Gear mesh keeps changing on XMAXX.

[Mrgoodwrench35]

I've had this weird problem with my Xmaxx for a while now and can't seem to figure out the cause. Been ongoing for months and it's starting to drive me a little nutty. I'm a master technician by trade and have a pretty good understanding of all things mechanical (or so I thought) but this is kicking my butt.

My problem is that when I set the gear mesh by adjusting motor position on my M2C mount, the mesh changes as I roll the motor. Gets tighter and looser with rotation.

I first noticed this after I jumped about 10 ft in the air (pretty tame in comparison to some of the jumps it's lived through) and landed in what I thought was a soft grassy field. Turns out, covered up by the grass was a very large rock which I landed perfectly on top of. The impact damaged the rear bulkhead, more specifically, the section that houses the center diff.

After the impact, I drive off, everything seems fine other than an odd sound. Turns out the sound was a piece of the damaged bulkhead rubbing on the center diff teeth where the bulkhead was pushed into it on impact.

I replaced the rear bulkhead with a new Traxxas part, reassemble, and find this gear mesh issue I described above. I assume something else bent on impact, tear back down to inspect and see nothing wrong.

I've since replaced the spur assembly, kush drive shaft and all, rear diff, center diff, and last night tried a different motor to no avail. Mesh still changes with rotation. Judging by the frequency of change with rotation, the problem seems to be in the center diff area as the rate of change matches up to center diff rotation.

Forgot to mention I've also replaced center drive shaft assembly with M2C 2 piece shaft. There is a good amount of flex room in this center drive shaft setup so I'm certain that the issue isn't coming from the front diff.

I'm running out of ideas. Anyone have any input? I can provide pics or video although I'm not sure that would help.

 

[tntpoof]

i only read the first 2 line of this post. you cold have 2 issues. bent motor shaft ot bearing are off as in if you measure bearing from inside shaft mount edge it will be different when you rotate bearing

 

[Greywolf74]

My first thought was also bent motor shaft. Also check motor bearings as TNT mentioned and check your motor mount and mounting screws.

 

[Mrgoodwrench35]

All the bearings in the rear section were replaced as well as the motor. I was running a hobbywing 1100 kv and swapped in an arrma spectrum motor just to eliminate the motor shaft.

 

[Greywolf74]

All the bearings in the rear section were replaced as well as the motor. I was running a hobbywing 1100 kv and swapped in an arrma spectrum motor just to eliminate the motor shaft.

have you confirmed that pinion and spur arent damaged?

 

[Mrgoodwrench35]

Did I mention this problem shows itself right away. Not that I run the car and the mesh changes. Just rotating it by hand I can watch the mesh go tight and loose, back to tight.
Pinion and spur were replaced as well.
This is driving me batty

 

[Greywolf74]

Did I mention this problem shows itself right away. Not that I run the car and the mesh changes. Just rotating it by hand I can watch the mesh go tight and loose, back to tight.
Pinion and spur were replaced as well.
This is driving me batty

Hmmmmm, I swear I had a kit at one point that was doing that same thing but I dont remember what it was or how I fixed it. I want to say (IIRC) that in my case the fluctuation wasn't enough to cause an issue so I just picked a middle ground with the mesh and ran it like that. I'm not sure how much fluctuation yours is getting. Any chance you can get a short video clip of what its doing? Maybe someone might spot something that you're overlooking? Maybe not but worth a shot. No other suggestions are coming to mind.

 

[Mrgoodwrench35]

The mesh change is severe enough that if I set the mesh correctly at its tightest point, then it becomes too loose at its loosest point.

I know my description is lengthy. I tried to only include details that were important. I'm sure it's just something silly I am overlooking. I'm just stumped.

I'll take a video of what's happing later and see if that helps.

I considered taking it to my LHS and seeing what they think but frankly, the majority of guys that work there are idiots that don't seem to like or participate much in the hobby. They are rarely helpful.

