Tuesday, 18 January 2011

Building a ball diff

Racing 8th scale for last few years (and with a break from RC for a few years before that) it was a shock to the system when I had to build a ball diff for the Ansmann. I figure the last one of those I built was about 2001 in an HPI touring car. Before that I'd built them into everything from 2wd and 4wd off-roaders, 'wider' touring cars (remember them?) and my favorite Pro10 pan cars. I'll say straight off the bat that I don't consider myself any kind of expert at this, and I may well not be doing this the best way, but here's part one of my diff building guide. If you want to put me right on any of this, feel free to comment below... I'll be happy to hear from you!

Diff ring prep
If you're using top of the line diff rings, like those from BFast for instance, you may be able to skip this step. The rings in the picture below are stock Ansmann, and could do with a little bit of prep work before use.

The left ring is being preped, the right ring is raw from the box

I like to make sure that the working surface of the ring is as flat and smooth as possible, but not polished. I figure the flat surface ensure an even pressure across all of the balls, working them evenly and avoiding tight spots. To my mind this gives the balls an easier time which should improve lifespan and time between rebuilds, and should also help to get a nice smooth diff action. I don't polish the rings however, as a little bit of texture gives the balls something to grip as the diff spins.

I take each ring in turn, mount it to an out-drive and then gently work it over wet and dry paper fastened down to a nice flat sheet. I work the piece in sweeping cycles, rotating it as I go to get an even finish, and work from 600 through to 1500 grit. What I'm looking for is a near mirror finish, hopefully the picture above shows this.

Once I'm done I use a good degreaser to clean off the rings and out-drives. Wynn's Clutch and Break cleaner is pretty good as it come in a spray can and so as well as clearing any grease it also blows off any debris from the sanding process.

Basic Assembly
Now the rings are ready to build into the diff, and everything is cleaned up I'll start the assembly. First up a bearing is fitted to each out-drive, and then the rings are fitted. I'll now give the rings a very light coat of diff lube on both sides by smearing it around the ring using my finger.

Taking one of the out-drives and standing it on end I can place a ring (preped side up) on it, and then the main gear, ready to receive the diff balls. In this instance I'm using the kit steel balls as this is a spare unit and they were all I had to hand. For choice I would use carbide balls from someone like Rudebits or RCLazy as they well help build a harder-wearing, smoother longer lasting diff. Just make sure you fit the right number and size!

To lube the balls I place a small amount of diff lube in the palm of my hand followed by the balls. I then very gently roll the balls around in the lube using a finger, being careful not to let the balls clatter into each other.

Putting a light coat of diff-lube onto the main balls

Once lubed, the balls are ready to load into the diff. This is the fun part if you have big, thick fingers like me as getting the balls into the holes can be a pain. I use a pair of plastic tweezers and quite a lot of foul language, and in the end I seem to get the job done. Back in the day of Pro10 racing we used to decrease drag in the diff by fitting every other ball, but that seems out of fashion now from what I can tell, and to be fair we did end up rebuilding diffs a lot back then, so now I'll completely fill the diff.

The diff gear with bearing and fully loaded with diff balls

The Thrust Bearing assembly
Thrust bearings come in two main types these days, caged and uncaged. The Ansmann item shown below is of the caged type, where the diff balls are contained in a cage and sandwiched between two races. The alternative design is very similar, but the balls are separate and not contained in a cage. I find the latter considerably harder to build, but they do seem to last longer in my experience.

Diff screw and nut, thrust bearing parts and grease. Note the caged bearing and races.

To assemble the bearing is pretty easy, I slip one race onto the diff screw, ensuring the concave track around the race is facing up. I then load the race with a good dollop of Team Losi Hi-Pressure Grease and drop the caged bearing on. Another layer of grease is added, and finally the top race, track facing down. Finally I'll wipe off any excess grease, although I do leave plenty on there in this case as the Ansmann diff seems to like it.

Non-caged thrust bearings are similar, but trickier to work with as there is no cage to hold the balls, you end up relying on the grease to hold them in the race whilst building the diff. The thrust balls are also tiny!

Bringing it all together
Now it's time to finish up building the diff. Fit the top out-drive and ring to the lower half/gear assembly put together earlier, and the drop in the spring and dif nut. If the case of the Ansmann the diff the nut is held in a plastic piece that compresses the diff spring, and has a pair of wings that lock into the out-drive to prevent it spinning during adjustment of the diff.

Fit the diff screw in from the opposite side ensuring the thrust washer is in place, and the tighten the diff screw into the nut. You want to aim for the diff to be just tight enough that the gear will not turn if the out-drives are held stationary. I check this by fitting hex wrenches into each of the out-drive slots and holding them in one hand and trying to turn the gear. If it turns easily, tighten it a bit at a time until it no longer turns. If it won't turn when you first try, back the screw out a bit at a time until it does, and then tighten it just a little to stop any movement. Now gently check the diff action by hand an make sure it all feels good and that the diff is working as it should.

The diff screw and a diff ring fitted to an out-drive ready to attach to the other half


In a future post I'll cover how a break in and make final adjustments to my diff, and also look at the role and adjustment of the slipper clutch.

If you want to comment on anything in this post, please use the comment link below.

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