Eating gears ... is there a reason?

[Petroholic]

Had my RR Evo3 for about 6 months, and not had this happen until quite recently. The car is stock apart from replacing the rather yucky stock air-filter with a complete Baja filter setup, and the recent addition of a jetpro copy tailpipe. Servo trays and centre diff supports are all alloy too, but the gear carrier is still stock.

Ran like this since christmas with no problems, until a few weeks ago after fitting hte pipe and getting the tuning right. First run with the pipe was fine, but the tuning was off and she was well down on power. It was just a quick run to make sure everything was sealed up and the mounts I had made were OK. Second run, I tuned the thing properly and she was going great until I lost drive. The 2 smaller plastic gears were mashed. I checked the shafts and they were quite warm, so I have added some extra venting to the shell as a precaution, and got hold af a pair of replacement gears ( thanks Clive ). I have also got a set of steel gears now which I want to fit, but wanted to test drive first with the plastic to make sure all was well.

Took her out this morning, and gave her a shakedown run with the replacement plastic gears. Had checked the transmission was ok as far as I could, and could turn the drive to the centre diff by hand easily. Same with the middle shaft. Everything felt OK, so off I went to the park for a test run. 10 minutes of sideways sliding and 4-wheel drifting on some nice dampish loose ground, throwing debris around and generally 'giving it some' and drive went again. Off with the shell .... and another set of gears had gone to the great gear carrier in the sky....

Now....it could be the extra power from the pipe/carb combo that has overstressed the gears, or it may be something else doing this.... but I want to try and find out what might be causing this before I put on the steel gears I have. I know they are much stronger than the plastic ones....but if I have a failed bearing or transmission problem at the root of my stripped gears, then I could end up wrecking the steel ones too...and that I just cannot afford to do.

Help.....

[dgav407]

The plastic gears do benefit from a few minutes of slow running to bed them in...don't worry I will sent you another set in the week. 

Check the bearings on the gear carrier plate, especially the ones in the middle. If there is too much play the central layshaft will move about underload and potentially cause the inner plastic gears to run tight and eventually fail. If you need any bearings I have some you can use.

The most important thing is to ensure there is sufficient air flow under the bodyshell to keep the temperature down, running the engine slightly rich helps as well.

Clive

[Petroholic]

Temperature wise, it's nice in there I have removed a bit more window, and the gears and shafts were all cool to the touch. Last time, they were a bit warm! The plastic gears were a bit warm, but that was where the edges had been sliding over each other. The centres were not warm to the touch.

I will give the bearings a good check over, and also make sure the things don't have any play. The meshing seemed OK when on the bench...but there is no guarantee it stayed that way under power.

Next test will be run with the shell off.... which I CAN do now the ground has started to dry a bit.

This is a bit frustrating, but I will get to the bottom of this. It's doubly frustrating to have the car running so well and not be able to use it. Ah well Thanks again for the help Clive!

[monotheist]

What ratio of of gears are you using fo A, B C & D positions? Just wondering if your using a slightly incorrect ratio causing the mesh to be slightly out. Remember  gears A & B should always have a total tooth count of 63 and C & D 49. Only once have I ever stripped a set of plastc gears and that was when I accidentally ran an incorect ratio.

[Petroholic]

Ratios in there were 27, 36, 24, 25 ..which is how it came when I bought it. Ran it like that for a few months before it ate it's first set of gears.

Having stripped down the car, I did find a bit of play in the centre layshaft bearings. The shaft was visibly moving from side to side when pulled and pushed. Probably about 1mm shift each way ... engough to make a difference to the meshing. Pull a gear 1mm out of the right mesh and it won't help! Also, bearings like this should turn pretty much silently if in good shape....not make nasty scraping noices.. Not loud ones...but scraping. I reckon dirt or moisture has got into the bearings and they are on the way out.

Have another set of bearings on the way and will put it back together with those and give it another try. Didn't bother with 'official' bearings as the MCD ones are stupid money. Instead have bought 2RS double rubber shielded bearings which are sealed for life. Fingers crossed, but I think this is the root cause. Thanks to Dgav407 and HarryHackenbacker for help diagnosing the problem!

[Petroholic]

Thanks for the help ( both parts and e-mail advice froom DGav and Harry ) coz it looks like I now have a solution.

With the new bearings installed, I took it for a run this weekend and all went well. I ran it gently for the first 1/4 tank just to make sure everything was bedded in. I kept hte shell off so I could watch the gears and pick up any early signs of problems, and I stopped regularly to inspect. After that, the shell went back on and the speed started to increase. After about 1/2 tank gone, I started to give it some full throttle and got her sliding again. The ground was quite dry and dusty, and by the time I was feeling more confident we had donuts and sliding with 4 wheel drift, flicking into turns under power and all the usual fun. By the time I was done with tank number one, the gears were unmarked and I was a happy bunny. I didn;t get much more running done, as I paid the price for running without shell in the dust, and clogged up the airfilter. Normally I carry spares ( must get an outerwear cover fitted....but they didn't have the colour I wanted in stock!) so I can just swap the outer on the Baja filter I use, but no matter. I ran her long enough to know the gears were OK. With the old bearings installed, I got no more than 10 minutes run time before hte gears destructed.

So...the problem was actually quite simple. the centre layshaft bearings, particularly the outer one, had worn enough to allow the shaft to move and put the gears out of mesh. With the cogs only half making contact, any hard on the throttle driving was enough to overstress them and mash the teeth down. It's something I will make sure I check often as it's a 5 minute job to swap the outer bearing. Next time I have the motor out, I might also take a Dremel to the outer casing and relieve it a bit so I can slide out the centre layshaft without having to loosen the motor. It's in the way otherwise!