Repairing a chipped gear up against a edge
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  1. #1
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    Default Repairing a chipped gear up against a edge

    Hello all. I'm a entry level person. I recently acquired my first bridgeport and lathe. Anyways I have a gear that's part of a complex shaft that has 6 damaged teeth out of 14. Its for a tractor pto. Im planning on tig welding some filler on tp build it up. Machining it is the issue. I can't use a rotary cutter because the edge. A new part is on backorder for 2 months and keeping the machine down is out of the question. Anyways anyone have any tips or tricks?






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    Since you don't have a gear shaper handy, I am going to suggest hand filing.

    You will probably get lots of advice about how welding is going to destroy any case hardening of the teeth. It's all true.

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    Looks like it was caused by misalignment. If you look at the top photo, see the area in the tooth towards the viewer ? the little splotch towards the root, in sort of the same place where the blown-off area in the next tooth starts ? It's gonna go there next.

    If it's overloading caused by misalignment then welding material on isn't going to fix anything. Not for long, anyhow. And you'll soften up the teeth everywhere ... which might be better, in your case. Wearing out fast is better than having chunks come off ?

    I'd just run it as-is with light loads until you get new parts if at all possible. Maybe some synthetic gear oil ? That stuff works well. Looks like it isn't really chipped, the case has come off in a big chunk from localized overloading due to the teeth being misaligned.

    Course, that's just a guess from looking at a photo but ... when you get the new parts I'd for-sure check the shaft alignment AND BEARINGS (many if not most gear failures are actually bearing failures).

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    What is the tractor? Have you tried tractor salvage yards?

    Personally, I'd just put it back together as is if I had to have the tractor. Then order the part and repair it as soon as it arrives. That failure suggests your part is plumb shot, and welding is likely to make it worse.

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    Dremel tool and a handful of wheels.

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    Are the mating parts damaged also?

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    If you TIG weld it use 312 stainless rod, 312 ONLY! If you cannot find filler rod then buy some Harris Super Missile Weld stick rod and knock off the flux.

    Preheat slowly to just getting brown and weld it up.

    Cool slowly then grind with appropriate equipment. Use the matching gear and Dychem to check for high spots. Done this many times. It's not a forever fix but gets you going.

    From the looks of the wear the gears have been out off alignment, has that been corrected?

    Ed.

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    Quote Originally Posted by atex57 View Post
    If you TIG weld it use 312 stainless rod, 312 ONLY! If you cannot find filler rod then buy some Harris Super Missile Weld stick rod and knock off the flux.

    Preheat slowly to just getting brown and weld it up.

    Cool slowly then grind with appropriate equipment. Use the matching gear and Dychem to check for high spots. Done this many times. It's not a forever fix but gets you going.

    From the looks of the wear the gears have been out off alignment, has that been corrected?

    Ed.
    Stainless rod? Doesn't look like rust is the problem. If not bad bearings then the case is not bored correctly. Stainless rod ain't gonna help either of those conditions.

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    Looks like an important shaft to have done correctly.

    If you can't perfectly shape the replacement metal to match the profile of what is there, you're just wasting your time. If the repair has high spots, it will take too much point load and likely break something. If it has low spots....well, it already has those, so 'filling them in a bit' doesn't do shit.

    And if it isn't the same hardness as what's there, you're just wasting your time. Probably all the teeth have fatigue checking if the gear has been running crooked, so expect more flaking off to occur even without touching it.
    Last edited by HuFlungDung; 06-14-2018 at 09:24 AM.

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    Whatever you do you should also get the part that mates to that on order too.

    Even if it isn't totally destroyed right now, it probably will be after two months of whatever fix you come up with. Which in itself isn't too unreasonable, I've certainly had to allow damage to parts of an assembly that would have otherwise been fine had I not had to keep it running whatever way possible.

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    I welded up a cracked truck diff pinion,as we were in the middle of nowhere,and couldnt leave the job.The weld held for about 1000 miles,but cracking occured where the 312/weldall alloy merged into the hard case.The pinion was preheated to smoke oil.Still,we did the job,drove the truck back to depot,and fitted replacement parts.The oldtimers used to say that to weld up gearteeth,you should grind the entire tooth down into the root,then build up with weld metal.Plenty of dozer bull gears welded up to keep working,for a while.

