Accurate bearing pre-load for Final Drives

When rebuilding the finals on my OC-46 project I referenced the manual on setting up the bearing pre-load for the drive shaft and ring gear bearings. What I discovered was that the process was a little more susceptible to interpretation than I felt comfortable with. The manual suggests using a fish scale on the ring gear to measure it's rotational drag in pounds. This is somewhat acceptable but" ball parkish" to say the least. I found it very difficult to maintain the same angle on the scale hooked to the ring gear tooth as I rotated the gear leading it with the scale making it very very difficult to get a repeatable reading. As for the drive shaft bearing preload it simply suggest to "feel by hand" 2-4 lbs rotational drag. I really hadn't a clue what 2-4 lbs of resistance felt like when twisting a 1.75 inch shaft???

Since it has been a long time since I last had my wrist calibrated I elected to employ another means of measuring the rotating resistance.
In the past I've set up a few automotive differentials and had to set up cup and cone bearing pre-loads for the pinion shafts. This was always done with a needle or dial type torque wrench measured in inch lbs. of rotating torque. I was positive this was the process I wanted to use on the final drives also. All I needed was a couple of adapters to plug my torque wrench into to rotate the bearings.

The first thing I had to do was convert the fish scale specs to inch lbs of rotating torque.The book calls out for a rotating resistance of between 2- 4 lbs so I shimmed up one of the ring gears until I could get a good reading of about 3 lbs.

Then I fabbed up an adapter for the torque wrench. The adapter consist of a couple stud extensions and a spreader bar that slips onto two opposite studs on the ring gear hub and a 1/4" square hole in the center for the torque wrench.

The stud extensions are a couple of spare studs screwed and loctited half way into a couple spare nuts. The spreader bar just slips over these.

Now I could measure how many inch lbs of rotating torque is equal to 3 lbs of rotating resistance. It turns out to be about 17.5-18 inch lbs

I also previously recorded 2.6 lbs on the fish scale equaled between 11-12 inch lb.s of torque. I noted that these numbers fall in line with the specs for the pre-load of the automotive differential pinion bearings so I was very comfortable with my numbers.

At this point I was ready to remove the finals housing from the crawler to set up the drive shaft bearings. The adapter for this was simply a piece of 1.75" tubing, 3/16th" wall, giving me an internal dia. of 1.375". This fit perfectly with no slop. I then drilled and tapped the side on one end for two opposing 10-32 set screws (one would have worked just as well) to engage the splines on the shaft and welded a washer and 1/4" socket to the other end to adapt the torque wrench.

I elected to set the pre-load on the dive shaft at the lower 12 inch lb setting which equates to about 2.6 fish scale, calibrated wrist spec. I justified this because just as with the different size automotive differential pinion bearings. The smaller bearings always called out for less pre-load. Just a good feeling I guess.

One last note: The manual instructs to set up the ring gear first, then back the nut off until there is no resistance, then install the drive shaft and set it up with the ring gear still installed and interacting with the pinion gear. I elected to do them totally independent of each other so there was no additional drag from gear interaction to foul my torque wrench readings. Also it was much easier to do the ring gear with the finals installed on the crawler. I suggest that you remove the finals to do the shafts then pull them out of the finals after you get a good pre-load reading, measure and record the shim packs and set them in a safe place, then install the final housings and set up the ring gear, re-installing the shafts after the ring gear is done and set.

Also note: If you change direction of rotation to get a scale reading you'll notice much less resistance for the first 1/8th or so revolution of rotation. This is because the rollers haven't loaded in that direction yet until the space between the rollers, cage and race are taken up. I rotated a full revolution before I started reading the scale on the torque wrench.

I guess this all might seem a little more than is really needed and that "going by feel" is really close enough. I know I'll sleep better knowing I didn't leave them too loose or get them too tight. I do know that the bearings I pulled out of these finals had evidence of too much preload so for all it's worth I hope this might be helpful to some of you.

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