SAE oil viscosity vs. Centipoise

[klyde]

I have a viscosity chart that shows SAE 50wt engine oil viscosity varying from 175 to 280 cSt @ 40 C. Why is that? Also the chart depicts SAE 90wt gear oil varying even more at the same temp??

 

[Xtrema]

Viscosity grades are simply a range of possible thicknesses of an oil. You could think of them as you might the tolerance on a resistor labeled 1000 ohms. Would you be surprised that one resistor was 990 and another was 1010 ohms? It's the same kind of thing. Oils with the same SAE weight fall into a range of centistoke or SUS values.

At cold temperatures, oil viscosity can vary much more for a given SAE weight than at warm temperatures. You may not think 40 C is cold, and of course it isn't to a human. But, to an oil designed to operate at 125 C, 40 C is 'cold.' Depending on the oil formulation, two oils with exactly the same viscosity at 100 C might be very different at 40 C.

Gear oil and crankcase oil and manual transmission oil have different SAE weights for similar viscosity.

 

[25362]

The SAE classification for the "50" motor oil and the "90" transmission oil grades are defined at 100^oC, as 16.3-21.9 cS and 13.5-24.0 cSt, respectively.

As Xtrema says at the lower temperature of 40^oC (as adopted by the ISO system, and in the US by ASLE, for all industrial lubrication fluids) and considering the different viscosity indices of these monograde oils, the spread in viscosities at 40^oC becomes larger.

 

[sprintcar]

klyde -
I've got a chart that shows relative viscosity between ISO VG, AGMA, SAE crankshaft and gear - don't remember where it came from, but I can email it to you in Word format

send me your email address if you'd like it.

Keep the wheels on the ground
Bob
showshine@aol.com

 

[klyde]

sprintcar: Thanks for the offer, but I found a chart in a bearing handbook. It gave me cause for the question. I got into the game because I had changed my blade grip bearings in my helicopter from grease to oil and the bearing handbook said to use a high viscosity oil due to the high centrifugal loads. It then said to use a low viscosity oil due to the fact that the bearings were not rotating but just oscilating back and forth. Then I started looking at oil viscosities. That is why I got chart happy.
Can't keep the wheels on the ground.
klyde
sfkf@iwvisp.com

 

[25362]

To klyde: please tell us what's the reason for the thread heading referring to centipoises, while the text of the question mentions only cSt.