Clock with sprockets and chains instead of gears and pinions

[Stephen Fornelius]

I was playing around with GT, and flashed on a clock wherein all the internal shafts are sprocket and chain driven.  then the clock will be assembled using ball bearings and acrylic plates so the mechanism can be seen. 

I'm thinking a #25 chain so I could use 1/8" aluminum for the sprockets and stainless for the chain.

The question I have is simple, but I need validation on my thoughts:  The ratio between the outside diameter of a wheel and pinion is fixed.  Is the ratio between the pitch circles proportional as well?  So if the diameters are a ratio of say 1:20, then the pitch ratio is the same?

The reason for the question is that, for obvious reasons, the sprockets are going to be about 3x the size of the original wheels, so I need to know if the pinion sprocket would be the same 3x to keep the ratio the same.

[JustinO]

The important thing is the ratio of the number of teeth. The diameter ratios and other geometry is important for the efficiency of the mechanism. Gears typically have the same ratio of pitch circle radiuses as they do ratio of teeth, although there are exceptions.  Sprockets work pretty much the same way. If the chain runs over your sprockets nicely, and your sprockets have the correct ratio of teeth, you should be good.

For clocks; chains and sprockets may not use the energy from your prime mover (spring, weight, or motor) as efficiently as gears might.

Here is a link to pictures of a robotic arm driven with chains -- I have a couple of these:
http://www.theoldrobots.com/rhinoarm.html

[Stephen Fornelius]

OK, that's one of the things I was wondering about.  I can make the same tooth to tooth ratio and the sprockets will be about the same size (proportionally per GT) as I would imagine.  So I'm going to plot tooth to tooth and take what diameters come out.  That sound right?

[BobL]

Thanks Justin... ;)

[JustinO]

One other thing .... the sprockets will all turn the same direction! Maybe you'll want one gear pair somewhere in the middle of the works so you have sprockets turning both directions.

[Stephen Fornelius]

Hi Justin, thanks for the info.

That's an interesting idea, but 1st things 1st!.  I worked on the sprockets yesterday and only have one design glitch so far - the main wheels is larger than what I can cut on my TAIG mill.  Sooooo - I'm going to index it so I cut 1/2 and then remount it and cut the opposite half.  The smaller sprockets I cut yesterday turned out fine.  I'll order some chain today.

I did find that I have to remove the center hole, drill the blank and the spoil plate with a 10-32 Allen head, and then bolt that though into a t-nut into a bed slot.  Otherwise when the 2nd roughing cut is made, the whole gear pops out of the scrap.  Also noticed that the center hole doesn't interpolate round, so I'll mount it in the lathe and bore the mounting hole true to fit a collar.  Option "b" would be to glue the work to another piece of aluminum to use as a spoil board and then do a separate clean up on the center hole.

Using BobCAD-CAM to generate the machining, but I may try a "direct from GT CNC" to see if I can work with that without the additional steps.