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Another Maths type formula please

Community Veteran
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Registered: 06-11-2007

Another Maths type formula please

I need to work out how to make a "slow motion drive" for a radio dial, using a pulley,  on the tuning knob, OR on the tuning capacitor
The tuning capacitor has a quarter inch shaft....... and the tuning knob will also have a quarter inch shaft. 
I am contemplating using a pulley about two inches in diameter, connected to either shaft (depending on which one has the pulley) via a special type of drive cord, and a spring to tension it.
The tuning capacitor shaft only rotates 180 degrees, to give the full range of tuning capability. so I need to slow down it`s rate of turn, ......as example for 6 (or more) turns, of the tuning knob the tuning capacitor shaft only turns a small amount (say about 5 degrees or so)...
Which shaft needs the pulley?  (Trial and error will decide the actual size of the pulley )
any advice, drawings, pics would be useful, please.  Wink

12 REPLIES
Community Veteran
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Registered: 02-02-2008

Re: Another Maths type formula please

Put the big pulley on the shaft you want to turn "slowly" (measured as degrees rotation).
Community Veteran
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Re: Another Maths type formula please

The easy answer - which you won't like - is it won't do what you want.
An effective transmission ratio of about 432 to 1 cannot be achieved using a single pulley system
Calculation
6 turns = 2160 deg converted to 5 deg rotation is an overall ratio of 432 to 1
The best you can get from a single pulley pair is about 12 to 1
Community Veteran
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Re: Another Maths type formula please

Thanks for the info guys
@jim..... I`d need a very big pulley then  Cheesy
actually... 12 : 1 might be enough.... it will be mostly trial and error to get it to work.... as I need to make a "linear" tuning scale the length will be decided by the actual "travel" horizontally of the pointer, and the available space of  the radio front panel.
( I did make one a long time ago, when I had a shed, [ oh I do miss my shed ! ] and much old radio junk in boxes..... plus, of course the time to mess about doing it ! )  Grin
Community Veteran
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Re: Another Maths type formula please

I get the same answer, to turn the capacitor 180 degrees with six turns of the tuning knob, then the pulley on the capacitor shaft needs to be twelve times the diameter of the pulley on the tuning knob shaft.
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Re: Another Maths type formula please

which I work out as 12 X 0.25"  would give a 3" pulley on the tuning shaft.
Thanks..... now, next question .... how long a travel would that give on a horizontal run? (approx)
Community Veteran
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Re: Another Maths type formula please

If you are taking the horizontal pointer cord from the 3" pulley, then the linear movement would be 4.71"
However I am skeptical whether you could actually drive a pulley from a cord/belt wrapped directly onto a 1/4" tuning shaft without adding either a toothed or 'V' shaped wheel of some sort, to give it some friction to stop it slipping.
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Re: Another Maths type formula please

Yes it is possible to drive the 3"  pulley from the .25" shaft....  there is usually a spring on the pulley end  (or very near) to tension the cord, which is passed around the tuning knob drive shaft 2 times to give it "grip".... I would have to arrange some guide pulleys to make the horizontal "track" for the pointer to run , but this is no problem...
Thanks for all your input guys. Much appreciated.  Smiley
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Re: Another Maths type formula please

If I were doing it I wouldn't use pulley wheels, I'd dig out my old Mechano set which is lurking somewhere and use a large cog and a worm gear. The ratio on that is one to the number of teeth on the cog.
jelv (a.k.a Spoon Whittler)
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Community Veteran
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Re: Another Maths type formula please

Alternatively you could use a pre-made "Vernier Drive" or "Reduction Drive" which are cheap enough to buy on eBay or from an electronics shop.
For those that don't know, a Vernier or Reduction drive is usually a miniature planetary drive type gearbox which forms part of the tuning knob shaft, so that for each turn of the front panel knob, the output shaft (socket) rotates at a lower ratio, often between a quarter and tenth of the rotation of the knob.
Therefore you would fit the tuning capacitor shaft directly into the back of the reduction drive, and that would give a nice direct drive feel rather than the less positive feel that you will get from pulleys.  However you would still need to add some sort of additional pulley arrangement to the knob shaft in order to drive a linear tuning pointer.

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Re: Another Maths type formula please

Yes, I think I might have a couple of those type of drives.... I seem to remember putting the bottom one on the tuning knob drive on the last radio I built.
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Re: Another Maths type formula please

Community Veteran
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Re: Another Maths type formula please

I haven`t started to build this particular radio yet.... but, Yes.... I did get the string and pulley system to work on another short wave valve radio I built..... I made it from scrap materials, even used an old 405 line tv wooden cabinet (top 8 inches) to make the cabinet.... at the time, I was not restricted to size, so I just used what I had.... this one, I need to be careful about completed size, hence my original question.
Thanks for the interest