Mojave Issue(s) with 50T spur gear and 16T pinion

[5mall5nail5]

Hey all - been bashing my Mojave with great success for quite a while. A bit ago, I was bashing, and then hear a bad "grrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr" and saw some bits hit the road. I think what happened was the e-clip fell off the pinion gear and or the grub screw backed out, as the pinion came right off in hand, but first it destroyed all the teeth on the stock spur gear.

So, I went and ordered a new 16T pinion gear (ARRMA ARA310963) and e-clip along with a Hot Racing Steel Mod 1 Spur Gear ARRMA 1/8 AON50T. I went and swapped the spur and pinion on, but forgot to tighten then grub screw (sigh) in the pinion and it chomped like 3 - 4 teeth off of the spur gear. SO, I ordered a NEW spur gear along with the Hot Racing aluminum center diff housing stuff because the plastic side was stripped out from installing/removing so many times.

I've got it all installed, the pinion is installed with the grub screw loctite'd and tightened, the e-clip is on, the sleeve is on the shaft, etc. Everything sounds great at like 10% throttle with the wheels off the ground, but as soon as I go like 50% throttle it makes a nasty grinding noise. I broke it all down again and even removed the pinion gear to check the shaft on the motor - it seems straight. I powered the motor with the pinion off and it doesn't wobble or anything. I don't know what is up at this point. Should I order the stock spur gear instead? Everything is labeled "Mod 1" but something just sounds off under high load and I am sure it'll chew up teeth if I put it on the ground and hoon.

Can anyone recommend any next steps? Thanks!

 

[Primarrma]

Have you checked your bearings on the diff cup?

Also what is the condition of your motor mount, is it locking in properly are you getting any movement if you try to rock the can side to side or up and down?

If you install the driveline without the pinion in place and spin the wheels is there any binding or clicking?

If you spin the motor with it not in touch with the spur is it smooth, give it half throttle and listen for any noises?

Are your driveshafts straight?

If you just run the pinion and spur gear together in your fingers do they mesh nicely, or do they catch at any point? Do this with them not attached to the car just run the spur around the pinion or the pinion round the spur?

Also make sure that the grub screw isn't hitting the spur gear, when you mount it all up push the spur back and forth and make sure it can't contact the grub screw.

There should always be a little bit of slack between the spur and pinion, I like to rock the spur gear back and forth, you can do this from underneath the chassis. You should hear a tiny knocking noise when you rock it, do this the whole way round the gear. Rock it, spin it a little, rock it, spin it a little more, rock it and so on, if you have a spot that you don't hear the knocking, you'll have to loosen the mesh I tiny bit. Then test it again.

 

[Sargent_Horse]

Man. I have never seen a spur gear so destroyed. @Notorious J will like this one.

Keep in mind your pinion may be damaged from destroying the 2nd spur gear.

 

[joeshow750]

Damn, just nom noming that spur gear like the cookie monster. Seems like something is moving...here are some possibilities: motor mount has play on the slider, diff bearings are blown/extremely worn down, maybe the plastic that holds the diff bearings down from the top is/are melted or worn allowing the diff to move, the center piece to the diff not fully tightened down, loose screws underneath that go to the actual diff mounting brackets, loose screws underneath to the motor mount bracket, waaay too tight of gear mesh, blown motor bearings.

I would put it on a stand and run it slowly, gradually increasing speed, and see if I can see what's happening.

Edit: One more thing, I would put your diff in your aftermarket mount, slide the motor back off of the spur, remove your two center driveshafts, tighten down the top cover as your normally would, and make sure your diff spins freely in the mount. I have and aftermarket diff mount/motor mount, and I did have to do a fair amount of filing in a few spots to make it spin freely....I wouldn't think it would be enough to brake off spur teeth, but who knows if it's completely bound up.

 

[Engineer]

Ouch. Looks like your spur gear has been doing some meth.

Something is moving way more than it should - definitely check for bad center diff bearings and that the top cover is fully seated (not just on) so that there is no up/down play on the center diff.

 

[5mall5nail5]

Have you checked your bearings on the diff cup?

