Gear partially stripped?

[Kol__ 3377]

Hey guys,

Recently stripped down and rebuilt a fairly heavily modified Madcap, the first real RC tinkering and rebuilding I've for a couple of decades. I rebuilt the diff and gearbox. Upon inspection the largest rear cog (someone can tell me this correct name I'm sure) which the pinion gear connects with is I think damaged?...

Do we think perhaps a past fitted pinion gear slipped back and took the edge off one side of the gear. The damage does look awfully accurate though, the 'cut' so to speak is very sharp and consistent, the rounded inside edges you can see in the pic above are just lined with grease. I'm a bit puzzled on this one

 

[Champ85 609]

Yes, it looks like the pinion gear was not aligned correctly so only a couple of millimeters of pinion teeth was engaged with the spur gear teeth, plus the pinion was not fully meshed with the spur so only the tips of the teeth were engaged.  With a powerful enough motor and the right initiating factor (a jump or sudden shift from reverse to forward, etc.), the pinion stripped the tips of the teeth of the spur gear and as the car was moving, the rest of the spur gear teeth were carved away as the pinion acted like a saw blade, spinning much faster than the spur was rotating.  The result is what you see.

The good news is that I think the spur gear is still good enough to be used.  Just make sure the pinion is fully engaged with the spur for the entire width of the spur teeth and also make sure the pinion is as close to the spur as possible so the teeth mesh fully while making sure there is just a hint of play (movement) still possible by the spur when you move it back and forth by hand.  You want just a fraction of a millimeter of space between the pinion and spur teeth.  When meshed correctly, it should not be loud or grindy sounding. Some people use a piece of paper between the pinion and spur, but I find this often leaves too much space between them, so I just do it by eye and sound.  Sometimes it takes multiple adjustments to find just the right mesh.  And of course, grease them lightly.

[hIghQ 149]

Some teeth only seem to be missing only a very small part, not even at the very edge. Might have been a tiny little stone got stuck in the pinion and then starting wreaking some havoc?

[Willy iine 18805]

Looks like sand got in the mesh.  

[Kol__ 3377]

Thanks guys, yeah I thought it didn't look right!

It is serviceable and appears to be working ok. Good bit of info about the pinion to spur gear set up there @Champ85, I wasn't aware there needed to be a smidge of play between them. I was snugging the pinion up as close as poss, I'll now review and adjust, cheers

I think a new spur gear will be on my to buy list, just to get it back how it should be!

[blu-tak 3]

3 hours ago, Kol__ said:

I think a new spur gear will be on my to buy list, just to get it back how it should be!

Others have said it's saveable. I would follow their advice.

Carefully re-mesh and give it a run.

[Kol__ 3377]

Agree @blu-tak, it's certainly useable and has been running ok. However, for me that's not quite good enough long term. I'm not too precious about this particular car, but I'd like to have it all tip top underneath the hoodCheers

[hIghQ 149]

4 hours ago, Kol__ said:

Good bit of info about the pinion to spur gear set up there @Champ85, I wasn't aware there needed to be a smidge of play between them. I was snugging the pinion up as close as poss, I'll now review and adjust, cheers

That's actually quite an important lesson you have learned there. I still get asked by some of my club mates to check their gear play (especially on their on-road cars with the very fine 64dp gearing) for them.

[Silver-Can 402]

It's not clear from the photo but I would check/replace your pinion gear too, especially if it's the soft tamiya alloy original. 

On a side note, when I did a light resto job on my madcap 5+ years ago, finding new spur gears was somewhat tricky. 

[Big Jon 684]

Chipping like that occurs when the set screw contacts the spur. That super appears to be well worn, but the damage is probably from the set screw. It must have sounded awful.