school me

[l921428x]

on oil weights and viscosity. what does 10w30, 30sae, 5w20..............
mean? i heard yesterday that the numbers have to do with temperature.
what exactly is viscosity.

[stinker]

what exactly is viscosity.

http://en.wikipedia.org/wiki/Viscosity

In simplistic terms, high viscosity means thicker fluid(flows harder, more resistance to pressure stress) low viscosity means thinner fluid(flows easier, less resistance to pressure stress).

Superfluids have zero viscosity. You can literally watch them flow (vertical climb) upwards on the inside walls of a glass jar that's just sitting there untouched and pour out over and out the top for what appears to be no reason. Their own weight is enough to cause them to start flowing across any surface in a manner that looks like it's defying gravity. Very cool stuff.

[Gunreference1]

Some good information on engine oil viscosity at the link below.

http://www.upmpg.com/tech_articles/motoroil_viscosity/

Hope this helps!

Steve

[Krupski]

on oil weights and viscosity. what does 10w30, 30sae, 5w20..............
mean? i heard yesterday that the numbers have to do with temperature.
what exactly is viscosity.

OK, first "SAE" is "Society of Automotive Engineers" - a standards organization (kinda like ANSI, ISO, CSA, UL, etc...). They devised the oil rating system.

The numbers refer to the viscosity of the oil (how thick it is). Maple syrup has a higher viscosity than water, for example.

Viscosity is measured my pouring a test liquid through a calibrated orifice and seeing how long it takes for a certain amount to flow through.

Plain oil (single viscosity - like "30 weight" or "SAE30") is just "unmodified" oil and the number refers to it's viscosity at a certain standard test temperature.

"50 weight" oil is thicker than "10 weight" oil. Although both would work in your car engine, the thicker 50 weight would require more power to pump around the engine as well as creating more "drag" as the engine parts tried to slide past the thick oil, so gas mileage would go down with 50 weight. Also, when the engine was cold, the 50 weight oil would be so thick that the starter motor may not even be able to crank the engine fast enough to start!

"Multi-Vis(cosity)" oils like "10W-30" or "5W-20" have additives (basically "liquid plastics") that change viscosity inversely with temperature. So, "10W-30" oil, for example, will be no thicker than 10 weight when cold, and no thinner than 30 weight when hot. This kind of oil allows the car to start easily and also lubricate well and produce minimal drag losses when hot.

Car oil needs to be changed regularly for three reasons:

(1) Water and sulfur from engine blow-by and condensation after the engine cools down gradually form sulphuric acid in the oil, which is obviously bad for metal parts.

(2) Carbon and other combustion debris from blow-by gradually make the oil "dirty" and "abrasive". Abrasive oil is obviously not good.

(3) The VISCOSITY MODIFIERS (mentioned above) break down over time from heat and shear (friction) forces. The "10W-30" oil gradually degrades into "Nothing W-30".



Class dismissed.

[Krupski]

Superfluids have zero viscosity. You can literally watch them flow (vertical climb) upwards on the inside walls of a glass jar that's just sitting there untouched and pour out over and out the top for what appears to be no reason. Their own weight is enough to cause them to start flowing across any surface in a manner that looks like it's defying gravity. Very cool stuff.

Supercritical helium is weird, amazing stuff. It's not a gas or liquid. It has the density of liquid, but it fills it's container completely like a gas. And it has ZERO viscosity.


http://www.youtube.com/watch?v=2Z6UJbwxBZI

[El Laton Caliente]

"Multi-Vis(cosity)" oils like "10W-30" or "5W-20" have additives (basically "liquid plastics") that change viscosity inversely with temperature. So, "10W-30" oil, for example, will be no thicker than 30 weight when cold, and no thinner than 10 weight when hot. This kind of oil allows the car to start easily (cold oil isn't too think) and also lubricate well and produce minimal drag losses when hot (oil isn't too thin).


Are you sure on this? I always thought the two weights meant it acted as 10 weight at starting temp and 30 weight at running temp...

The other consideration is the letter grade of the oil. The higher the letter the more temp the oil can handle before breaking down. Diesels, because of higher temps should never run less than a "G" grade oil. Gas engines can run "D" to "F" grades.

[Krupski]

Are you sure on this? I always thought the two weights meant it acted as 10 weight at starting temp and 30 weight at running temp...

The other consideration is the letter grade of the oil. The higher the letter the more temp the oil can handle before breaking down. Diesels, because of higher temps should never run less than a "G" grade oil. Gas engines can run "D" to "F" grades.

Yup, I said it ass-backwards. I'll edit the post to correct it. Thanks for finding the mistake.