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Motor for a gyroscope.

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Robin Graham28/12/2020 23:33:08
945 forum posts
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I have been asked to make a gyroscope, better than the 'toy' ones (but perhaps not to aerospace specs!)

I think I can do the machining OK, but I'm wondering about how to spin it up.

I came across a commercial site which offers gyroscopes stared by a motor which is powered by 4xAA batteries and achieves 12,000 rpm, but they don't seem to sell the motor alone, and I've drawn a blank in searching for something similar - probably because I don't know what I'm asking for.

Can anyone help?

Robin.

duncan webster28/12/2020 23:42:00
3947 forum posts
63 photos

Absolutely not what you asked for, but how about half round teeth in the rotor periphery and an air jet? Turbine guy who writes in this forum should have some good suggestions

Michael Gilligan28/12/2020 23:48:00
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20113 forum posts
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Funnily enough, Duncan ... we used gas motors in the Rapier

... See 1982 on this timeline : **LINK**

https://www.siliconsensing.com/technology/the-gyro-100-years-on/

MichaelG.

Martin Connelly29/12/2020 08:52:58
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2125 forum posts
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I think some gliders have artificial horizons that have air driven gyros. Airflow through a venturi creates a vacuum that results in the necessary pressure differential to cause air flow. This is from memory and over 40 years ago so may be dodgy info.

Martin C

Edited By Martin Connelly on 29/12/2020 08:53:33

Michael Gilligan29/12/2020 09:20:01
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20113 forum posts
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I’ve just looked at your linked site, Robin

That seems to be a basic little electric motor, with a coupling attached.

... The trick will be engineering the gyroscope and the coupling to spin-up with so little input.

MichaelG.

Adam Mara29/12/2020 09:47:50
167 forum posts
3 photos

My first thought reading the OP was air driven, which was backed up by the following comments!

Shortly after WW2 I was given a WD surplus Mk XIV Bombsight Computer as a birthday present, and it certainly contained an air driven gyro, and lots of other interesting bits and pieces, and still have a few bits left in my 'might be useful' draws!

SillyOldDuffer29/12/2020 10:10:42
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8516 forum posts
1915 photos
Posted by Michael Gilligan on 29/12/2020 09:20:01:

I’ve just looked at your linked site, Robin

That seems to be a basic little electric motor, with a coupling attached.

... The trick will be engineering the gyroscope and the coupling to spin-up with so little input.

MichaelG.

+1 to 'basic little electric motor'. Off load most basic DC motors spin at 10000rpm or more, which is usually too much, so better motors often contain a reduction gearbox. The trick is to find a motor without a built-in gearbox! How about this ebay example from China via Yorkshire (Might be over the top for a small Gyro - 20000rpm at 9V 2A)

For testing, a Dremel or cheaper alternative would do.

Dave

Michael Gilligan29/12/2020 10:31:43
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20113 forum posts
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Good illustrations and specs in this document: **LINK**

https://www.gyroscope.com/images/super/SuperManual9C-EN.pdf

MichaelG.

Peter Spink29/12/2020 10:35:23
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41 photos
Posted by duncan webster on 28/12/2020 23:42:00:

Absolutely not what you asked for, but how about half round teeth in the rotor periphery and an air jet? Turbine guy who writes in this forum should have some good suggestions

Like this?

Gyro 2.jpeg


Gyro 1.jpeg

Gyro 3.jpeg

I always assumed this was intended for a missile where the gyro would be spun up during the launch period and then the nozzle would retract when up to speed.
Goes at quite a rate on the compressor and spins for several minutes after nozzle pulled out.

Frances IoM29/12/2020 10:36:09
1250 forum posts
28 photos
could the gyro rotor not consist of an annulus of alternating polarity small rare earth magnets - the start can be given by a pair of coils placed over the rotor + driven in same way as brushless motors - pull the coils away when speed is obtained
John Haine29/12/2020 10:39:48
4644 forum posts
273 photos

Robin, could you clarify if you want something to start the gyro but then let it spin down (a replacement for the string in the toy ones) or a built-in motor to keep it spinning?

If the first it looks like the site you mentioned shows how to do it - one end ot the shaft engaged with a keyed end on the motor shaft, the motor can either be held in place to spin it up or attached with a couple of screws.

Another approach would be to have a part of the shaft which is a larger diameter (not the wheel itself which will presumably be shielded), to which you can apply a friction wheel with a rubber tyre, driven by something like a Dremel.

If you want a continuously running one why not just use an outrunner motor?  They will run at high speed, and have an external rotor revolving around the stator with probably quite a large MoI, and must include reasonable bearings.

Edited By John Haine on 29/12/2020 10:54:22

Robin Graham31/12/2020 00:17:50
945 forum posts
295 photos

Thanks for replies. To clarify, I was looking for for something to spin the gyroscope up rather than for continuous operation. The product I linked to claims a run time of up to 25 minutes starting at 12,000 rpm. I doubt that I can match their machining standards (they say that their discs are ' balanced to an impressive 250th of a gram accuracy', whatever that means) but I'd be happy with 5-10 minutes. To get up to 12,000 rpm with the 5mm shaft I'm planning by using a piece of string would mean (thanks to a fortuitous cancellation of numerical factors!) pulling the string with a final speed of precisely pi metres per second, which would be hard to do I think.

I can test it with a Dremel type tool for sure, but the person who asked me to make it is unlikely to have such a thing. The motor SoD linked to looks like the sort of thing I want - 12,000 rpm @ 6V/1.9A. That would be four AA's in series - but would I get 1.9A from that arrangement? A quick look suggests that the internal resistance of an alkaline AA is about 0.15 ohms, so in theory they should deliver 10A shorted. Really? That surprises me.

