GearHeads Forum

Hi all,

I am stymied at my lack of ability to cut a 7" gear with my TAIG - soooooo:

If the first gear in a clock has 84 teeth - why?  Why can't it be some other number divisible by the pinion it drives like 64 teeth?

Should I go the other direction and work the escapement 1st so I know that I have a seconds beat pendulum and then figure out the ratios I need to get down to the 1st wheel?  I looked at a few movements today and they all have a large (but not necessarily 84 tooth) 1st wheel.  But it still doesn't answer the question "why"? 

Any input would be greatly appreciated.

Steve

Steve:

  There is no requirement for a large wheel. Clocks are all about energy. One wishes a clock to run for years on a single wind. Never happens, but thats what one wishes for.
  So a clock design is always about energy and friction. "Keep it simple stupid" could have been written by the original clock designers, even though
at the time it was a wonder of the world due to their complexity, but underlying all that was the concept of energy conservation. Large wheels
keep the friction down and allow the spring or weight to be smaller, it allows for longer runtimes, and pisses you off when you have to make one.

  BUT, as a hobbiest it doesnt have to be that way, youll just find it takes more wheels and pinions and it wont run as long on whatever energy you use. If you use a slow motor then it wouldnt matter at all other than wearing out earlier. Remember all such devices have friction points, mainly the bearing on the shafts as well as the tooth contact. Its why you dont typically use involutes as well, you use the cycloidal teeth, they barely touch
so they reduce friction. Hypocycloidic gears, when I finally get to them will allow much higher reductions with smaller gears, ( Google cycloidic planetaries for those).

    But you know, building a clock or a ticker isnt always about doing it right, Im as proud of my old wooden clock that barely runs due
to friction as I am of my ticker that runs very well. I do it to learn and make a work of Art.. ( No pun intended. )

  So dont feel you HAVE to use large wheels, use whatever you CAN make, it will still work. Use cycloidic teeth or cage gears to their
best advantage, start with the escapement, its always the best place, then work backwards to the weight or spring, then again forwards from the escapement to your hands..feet..whatever will display the time, if indeed a display is necessary. I'm a huge proponent of gear works that just..well..run, they dont even need to do anything other than look good.
 
  In fact, I just took down my first wood clock and will now ( at my wifes request) turn it into a plant holder. So never feel constrained
by convention..

( Heres a photo of my old wall clock as an example, Ill be adding a small table to the top to hold a plant.. )


Just my advice..
Art




           

Hi Art,

Thanks for the input on the wheel size.  I actually tried to find out what a company would charge to make me that sprocket, but the bid came out as $2000.  TWO THOUSAND for something that takes about 30 minutes on my TAIG.  They said they used a vertical machining center and a 7" dia aluminium billet and hobbed the sprocket.  WAY too much work and so that's why I brought up the question.

I'll start from the escapement now and go back down.  That shouldn't be hard, but getting the motion works figured out is going to be a little more difficult.  But at least it will tick!

Thanks again,

Steve

Steve:

>>But at least it will tick!

And thats all that counts..

Art

Art,

I took my Clockmaker's Beat Book, and plotted the 100 different listed beat/pendulum length.  Then I picked a pendulum length I liked.

I then took the calculations to find the beat and worked backwards from the escape wheel, to find a pattern of wheels and sprockets I could machine with Gearotic.

The pattern comes out like:

EW  EW pinion  4thW  4thWpinion    3rdW  3rdWpinion    Center Wheel
30        12        40          12              50        12                  60

This should give a beat of 4167 with a pendulum of about 29 inches (maybe a wall clock and not a mantle clock).

Do you know why the 2nd wheel isn't in the calculation?  I'm planning on a 70 tooth 2nd wheel and probably the same for the 1st wheel (I can cut this size on the modified TAIG).  I think the graduated sizes will give a pleasing appearance. 

Steve

Steve:

Im afraid I never saw the book. Perhaps the second wheel is on the weight side?
Im not a great clock maker, Im happy with the ticking so accuracy has never been my thing in clocks,
so Id take the books advice over mine anyday.



Art

Hi Stephen,
I fed your data into my own excel gear train calculator.
Most of the info I have read uses centre wheel to indicate the wheel that will rotate once per hour and that is the wheel you use to calculate the time train (Clock Hands) as opposed to the running train.

In order to get your gear train numbers to give me a one hour center wheel I have to use about a 0.86 period pendulum.  I think the length of your pendulum will be a little longer than you indicate IF you want the centre wheel to rotate once per hour. 

The gear train beyond the centre wheel will provide the length of time between windings.  i.e. if you want to wind every twelve hours you'll need a 12:1 ratio after the centre wheel.

John

John,

Thanks for clearing that up for me.  I'm going to start cutting sprockets this weekend. 

I think that the "center" wheel is also the "2nd" wheel, so that one will need a longer arbor so I can make it turn the minute hand.  Then, as tradition, an idler wheel running to the hour wheel.  I'm not too worried about the period of the pendulum yet.  I've had a couple of clocks to repair where I had to guess-and-check the pendulum.  In one case, someone had put a grandfather movement (114 CM pendulum) into a wall clock so the pendulum was WAY too long for the casework.  I put in a 66 CM movement and it looked a lot better.

Steve