Ramblings on building differntials


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Golden Jubilee
Golden Jubilee

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Post Sat Nov 04, 2017 9:44 am

Ramblings on building differntials

Over the course of my life being a mechanic and now as an owner/operator of a truck, I have rebuilt so many diffs that I can't count the number. The biggest difference is now I am building them for my own use, and they are mostly old enough that I am not the 1st one to break them open.
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.

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Post Sat Nov 04, 2017 11:03 am

Re: Ramblings on building differntials

Informational and food for thought. I used to deal with some of this when I was an army Mechanic. So long ago.
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Golden Jubilee
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Post Thu Nov 23, 2017 12:14 pm

Re: Ramblings on building differntials

My boss runs dump trucks/pups and lowboys in the woods working on steep and often soft roads. We usually break teeth long before bearings become an issue in 40k rears and the trucks with 46k rears almost never have a problem. We've got a gear shop nearby that does good work.
'52 L-160 dump truck
'57 RD-405
'58 Allis Chalmers D grader

Golden Jubilee
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Post Fri Nov 24, 2017 11:27 am

Re: Ramblings on building differntials

Do all pinion shafts have a "crush sleeve", CB? If not how was that overcome in early diffs? Should crush sleeves always be replaced? On pinion seal replacement my shop usually does not.
Ebay has any number of old diff. setting tools for sale. Usually Kent-Moore or another "dealer" tool. How do you set up pinion pre-load and pinion to ring gear adjustment?
Great stuff! Tell us more!
My last two diff stories are related to the owners wanting to chrome out the housings and the added copper/nickel/chrome plating throwing all the tolerances off. A mess!

Golden Jubilee
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Post Fri Nov 24, 2017 4:47 pm

Re: Ramblings on building differntials

Heavy diffs use a solid spacer(selectable fit) in place of a crush sleeve. You put the bearings in the pinion bearing housing (a separate piece that bolts into the center section). so if you have a press handy, you don't even need to put the bearings on the pinion to select the correct spacer. You assemble the bearings with the spacer in between and press to the spec'd tonnage, and check the rotating resistance. Once the correct spacer in known you can assemble the bearing on the pinion.
You don't use shims behind the bearings like on light diff's the whole pinion carrier is shimmed to the center section to set pinion depth.
Although not 100% correct, crush collars can be expanded by placing on a iron rod and striking the center of the expanded portion to make the collar longer again and allow it to be crushed on assembly.
On heavy diffs you set for best tooth pattern and don't worry so much about getting it quiet. You can do the same on lighter diffs if you can stand the noise.
In most cases, if you are just replacing bearings, if you go back with the spacers in the same location, same size as came out you'll be OK. If you are changing gears you can work backwards and find base settings from the thickness of spacers and the +/- setting etched in the old gear then work forward from there with the new gearset setting.
I ran heavy haul with 40K diff's and while I was able to wear them out, they never failed. I knew from the play that they were shot and pulled from service before they failed.
45K diffs and 40 K diffs are the same center section with different side gears, 34K and 38K are also the same as the 40 and 45's, again with different thickness housings and side gear splines. 46K are the same as 50 and 56K so have a bigger ring and pinion. My 40K's have a 16 or 16.5" ring gear where as the bigger ones are 17, 18 or bigger.
What causes teeth sheading is breaking and than suddenly making traction (shock-load) my 38K rears had no trouble pulling 170K lbs on asphalt, but ice or mud would easily tore them up if I allowed a wheel to spin than catch.
Real heavy haul diffs use planetary hubs for the final reduction so the ring and pinion only do intermediate reduction and the gearset and axle shafts don't have to be as big, the hub does the final reduction so nothing has to carry the full torque except the hub and wheel.
It is why 2 spd rears are preferable to and aux trans or deep reduction 18 spd, where all reduction passes thru the whole drive line.
If I were buying new and could afford to do so, all my trucks would be fitted with 2 spd tandems. I love the set of 2spd I built for my Marmon.

Golden Jubilee
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Post Sun Nov 26, 2017 12:41 am

Re: Ramblings on building differntials

cornbinder89 wrote:Heavy diffs use a solid spacer(selectable fit) in place of a crush sleeve. You put the bearings in the pinion bearing housing (a separate piece that bolts into the center section). so if you have a press handy, you don't even need to put the bearings on the pinion to select the correct spacer. You assemble the bearings with the spacer in between and press to the spec'd tonnage, and check the rotating resistance. Once the correct spacer in known you can assemble the bearing on the pinion. . . .

Wow, great tip! I hope you're around if/when I ever need to repair my Loadstar axles. I'm sure there isn't any mechanic in my city I would trust doing that work.
I've only rebuilt a couple in my lifetime - just little 1-ton differentials. The last time, probably 30 years ago, it was a Dana 70 front axle I needed to change the r & p on to a 5.13 ratio to match the 5.13 in the rear on my truck. At that time I had a publication from Spicer about 20 pages long that showed every step with photographs. I still have the heavy spreader bars they said to use to expand to insert the side shims. I remember later someone telling me they use a set of bearings that have been cut out in the center a few thou so they could slide them on and off until they get the pattern exactly right with the shims, then they press the good bearings on. I don't know if that is true, or would work, I just remember being told that.