I'm to the point where I'm about to buy another Xmaxx just to swap parts out one at a time to prove out the issue. Crazy right? But that's where I'm at

 

[Greywolf74]

The mesh change is severe enough that if I set the mesh correctly at its tightest point, then it becomes too loose at its loosest point.

I know my description is lengthy. I tried to only include details that were important. I'm sure it's just something silly I am overlooking. I'm just stumped.

I'll take a video of what's happing later and see if that helps.

I considered taking it to my LHS and seeing what they think but frankly, the majority of guys that work there are idiots that don't seem to like or participate much in the hobby. They are rarely helpful.

I'm to the point where I'm about to buy another Xmaxx just to swap parts out one at a time to prove out the issue. Crazy right? But that's where I'm at

desperate times call for desperate measures lol. i wish I had an idea but I'm coming up blank. Something is allowing the motor and/or pinion to move side to side or something is letting the spur gear do that. What I can't say and I've never owned an xmaxx so I dont know the rig very well.

Have you considered tearing the whole thing down? I know youve replaced quite a few things but maybe a complete tear down may reveal some damage somewhere that isnt being seen?

 

[WickedFog]

What I would do... as an ex machinist, is get a test indicator and find exactly where the runnout is occuring. It could be a number of things, but you have replaced everything I would suspect to be the culprit.

Is their any play of your pinion to the motor shaft? If there is, tightening the set screw would push that gear off-concentricity with the motor shaft. But you said it seems to coincide with the spur rotation, so you will need to chase it down.

I would remove the spur and reassemble without it if possible. See if you can detect any shaft wobble. If so, where does it stem from? As mentioned, are the bearings good?

 

[Mrgoodwrench35]

What I would do... as an ex machinist, is get a test indicator and find exactly where the runnout is occuring. It could be a number of things, but you have replaced everything I would suspect to be the culprit

I wanted to do the same thing. The runout gauges I have are for larger scale machines so I checked what I could. The runout condition is occurring at the spur shaft. The gauge I have is too big to go any deeper into the drive train. The pinion, spur, spur kush drive housing, shaft, pins and bearings all new. Some twice over. I speculate that it's coming from the center diff area as the peak of the mesh change at the spur occurs at the same white dot I marked on a center diff tooth.

Is their any play of your pinion to the motor shaft? If there is, tightening the set screw would push that gear off-concentricity with the motor shaft. But you said it seems to coincide with the spur rotation, so you will need to chase it down.

None. Both motors I've tried (castle 1100 and hobbywing 1100kv both have D cut shaft. I've tried arrma pinions that are D cut and an assortment of other non D cut pinions with the same result.

All the bearings I've replaced with a fast eddy set and the problem was there before and after.

I've had the truck fully disassembled enough times I could put on a clinic. I jump dumb stuff and break it a lot so I'm pretty familiar with the repair side.

I've replaced the center diff, rear diff, rear ring and pinion. I have enough spare parts to almost fully assemble a rear bulkhead.

 

[WickedFog]

That's a real brain teaser then. Can you post some vids?

 

[Mrgoodwrench35]

I posted a video here

 

[Donnie533]

I would guess that the transmission input shaft is bent (part # 7786X - Input shaft, transmission (cush drive)).
This is the shaft that the spur gear is attached - your problem is definitely rated to the spur and not the motor shaft.

 

[Mrgoodwrench35]

I thought the same thing. Seems like the most likely culprit as it seems it would bend pretty easy. I replaced it as an assembly early on when I first noticed the problem.

I'm going to tear it down again when I get home tonight and see what I can figure out.

 

[Donnie533]

I thought the same thing. Seems like the most likely culprit as it seems it would bend pretty easy. I replaced it as an assembly early on when I first noticed the problem.

I'm going to tear it down again when I get home tonight and see what I can figure out.

check the bearings on this shaft as well. I found the rear bearing on the shaft had failed and caused the bulkhead and cover to melt a bit, which threw out the mesh. Bulkhead could be cracked as well. I had no problems after replacing bulkhead and cover.

