Belt drive

Another televisor build using club circuits. Geared towards identifying where to get all the materials, tools, how to 3D print stuff, and troubleshoot.

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Belt drive

Postby Andrew Davie » Tue Feb 14, 2017 1:12 pm

Thinking about what to use to drive the Nipkow disk from the cassette motor - my original televisor used a rubber band, of all things. This is about the worst possible way to do it, because the rubber band dampens any effect the motor might have in accurately controlling disk speed. At work I have some surplus O-rings used for sealing a robotic submarine hull. Anyway, although rather "long" (i.e., large diameter loop), they seem about right in terms of fitting in the pulley mechanisms I have on my motor/disk.

In particular, the diameter of the ring profile is 2.6 mm and the diameter of the ring itself about 130 mm.

I was also thinking of a toothed belt drive as a possibility. This would require some toothed wheels on both the disk and motor shaft - will look into that possibility as it seems to provide the most direct coupling without any stretching involved. If I'm going to put toothed belt wheels on, then I need to be looking at shaft diameters and avaialble tooth wheel sizes. Actually, these are candidates for 3D printing, although the wear could be a bit much. If I go with a large wheel on the disk, then the motor's going to have to spin much faster. Large on the motor, small on the disk, and the motor doesn't have to spin fast at all.

So there's probably an optimal speed to spin the motor, and I could/should choose a toothed wheel ratio (if I go that way) so that at 750 rpm (12.5 frames/second) the motor is happy. The length of the toothed band won't matter so much - it's

a) can I get toothed wheels for the shaft sizes I have
b) can I get (roughly) appropriately sized wheels to optimise motor speed
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Andrew Davie
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Re: Belt drive

Postby Andrew Davie » Tue Feb 14, 2017 1:54 pm

I had a bit of a look-around trying to find the speed at which cassette motors are happy. No idea, really. So thinking about my original televisor, the belt wheel on the motor is about half the diameter of the wheel on the disk - so the cassette motor is going to be running quite a bit quicker than 750 rpm the disk requires. Let's say (if it's indeed half diameter) that the cassette's motor is spinning around 1500 rpm. That sounds like a reasonable speed to me, but just a guess. I suppose there's also the consideration of torque and momentum.

Thinking about how a spinning Nipkow/motor combination is going to work when going too fast. Circuitry is going to slow down the motor, but intertia on the fairly massive disk is going to resist that. So we want a fair bit of 'drag' on that motor to assist with the slowdown. How is that achieved? Do motors spinning slowly have more resistance to being sped up by external forces, or is it the other way around... do motors spinning fast provide a better breaking effect. I suspect the former. That is, a slower-speed motor is going to have more dampening effect on excess rotational speed than a faster-one does.

Well, I can always experiment, and I guess there's no real reason not to get a few different sized toothed wheels and play and see.

A smaller wheel on the cassette motor and larger on the nipkow is going to help it get the Nipkow disk rotating from zero. And since that's what my original televisor has, I'll go with that setup. And I'll get a variety of toothed wheels to test.
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Andrew Davie
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Re: Belt drive

Postby Andrew Davie » Tue Feb 14, 2017 1:56 pm

For getting toothed belt wheels I have to be considering

a) inner shaft diameter
b) bellt width
c) tooth spacing on the belt
d) external diameter

For the belt I have to consider

a) material
b) tooth spacing
c) belt length
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Andrew Davie
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Re: Belt drive

Postby Harry Dalek » Sat Feb 18, 2017 5:18 pm

HI Andrew have to get use to this construction diaries i am bit thick so may take me most of the year :shock:

But on belt drives you have the option of increasing speed with different size pulley wheels for me it all depends on what i have handy ... the big pulley wheel on the motor driving the a smaller wheel with the disk so....motor one twice the size of the disk one twice the speed and so on .But for NBTV 32 line i think any old dc motor direct drive is fast enough.
Also you have the other option where you mount the disk on some thing good like a vcr head or old HD drive where you know there will be no wobble you can leave the wobbling to the belt and the dc motor a touch .
Yes rubber bands are not good scrap an old vcr a very old vcr or old cassette player you can find belts in there if you saw a beta vcr top loaders at your reuse shop they have them a few in fact .
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