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Styler
14-04-2012, 12:33 AM
Hi all

I hope some clever people can help me...

In a traditional worm drive, the worm (the driver) rotates about its own axis (which is fixed in place). This causes the worm gear (the driven) to rotate about its axis. Is it possible to fix the worm gear and allow the worm axis to rotate about the worm gear? In other words, if you lifted a worm drive up, held it by its worm gear and turned the worm motor on... would the worm "climb" around the circumference of the worm gear? Obviously, the worm would have to be attached to some sort of arm that connected to the worm gear axis.

On a similar note...

A rack and pinion is usually setup in two ways:

1) The pinion rotates and causes the rack to move linearly.
2) The rack moves linearly to rotate the pinion.

However, if the pinion is fixed, would a linear movement of the rack cause the lengthwise axis of the rack to rotate about the pinion (assuming it had a design that allowed it to do so)?

I've attached an image that hopefully explains these setups. Are they possible or am I making a fundamental error here? Your help is appreciated. :)

alocky
14-04-2012, 12:58 AM
No need for clever people, I can answer that. Yes, the worm will move around the worm wheel if the worm wheel is fixed and there is freedom for the worm and its motor to move. However, your rack and pinion won't work. If you push against the rack it will just break the tangential teeth off the pinion. If you put some kind of bearing behind the rack on a radius arm holding it against the pinion and turned the radius arm it would move the rack around the pinion; don't know why you'd want to do that.
Cheers,
Andrew.

jenchris
14-04-2012, 12:42 PM
yes you could roll the bar around the wheel, can't think of an instance where that would be needed though. Bit like one of those window winders I guess.

Climbing a worm round a wheel? I think tractor diffs are a bit similar to that. Though the worm is hobbed at 45 degrees and tapered a bit! No that drives in the other direction doesn't it? Reckon if you tied the wheels down, your tractor would rotate round the wheels - that's the effect.

Still and all, the gears will always work, just can't really find a reason for it to be made that way though.

Rod
14-04-2012, 01:04 PM
My scope as described here:

Is driven the way you describe on the azimuth axis. You will find a lot of bartelized dobs driven that way. I think it is easier to do it that way on an altaz mount.

Rod.

Styler
14-04-2012, 11:16 PM
Thanks for all the replies so far. Rod - thanks for the pics of your telescope. Its good to see that the worm around the gear idea works in practice. jenchris - yup thats sort of what I'm trying to do. alocky - thanks. I'm still not convinced that the one would work and the other would not. Surely if they can both convert a linear force into a rotational force they would both be able to spin around the fixed gear? I'm still scratching my head...

As for the rack and pinion, I've revised the diagram. I tried to do a force diagram analysis... If the motor moves the rack to the left then there will be a resultant force to the right on the red frame. To my thinking this will make the red frame rotate (with the rack) anti-clockwise around the pinion. Is this correct?

The reason for wanting to do it this way? In the application I'm thinking of, it would use less materials plus I can mount items to the red frame rather than a smaller gear (or another part attached to the gear). It might not end up being practical but its worth looking into. :)

mplanet62
15-04-2012, 04:32 PM
A toothed rack is essentially a toothed wheel of endless radius. Therefore, the ratio of torque transmission is infinity. What you offer is just this. While it is possible to move the rack in linear fashion with this gear (most of modern car steerings work this way), utilizing rotational component would be not possible - the way it's not possible or extremely hard for many worms to be rotated by wheel. With the purpose of simplifying your analysis forget about teeth for a while - replace the gear with rollers and beams (no mutual slippage). The system on your drawing may work - but synchronization of both motors would be true pain. Also, you need a toothed wheel, two racks, two motors pulling the racks with some extra gear. Hardly simpler than a simple worm gear.
Just my professional 2c on this problem.