 

[WickedFog]

After watching the vid, it's hard to see any wobble. The mesh is clearly changing, but it's hard to say if it is all coming from the spur.

The first thing I would do is put a dial indicator on the pinion teeth, and check how much run out it naturally has with the motor pulled away from the spur. Even just a few thousandths of run out there can be a bit problematic, and very hard to see.

Then, with the spur disengaged from the pinion, reapeat the run out test on the spur.

Then repeat both tests on each gear with them meshed.

You may have a slight bit of run out combining from a few different places.

A test indicator with a mag base would be the best way to sort this out, but decent ones are over $250. But they do come in very handy for stuff like this. So here is a cheap setup...

KHCRAFT Professional Dial Test Indicator 0-0.03"x0.0005" Steel Hardened All Metal Components with 2 Steel Dovetail Clamps 3/8'' and 5/32'' in Storage Case https://a.co/d/a8T77RW

HFS (R) Magnetic Base Adjustable Metal Test Indicator Holder Digital Level 14" - Tool Stand https://a.co/d/4uBnMfX

 

[Mrgoodwrench35]

The first thing I would do is put a dial indicator on the pinion teeth, and check how much run out it naturally has with the motor pulled away from the spur. Even just a few thousandths of run out there can be a bit problematic, and very hard to see.

Then, with the spur disengaged from the pinion, reapeat the run out test on the spur.

Then repeat both tests on each gear with them meshed

This is what I did after replacing the spur shaft and it still wobbled. Read at the spur, motor removed, and the deviation is present. I can't read it any further back unless I cut up a bearing cap.

Keep in mind the bulkhead is new, twice.

I have a bunch of Snap On dial test equipment made for 1/1 gas engines. I think I could make it work if I 'modified' the spur shaft cover. I have an extra plastic one. But I also thought throwing a few parts at it would be easier. Turns out, not so much

I really appreciate everyone's input here. Such a simple little problem has kept me from playing with this beast for a month or better.

 

[WickedFog]

Would be kinda discerning to find out the spur itself has run out between its center bore to the outside diameter. At this point, I would say remove the spur from the assembly and test the shaft run out as it is installed in the vehicle. If you have to buy a sacrificial cover to hack up to check different areas of the assembly, that's where I'd be heading.

 

[Mrgoodwrench35]

Would be kinda discerning to find out the spur itself has run out between its center bore to the outside diameter. At this point, I would say remove the spur from the assembly and test the shaft run out as it is installed in the vehicle. If you have to buy a sacrificial cover to hack up to check different areas of the assembly, that's where I'd be heading.

The spur has a little bit of runout due to the kush assembly being, in my opinion, poorly designed. But that play has always been there and its manageable.

I ended up mounting up the spur assembly in a vice first to verify it wasn't the culprit. All good there. I pulled out another new bulkhead that I forgot I had and mounted the spur assembly into it with nothing else installed, factory plastic cover cut open so I could read shaft runout and nothing there.

I then installed 1 assembly at a time into the new bulkhead (center diff, rear diff, then the motor) and read runout at the shaft after each new piece was added. Guess what, no runout. Everything works fine when I install all the same parts into a new bulkhead.

At that point, I'm confused. The bulkhead I had used before was brand new in the plastic when I installed everything and noticed the gear lash issue.
So I put all the components back into the suspect bulkhead and sure enough, runout at the spur shaft is present.

So I got a bad brand new bulkhead? What?!?!? That's a first for me. I've looked it over and visually see nothing wrong with it. This 20 dollar part robbed a whole lot of my time.

Reguardless, the beast lives. I'll be taking it out (and probably breaking it again) at some point today.

Thank you all for the insight and helpful suggestions. Its all very much appreciated. If any of you are ever in KCMO, hit me up, beers on me.