MTFCA Forum

So this is the second time this has happened to me... so I can’t be the only one?!?!

The first time was in my speedster, and so I figured it had something to do with all the burn outs and hill climbs and my oversize right foot. But this time it was in a stock touring car??? Granted it’s an A crank motor with some “improvements”... but this motor has only been it the car for 100+ miles
I would love to say that it was all my fault, and my motors are to blame... but it seems like there is some other variable here that I am missing...?

Common threads here are that these both had the 39 tooth 3:1 ring gears, all the ring gear bolts were broken, and a Ruxtell axle. Not sure who rebuilt this Ruxtell, but both were new Mark Auto gears with new bolts

I can’t help but believe that it has something to do with the counterbored ring gear, and the resulting reduction of thread engagement on the bolts. The threads in the ring gear are relatively undamaged, while the bolts are absolutely destroyed. I have found broken ring gear bolts in my speedster Ruxtell before (always with 3:1 gears), and took some pretty radical precautions to correct the issue (now with 3.63:1 gears). The touring car will go back together with 3.63:1 gears too

HOLY CR@P BATMAN! I think your investigation of cause is about right Kevin. If shims were used could have caused harmonics if not fully coated with a Loc-tite type of compound. Just an observation, trying to help.

Hank

Kevin
It would be interesting to do some testing of the ring gear bolts. I have been suspicious of some of the reproduction bolts offered for the T, which is why I use name brand Grade 8 bolts for connecting rods similar when I install a BIG crank in a T block for main bearing bolts.
I appreciate you posting about this as I have just built a 13-40 Ruckstell that want to install a powerful engine to. I might change out these bolts!!

I agree BIG TIME with Les! Faulty bolts I watch like a hawk.

I know the Ruxtell ring gear bolts are shouldered and are notably “special”... but the thread is standard, and the shoulder is meant to pilot the differential bell/planetary gear/thrust plate assembly. I seem to recall that Chaffin’s began correctly reproducing these bolts years ago, as the replacements at that time were inferior. I don’t necessarily suspect that the “bolt” in its original design or heat treat is failing, but more so suspect that the reduction of thread engagement could be the issue. Could it be that reducing the thread engagement could cause the threads to yield prematurely...? Maybe the problem is really happening at assembly?!

Look at how the tooth root is gashed into some of the bolt holes. Is it deep or does it just look worse in photos than it really is? On a standard 40-tooth gear they don't do that. Is the fillet at the root sharp? Do you think the problem originated at the hole near 12-o'clock that appears to be torn? Is there any evidence on the backside of the gear that it was rubbing on the carrier before it failed? It's interesting. Thanks for sharing.

Walter Higgins wrote: ↑

Mon Jun 08, 2020 8:18 pm

On a standard 40-tooth gear they don't do that

The 39 tooth ring gear is noticeabley thinner, and causes the gear teeth to intersect with the bolt holes. And it is not uncommon to grind down a few bolts to clear the pinion either. I do not believe that this is the failure mechanism tho. Once the bolts fail...the ring gear is at the mercy of fate...

I have stripped the teeth off a few but have never seen that. Nice work!

Seriously, sorry for your loss.
Rich

I would say that something you are doing, or something to do with your power train is causing the ring gear to flex and eventually to snap. I've heard of lost teeth, but the cracking of the gear in so many places would tell me it is not being supported equally all the way around.
I know of one car which used to be in the San Diego area before the owner passed away which had a clutch like a Model A and at least 3 speed transmission with Ruckstell. He also had an overhead conversion to the engine and the car was very fast and pulled hills very well. When the stock T's were going uphill about 25 MPH, he would pull around them at about 45. Twice that I know of he lost the ring gear. Once was in a very embarrasing situation. He had the mayor or governor in his car when the Coronado Bridge was dedicated. And it happened during that ride and he had to be towed! Anyway, if you increase the power of the engine and gearing, for power and speed, you are putting an additional strain on the rear end.

Norm

Kevin Pharis wrote: ↑

Mon Jun 08, 2020 8:51 pm

The 39 tooth ring gear is noticeabley thinner, and causes the gear teeth to intersect with the bolt holes. And it is not uncommon to grind down a few bolts to clear the pinion either. I do not believe that this is the failure mechanism tho. Once the bolts fail...the ring gear is at the mercy of fate...

I've never had one of these to study so please humor me -- what is the point of the threaded holes being counterbored? It looks like there are only two or three threads.

Walter Higgins wrote: ↑

Mon Jun 08, 2020 9:14 pm

I've never had one of these to study so please humor me -- what is the point of the threaded holes being counterbored?

The entire ring gear has a counterbore to allow the gear teeth to overhang over the carrier and allow additional room for the larger 13 tooth pinion. The individual holes are not counterbored

If you have a race car, you drive it hard on Saturday night and work on it every other night of the week...

Is it possible that the root of the issue is that the parts are operating significantly outside of their original design parameter?

Original Ruckstel 3 to 1 ring gear bolts.
When the last one let go the ring gear simply moved away from the pinion gear.
All that happened is that I had "No Go"
And of course "No Brakes"
 

Rich Eagle wrote: ↑

Mon Jun 08, 2020 9:00 pm

I have stripped the teeth off a few but have never seen that. Nice work!

I would have proudly displayed overpowered mechanical carnage. But sadly... this is not the case

BE_ZERO_BE wrote: ↑

Mon Jun 08, 2020 9:33 pm

Original Ruckstel 3 to 1 ring gear bolts.
When the last one let go the ring gear simply moved away from the pinion gear.
All that happened is that I had "No Go"
And of course "No Brakes"

AH HA!!! I’m not alone!

I’m intrigued to see that all your bolts are both stripped and broken..?! My bolts weren’t all that different... exept much less consistent failure. Some broken at the thread, some broken up mid shank, and some simply stripped

I wonder if using studs and nuts might work better. Avoid torquing the studs into the gear. Potentially Loktite the studs into the ring gear. I haven’t explored this idea yet!

When my speedster shattered the first ring gear, I was determined to never have this issue again. So I machined an extra set of threaded holes in the ring gear, and drilled and counterbored for socket head cap screws in the bell. It may be a touch of overkill, but at the time I saw the long bolts holding the planetary assembly together as part of the problem. Now the ring gear is attached to the bell with short bolts, and the long bolts simply hold together the planetary assembly. If nothing else... if the long bolts break again, the planetarys will fall apart, but I won’t break the ring gear!

I remembered a discussion about this several years ago so I went searching this is what I found. Jim

http://www.mtfca.com/discus/messages/29 ... 1187143172