View Full Version : Temporary gear repairs

01-30-2012, 03:20 AM
I have a cat30 right angle drive and it has damaged gears one has five teeth missing and the other about seven appear to be well chewed.
I can obtain replacements from the manufacturer but the lead time is horrendous, I need to complete a small job for a customer and I was wondering if the damaged sections could just be built up with a mig and then carefully ground to shape .
I realise that they wouldnt be hard but if I only take light cuts it should let me finish the bush.
I bought this unit second hand and have no idea of its history except it looks like someone really "crashed" it.

I have only repaired large gears previously and they worked in industrial situations they are easier to weld and cut to shape.

These are about an inch and a quarter in diameter with about an inch of gear in width .

Any thoughts ?

01-30-2012, 03:47 AM
It may be beyond the scope of what's required for you to get your job done, but I've repaired gears for people before by TIG welding up the damaged teeth, then making a fixture to pick up the gear by a couple of the good teeth, and wire edm'ing the damaged section back into the correct shape.

If done correctly, the result can be as good as the original gear. If you happen to have that resource at your disposal, it may be an option.


01-30-2012, 05:43 AM
Unless finish is gonna be critical, Go for it. Lots of people have done that and had to file the teeth into shape. We have achient gears with square teeth still turning today.

It might not be the quietest gear drive, Or the longest lasting, But it should work just fine untill a better replacement can be obtained (if needed)

01-30-2012, 08:26 AM
I fixed a gear on a vertical bandsaw's gearcase back in 1997. Still working fine. Here's what I did:

Note: just one tooth was missing in this case

This text describes in detail how exactly to fix a broken gear tooth on a helical gear with a couple of bucks worth of JB Weld and about ten cents worth of set screws.

I decided to drill and tap 3 holes, put in set screws, rough grind the set screws to approximate shape but slightly smaller, and to cast in place epoxy putty to form a new gear tooth over the 3 pins made from set screws. I figured the epoxy putty would bind to the remaining threads of the set screws, and that the tough metal the set screws are made of would sufficiently strengthen the new tooth.

I mounted the gear on its shaft, which was 3/4" in diameter, and put the shaft into a 5C collet fixture.

I put the collet fixture in the bench vise. I took a very small fine file and filed the broken tooth off flat. The file I used had no teeth on either side, so it didn't mar the unbroken teeth.

After filing the tooth flat, I scribed a line down the middle of the tooth. This line was 10 degrees from the axis of the gear, which is the helix angle. Then, I center-popped 3 centers by eye. I then took the fixture over to the drill press, where it bolted down to the table. I located the holes with a wiggler, and drilled and tapped for (in my case) 4-40 set screws. This actually was the most difficult part. I find it difficult to get a very small drill bit to start where I want it, and this time was just the same way. Regardless, I got 3 usable holes, and they tapped just fine. I always use high quality taps - they cut better, and breaking a tap in this application would have been extremely difficult to recover from.

I then Loctited in the 3 set screws. I only tapped them about 3 threads deep (maybe 1 diameter) because I didn't want to weaken the gear body.

Then I cleaned some teeth on the opposite side from the break until they were as clean as I could get them. I cut a couple pieces of scrap black sheet metal, maybe 1" x 5", approximately 1/16" thick. Not too flexible. I cut out some rectangles of wax paper, and carefully folded them around the metal strips, taping them up the back side. Then I took the gear outside and lit my oxyacetylene torch, with just a little bit of oxygen, which generates a very yellow, sooty flame. I played the flame directly over the gear teeth and got a lovely velvety black soot layer. Then I took the gear back inside, and clamped the metal strips over the cleaned, sooted teeth, and mixed up some JB Weld, and gooped it in with a little piece of sheet brass cut just a little thinner than the gear. I covered about 5 teeth. The JB Weld says it sets up in 4-6 hours and cures overnight. After 6 hours, I put the gear in the vise, took a suitable block of metal, and gave the molding a robust tap from the side. It shattered!

Chastened, I cleaned up the teeth again, and tried again. What I figure I did wrong was to put on too much JB Weld, worked it in too much instead of letting it run in, and tried to remove it before it was strong enough. Putting on too much caused it to flow sideways over more teeth, getting a better grip. Working it in overmuch mechanically removes the soot, which acts as a releasing agent. Anyway, the second time around, I got a piece which spanned 4 teeth and came out perfect.

Then I got out the Dremel tool and a solid carbide burr, and went to work on the set screws. I didn't care very much about the final shape, I just wanted to leave as much meat as I could while allowing a reasonable amount of epoxy to cover. I used the mold to test the fit until I was satisfied that the pins were ground down enough.

Then, I took the same sheet metal strips I used when I made the mold, and again applied wax paper, this time to only one of them. I "sooted" the mold I'd made, again with the torch, and clamped the one side piece back onto the gear. I applied some JB Weld, then set the gear on its side (the 5C fixture has a 90 degree body, so you can work the piece either way) with the strip down. I had discovered that the JB Weld flows with gravity before setting up, and I wanted to use that. So, I put a "bottom" on with the wax-papered metal strip. Then I carefully pressed the sooted mold over the JB Weld, clamping it into place. It was gratifying to see the surplus putty run out the top, forming a nice little drop which would then tend to run down and fill any interstices before hardening. The next day, I took off all the clamps and the side piece, and carefully filed away the small excess amount of JB Weld. The advantage of letting it go out the side is that the side of the gear is a flat surface, which is very easy to machine. One of the guys who had given me advice on the Internet had suggested that you drill holes in the top of the mold, and put both sides on. I didn't want to fiddle around with cutting off any "sprues", or having them in any significant way impede me removing the mold.

