Thread: Advice on fixing broken gear teeth?

Originally Posted by onthebeachalone

Having repaired and re-commissioned our old MARS Atlas lathe (with help from this forum), and dismantled and de-commissioned our even older lathe I am now attempting to repair an old mechanical body jack that we need for a project.

The problem is that some of the gear teeth are broken. I suppose the obvious approach is to insert screw studs, braze over them, then file to shape as has been done in the second picture below. I think we could probably manage to insert the screw studs, but we don't really have the brazing skills to tackle that part of the task..

Bevel Pinion a.JPG Bevel Pinion b.JPG Bevel Crown and Pinion.jpg Pinion.JPG

Any suggestions/comments appreciated.

Find a proper set of replacement gears or scrap it.

I would never repair anything with that sort of damage that's going to be used as lifting gear. OK you might not be going to exceed a light load but unless you're going to cannibalise the thing so it can't be, I still wouldn't do it.

If it's not for lifting gear, clean and build up with braze then file to fit.

PDW

Hi
I too would say if used to lift then ask if it is worth it. Particularly explaining to coroner or WHS etc.
That said some old jacks use a " nut" on a threaded post to lift and the angle of the thread means it supports weight without undoing regardless of weight on the post. In that case it would be worth asking why it broke.

Then consider brazing (with brass filler) or high nickel stick weld or silver solder a new wedge in and then recut teeth or file / grind teeth profile.
Search www and you will find guides for each method

I have a broken gear I will soon need to repair but no safety risk if it fails. Probably a good thing with my current skill level.

James


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Hard bronze would do a good repair (high strength) if done correctly, but I'm with the others and say forget it if it's lifting equipment.

Just too dangerous, even with stands.

My father was a mechanic and there's no way he would consider what you're thinking about.

Plenty of people get crushed under cars.

Cheers

Rob

I'd probably repair it. Done correctly the repair should be at least as strong as the original tooth. The OP said it it was a mechanical jack that they wanted to use for a project. He didn't say the project was lifting cars. If its a screw type jack, where the bevel gear turns a nut, the bevel gear itself would not be supporting the load on the jack. The nut would be doing that.
Having said that, this gear obviously attaches to a shaft of some sort with a key. It would be interesting to see some pics of the jack itself.

bollie7

Originally Posted by bollie7

I'd probably repair it. Done correctly the repair should be at least as strong as the original tooth. The OP said it it was a mechanical jack that they wanted to use for a project. He didn't say the project was lifting cars. If its a screw type jack, where the bevel gear turns a nut, the bevel gear itself would not be supporting the load on the jack. The nut would be doing that.
Having said that, this gear obviously attaches to a shaft of some sort with a key. It would be interesting to see some pics of the jack itself.

bollie7

Key words -

'done properly'
'should be as strong as the original tooth'

Who is going to guarantee the above?

I did qualify my initial comment to the effect that if it was going to be cannibalised so as not to be used for lifting, brazing up the missing bits would be ok. Brazing & cast iron play very nicely together.

I stand by what I said. If there was any reasonable possibility of it being used to lift heavy objects, I wouldn't repair it, I'd scrap it. I have a lot of lifting gear, yesterday I was lifting a $17,000 engine up and lowering it into my boat. Couple of friends giving me a hand via constructive criticism. 1 tonne chain block, 2 of 250kg lever blocks for positional control and all slings in good condition rated for 4X the load. No dramas which is just how I like it.

The cost of decent lifting gear is totally trivial compared with the foreseeable consequences of failure of any 1 item.

PDW

Originally Posted by PDW

I stand by what I said. If there was any reasonable possibility of it being used to lift heavy objects, I wouldn't repair it,

I agree, if the gear was doing the lifting and taking the load, but all the screw jacks that I have seen (and I certainly don't claim to be an expert, or claim to have seen every variation of screw jack) that have a bevel gear set in them, the gear set only turns a nut which does the actual lifting and takes the load. So it would make no difference if a tooth had been repaired. The jack would not let go if a tooth broke on the gear. That would just make it difficult, if not impossible, to turn the gear and thus the nut.

All I'm saying is the OP hasn't given enough detail as to the use that he wants to put the jack to, so repairing the gear might be fine for the application he wants to use the jack in.

bollie7

Looking at the book ivan law, making a bevel gear will make my worm gear project look a piece of cake!Do you have the ability or would you entertain the idea of making another?Simon

Stock Gears and Racks - Gear Racks, Spur Gears, Mitre Gears, Bevel Gears | Ronson Gears Australia

Edit: This link is more direct:

KHK Web Catalog Dimension table

Originally Posted by simonl

Looking at the book ivan law, making a bevel gear will make my worm gear project look a piece of cake!Do you have the ability or would you entertain the idea of making another?Simon

I once made a set of 3:1 bevel gears.

Once.

20+ years ago. The pages in my machining texts still have slips of paper with calculations scrawled all over them as bookmarks to the bevel gear pages.

Buying them is a lot simpler, easier and they work better too. The ones I made still work - they're in a lathe apron so very slow speeds and not a huge amount of torque transfer.

I'd rather make worms & worm wheels.

PDW

I have repaired a fair few gears with studs. As above the bevel usually drives the lift nut. Should be fine.

Hmmmm I hadn't thought of buying ready made gears, or even considered it as a possibility. I'll check the dimensions and see if there's something in the catalog that will do the job. I presume I would have to buy a pair.

The arrangements with these jacks is that the bevel gear reduction in question drives another pair of reduction gears, which drive worm. I'm sure the 'worm would not turn' with a load on even if all the teeth fell off the bevel gears.

Thanks for the posts.