[How-To] Back gear repair

[Brento]

Ok so i am in a little bit of a pickle. I have a tooth gone on my back gear for my SB9A. I would weld it but i have no argon at work. I have no gas for torches to braze. I saw Jim F posted about a back gear repair that is on ebay but i would rather do that as a last resort. What i have in my disposal are little 1 pound propane tanks and the Map torch set for like the small bottles. My question is will that be enough heat to braze a tooth? Ive heard of the JB weld trick but i dont really have time for that as well.

Edit: i also have a little 110 arc welder that i have never used.

 

[markba633csi]

What I suspect is you'll have trouble getting the piece hot enough because of it's mass
But you can try.
I would opt for the Ebay kit, it looks pretty spiffo-matic

 

[Bi11Hudson]

Gear pitch would affect the following description to an extent. My machine has 16 DP gears on most of the train. I'm not sure what the back gears are. Further, I have seen this done on larger gears but my only personal experience is on small gears, 28 DP and smaller. On top of that, my application is for lighter loads. I don't know how it would stand up to heavy loads.

This instruction is a purely mechanical repair. No torch work or welding is used. The first step on removal of the gear is examination of the gear for cracks in the web or hub of the gear. If there is any anomoly, I immediately withdraw from the conversation. The first step is to mill down the tooth a 'fuzz' below its' root. If a milling machine is available, it likely would make a smoother path. But hand filing is sort of a 'fall back, old standby' situation. The gear is drilled and tapped for an appropriate screw size. Three or four screws, or more depending on width. For my 16 DP gears, I would use a 3/16 screw, Nr 10-24 or 32. As much a matter of what screws I had on hand, 1/8 inch or 3-3.5mm would do as well. Hardened or high strength screws are preferable.

The metal that the gear is made of would determine the best metal for repair. For a cast iron or brass gear, I would use brass. For a ZAMAK gear, aluminium would be a better choice. A steel gear would be best built back with mild steel. Make a rectangular piece like key stock to fit the milled flat. The flat can be milled or filed. I'm pretty good for patience and a fair hand at filing. That would be my choice in general.

Attach the key stock with the screws tapped into the gear. Final fitting of the keystock must be done carefully, assuring a solid seat. A plastic steel such as JB Weld would make a smoother fit, but is not vital. Then filing the tooth profile to match the existing gear. There is room for small mistakes, provided the repair tooth has the overall profile desired. If a milling machine is available, a profile cutter will give better results. But hand shaping (filing) is the way my small gears are done.

As well as watching underway repairs in the engine room in times long past. The ship didn't have a milling machine, it was all hand work. We were at sea for 6 months at a time, any repairs were done under weigh. The screw heads will be distorted by the filing. Make sure they are tight and then forget them. Just an integral part of the repair, they will be a 'total loss' as far as removing them.

.

 

[DavidR8]

I have seen a method similar to what @Bi11Hudson describes and it looked very solid and worked perfectly.
What I saw was a gear drilled and tapped to accept a row of three socket head cap screws in place of the missing tooth. The heads were filed down to match the profile of the tooth. Worked perfectly and there was absolutely no difference in the operation of the lathe.

 

[benmychree]

Do the e bay fix, welding on cast iron will doubtless result in an hard un machinable weld.

 

[Brento]

I like the key stock idea but the issue comes to it being a small gear. Would be tight to get a screw in some keystock for that. I have a litttle bit to think about it but not alot.

 

[ericc]

Why are you in a rush? This can end up causing unwanted consequences. Take your time and try out a few things.

When I was totally short on resources, I would come up with all these schemes to save money and do things with less. The little consumer grade propane torch will work if you can focus and contain the heat. With so little heat, there are not too many choices. Pretty much, you're stuck with silver solder. I used a mixture of grass clippings, clay from digging fence posts, and junk perlite to make a little barricade. Bury the rest of the gear in sand and just expose the top that you need to braze. Some scrap from a food can could come in useful. Unfortunately, you will have to pay for the silver solder and the flux. Don't skimp on flux. You want that low temperature paste stuff. For me, I was lucky that the local university was getting rid of all their cadmium containing silver solder due to the toxicity hazard. Don't complain if you are broke. The vapor pressure is very low at propane torch temperatures. Practice at least once.

 

[Bi11Hudson]

I like the key stock idea but the issue comes to it being a small gear. Would be tight to get a screw in some keystock for that. I have a litttle bit to think about it but not alot.

For my purposes, I have used the system on small gears using as small as 2-56 (0.086") screws.The larger gears on the ship, I have seen as large as 1/2-20 bolts used. The system works well. I stock brass key stock as small as 1/8 square and can file smaller if needed. I think I still have some 1/16 key, but that would require a Nr 00 (0.047") screw or smaller. I don't think I'm that patient any more.

.

 

[Manual Mac]

SB9 small back gear teeth replacement.
repaired 60-70 yrs ago

 

[Brento]

My rush is due to work coming in the door in a few days and this is an after hours job. I can look into the keystock idea but my fear is how small of a screw i am going to need which i do not have. @Manual Mac how did you repair yours?