I know I will need to replace the pinion seal in my diff as its beginning to seep oil. Can I just remove the u-joint yoke, replace the seal then re-torque the u-joint yoke or will this upset any adjustment (pinion depth or bearing preload)? My truck is a '69 Loadstar 1700 with only 15,000 original miles. I've never worked as a mechanic - I'm just a self-taught monkey who finds if I'm taught how, can do a better job than having someone else do the work for me on my trucks.

Thanks.

Golden Jubilee
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Post Sun Nov 26, 2017 9:55 am

Re: Ramblings on building differntials

In most cases, unless you are changing the gearing, its just a matter of putting the spacers back where they came. Medium and heavy truck diffs don't use a spreader, the center drop-out type, they don't use bearing spacers either (except the pinion bearing) they use threaded bearing adjusters, so it is easy to set back-lash and pre-load. The adjusters are move with a big bar inserted into the notched adjuster ring.
To set-up a big diff, you take a stab at what pinion bearing housing shims you think you will need (again, go back with what came out) install the carrier with the ring gear and set to 0 pre load with the correct back lash and roll the gear pattern and see what you have, If not correct, remove the carrier, and the pinion with its housing, if the pinion was too deep then add shims , too far out remove shims, re-assemble and roll another pattern. Once the pattern and back-lash are correct (remember the carrier pre-load is at 0 to make thing easy) you leave the bearing adjuster on the ring gear side of the ring gear carrier where it is, and tighten the one on the pinion side to pre-load the bearings. There are many ways to measure the pre-load, one of which is by how much it spreads the "shoes" where the bearing saddles slide into the rear case, but if you have done a few, you can gauge it by feel. Most often it only needs 1 notch from 0, may be 2 to get plenty of pre-load. After pre load is set, then re-roll a tooth pattern and check nothing has changed.
On a pinion seal replacement, take a paint stick and mark the nut and head of the pinion, remove the nut and flange, replace the seal, re-install the nut to the pain mark. This is for pinions with crush collars(light duty stuff), for medium and heavy diffs, just tighten the nut back to spec, the solid spacer will not allow the pre-load to change.
Tip, if you have a lathe and some tubing stock, you can replace a crush collar with a solid spacer and it will be good for the life of the gearset. Measure the crush collar and make the spacer a few thousandths bigger to start, a few trial and error to get it to the pre-load required, skimming a little off the face until it give the correct pre load. Crush collars are used to make it fast and easy for them to assemble on an assembly line, they are not required to be crushable for any other reason. If you had the time, you could make up several spacers, varying the thickness by one thousandth and just try a few until the correct pre-load. It is not recommended to make the spacer smaller, then shim it out with shim-stock and the loads are high and may tear the shim-stock in operation.

Golden Jubilee
Golden Jubilee

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Post Sun Dec 17, 2017 11:54 pm

Re: Ramblings on building differntials

Hi,
Interesting, as always. There's just no substitute for experience.
In another post there was discussion in what I had to do, and the seals I would need, to convert my RA-25 axle in my LS 1600 so the diff oil would lube the wheel hub bearings too. That way I would not have to repack them.
My question is this; will the larger axle in my LS 1700 also need to be converted so the diff oil lubes its wheel bearings too? This axle is a single speed and uses Dayton type hubs. It's considerably larger than the RA-25 axle, but I can't tell you the axle model or code number because there is no line setting ticket available for the truck, per the Wisconsin Historical Society. The truck was apparently manufactured in Canada for a fire department, then became owned by a rancher in Montana before I purchased it. Is there anything published anywhere that shows axle model identification by the appearance of the axle or its housing webbing?
Thanks.

Golden Jubilee
Golden Jubilee

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Post Mon Dec 18, 2017 9:25 pm

Re: Ramblings on building differntials

IF you can get an oil seal for the hub, then remove the rubber seal on the nut and the oil will flow out the tubes to the hub.

Golden Jubilee
Golden Jubilee

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Post Tue Dec 19, 2017 2:16 am

Re: Ramblings on building differntials

Seems simple enough ... in theory anyway.

On tapered roller bearings, such as wheel bearings, for example, I was always taught to preload the bearings. For example, tighten the wheel nut to 50 ft lbs while spinning the wheel, back off then re-tighten again to about 15 ft lbs for cars and 20 to 25 ft lbs for trucks, depending on the size. I was told it was better to have too much pre-load rather than not enough. And the example given me was a piece of chalk tapped on a blackboard, simulating the free-play in a set of bearings, will soon crumble but that same piece of chalk can be pressed against the blackboard with force and not crumble. And he took the same approach to diffs - too much preload was much better than zero preload.

Because of my limited experience, I like to use a torque wrench for everything I can. How much final pre-load do you recommend for wheel bearings? And based on your experience, what is the optimal diff pre-load for side bearings and pinion bearings? I'm sure I will never use my LS 1700 to its full weight-carrying capacity, but I do plan on taking it on long trips and want the piece of mind that the thing won't explode apart on a 10 hour, 65 mph trip. I suspect my Spicer aux overdrive trans will push it at faster speeds than it was originally intended.
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