Author Topic: Repairing rounded gear dogs  (Read 4115 times)

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Offline vmxrider

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Repairing rounded gear dogs
« on: December 15, 2009, 10:44:49 am »
Is this possible? Bike is jumping out of gear under load, so assume dogs and/or shift forks are buggered. Thinking of hard welding the rounded corners and grinding back to original spec with a bit of undercut. Anyone done this with any success and reliability. 

Offline Colin Jay

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Re: Repairing rounded gear dogs
« Reply #1 on: December 15, 2009, 11:52:10 am »
I haven't welded gears, but back in the 80's I repaired a few gearboxes which had rounded off dogs by grinding a slight undercut on to them.  I can't remember any real problems with them after the gears were undercut. I can't remember how many degrees of undercut I put on the dogs, but if you undercut them to far you can have trouble with them not wanting to disenage easily, which can result in the action of the gear slectors feeling a bit stiff, especially if you are trying to do quick shifts without the clutch.

To grind the dogs, I made a jig which allowed me to mount the gears into a chainswaw chain sharpening grinder. This allowed me to set the depth, and angle of grinding in two planes. You have to be quite accurate and a fair degree of care needs to be taken when undercutting the dogs to make sure that all the dogs on a particular gear end up the same size and shape and that the match up accurately with the dog on the gear that they engage with. If the job isn't done correctly, you will end up with the distinct possibility of just just one dog taking all of the load. You can then end up with the overloaded dog braking off of the gear and if you are unluck jamming the gears and trashing the entire gear set.

CJ 
Why do things the easy way, when with a bit of effort you can really make it difficult for yourself!!

DR

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Re: Repairing rounded gear dogs
« Reply #2 on: December 15, 2009, 01:12:13 pm »
I'd be taking the gears to a reputable tranny shop and getting the dogs backcut. Welding or building up the rounded corners sounds like a lot of work for nothing. The dogs themselves do not have to be a set width, it is the leading edges that is of importance so therefor undercutting or back cutting would be far easier. Depending on the model it may also be possible to find nos gears for less than the price of a few hours labour. I had a few gears on my rm400 done many many years ago and they've been great ever since. Replacing the fork if it shows any sign of wear or scoring is a must also.
« Last Edit: December 15, 2009, 01:13:56 pm by Doc »

Offline Lozza

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Re: Repairing rounded gear dogs
« Reply #3 on: December 15, 2009, 03:54:15 pm »
Can have the dog and the mating slot undercut by a gearcutter for $75. Welding gears is not recommended at all. All 'points' on the dogs and mating slots must be equaly spaced, very hard without a rotary table/cnc .Otherwise there is 'point loading' ie 1 dog or slot is slightly forward of the others takes all the load and rounds off.
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Offline vmxrider

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Re: Repairing rounded gear dogs
« Reply #4 on: December 15, 2009, 08:48:31 pm »
Hmmmmm, think I'll be chasing down some replacement gears. Still nice to know that as a last resort reworking  buggered dogs is a possibilty. 

Offline jimg1au

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Re: Repairing rounded gear dogs
« Reply #5 on: December 15, 2009, 09:56:39 pm »
HI
i have back cut (undecut) second gear on 3 xs1100 yamahas with no problems.to this day a mate is still rideing around after 50k klms on the gears.xs11.com web page has a detailed drawing of what to do.some were on my home pc i have the info to undercut cr250 gears as well
cheers
jim

Offline holeshot buddy

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Re: Repairing rounded gear dogs
« Reply #6 on: December 17, 2009, 12:06:58 am »
where are you located i know where you can get
the gearbox undercut in victoria and queensland
its not that expensive
i got my 125 box done for a couple of hundred bucks ;)
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