Understanding gear oil ratings.

[rickyquicky]

The following copy/paste is from another website as I try to get my head around what gear oil ratings mean...

• Multigrade gear oils

Viscosity of gear oils may be stabilized by polymeric additives (viscosity index improvers). Viscosity of such gear oils is specified at both high and low temperature. These oils are called multigrades and they are designated by two numbers and the letter “W” (SAE 75W-90, SAE 80W-90, SAE 85W-140 etc.). The first number of the designation specify the oil viscosity at cold temperature, the second number specifies the oil viscosity at high temperature.
For example: SAE 85W-140 oil has a low temperature viscosity similar to that of SAE 85W, but it has a high temperature viscosity similar to that of SAE 140.
Multigrade gear oils are used in a wide temperature range.

I thought I understood it, as the first number simply referred to viscosity when cold, the second number when the oil was hot.

Apparently a higher number means a higher viscosity (basically a thicker fluid).

Now what I am really struggling to understand is how the oil can have a HIGHER number when hot, surely as oils/fluids get hotter they get thinner??

Can anyone teach me something please!

 

[Fred@BTM]

Same as engine oil, its the viscosity rating, it doesn't get thicker as you heat it lol! It just doesn't thin out as much basically. Ie a 75w80 and a 75w90 you'd have to heat the 90 more to make it the same viscosity as the 80

 

[rickyquicky]

Same as engine oil, its the viscosity rating, it doesn't get thicker as you heat it lol! It just doesn't thin out as much basically. Ie a 75w80 and a 75w90 you'd have to heat the 90 more to make it the same viscosity as the 80

Thanks for the reply, I know that oil gets thinner as it heats up obviously, but my understanding of the numbers meant that the 'hot rating' of the oil was saying it was higher viscosity!

So how do the first number and second number relate to the other then, if the first is cold, and the 2nd is hot then why does the second number have a higher figure?

Surely the second 'hot rating' should be lower as the oil will be thinner and therefore have a lower viscosity.

My understanding (lack of!) of this is obviously well off then, those numbers must not represent the way I think they do...

I understand that the numbers represent cold and hot viscosities. I think that the higher the number, the higher the viscosity, so what am I missing here! Why would the second number be higher if it represents viscosity than the cold rating?

 

[Fred@BTM]

only at temp though, higher the number the higher viscosity at a given temp, first number is at 0degC, second number is at 100degC, i forget how viscosity is measured, its been a few years since my brain had to do clever stuff, ask oilman

 

[Fred@BTM]

google - http://www.kewengineering.co.uk/Auto_oils/oil_viscosity_explained.htm

 

[rickyquicky]

google - http://www.kewengineering.co.uk/Auto_oils/oil_viscosity_explained.htm

Cheers Fred, an interesting read.

Still don't understand why the second number is higher though!