Hardfacing used sintered gear dogs

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Hi, it's a tough one, if I could fix it by buying new gears I would do it but they fail in a short order.The dogs are rounded under pressure and wear so what I want to do is rebuild the dog face aera, hardface it and undercut the gear dogs so it must remain soft enough to be somehow machinable, soft enough not to chip but hard enough to stay in shape.I read those powder metals are porous so I expect a fair amount of oil (carbon)So I have read and read...but the more I read the more confused I get.What about TIG welding with 309L, I read it does not crack on sintered steel, then do another pass with some hardface rod of medium hardness like those made to fill bearing races ??Weld directly with a hardface rod, if so what content, I know I have far less trouble welding dirty steel with rods that have more copper in it.Thanks for taking timeEDIT : I'm adding a web image of the same gear with matching damageLast edited by gboezio; 10-22-2013 at 11:19 AM.Reason: add image
Reply:You'll need to bake that gear in an oven at a high enough temperature to burn off all the oil absorbed into the pores left in the metal by the sintering process.  If the steel in the gear has undergone and post sintering heat treatment or surface hardening, you may destroy that property.  If the gear teeth are hardened by carburizing, or nitriding, you may find it impossible to weld on them without cracking.  Without knowing anything about the steel chemistry, using TIG and 309 filler metal is a good place to start.  Yes, buttering the sintered part with 309 is a good way to prep for applying a layer of hardfacing.  I'd check with the filler metal manufacturer and make certain that whatever hard-facing you chose is machinable.  Many are not.Something else to consider.  1 - baking all of the oil out of the sintered gear and hardfacing may defeat some of the natural lubrication that is built into the gear.  That oil absorbed into the gear serves to lubricate the gear teeth if they are run in a dry environment.  If the gear is oiled, then you have less to worry about.2 - hardfacing this gear may cause the mating gear to wear prematurely instead.  You may be shifting your problem to the next weakest link in the chain.  Typically mating surfaces are designed to have about the same hardness.  That way both wear equally and evenly in use.  Making one surface much harder will simply cause the other surface to wear even faster.  So unless the system you're working on has a design flaw built in, be wary that you might do more harm than good by trying to hardface this gear.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:Originally Posted by A_DAB_will_dohardfacing this gear may cause the mating gear to wear prematurely instead.  You may be shifting your problem to the next weakest link in the chain.
Reply:Thanks for answers, I have planned to do both sides since both dogs set are worn and when undercutting one side the other must be done to use 100% of the contact aera.I like the baking idea, they may have recieved a post machining heat treatment, maybe I can preheat to 300°F, strike a low power arc to burn off the residue, then bring back to 300°F, quickly weld with 309L, bring back to 300°F then hardface a final pass, this way I should not distort the inside and outside machined surfaces and if there's a heat treatment it should remain intact to about 500°F.It's inside a quad transmission filled with motor oil running to about 200°F
Reply:He is not welding the gear teeth. If you look around the center hub of the gear there are 4 dogs that engage another part of the transmission. What looks like is happening is that there is some tolerance between the two parts and the corner of the dog is taking the majority of the stress instead of it being distributed along the whole contact area between the two parts. I am assuming that the mating part also has some wear or is out of spec which would explain why these gears are not lasting very long, either that or this thing is being pushed way beyond its stock capacity.Can you give us a little more info on what the parts in question are doing, just for our own curiosity, we may be able to help determine why parts are wearing so fast.
Reply:Originally Posted by ironmangqHe is not welding the gear teeth. If you look around the center hub of the gear there are 4 dogs that engage another part of the transmission. What looks like is happening is that there is some tolerance between the two parts and the corner of the dog is taking the majority of the stress instead of it being distributed along the whole contact area between the two parts. I am assuming that the mating part also has some wear or is out of spec which would explain why these gears are not lasting very long, either that or this thing is being pushed way beyond its stock capacity.Can you give us a little more info on what the parts in question are doing, just for our own curiosity, we may be able to help determine why parts are wearing so fast.
Reply:Ahh, I see now.  Thanks for explaining. Originally Posted by ironmangqHe is not welding the gear teeth. If you look around the center hub of the gear there are 4 dogs that engage another part of the transmission. What looks like is happening is that there is some tolerance between the two parts and the corner of the dog is taking the majority of the stress instead of it being distributed along the whole contact area between the two parts. I am assuming that the mating part also has some wear or is out of spec which would explain why these gears are not lasting very long, either that or this thing is being pushed way beyond its stock capacity.Can you give us a little more info on what the parts in question are doing, just for our own curiosity, we may be able to help determine why parts are wearing so fast.
