From: elliott@serv.net@INTERNET on Fri, Jan 10, 1997 10:46 PM
On Fri, 10 Jan 1997, Dave Clement-LDC009 wrote:
>> The motivation behind using ATF or motor oil in gear boxes is fuel
mileage.
>> Thinner oil produces less drag resulting in more mpg's. I found out the
hard
>> way that putting heavier oil in a gear box designed for a lighter oil
does
>> not work well. The gear box in my Shelby Daytona specs 5W-30, being that
I
>> have a mental thing that heavier is better I replaced the oil with 20W-50
>> and the gear box shifted like sh*. I was back under the car the next day
>> draining it and refilling with 5W-30.
>>
>> NOTE: As I recall from MX days the Bel-Ray and Spectro gear box oils were
>> still a fairly light weight, on the order of 30W to 50W. Also, there are
no
>> syncros in a motorcyce geasr box.
>>
>> Dave Clement
>
>Somewhere I've got a viscosity chart that shows the equivalence between
>SAE engine oil and gear oil numbers with Saybolt Universal Seconds (SUS).
>As I remember SAE 30 engine oil is about the same as SAE 90 gear lube in
>SUS, why the SAE viscosity numbers are different is beyond me, maybe to
>keep people from using gear oil in engines and wrecking the plain
>bearings. As I remember the Bel-Ray oil that was supposed to replace
>10W-40 in the PE 175's gearbox was 75W. It was pretty thin and greatly
>improved the shifting, snick, snick, snick instead of crunch, crunch,
>crunch.
>
>From trying to pour straight SAE 40 or 50 wt engine oil and SAE 75 to 90
>gear lube at similar temps I can testify that the engine oils were a lot
>more viscous than the gear lubes.
It's been a lot of years since I have been involved with fluid dynamics but
my recallection of the SAE viscosity measurement system is that it's a
relative number assigned based on the viscosity at zero degrees F. Also, the
additive packages in the oil have a dramtic impact on the viscosity as you
move away form zero degrees. I think if you compare 50 weight motor oil to
75 weight gear oil at zero degrees the gear oil would be more viscus.
Dave
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