Perfect Involute Tooth Forms?

Interesting question from a builder:

I'm new to wood clocks, but have some mechanical engineering experience. I see that the tooth profiles on the pinions are nothing like those on the wheels they drive (and neither look like involute teeth). 

Why the difference in tooth profiles? It seems to me that the two ought to be the same. To add to the question, it seems to me the quasi-trapezoidal teeth on the driven wheels would be easier to cut accurately, suggesting the pinion teeth should be similar. What do you say?

Regards,

John

Aloha John, it is nice to hear from you.

Your engineering experience has taught you right.  It has taught you about high speed gearing at its finest.  

Thing is...clocks don't move fast so that level of perfection is not required.

As a matter of fact, in my own experiments, it's difficult to notice any difference at all with various tooth forms.

I discuss this at length in my book, Practical Guide.

Take a look at the various tooth forms that I use in my designs.  You'll see straight sided, involute, "random", rounded, and none of them seem to perform much better than any other.

You can make high speed, "perfectly" engineered teeth if you like, but do some experiments on your own and I think you'll convince yourself that pretty much any tooth form works well at low speeds.

The old tymers used to cut triangular tooth forms.  Worked fine for them.  Hacked them out with adz and axes.

Take a look at the teeth on this old clock on the cover of the National Association of Watch and Clock Collectors (NAWCC) journal.  Here is a perfectly fine running clock with possibly the poorest choice of tooth form available...triangles.  Ha!  

And yet................it has been working for centuries.

So, my advice is to make whatever tooth form strikes your fancy and compare it against the "perfect" involute tooth forms and see how much difference it makes in the running of your clock.

Enjoy!  Aloha.  Clayton