Thoughts on weld repair of cast gears 1

[KM]

We've got a machine that is almost 100 years old, to be scrapped at most 5 years hence, but to be operational until then. Intermittent service two or three times a month, for a couple of hours at a time, but lots of starts & stops.

The power transmission system is an open spur gear train where many gears have little bits out of them, pieces broken off, and so forth. Mostly on the mitre and bevel gears, the spur gears look mostly OK.

See pic attached. Will post a 2nd one too.

Looks like the original gears were cast, not sure what material, though. In the 1910s could they even do cast steel? I'm guessing it's cast iron.

Some previous repairs were done by welding 5 to 10 years ago, in some type of metal and grinding a new tooth profile. From what I've been reading and from conversations with custom gear fabricators, this is generally a bad idea. Introduces all sorts of weird stress concentrations, possibly micro-cracks leading to sudden failure, etc. etc. They all say this is a repair technique to stay away from.

Yet...the machine has been working. And I only need it to keep working for another 5 years, tops.

So, what kind of risks am I running if I do another weld repair now?

Could we braze-in material rather than weld, and would that be less likely to cause problems?

Constraints are that the machine can only be out of service for about a week to 10 days at a time.

All sorts of thoughts welcome.

 

[KM]

Another picture of the gears.

 

[KM]

And one last one for good measure.

 

[metengr]

Determine the material composition of ther cast gear, qualify a weld procedure, clean and apply a preheat and weld repair, re-profile the tooth, perform final surface NDT and return to service. Is this what you wanted to hear? If so, do it. Some weld repairs work others do not because of speed and loading, that is the risk with gears.

 

[KM]

metengr,

I don't think there is an all-or-nothing answer, and I realize my decision is going to be based on a balance of probabilities. So, no, that was not what I "wanted to hear." There is no particular solution that I "wanted to hear." I'd like a discussion on pros and cons, constraints, things to watch out for, relative probabilities of problems, and so forth.

 

[metengr]

KM;
Good, I am glad you did not have your mind made up. I would have a hard time giving you realistic probabilities when I don't even know the gear casting composition. If this indeed a weldable grade of cast steel, I would estimate a 50% chance of short term success. Since the gear is a casting, low service duty and low rpm welding will result in a cast structure, similar to the original cast gear material so I see no detriment.
The downside to this is a non-weldable gear material or the gear exhibits poor weldability. The result will be premature failure of the weld repaired gear toot and possible collateral damage to the gear.

The weak link will be the weld region in the gear base material. If you can undercut and locate the weld fusion line below the root, this will keep the stresses lower in the weld heat affected zone.

Another option may be to machine the gear teeth and some distance into the ring and manufacture a replacement gear ring that can be shrunk fit and welded. I have seen large gear fabricated in this manner. This could be a permanent solution.

 

[mfgenggear]

KM

This is my suggestion. if possible, again if possible take all the gears. to a bevel gear shop, if time & money permits
reverse engineer them. then try to only repair the gears that are absolutely need repairing.
or take those gears in the worst condition & reverse engineer them.

then if any of the gears fail,or get fubar during the repair procedure then have new one made as a set. so it will match.
Welding will destroy the involute profile of the bevels. do you know how to verify tooth pattern?

to remove small surface defects,reworking/repair
there is a chance that a bevel gear shop can grind them to remove very small amount of stock.


HTH

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[dvd]

How much would it cost in lost production if there was suddenly a failure? Millions? thousands? tens of dollars? Seems like the solution is based on the cost. This doesn't look extremely complicated to just replace with standard gearing and move on. It seems like the gears could be replaced with something from Martin Sprocket (or others, I just happen to know Martin). It's hard to infer a scale based on the pictures, but if you have 1000 lbs of gears, I'd be surprised. Let's say you're paying $10/lb so that's 10 grand (and I'm betting it's less $/lb and less lbs) plus installation costs, call it $20,000 total. I can see that it would take that much to disassemble, clean, repair, NDT, and re-assemble; all with the risk of failure. My vote is to take measurements, determine a solution with off-the-shelf replacement parts and then sit tight. You may never need to do anything, and if it does fail, you are ready to respond with a quick solution.

 

[KM]

Good hints, thanks all.

Note: Failure of the machine could mean liability for flooding a whole lot of private property. Not sure of the cost of that, but there would be more than a few zeros on the end of the number, I'm sure. The more lawyers the more zeroes.

If replacement gears are only in the ballpark +/-$10/lb and if welding is really rather risky, then replacement may indeed be the best approach.

