Re: Re: Re: Re: The full story on transmission/hydraulic fluid for use in N series tractors

Posted by Need to clarify this - llamas on March 30, 2000 at 12:49:18 from (192.62.142.21):

In Reply to: Re: Re: Re: The full story on transmission/hydraulic fluid for use in N series tractors posted by Nolan on March 30, 2000 at 07:10:16:

Fistly, it's important not to confuse viscosity index (a measure of the change of viscosity with temperature) with viscosity number. The two mean very different things.

Secondly, a lot of folks have said (in some form of words or other) that the SAE viscosity number scales are "different" for gear oils and crankcase oils. By outcomes, this statement is true, but saying it that way might lead someone to think that there are two different scales when there are not.

SAE viscosity is SAE viscosity. What's different is the temperatures at which viscosities are measured. Viscosity reduces as oil gets warmer - how much it reduces is defined by its viscosity index. Oils defined as "gear oils" are specified by viscosity at 150°F. Oils defined as "crankcase oils" are specified by viscosity at 210°F. So,for example, a gear oil with SAE viscosity 90 , at a given temperature, will have the same viscosity as a crankcase oil with SAE viscosity 40, at the same temperature.

Put another way, heat SAE 90 viscosity gear oil to 210°F and it will flow like SAE 40 weight crankcase oil.

The basic difference between "gear oils" and "crankcase oils" is that you can't get plain old mineral oil to do what's required at crankcase temperatures without additives - additives which stabilize the viscosity of the oil and keep it high enough at high temperatures to do its work. If you needed 40 weight oil in a motor at operating temperature, for example, without viscosity enhancers, it would be molasses in the motor when it's cold. Viscosity enhancers reduce the amount by which viscosity changes with temperature (improve the viscosity index).

Gear oils may have some viscosity index additives, but they're relly not required because the temeperature differences are not nearly as great as they are for crankcase oil. Gear oils also have another much more critical function, and that's taking care of very high pressure contact/sliding friction - between gears, especially hypoid and bevel final drive gears - at high torque and low speed. To this end, they contain lots of friction enhancing additives - sulfur and phosphor compounds. Crankcase oils need much less of this because most of the lubrication in a motor is hydrodynamic - the surfaces don't touch like they do in a transmission. The downside of these additives is that they can be corrosive (especially to bronze and alloy bearings), and they should never be used in a crankcase, even if the viscosity has been "corrected" for temeperature difference.

So, in summary, gear oils and crankcase oils are measured under different conditions. It may be useful to understand the scale differences - what's the viscosity of an SAE 90 gear oil in terms of a crankcase oil number - but it doesn't tell you very much. It's more important that you be sure that you have the right stuff - look beyond just the SAE viscosity number.

llater,

llamas

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• Re: Re: Re: Re: Re: The full story on transmission/hydraulic fluid for use in N series tractors Nolan, an addendum 04:05:32 03/31/00 (0)

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