Also what is the condition of your motor mount, is it locking in properly are you getting any movement if you try to rock the can side to side or up and down?

If you install the driveline without the pinion in place and spin the wheels is there any binding or clicking?

If you spin the motor with it not in touch with the spur is it smooth, give it half throttle and listen for any noises?

Are your driveshafts straight?

If you just run the pinion and spur gear together in your fingers do they mesh nicely, or do they catch at any point? Do this with them not attached to the car just run the spur around the pinion or the pinion round the spur?

Also make sure that the grub screw isn't hitting the spur gear, when you mount it all up push the spur back and forth and make sure it can't contact the grub screw.

There should always be a little bit of slack between the spur and pinion, I like to rock the spur gear back and forth, you can do this from underneath the chassis. You should hear a tiny knocking noise when you rock it, do this the whole way round the gear. Rock it, spin it a little, rock it, spin it a little more, rock it and so on, if you have a spot that you don't hear the knocking, you'll have to loosen the mesh I tiny bit. Then test it again.

Thanks everyone for the feedback. Going to try and reply in order to get some help!

The bearings have been replaced with Fast Eddy bearings on the diff so they are good to go, checked and running true and free.

No clicking or anything with the pinion out of play. Everything seems smooth. The driveshafts have a little wobble in them but nothing offensive.

The two gears mesh nicely but I think I discovered an issue, will post pics below.

Grub screw is recessed below the pinion and doesn't contact the spur gear. It's now also stripped a bit, so I have not figure out how to get that out... sigh.

So doing your test, but out of the car (I held the spur in the aluminum HR diff center, so the spur/pinion/motor are like a unit outside of the truck, there seems to be varying mesh as I rotate it. I noticed this somewhat with it in the vehicle but once removed it was more obvious. Pics below.

Man. I have never seen a spur gear so destroyed. @Notorious J will like this one.

Keep in mind your pinion may be damaged from destroying the 2nd spur gear.

I had replaced the pinion once already for that reason. I have the 20T pinion to try and check with next to see how that does in terms of alignment.

Damn, just nom noming that spur gear like the cookie monster. Seems like something is moving...here are some possibilities: motor mount has play on the slider, diff bearings are blown/extremely worn down, maybe the plastic that holds the diff bearings down from the top is/are melted or worn allowing the diff to move, the center piece to the diff not fully tightened down, loose screws underneath that go to the actual diff mounting brackets, loose screws underneath to the motor mount bracket, waaay too tight of gear mesh, blown motor bearings.

I would put it on a stand and run it slowly, gradually increasing speed, and see if I can see what's happening.

Edit: One more thing, I would put your diff in your aftermarket mount, slide the motor back off of the spur, remove your two center driveshafts, tighten down the top cover as your normally would, and make sure your diff spins freely in the mount. I have and aftermarket diff mount/motor mount, and I did have to do a fair amount of filing in a few spots to make it spin freely....I wouldn't think it would be enough to brake off spur teeth, but who knows if it's completely bound up.

Good call - I did notice the screws that mount axially to the motor that hold it to the motor bracket slider thing, were a little loose. I ended up removing them and replacing with M4 stainless torx plus fasteners I have on hand, all tight now, but still some run-out in terms of mesh.

When I put it on a stand and run it slowly/25% throttle things seem sane, but when I give it the beans, it grinds or makes a noise I do not think is ideal.

Diff spins very well and is held fairly tight in the aftermarket diff mount. The bearing movement in the aftermarket diff mount is next to zero and also spins true and free.

Ouch. Looks like your spur gear has been doing some meth.

Something is moving way more than it should - definitely check for bad center diff bearings and that the top cover is fully seated (not just on) so that there is no up/down play on the center diff.

I agree. And, I think I may be on to something.

