Golden Jubilee
Posts: 5177
Joined: Thu Nov 29, 2012 9:28 pm
Location: Lyman, IA
Ramblings on building differntials
I am seeing a pattern on these older diffs, many the bearing are really loose. In one case I bought a "core" diff that had just been "rebuilt" and had failed due to this. Fortunately it was the power divider that failed 1st and the ring and pinion were unharmed.
Most mechanic know that when setting up wheel bearing, you don't want them too tight. I doubt many have put much thought into why. Taper roller bearings have a built in corundum, The inner race is smaller than the outer, and the distance the roller must travel is less at inner end of the roller that outer. If you draw out the centerlines of the cone, cup and roller, they are all slightly different. This means there is some friction and slippage with the bearing. Wheel bearings primarily take axle loads with only minor thrust loads so can be set to less then zero preload. This minimizes friction and allows them to run in less than ideal lube. Personally I like to set my wheel bearing as close to 0 as possible, but they will give satisfactory service over a large range of settings.
Diffs on the other hand, are highly loaded. Primarily pinion bearings, but to some extent all bearings in a differential are highly loaded in all directions. Add to that, the forces on the ring and pinion are so high that there is some deflection in the ring and pinion. Heavy diffs often have a "spigot bearing" on the pinion inside of pinion gear and many ring gears use a rubbing block on the back side of the ring gear to prevent the gears from trying to deflect away from the each other on heavy loads.
The bearings supporting the differential case must hold the ring gear firmly in alignment with the pinion all while the pinion is trying to push it sideways out of the way! The bearing saddles are large castings and on big truck diffs are webbed 1/3rd the way around the center section, add to that, the bearing caps are fitted with "shoes" that fit into the rear of the axles housings to prevent the saddles from deflection under heavy loads.
Even with all this girdling, there is some deflection, so to keep the bearings from being loose under load, they need to pre-loaded in the relaxed state. Pinion bearings are especially vulnerable, if allowed to be loose, will hammer back and forth when the load goes from power on to power off, rapidly damaging the bearings and gears.
Surprisingly the preload rotational forces don't vary with size of the diff, it is in the 28-32 inch/lb range regardless.
Another reason for the pre load is if set to 0 once the bearing wears a bit it will be loose.
So why am I seeing all these loose bearing? I took my pinion to the local gear shop to have the bearings replace (bearings are not something most autoparts stores stock, and take a press which I don't have). This shop has a good reputation, but they returned the gear with insufficient pre-load.
If you look at a bell curve of bearing failure, it will be skewed toward the tight side, so it is much easier to ruin bearings by being too tight than too loose. The only explanation that I can come up with for these loose bearing is that if you put them in too tight, the diff will fail in a very short time, if you put them in too loose, it will survive long enough to get out of warnentte, so better to error on the loose side. I think it is better spend the time to set up correctly, and not to error at all .
I would so love to be able to "farm out" some of this work, but the times I have tried have ended in disaster. I had one diff explode on I-55 in Illinois all rolling along on the level.