Gear repair, advice needed.

[Com_VC]

Hi,

I have a stripped gear on the armature of my bosch straight grinder that needs repairing. I am just looking for some advice on what would be the best way to go about this.

Unfortunatly bosch designed this as a one piece unit for some reason.

Also if anyone knows what the name of the style of gear is called?

I have attached a couple of photos to show the damage.

Thanks

[Ropetangler]

I would say that it is not feasible to repair. You may be able to get a new armature, but if not I doubt that repairing will make economic sense. The gear type is a helical spur gear, and even if you can press the shaft off the armature, making a new one would not be easy, and I imagine that pressing the shaft back into the armature, and keeping it all straight and in balance, will test you well and truly.
Interestingly it appears to have failed possibly because only a short portion of it was engaged, so if a new armature can be obtained, it may be possible to shim it so that more of the helical gear is taking the load, not just the very end portion. I freely admit to being well outside my field of expertise here, but if I'm wrong, someone will chime in and put us both straight.:wink:
It seems like you may be in for some credit card therapy in the near future, Regards,
Rob.

[Michael G]

It's probably one piece for strength; it looks like some form of helical gear. Although it is sad to say, I suspect trying to fix it would cost more than a replacement grinder. I think you would have to try and weld up the end of the shaft without warping it and then try and recut the teeth.

Michael

[Com_VC]

Yeah they are not cheap so it's why I thought about repair, I think they are around the $700 mark.

Yeah I don't know why so little of the gear is engaged. Must be a bad design. I don't think it has been abused because the rest of the grinder looks like new.

I thought about some form of welding. I was thinking about maybe setting it up in a motorised rotary positioning fixture and then maybe tig welding or spray welding it.

The main issue for me would be recutting the gear.

[jhovel]

I'm sorry to tell you that you are almost certainly out of luck....
The repair would cost a LOT more than a new armature. You owuld have to either make a whole new armature spindle, press the old one out and fir the new one and dynamically balance the armature. That assumes you have spiral/helical gear cutting machinery (a 4 axis CNC could do it) and hardening equipment. Alternatively a 4 axis CNC gear grinding machine and using hardened steel may work as well.

The other thing is that your matching crown wheel is now stuffed as well I'm afraid. It may have only worn a little because it has a lot more teeth, but if you re-use it, it will make short work of the new armature and take its 5 teeth off again.

If the grinder is still really good and valuable to you, get a price for a new armature and crown wheel and compare that to the price of a replacement grinder.

The other important question is 'why did it wear out?'
Did the gearbox grease come out or was never put in? Did you cut something that vibrated or chattered the wheel viciously (done that with circular saw myself) or was it a VERY cheap grinder and it has lived out its design lifespan....

Just my thoughts on the topics - not expert advice.
Cheers,
Joe

[Ropetangler]

I'm not saying that it is impossible to repair Com VC, but if the gear was straight cut, it would still be awkward due to the small size, but once you managed to make your gear cutter, the rest may be do-able, but with the helical gear, it is more complicated, and not many of us amateurs will be up to the task I would think. Are you certain that Bosch do not supply new armatures for your grinder, or it may be worth talking to one of their reps to see if they have another which is close enough to be adapted. It may even be possible to cut the old gear from the armature shaft and Tig another shaft end on (with an identical helical gear already cut). All this though while possible, will throw many challenges your way.
If you do end up with it repaired or replaced, I would see if there are shim washers which would control shaft end float, and if so, you may be able to shim it so that more gear is engaged, ie. shim the shaft so that you push more of the helical gear into engagement with its mating gear, so that the load is spread over more than just 3 or 4 mm of gear. Good luck, it sounds like you will need plenty of it to repair your grinder. Regards,
Rob.

[Bryan]

I guess you could press/pin/weld a new end on the shaft and cut straight teeth on it. Have to replace the mating gear as well of course. Might be a bit noisy....

[MuellerNick]

Making a helical gear "is not that complicated"[tm].
But welding something onto the shaft without weld-stressing the center-bore out of line is impossible. The armature will be out of balance.

At 5:16, I show how that works: YouTube (http://www.youtube.com/watch?v=SJQtx80euGM)
You "only"[tm] need a CNC-mill and a forth axis. I wrote the program with the built in language (G-code-extension) in LinuxCNC.


Nick

[nearnexus]

Looking at that armature and the stripped teeth, you'd have to say that is pretty crappy design.

It's obvious what's happened - the unsupported gear shaft is engaging right at the worst possible point, has flexed under load and has skipped/torn up the teeth as a result.

If the gear had engaged close by the bearing (where the teeth are unused LOL - intact) this probably would not have happened.

Bosh eh? I hope the rest of their stuff is better than this.

This not the first time i've seen this sort of crummy setup - Black and Decker drills had something similar.

You will have to replace the armature and the mating drive gear to fix it. Probably not economic and who's to say it won't happen again.. Bin it.

This type of pinion gear is the usual setup, is machined straight into the armature shaft, as seen, but in better units has a bearing support each side of the gear contact area. It's a throw away item.

To try and repair the stripped gear would be just about impossible. You cannot afford to reduce the already weak shaft diameter, and any welding will almost certainly harden the shaft making it brittle. Any flex would then almost certainly fracture the armature.

Rob

[PDW]

Hi,

I have a stripped gear on the armature of my bosch straight grinder that needs repairing. I am just looking for some advice on what would be the best way to go about this.

You are SOL. Bin it now and save yourself a lot of time & trouble.

I've a lot of gear for gearcutting etc and I wouldn't even attempt this for 3X the cost of a new unit. The chances of a lasting repair are vanishingly small.

I had to bin a Metabo grinder I was particularly fond of for a similar reason.

PDW

[Com_VC]

Thanks for all the replys.

I think I may give it a miss, though I probably won't throw it out just yet as maybe I could use it for parts in the future.

I don't know if it would be worth contacting bosch about it, maybe they will give me a a cheaper price for parts. The thing would almost pass as a new item.

[nearnexus]

You've got nothing to lose.

If it's an expensive item and it craps out like it has due to poor design then , yeah have a go at Bosch.

They may(probably will) come back at you and say you've abused it - but it's worth a try. Make sure to point out that the gear stub is not dis-coloured (blue) from heat/associated with abuse. It's failed from the cheesy steel used and the crummy design.

I've had a few wins over the years by complaining about crappy products.

The only company that ever gave me the total cold shoulder was Electrolux, who own kelvinator, Weedeater and lots of other white goods and electrical stuff.

I would never ever buy another product from their company again. They totally suck on service calls - and some of the design is shocking.

How about a leaf blower/ sucker that pulls the gritty, wet air through the armature enclose when sucking into the dust bag. Yep, I couldn't believe it either but that's a fact on the mighty (sic) Weedeater electric blower vac.

No wonder they say not to use in wet conditions.

To top it off the armature runs in plain bushes, the grit siezes them up and they tear themselves out of the plastic housing.

Total and utter rubbish.

Rob