I don't know if its a flaw in the spur gear or if the motor shaft is bent or motor bearings are toast, but check this out:

With the "16T" upside down (for indexing reference), check out the tooth gap between mesh:

Notice that there is a decent air-gap between the pinion and spur. Notice too that the pinion teeth seem a little more rounded than I would expect? Perhaps this is from some iteration of things being loose/out of whack, not sure. Now, not moving the motor at all, I rotate the pinion gear about 180 degrees:

The mesh gap is much tighter. I think this is my issue. However, I can't tell what's causing it - but as someone mentioned above, the mesh is moving as it rotates. It seems to be in 180 deg phase to the pinion, so more likely the pinion than the spur gear side as I doubt the spur gear moves in and out of
spec every like 20 degrees of rotation.

By the way, the images above in this post, the differential is fully held by the opposite side bearing bracket. The near (camera) side attahes to the chassis but the other side bottom is part of the motor bracket assembly and so the lid is clamping the diff in place relatively well/tight so the drift/gap movement is not the diff moving around as I am holding it firmly in place.

I need to remove a stripped grub screw (any pointers other than drilling?) and running the motor w/o the pinion and seeing if the motor shaft/bearings are the culprit or what.

Thanks for the replies all - have a 3 year old dying to play again and a 2 week old freshy kiddo, so I've been meaning to reply but haven't had the ability. Appreciate the feedback.

 

[SrC]

Bad quality spur gear. It is not CNC'ed Centric to the diff or you have a bent motor armature shaft. One or the other, or even both is possible.
Mesh in the second picture looks to be perfect and what you want as an example.
Edit:
Spur gears and pinions are not always machined precise, but close enough. The above spur seems Way off though.

I just drill out pin. grubs. It rarely happens for me. I never bricked a pinion in the process, but it is likely to. Need a fine drill bit down the center. You might spare the threads. Or its bricked. Just don't damage the armature's flat while drilling.

 

[Primarrma]

if you run the motor and watch the shaft it will be clear if it isn't true, if you grab the motor shaft and force it left right up down, you will most likely feel the shaft move, it shouldn't. If it is consistent it sounds like the motor shaft is bent?

 

[joeshow750]

I mean typically if its the motor bearings you'd be able to wiggle the shaft at least a little. So if you can't do that then it's probably a bent motor shaft...you can pop it open and look. I wouldn't think it would be a out of tolerance manufactured spur, since it happened with more than one, but crazier things have happened I guess.

 

[5mall5nail5]

Bad quality spur gear. It is not CNC'ed Centric to the diff or you have a bent motor armature shaft. One or the other, or even both is possible.
Mesh in the second picture looks to be perfect and what you want as an example.
Edit:
Spur gears and pinions are not always machined precise, but close enough. The above spur seems Way off though.

I just drill out pin. grubs. It rarely happens for me. I never bricked a pinion in the process, but it is likely to. Need a fine drill bit down the center. You might spare the threads. Or its bricked. Just don't damage the armature's flat while drilling.

Yeah, it seems the 16T spur is just bad. Going to email Horizon Hobby/Arrma to see if they'll send a replacement. I managed to drill the grub screw out (thank god for having a machinist drill bit set with a million incremental bits). I did slightly dimple the shaft despite setting the plunge depth but it's very minor. I haven't bothered to pick the threaded sleeve portion out of the pinion yet that remains because... well, check the video below.

if you run the motor and watch the shaft it will be clear if it isn't true, if you grab the motor shaft and force it left right up down, you will most likely feel the shaft move, it shouldn't. If it is consistent it sounds like the motor shaft is bent?

I stripped the motor down to just the shaft and powered it up - the flat portion of the shaft is too big for me to get any thing on to measure run out but I snuck a piece of 1/16" stainless welding rod on it and cut it about 8" long - there's no to minimal flicking at the other end of the welding rod suggesting it's not "bumping" on non-concentric shaft.

I mean typically if its the motor bearings you'd be able to wiggle the shaft at least a little. So if you can't do that then it's probably a bent motor shaft...you can pop it open and look. I wouldn't think it would be a out of tolerance manufactured spur, since it happened with more than one, but crazier things have happened I guess.

