same time though the tube they are very different and are used in different applications.
hoe that helps
bob
now, look at this chart visc chart , at 100c cst the motor oil is sae 20wt sae motoroil but if it is a gear oil at 100c cst it is a 80wt gear oil.
now that means the base oil if a motor oil or a gear lube goes though the tube at a speed of 8.5cst(which the technician has a second chart that coverts to a cst chart)
since both run at the same time they are the same except that... motor oil and gear oil have different additives and are different between the two. so just because they move the same time though the tube they are very different and are used in different applications.
hoe that helps
bob
Thanks. Just a totally different animal. Well at least they assign a grade to it, unlike ATF.
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