salvaging a gear

[John T]

I have a clock that I built using solid maple instead of the usual Baltic Birch plywood.  I wouldn't recommend solid hardwood for many reasons not the lease of which is that the teeth on the gears are "fragile".  Under normal operating conditions they are fine but when there is a upset things can go wrong.
The gear shown below has had a couple of teeth broken by the winder and trying to re-attach them while maintaining the original alignment proved next to impossible - so while the gears seemed to mesh then were enough off to stop the clock.
My solution was first to cut the teeth in Baltic Birch with the rim 1/2 the original gear.
Next I took all the teeth off the hardwood gear and half of its rim.
There was a fair amount of careful fitting but the final gear had the hardwood features and perfect teeth.

The clock now runs reliably.

[tweakie]

Looks like an extremely careful bit of fitting indeed - an excellent solution.

[steve323]

That is a very nice looking restoration.  It looks much better than a generic baltic birch gear.

Ont thing I notice about the gear is that the teeth are extremely tall.  They would be a lot stronger if the dedendum didn't need to be so deep.  Rounded inside corners could also help.  It may not matter much in BB, but maple would see an improvement.

Steve

[John T]

Hi Steve,
The clock is very traditional in nature and as such uses epicycloidal tooth form, the clockmakers standard.  At the same time you are correct in that an involute form would be a little stronger.

The gears are cut from a glued up segments that aim to have the grain of the wood run radially (not the case in the broken tooth). Unfortunately to have all segments run their grain radially is hugely wasteful of material - OK if you have a surplus of hardwood - unfortunately i didn't. 

Anyway your point is well taken - thanks very much.

John

[steve323]

Hi John,

That makes sense.  The epicycloidal tooth pattern looks great and it should be plenty strong in brass.

Here is a wood gear with involute teeth in cherry.  The teeth seem strong enough.  I used a 1/8" router bit that naturally leases rounded shoulders.

Steve

[John T]

Nice work. Best of luck!  We can all use that.

[ArtF]

I think that was an excellent solution. Makes it look more artistic anyway. :)

  Pinions are a bit less touch, and it is easier if you use plys of some sort, I have
on occarion just glued up two 1/4" layers of wood crossgrain to help on strength,
but usually pinions are used on low energy wheels with an involute on the weigth wheel
for better strength.

  In any event, it was a good solution to a problem we all face at some point. :)

Art