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speedy_w_beans

Experimental TT01 Sealed Gear Diff

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In anticipation of a future TT01R Type E rally conversion project, I've put a few hours into thinking about how to seal the TT01 gear diff so different silicone oils can be used instead of packing the diff with more or less anti-wear grease. The following pictures and description are for a version 1 prototype, but I think it's showing enough promise to share with the community. Feel free to comment back; I'm very open to feedback and suggestions.

In the first photo the sealed gear diff components include:

- Ring gear with enlarged center hole

- Standard TT01 bevel gears and spider shafts

- Diff cup with enlarged center hole

- Drive cups with machined recesses for O-rings

- Gasket for sealing the ring gear, diff cup, and screws

The TT01 ring gear and diff cup I had measured 0.312" OD and 0.200" ID where the drive cup shaft is inserted. The drive cup shafts measured 0.195" OD and 0.135" across the flats that key into the bevel gears. This only provides about 0.005" of clearance between the hole and the shaft, not enough for a regular Tamiya damper O-ring. I went on the hunt for some thinner O-rings at my local hobby shop and hardware store, but there wasn't anything readily available. As a compromise, I opted to use the regular Tamiya damper O-rings as shaft seals, but that required removing extra material from the shafts and gear/cup holes so the O-ring could fit reasonably well.

The second photo shows a TT01R drive cup with a machined recess for the O-ring. Since the Tamiya O-rings measured about 0.072" in thickness, I needed to create much more than 0.005" of clearance between the shaft and hole. Eventually I settled on a recess that measured 0.135" OD and 0.100" wide, centered between the cups and flats. I figured if the flats measured 0.135" across, then reducing the shaft diameter from 0.195" to 0.135" for the O-ring should be acceptable without compromising too much shear strength (yes, there is less cross sectional shaft area for the O-ring as compared to the flats, but it seemed worth trying like this). The clearance was then 0.0025" + 0.030" per side of the shaft. I tried compressing the O-ring, but it was almost impossible to fit in the gear or cup hole. So, the next step was drilling out the holes with a 15/64" drill bit, increasing the hole diameter from 0.200" to about 0.232" (measured). This added about 0.016" of clearance to each side of the shaft while reducing the wall thickness of the hole from 0.056" to 0.040". This didn't seem like too much material to remove, and it provided an overall clearance of 0.0485" per side between the shaft and hole. I figured if the O-ring compressed from 0.072" to about 0.048", it would probably grow sideways to about 0.096" width, which would fill the recess and cause it to stay centered during operation. My second attempt to install a shaft in a hole was successful, but it took a little pushing with a small hex key and working around the perimeter of the O-ring to get it to slide inside the hole. Lubed with #500 shock oil, the movement seemed smooth.

The third photo shows an assembled diff. With a shaft, O-ring, bevel gear, and spider gears installed in the diff cup, I filled it about 3/4 way with #7000 weight gear diff oil. Compared to #400 weight shock oil, this stuff was really sticky and slow to flow. I had cut a crude gasket out of a nitrile glove (measured only 0.0025" thick), but it was difficult to handle and would move out of position each time I tried to put the ring gear/shaft/O-ring/bevel assembly on top of the diff cup assembly. The fingers on the ring gear that mate with the slots on the diff cup would deform the gasket and cause it to move, which meant I wouldn't get a good seal if I installed the screws at that point. After a few attempts I gave up, and decided to make a paper gasket by photocopying another diff cup and then cutting out the detailed image. Since the ring gear was already wet with diff fluid, I stuck the paper gasket to it and pre-positioned the slots so I only had to align the gear to the cup. It all went together much more easily with a detailed paper gasket. A few screws later, and everything was tightly assembled.

Right now with #7000 fluid the diff feels like a freshly built standard TT01 diff that's been heavily packed with AW grease. I'm expecting the diff won't loosen up with time, though, since the fluid will continue to flow around the gears instead of getting squeezed and pushed out of the way like the grease. I plan to build another one with #3000 fluid to see if there is a perceivable difference in diff action. I've had the diff sitting in front of me, lying on its side, for a few hours and haven't found any fluid underneath it yet. It probably needs to sit for a few days and then driven hard just to be sure.