Bearings were good - I pushed pulled axially, and then laterally in all directions. Once I put the 20T pinion on the run out was MUCH improved. Still had a slight low spot in the meshing but I put a screw driver under the pinion and jacked it up just slightly (bending the shaft very minutely) and it's perfect now. So either the shaft had a little bit of a bend toward the end (where the flat starts) that I couldn't really see or measure, or maybe I just seated the pinion better. Either way... the truck drives! It's on the 20T for now until I figure out getting a replacement 16T, but I went full throttle and it makes clean motor noise like before. It's a little misleading because on the bench it seems to make quite a lot of gear mesh noise over like 60% throttle, but also there's a lot of harmonics at that point and stuff vibrating so it's hard to say - it's not like I was doing this regularly to test.

Once I have the drive shafts inserted and the truck under its own power everything seems resolved.

Thanks all!

 

[Primarrma]

Great to hear you've sorted it out.

 

[5mall5nail5]

Great to hear you've sorted it out.

OK so maybe pre-mature. Took the truck out with the 20T. Had a high RPM motor fan on it (that failed, too...) and parked it. Went to push it into the garage once it cooled and it won't move. Motor is locked. I spaced the mesh out and can no longer turn the pinion by hand.

Sigh.

 

[Primarrma]

Seized motor bearing?

Have you checked the front and rear diffs are running free?

 

[Sargent_Horse]

OK so maybe pre-mature. Took the truck out with the 20T. Had a high RPM motor fan on it (that failed, too...) and parked it. Went to push it into the garage once it cooled and it won't move. Motor is locked. I spaced the mesh out and can no longer turn the pinion by hand.

Sigh.

How hot was the motor via temp gun? Could be a seized bearing or the motor may have overheated due to running 20T and a failed fan. Wouldn't be shocked if that was the case.

 

[5mall5nail5]

How hot was the motor via temp gun? Could be a seized bearing or the motor may have overheated due to running 20T and a failed fan. Wouldn't be shocked if that was the case.

Not sure how hot - I ran it up and down the street for about 10 - 15 mins, no jumps/grass/etc. just clean pulls up and down the street admiring how normal the gear mesh sounded and then a neighbor came over and chatted for 20 mins.

Seized motor bearing?

Have you checked the front and rear diffs are running free?

I am going to open the motor today. The diffs are good - everything spins great now that I spaced the motor away from the spur gear. The pinion is seized in place.



Are these Spektrum motors rebuildable? If not, what should I buy?

 

[Sargent_Horse]

Not sure how hot - I ran it up and down the street for about 10 - 15 mins, no jumps/grass/etc. just clean pulls up and down the street admiring how normal the gear mesh sounded and then a neighbor came over and chatted for 20 mins.

I am going to open the motor today. The diffs are good - everything spins great now that I spaced the motor away from the spur gear. The pinion is seized in place.



Are these Spektrum motors rebuildable? If not, what should I buy?

If it got hot enough to disintegrate the motor then it can't be rebuilt. They just don't make motor parts really. Cheapest option would be to just get the same motor from JRC. You could also switch to something like a Hobbywing 1650kV motor if you want the extra power, but it costs more.

 

[5mall5nail5]

If it got hot enough to disintegrate the motor then it can't be rebuilt. They just don't make motor parts really. Cheapest option would be to just get the same motor from JRC. You could also switch to something like a Hobbywing 1650kV motor if you want the extra power, but it costs more.

Wow - just found Jennys RC site. That's JRC right? The motor is like $50 or so. Why the heck was I seeing it at like $150 elsewhere? Was that with ESC I wonder? Crazy!

 

[Robbbbbb66]

Because Jennys tears down new rc cars and sells them for parts. Yes JRC is short for Jennys rc

jennys

amain

 

[5mall5nail5]

Wow that is epic - what a deal. I just ordered a replacement motor and pinion gears for like $60. I ordered. Is there any reason I wouldn't want to do the JRC option though? I can't see why not.

 

[SrC]

When it comes to pinions, try to source the Tekno's. Also RRP, and Saga Customs come to mind. Better tolerances. Yes you pay more. But...... get what you pay for.
JRC has OE pinions very cheap, and could work, just that the stocker pinions have been a mixed bag for me. Like a box of Chocolate. You never know what you will get.
Just as @5mall5nail5 demonstrated.