Anyway, the next day, the mold came off cleanly, leaving a perfectly-formed tooth. It was actually not even very obvious, since the soot bonded with the epoxy to turn it black just like the gear.

Good luck, hope this helps, the above would work on 5 teeth as well as one.


01-30-2012, 08:27 AM
And before I did that gear fix I did research (as you are doing now) and found that more guys built the gear teeth up with braze than MIG weld. YMMV

Shuswap Pat
01-30-2012, 08:56 AM
Outboard Motor legs are a source for small hardened bevel gears. Used somw to fix an old Van Norman mill.

01-30-2012, 09:26 AM
I have also TIGed teeth back on gears. Some are still in operation today. I used a dremel tool with a cut off wheel to reshape the teeth after working them.

Bill Pace
01-30-2012, 09:27 AM
A few months ago I had a Bridgeport type mill that some 'gorilla' had managed to rip out about 5 of the teeth on the knuckle the spindle is mounted on. These are pretty large teeth and the knuckle had a lot of mass. Having only the means to braze, thats what I did. Keeping that much cast hot enough to keep the braze flowing was difficult - I ended up using a cutting torch - but I did finally get some rough teeth profiled, finishing them off with a die grinder. The thought of tramming the spindle without the 'nod' ability ----

01-30-2012, 05:52 PM
Thanks for the replies , as I said large gears like dozer final drives or press drive gears are no problem .
These are small but very expensive gears the teeth are not straight and high current mig would hold , my problem is getting the shape correct.

Brazing would result in getting the rest too hot but its easy to work with.

Not interested in extremely fine finishes as it only has to last for about 1/2 hour then I will spend the $400 for each gear and fit new gears.
Its about two months from when i place and pay for the order before I even get them .
Thanks again

01-30-2012, 07:29 PM
I have ground the one or two bad teeth clear flat. Than use dowel pins, drill holes in the gear and use enough pins to go across. Fill inbetween the pins with mig welding and than hand grind or file to shape. Should work for a good while.

01-30-2012, 11:02 PM
Is it possible to access the opposite end of the driven shaft? You could weld on an extension, true it up on the lathe, and drive it with a 1/2 inch drill or something.

You might also be able to temporarily mount a pulley behind the tooling and rig up a motor and belt. You can weld, so it isn't much of a stretch to make up a motor mount that would clamp around a suitable spot.

Take one of the gears right out so it doesn't interfere with the external drive.

Just trying to think out of the box-

Paul Alciatore
01-31-2012, 02:23 AM
Would generic gears work? Sounds like they are fairly wide so perhaps two or more generic ones side by side to replace each of the individual ones that are damaged.

Forrest Addy
01-31-2012, 03:26 AM
Repair of damaged gears might be tempoting but I've found replacement with new even temporary gears leads to a smoother drive. The metallurgy is more consistant and responsive to heat treatment if the replacements are stock gears whose hubs are modified to suit. They may not be capable of as designed power transmission but if de-rated the smoother drive and knowledge that something dreatful is not happening in a heat affected zone leads to greater durability and maybe even confidence.

I strongly suggestt you take a quick look at srock gear supplier's offerings for candidate gears in modification range as temp replacements in your powered tooling.

I'm intreagued with the concept of a build up followed by a wire EDM of the tooth profile. If the resources are available and the weld repair of good quality I'm inclined to think this would be an option far superior to machining with a formed cutter, hand dressiing, etc. I'm assuming a wire EDM can be made to pick up and continure an involute profile merging with the intact teeth such that the new teeth are indectably consonant with the existing and that subsequent heat treatment renders the whole as durable as new.

That would be a dream. I've repaired many a broken gear and even with excrutiating care and attention, re-cuttng the entire profile a bit under the factory's on hobbers or bevel gear generators have had uneven results.

Good luck. 99% of the repairs to broken and damaged gears I've seen have been noisy second bests.

BTW, gears broken in their housing results in enormous stresses in the housings and often destroyed bearings. Have you checked housing bores, shafts, retainers, and other features for distortion?

01-31-2012, 08:40 AM
Oh yeah, pinning and bolts work too. I think it was mentioned here somewhere. Drill a few holes in the base of the gear, tap them, and screw in bolts, set screws, whatever and then shape them a bit to resemble a tooth.

02-01-2012, 03:07 AM
Forrest , thanks for the reply , however I have tried to source different types of gears locally but they are either way too big or suitable for a dremmel , I only want to cut a few thou out of a bearing seat to allow a new slightly large replacement to be fitted as the original is no longer available .
I have checked the shafts and housing for damage , none founf , the bearings are sealed so all I have to do is dismantle the drive and wash all of the crunchy bits out fit new/repaired gears and do 1 job then wait for genuine replacements.
VPT the gears are too small to drill and tap any thing into and the only way to build up the remains to near original is to mig enough metal onto the base and then cut/grind to match the original .

Either way it looks like I'll have an interesting weekend.
Thanks again to all who have replied , if I remember to take pics they will be posted for all to comment on.