A couple of follow-up questions:

1. If we go with replacements: Can gears be measured-up in-situ, or do they need to be taken out of service for such measuring? Our window of opportunity is only about a week out of service at any one time, although we could have more than one of these periods in the December-to-February period.

2. If we go with welding, composition of the gears is the key. So I was already thinking of getting some NDT done--to find out if there are fatigue problems in the making as well as the damage visible to the naked eye. I phoned around for prices, and phased array UT goes for about $2000 a day and this NDT method does not require cleaning the gears of lubricant; this is looking pretty feasible to me. Could an NDT firm determine composition of the gear material at the same time they're there for the UT? (I'm not sure how one goes about getting composition--is there an NDT way of doing it, or is the only way to cut out a piece of it and send it to a lab?).

3. Can one braze rather than weld? There'd be less heat, hence (I would think) less problems.... Is that even feasible to look at?

 

[metengr]

2. If we go with welding, composition of the gears is the key. So I was already thinking of getting some NDT done--to find out if there are fatigue problems in the making as well as the damage visible to the naked eye. I phoned around for prices, and phased array UT goes for about $2000 a day and this NDT method does not require cleaning the gears of lubricant; this is looking pretty feasible to me. Could an NDT firm determine composition of the gear material at the same time they're there for the UT? (I'm not sure how one goes about getting composition--is there an NDT way of doing it, or is the only way to cut out a piece of it and send it to a lab?).

I would not perform phased array UT. This is unnecessary for what you need to accomplish. I would clean the gear tooth surfaces and perform a wet fluorescent MT. There should be no subsurface defects other than original casting defects which have remained since day 1. PA UT is a very sensitive volumetric examination, you need to focus on surface defects.
Some NDT companies can use PMI to type the material. Unfortunately, it will not provide carbon content. I would remove a small piece from the gear ring and have it analyzed for a complete chemical composition. The cost of the chemical composition will be less than $200.

3. Can one braze rather than weld? There'd be less heat, hence (I would think) less problems.... Is that even feasible to look at?

I would not use brazing for this repair, period. It will not develop the strength you need for service.

 

[KM]

metengr,

Thanks---much appreciated.

 

[CoryPad]

How large are these parts? I think dvd's view on this matter is accurate.

 

[mfgenggear]

I was somewhat on the same wave length, but my suggestion was in case these are not std center distance or mounting distance gears.
then it will be difficult to replace them.The gears appear to be ancient lol.
but if it's std mounting distance then yes there is a possibility of changing gears out.

so then the gear sets needs mechanical inspection.
for center distance, mounting distance, & backlash.


Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[Tmoose]

I can't see much detail in the pictures.

Is the machine rotation reversible? Is a precise ratio needed?
I might look at replacing them with a "timing" belt drive

 

[MiketheEngineer]

Break down and buy a new gear. It will save endless teardowns and headaches.

Been there - done that and it was not pretty. Ended up buying a new gear anyway!!

 

[kingnero]

depending on the application, I am pro brazing.
relatively easy, cheap, and not as intrusive on the base metal as acrual welding.
Also no need to perform a chemistry analysis research (or what it's called in english).

You will need a welding engineer or other, experienced consultant about the type of brazing filler and prep, but I would definitely risk it, again depending on the application (not if it provides fresh air down a mile deep mine with no backup, for instance - in that case, immediate renewal of whole machine).
If the brazed teeth fail, you will still have the gear, with an option of welding.
If you weld and it fails, chances are much bigger it's gonna be entirely broken.

Brazing can be as strong as welding, I've seen brazed samples that were fractured during tests in the base material.

 

[mfgenggear]

OK guys I am a gear mfg. If someone ask me to weld or braze their gears. I would tell them their out of their minds.
I sure it's done, but I would not recommend it. not if it's critical.
I am done

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[kingnero]

While I agree and respect your opinion, I would also tell someone they're out of their mind to re-engineer a machine that's almost at the end of its lifetime with just few hours left to run, only because of a few broken or chipped teeth.

few hours in-house, maybe even in-situ work versus either regrinding/rebuilding the gears, changing to a belt drive or changing the whole drive system.

It's up to the OP to flip the coin as only he knows all the facts...

 

[mfgenggear]

Kingnero

I also respect you opinion. & yes the function of the machine would dictate the corrective action

Take Care

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[metengr]

As an experienced welding/metallurgical consultant I would still not recommend a braze repair for this application because it will not provide the necessary strength for re-building a chipped (missing) tooth. I would leave a chipped tooth as is before I would consider any braze repair.