The final photo shows how I machined the recesses in the shafts. This is a poor man's lathe; I clamped a drill to the workbench, chucked the drive cup, partially clamped the drill trigger, and used a square hobby file that measures 0.097" to 0.100" per side to grind away the material. I'd periodically stop and check with a caliper, adjust pressure, and continue. Even though it was very uncontrolled, I was able to maintain the target OD to within 0.001" to 0.002" across the width. After grinding away the material I'd make a pass with 400 grit sandpaper to semi-polish it.

Future improvements:

- I'd like to find more appropriate O-rings and adjust the clearances and tolerances to reduce the shaft side play and the friction between the shaft and hole. If I can find some O-rings that are 0.030" in thickness instead of 0.072", I could avoid drilling the gear and cup holes and just focus on machining the shafts with narrower and shallower recesses.

- The gasket made from a nitrile glove was too crude and made assembly difficult. I'm not sure if a paper gasket is going to work long term. If it works, I'll just keep photocopying diff cups and cutting out the images carefully. I may need an alternative; it's too early to tell.

- While the poor man's lathe worked for now, it sure would be nice to have something more controlled.

Thanks for reading; I hope you enjoyed this post. Comments and questions are always welcome.

-Paul

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Paul,

I think you've made a very good job of this considering the tools you have available. I probably would not be able to put recesses on a drive cup with much better accuracy with my micro lathe than you have managed with the power drill and file. It's always nice to see people trying this kind of innovation themselves. I look forward to hearing how it works in practice. I've just ordered some 1 way roller bearings to try and make a one way unit for my hotshot :)

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Looks like the paper gasket is seeping a little bit -- the ring gear teeth are starting to look just a little wet. There aren't any drops of oil yet, but this looks like something that will drip a drop or two over a period of weeks. I'll have to go back to the nitrile glove and take more care fabricating a real gasket. The O-rings on the shafts are sealing just fine, however.

Also, I ordered some different metric O-rings from "The O-Ring Store." I should be able to fit these on the current shafts and then use an unmodified ring gear and diff cup, improve the side play, and reduce the friction.

Version 2 is in progress...

-Paul

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a simpler way of sealing the ring gear/main dif case is to use Permatex ultra black RTV- it is the most oil resistant gasket maker made

I simply didn't put any lube in the dif and used large inner diameter washers on each yoke gear and it tightened right up, is loosening up good, and still has no slop after 6+ 30 minute runs with a 15t motor. I also switched the ring gear around by grinding out the tang in the chassis

both wheels spin free like a normal open dif if they need to but other wise it is locked

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Hi Mad & Zac,

Thanks for the encouragement and ideas. RTV is a good alternative; I went down the nitrile/paper gasket path just to make it easier to disassemble the diff in the future. As a matter of fact, I cut a new nitrile gasket two days ago and installed it, and there hasn't been a leak since then. So, I'm encouraged by the results with a nitrile gasket and O-rings on the shafts.

I ordered different O-rings two days ago as well, and much to my surprise they showed up in the mail today (Christmas Eve). Sometime early next week I should have a complete version 2 diff including machined drive cups with smaller recesses, new O-rings, new nitrile gasket, and standard ring gear/diff cup/bevel gears/spider gears. It should have less friction and less side play, and it shouldn't leak based on lessons learned with the version 1 diff. I'll post pics and details when it's complete.

-Paul

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Just a quick update. I built version 2 of the diff today and now have it sitting to test for leaks. The shafts have .020" deep by .050" wide recesses cut in them, and the plastic ring gear and diff housing are no longer modified in any way. I reused the same nitrile gasket and gear set inside the diff. The smaller O-rings with less compression have reduced the friction considerably, and now it's easy to tell the effects due to the diff fluid. The side play of the shafts is also reduced, but there is enough compression of the O-rings to continue sealing even under worst case conditions. I used 3 mm ID O-rings with 1 mm cross section. When stretched over the recesses, the cross section reduces a little bit but the shaft is still guaranteed to seal even when pressed to one side.

So, standard shafts with .020" deep x 0.050" wide recesses machined into them, 3 mm ID x 1 mm cross section O-rings, .0025" nitrile diff housing gasket, and standard ring gear/diff housing/bevel gear set should yield a usable, sealed gear diff for the TT01. The plan is to build a final set and use them in a TT01R Type E rally conversion project.

The only remaining enhancement I can think of right now is to very lightly countersink the ring gear and diff housing holes to make O-ring installation just a little easier.

-Paul

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