Posted by Vaughn [172.69.140.162] on Wednesday, October 18, 2023 at 09:06:32 :
This is my reasoning for using .002 thousand maximum wear limits on spur gear components. I have searched specs. for the After-Type Transmission (went into production in 1940) Transmission and found nothing. In the Military manuals for the � Ton WC, there are the drivetrain specs. for the Up-To-Type Transmission (went into production in 1933) where they identify the maximum wear limit as .002. Using that limit and the Timken Bearing Specification Catalog for needle/roller bearings for housing and shafts, I applied that to the component specs. in the PIF to give the benchmark number. This is as close as I could get, and it all seems to work.
Most guys will look at the components and if they don�t see any visible wear or galling on bearing surfaces, or they can�t find parts, they reassemble the trans. all back together resulting in the same problems with a disappointing rebuild and not knowing why the rebuild failed. I can�t stress enough the importance of micing the components to ensure they are not out of wear limits. I have taken apart transmissions where the countershaft gear is visibly worn and where the transmission knocked heavily in fourth gear with/without a load. Looking in the tunnel of the gear, I could see slight wear where the roller bearings reside and, on the shaft, as well. Remember, when you are in fourth gear, the driven gear coming into the transmission is still rotating the countershaft gear, and with significant wear it�s going to rattle and knock as it spins because rollers/needles are trying to lock up against each other due to the excessive wear.
From reading history on the T-234 Heavy Duty 2-1/2- or 3-ton truck used on the Burma Road in WW2, they were only able to achieve 25,000 miles before the truck was shot. The transmission was a Clark 5 Speed, synchro 4 & 5 gears from what I can tell, so spur were all remaining gears and most of the time they probably ran in 2nd or 3rd gears. This should give you an indication of how long a spur gear trans. might last although we don�t drive the grades and carry the loads these trucks did, it�s all in rotation. So, when you pick up a truck that has around that many miles or more, you better plan on replacing components or you�re going to be one of the guys that wonders why the transmission doesn�t sound any better after rebuilding it.
I just didn�t want anyone to think I�m pulling that number, �0.002� out of the air as it does seem to be a low number, but you must understand the design in gearing and bearings, I guess.
As far as oil, straight 140, or 75-140, 85-140 are all fine until you get to the 30-degree mark in temperature. Whatever oil you buy, purchase oil that is specified for protecting yellow metal, that�s important. Good luck rebuilding your transmission!
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