John - thanks for your suggestion of an outrunner motor. I'd never heard of such a thing, but have had a look and they seem to offer possibilities. I'll think on't!

Robin.

 

 

 

 

 

 

 

 

 

 

 

 

Edited By Robin Graham on 31/12/2020 00:18:59

Michael Gilligan31/12/2020 05:17:28
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20113 forum posts
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Posted by Robin Graham on 31/12/2020 00:17:50:

The motor SoD linked to looks like the sort of thing I want - 12,000 rpm @ 6V/1.9A. That would be four AA's in series - but would I get 1.9A from that arrangement? A quick look suggests that the internal resistance of an alkaline AA is about 0.15 ohms, so in theory they should deliver 10A shorted. Really? That surprises me.

[…]

.

Did you look at the numbers on p7 of the document that I linked [from your source], Robin ?

MichaelG.

Andrew Johnston31/12/2020 10:20:55
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6577 forum posts
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Posted by Martin Connelly on 29/12/2020 08:52:58:

I think some gliders have artificial horizons that have air driven gyros.

Powered aircraft often use air driven gyros as they're cheaper than electric gyros. However, sailplanes exclusively use electric gyros. Air driven ones create way too much drag. My small sailplane has a Ferranti horizon. As and when I replace it I'll go for an electronic horizon - solid state gyros and a LCD display. I taught myself to cloud fly on a turn and slip which is simpler, and cheaper, than a horizon, but more difficult to use.

Andrew

Roger Hart31/12/2020 13:20:38
143 forum posts
31 photos

I agree with the DC motor option, 4 AA cells will easily drive that.

I have made a few gyros and can say what does not work. One I made from an old disk drive motor, used a turnigy model plane motor driver + a 555 pulse generator and camera batteries. All in a double gimble. The idea being to show the slow rotation of the earth. Big snag was getting all that clutter on board the gyro to balance dynamically. It would always drift too much. Also machining the 3 inch wheel was not so easy for vibration-proof running. Also such a gyro would need a much larger diameter wheel in brass, the clutter away from the gyro (slip rings etc) and better symmetry.

Next try used the same wheel with ex disk drive ball bearings and a string pull. The string pull was no problem provided good string was used. You can easily pull 18" of wrapped string in 0.1 seconds. But the ball bearings I used did not seem any better than traditional point-and-cup bearings.

So far as I can see the gyro shown does not have a large wheel, the frame looks heavy in relation to the wheel size. It may run for 20 mins but I doubt it stands up all that long. By comparison an old style child's toy gyro had a 3" wheel made of lead (brass painted) with a fairly light steel frame. The key issue is the moment of inertia of the rotating wheel - large == better and large == expensive especially in brass and a lightish frame if the thing is to stand up. The question is what is the gyro for. Navigation gyros tend to be cylindrical lumps whilst demonstration gyros need most of the mass in the rim if they are to stand up. The two requirements are different.

Is there anything different about the gyro you intend to make? Good luck and enjoy.

Robin Graham01/01/2021 00:33:04
945 forum posts
295 photos

Thanks for further replies.

MichaelG - somehow I missed your 3rd contribution, and hadn't looked at your link. Mea culpa! I have now had a look - useful info, thanks for that.

Roger - thanks for sharing your your gyro making experiences.The one I want to make is for demonstration purposes - specifically as a teaching aid in talking about angular momentum. I confess that I hadn't given much thought to the profile of the disc, assuming that the greater the moment of inertia about Z (the spinning axis), the better. I now see that it is a bit more complicated than that if the aim is for the gyro to 'stand up'. My plan is to make the disc from 3" dia x 1 inch brass (expensive, but I have a lump in the cutoff bin) with an aluminium frame.

My thanks to other contributors also - the info about air-driven devices etc isn't really germane to my original question, but is very interesting.

Robin.

Danny M2Z01/01/2021 02:50:41
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963 forum posts
2 photos

When I used to teach radar technicians about gyroscopic precession in a ferrite isolator my demonstration model of choice was the front wheel from a bicycle with a pair of wooden handles attached to the hub.

I used to get the students to hold the handles while another would spin it, then told the student on the handles to twist the wheel.

They soon learned about gyroscopic precession.

Edited By Danny M2Z on 01/01/2021 02:51:37

Clive Hartland01/01/2021 08:46:14
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2812 forum posts
40 photos

When I worked on the Wild heerbrugg Gyro's the motors were 3 phase driven, made by Rellensman.

Obsolete now, the power was passed to the motor through very fine tapes.Battery powered

The accuracy was about 20 secs. for magnetic North and sets were run and measurements taken to ascertain the true heading and accuracy, very tedious to do.

In fact they were used to align the weapons system in aircraft. The maritime aircraft had a powerful light and it had to point where they wanted it see the target. One system salvaged from Blue streak was used by the artillery to self locate for indirect firing but again was very finicky to set up and use. That one ran in a container of liquid that needed heating and was susceptable to cold wind on one side. so had a shield around it.Run up time was something like 20 to 30 minutes.Again obsolete now.

duncan webster01/01/2021 20:31:51
3947 forum posts
63 photos
Posted by Robin Graham on 31/12/2020 00:17:50:

........machining standards (they say that their discs are ' balanced to an impressive 250th of a gram accuracy', whatever that means) but I'd be happy with 5-10 minutes. ........

It doesn't mean anything, imbalance is measured in gram*mm or any other mass * distance units

duncan webster01/01/2021 20:35:35
3947 forum posts
63 photos
double post deleted

Edited By duncan webster on 01/01/2021 20:36:00

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