Reply:Now that I know this is a transmission gear from an ATV, I'm willing to bet you a cup of coffee that the gear teeth are induction hardened or carburized, nitro-carburized, or nitrided.  If you want to know for sure, take a file to a scrap gear and see if it will cut.  I expect the file teeth will skip right off those gears.  If the file won't cut, you are in for a difficult time trying to fix these.  If the teeth are induction hardened, that will be localized, and you might find that the dogs are not.  In this case, you may have some success.  If the whole gear is case hardened, then you're going to have problems with cracking and spalling of the metal in and around the weld and HAZ.You'll have to bake the whole gear, not just heat it with a TIG torch.  First, I'd soak the gear in acetone and then run through a parts washer with a good degreaser.  Then you'll have to take the gear above the smoke point for motor oil(above 300F-400F) and hold it there for several hours to burn off all the remaining oil saturating that gear.   Just locally heating the dogs will only result in oil from another part of the gear wicking to the hot spot.  This will make for poor welding with porosity and a high probability of cracking. As far as distortion goes, I think you're going to be disappointed.  The localized heating from welding on those dogs is going to make for some distortion on at least the ID of the gear..You're talking about hardfacing a relatively small area and then machining it back to original dimensions, and backcutting the dogs to prevent the problem in the future....Are you sure it's cheaper to do all this work than it is to buy a new set of gears from Suzuki?Good Luck.  Let us know how this turns out. Originally Posted by gboezioThanks for answers, I have planned to do both sides since both dogs set are worn and when undercutting one side the other must be done to use 100% of the contact aera.I like the baking idea, they may have recieved a post machining heat treatment, maybe I can preheat to 300°F, strike a low power arc to burn off the residue, then bring back to 300°F, quickly weld with 309L, bring back to 300°F then hardface a final pass, this way I should not distort the inside and outside machined surfaces and if there's a heat treatment it should remain intact to about 500°F.It's inside a quad transmission filled with motor oil running to about 200°F
Reply:Thanks for a very helpful post, well we have two sets of damaged gears so far, we may give it a try to see the outcome, if it become junk at least we would wave learned something.An alternative may be to undercut a worn set and see how it holds.A good start would be to know what I have first, if sintered steel is case hardenable, then my bet is that it's the method of choice, there is almost no nitrided parts on those bikes.  I will go get them when I have a minute and try to scratch them.
Reply:I'd take it to a foundry and have a master mold cast from it. They will use that to make a lost wax mold and cast as many as you want out of a harder steel. You could even sell them to others having the same problem.Gizzmo
Reply:That is a strange design.To me, it looks like the design allows so much back-lash that the drive dogs get a severe pounding every time the machine goes from drive to coast. Naturally that quickly pounds the dogs out. In the picture it looks like power and coast sides both took about an equal pounding.While the backcut will help in one direction (either drive or coast) it won't help in the other.Does it need that much slack to crash-shift or something?I'd be tempted to look for a way to tighten it up.Good Luck
Reply:Originally Posted by gizzmoI'd take it to a foundry and have a master mold cast from it. They will use that to make a lost wax mold and cast as many as you want out of a harder steel. You could even sell them to others having the same problem.Gizzmo
Reply:What you want to do with minimal machining capabilities is nearly impossible so if I were you I would look for new parts.It is what it is.Welding is nothing...Machining is something.Even if you could do it....zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:Depending on how and why the rounding is happening this might be something helpful. I have a lot of big power drag racing motorcycles in my shop throughout the year. Their 4, 5 and 6 speeds share a similar design in the gear-dog configuration but they're back cut much different. Instead of the driving faces that are cut on your pictures a drag racing transmission has the dogs back cut on the tall part of the crowns. So instead of having to have the entire crown of one gear go completely past the crown on the mating gear then drop to catch the face of both, the race gears have the crowns ramped. This way it give them a better chance of engagement at a high rpm and less deadheading. In other words once one gears dogs just clear the mating gears dogs it can start to close up on it's gap separating the two as the tops or crowns are tapered. These bikes are all air shifted and usually between 50 and 90 milliseconds. Deadheading happens 1 out of 100 passes maybe and I've confetti'd my share of transmissions but rounding the dogs has never been the cause. A surface grinder and indexer is what we use to back cut them.
Reply:Originally Posted by zapsterWhat you want to do with minimal machining capabilities is nearly impossible so if I were you I would look for new parts.It is what it is.Welding is nothing...Machining is something.Even if you could do it....zap!
Reply:Something of this nature must be ground into shape..Carbide tools will just chip and shatter....zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:You will notice that the dog only partially engages inside of that gear 50% like you said but that has to be to allow enough clearance to prevent two gears engaging at once.  If it's just this gear that is causing you problems and you don't work the adjacent gear too hard you could consider re-shimming the transmission so that gear gets fully engaged and the other does not.  You can also re-time the shift drum as well.  That would be fairly easy to do using a piece of bar stock to make a new shift drum from scratch.
Reply:The dogs are not cut at an angle from top of trailing edge to the floor of the non drive side. On a big power foot shifted bike I like them back cut at around 15 degrees. On an air shifted bike 30 degrees is works good. Bending shift forks is not a problem on XL or FX transmissions. It does happen on Orient Express and Zippers race case gear boxes though but those are automatics that a rider shifts. Meaning it's in first gear until it's air shifted into 2nd. 2nd gear pulls it out of 1st, 3rd pulls it out of 2nd, 4th out of 3rd etc. They are never in between gears. You can roll out of 5th without disengaging the drive but that's it. otherwise it's a timing thing with getting off the throttle and pulling in the clutch to avoid bent forks.  If you want to try and change the profile of yours yourself either on the top or the drive side, Zap is right and you're not going to do it on a Bridgeport with an end mill of any kind. You can do it with a few stones though. I have done it with a similar gear just as you first posted but it wasn't on a tranny gear. I did it with a dovetail stone. I did it in Las Vegas in 2006 in the pits with a die grinder and a cut off wheel on an FX air shifted 5 speed but we were going rounds at the final race of the year. It worked and got us through the day but it was not the method of choice.