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    The tooth damage shown is caused by a piece of hard steel jammming in the teeth.....more than likely a broken or chipped bit from another gear....and must be located before running again.....check the magnetic plugs in the tranny.....................I would not attempt welding,you will make things worse.

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    Quote Originally Posted by john.k View Post
    The tooth damage shown is caused by a piece of hard steel jammming in the teeth....
    I do not think so.

    When you get a piece of steel jammed between teeth, it generally causes a tooth to crack all the way across and a portion blows off, leaving an edge that looks like a glass break.

    On this, it appears that a section maybe .050" deep separated from the base material and came off. That's consistent with a failure of the case.

    Generally that's caused by overloading. The load on the tooth surface exceeds the strength of the base material, you get subsurface cracks, the cracks eventually join up and a big chunk of case falls off.

    The bad wear pattern on the teeth also tends toward this conclusion. If the tooth is designed for 50,000 psi but only half the tooth is taking the load, then the area that's engaged sees 100,000 psi. Then the subsurface cracking starts, ending in case spalling.

    Which is what that tooth looks like to me ... but it's just a guess based on a photo ...

    Whether welding is a good fix or not, that's anybody's guess. Nothing is going to amke that thing like new, but is full tooth contact over a softer tooth better, or just leave it as-is and lower the load better ?

    If he runs it the way it is with the normal load, it's going to come apart very fast. If that one place blew the case off, then there's a bunch more all the way around the gear ready to go. Maybe welding would soften the teeth and give a wider contact area, so that might work better for a short-term fix. I've seen softer teeth just smeared all over, that might be better than breaking. Or maybe not. Maybe just leaving it alone and running it lightly would be best.

    It's a disaster looking for a place to happen no matter what

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    Quote Originally Posted by tdmidget View Post
    Stainless rod? Doesn't look like rust is the problem. If not bad bearings then the case is not bored correctly. Stainless rod ain't gonna help either of those conditions.
    The reason for 312 stainless is the it work hardens more and is higher strength than other stainless and seems to "clad" onto the base material without undercut.

    If you have a different suggestion I would love to hear it, I am always looking for better on these jobs, farmers around hear can get creative on destroying stuff.

    Ed.

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    I work on equipment for a living. My opinion is no welding keep the hardness you have dress down the rough stuff smooth and replace other bearings. Definet misalignment. hope the new part comes in soon Your in a tough spot for sure. One thing I would do for sure is plenty of magnets to catch debris. If the plugs got em great but some added to a strainer or some other place where they are easily checked is great.

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    Looks like our OP is MIA.

    It doesn't have to be misalignment, it could be a slight lead angle on the gears. A shaper that was tenths out of square, especially on both gears, could cause that pattern. Improper heat treatment can also cause lead angle changes. Since the damage is more prevalent on one tooth, the problem may be compounded because the pitch diameter isn't concentric to the bearing centers.

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    Quote Originally Posted by atex57 View Post
    The reason for 312 stainless is the it work hardens more and is higher strength than other stainless and seems to "clad" onto the base material without undercut.

    If you have a different suggestion I would love to hear it, I am always looking for better on these jobs, farmers around hear can get creative on destroying stuff.

    Ed.
    thank you for explaining the 312 rod , I have welded gear teeth before and hand shaped best I could , they worked , gotta do what you gotta do when the man says it has to run , make it run.

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    It will never work harden to anywhere close to the case that is/was there. The HAZ will soften what case is there and you will have a soft gear tooth.

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    Quote Originally Posted by tdmidget View Post
    It will never work harden to anywhere close to the case that is/was there.
    Nothing will actually fix this part, I think we all agree. It's a case of what can work best for a short-term limp-home mode. I've seen people use 312 and it does seem to work okay. It's more of a hard brazing method than a welding method, imo worth a shot here.

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    I tested play in the bearings today and used dykem to check the gear mesh. Everything was in check. The pto is in the same cavity as the trans and diff so no telling what got in it. It's been rebuilt before but no one knows what was done or when. After reading everyone's responses I agree with everyone that it's not worth messing with. I'm replacing the bearings, and located a used shaft in Iowa. There is a magnet in the plugs for the trans, they don't do any good when there already full though. I haven't been full time long enough to go through the equipment, id like to say this issue was one off but..... I have anti seize looking gear oil from another machine. Thanks for all the advice.


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