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

Hajo Dezelski's Review of "A Practical Guide to Wooden Wheeled Clock Design"

A Screenshot of Hajo's Review...to see the complete review, please click on link below

Hajo emails: "Hello Clayton,

I just published a short hint to your book and I hope that you don't mind that I linked to your photo of the cover. If you have objections please tell me and I will remove the link.

http://hajos-kontrapunkte.blogspot.de/2013/09/tock-tock-tock-wooden-clocks-clicking.html

All the best,
Hajo"

No objections at all, Hajo!  Thanks for the nice review.  Aloha, Clayton

An Original Clock by Andi Using "A Practical Guide to Wooden Wheeled Clock Design"

#8 by Andi, his original design!

Andi writes:  "Hi Clayton, just thought I would share my new clock design as based on the gearing you described in your Practical Guide.

The pictures attached show the clock mounted temporarily at work where I can fiddle with it. I have also put a video of it up on my YouTube site http://youtu.be/YOxQzlSRbdE

As you can see I took the “eight day” theme all the way with the design of the frame hence the name #8.

The amount of weight needed was at first quite disconcerting so rather than use a doubling pulley system I have just used a pulley which can be mounted anywhere on the wall, luckily for me my walls at home are actually just high enough that I can mount it to allow the drop for an eight day run time. I must admit all my previous clocks do tend to stay dormant unless I am in the mood to wind them. The once a week wind is fabulous, this is one clock that will be in permanent motion. Just need some space on a wall to mount it! Will have to talk nicely to the wife to move some pictures lol. 

The length of pendulum is 74in. I had originally thought to use your 3 click wind mechanism but forgot this when I cut the wind gear which has a “four spoke” design, so I had to design a 4 click mechanism and also beefed it up for the weight it was to support. I used a second rewind barrel geared 1:1 with the driven one but with half the diameter so the drop of the counter weight could be straight down. The dial is my first attempt at segmenting and I am very happy with the result, the numbers are 1/8 brass rod inserted end on which though took a while to do but looks very pleasing, worth the effort. As the weight needed to drive was so much the actual length of the weight tube restricted the running of the clock by almost a day! So, cut this into 3 and mounted into wooden end supports. Using the pulley above the clock helps to support the weight of the clock its self, a bonus. The shape of the frame just shouted for a chime to be mounted on the right so I used a short length of copper tube and using just a wooden hammer makes an understated “ding” but works nicely and isn’t too loud.

The original design was to have used a grasshopper escapement of my own but I had to revert back to the graham escapement due to the fact the swing needed of the pendulum was just a bit too much. I think this is due to the fact I tried to design the escapement to the dimensions I had for the frame i.e. the centre of the escapement is close to the escapement gear, with the tooth spacing I used it needed to be further away to reduce the angle of pendulum swing needed. All good learning though and it also got me to understand how the design of the graham escapement works. Perhaps on my new clock I can get the grasshopper to work.

Cheers for the book, a great read and an inspiration for my own workman like design ;)"

Andi.

Andi, that is SO Very Excellent!!!  I love the creativity you show in this brand new "Andi" design!  I love the wood you used for the frame and reading about how you overcame the little bumps in the path to creating this beautiful clock.

Yes, the grasshopper is not only one that usually has a huge swing to the pendulum, but it also is an energy hog.  The two grasshopper escape clock designs that I offer have about double the required drive weight of the Graham's...so not a good choice for an 8 day with high weight requirements already.  

I have a different kind of grasshopper escapement in my latest research clock.  The two grasshoppers that I now offer are "Push-Pull" mechanisms.  This new one is a Push-Push.  It is designed after the very first grasshopper that Harrison created.  He later graduated to using the push pull exclusively.

I've attached a short vid.  You'll see not only the weird "parallelogram" grasshopper, but also a balanced compound pendulum, Geneva wheel hour indicator that's run off of the revolving minute wheel.  It is also set up for a seconds hand, but that's not on it right now.  I had trouble keeping it running on the weight I thought it should have, and it lost its seconds hand in all the modifications.  When in reality I had to admit that ALL grasshoppers are energy hogs.  I hung more weight on it and it's been performing excellently...which is the long way around saying that you decision NOT to use a grasshopper in your 8-day was most assuredly a good one.

Thanks for the wonderful pix, vid and story of your build.  Well done(!), and congratulations.

Enjoy!