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O-ring coming off pulley when motor power is cut

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Maff10/11/2009 20:25:29
3 forum posts
Hi!
 
I am making a motorised zeotrope which involves a spinning disc connected to a motor.
 
I bought a MFA/COMO 940D 100:1 Motor-Gearbox from maplin and have set it up to drive the disc which is connected to a 6mm drive shaft (mounted with bearings top and bottom), 16mm alum pulley and a large o-ring (all from Maplin).
 
The disc spins perfectly until the power to the motor is cut, because the momentum of the disc keeps it spinning but resistance from the motor tries to stop it until the belt comes off.
 
How can I get aroud this? Can you buy a freewheel ratchet 'thing' (Sorry... I'm a bit new to model-making!)?
 
Many many thanks for any advice you can give,
 
Cheers!
Martin W10/11/2009 23:09:49
940 forum posts
30 photos
Hi
 
You could use a speed controller, the same sort of thing as used in model railways or model cars, then you can control the rate of speed up/ down and the viewing speed well. Or it may be possible to slacken the belt slightly so that its braking action is moderated but again it may just fall off under braking loads. This of course will in time wear the drive band out.
 
I think that the first suggestion is preferable as this limits the stresses put on the system to an acceptable level and should preserve the mechanism. Anyhow it could be fun varying the speed of the action from Charlie Chaplin film style to slow motion.
 
Cheers
 
Martin
russell10/11/2009 23:33:18
142 forum posts
could you put 'flanges' on the pulley? It sounds to me like the belt is slightly too thick for the groove in the pulley, so it can ride out. (or is the motor geared down by a worm drive. In which case you will absolutely need at least a clutch, because your disk will never be able to drive the motor...)
 
There are freewheel mechanisms, which do exactly what you describe. A ratchet would be noisy, but there is a mech. that uses a ball bearing in a ramp arrangment, so that if the shaft rotates one way the ball is pulled down and locks, the other way it lifts and releases.
 
(i pulled one out of an old photocopier)
 
hope these ramblings help.
 
russell
 
 
Maff10/11/2009 23:57:18
3 forum posts
Cheers for the responses guys!
 
 Unfortunately I won't be able to vary the speed because I won't be operating this manually (I've set up a coin chute where the coin trips a contact which triggers a 30-second time delay switch - it's for a christmas charity window display).
 
I tried tightening the pulley before I read these posts which worked but I imagine it might wear the motor and belt quicker. Will try slackening belt and the flanges thing tomorrow.
 
Any ideas on where I can buy that freewheel thing?
 
Thanks again!
GoCreate11/11/2009 00:07:50
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387 forum posts
119 photos
Hi
My thoughts
'O' rings are an elastomer and very elastic, under a heavy breaking load induced due to the momentum of parts rotating, one side of the 'O' ring could be stretching significantly, as this happens the opposite side will become slacker and more vulnerable to coming off the pulley. It would be better to use a stiffer belt material such as polyurethane. You can get small section polyurethane cord  which you cut to length then mould the ends together, one way to do this is to cut the ends square, heat a thin piece of metal to a temperature that softens the polyurethane, place the ends either side long enough for the polyurethane to begin to soften, quickly remove the metal and push the ends together.

I you want to stick with an 'O;ring then a thicker section may stretch less under the breaking load and stay on, other than that the roller clutch or pulley flanges suggested above should cure the problem.

If the 'O' ring you are using is stretching significantly under breaking load it could be prone to failure and therefore, if using the same 'O' ring a roller clutch would be the better solution.

Some small machine tools use small section polyurethane belts, I thick the Taig is one, may be one of these will fit.

Nigel
russell11/11/2009 02:06:27
142 forum posts
i think what you need is a 'sprag clutch' - see www.renolds.com (who seem to be in UK)
 
Roller-ramp, or one-way roller bearings do a similar job. No doubt there are design criteria which would dictate what is best, but i'd contact a local bearing supplier.
 
regards
 
russell
 
 
John Haine11/11/2009 09:17:40
5563 forum posts
322 photos

Edited By David Clark 1 on 11/11/2009 09:56:26

Martin Cottrell11/11/2009 17:38:16
297 forum posts
18 photos
Hi Maff,
 
I fly (crash mainly!!) model helicopters and the main rotor drive utilises what is called a 'one way bearing' which allows the motor to drive the rotor blades in the correct direction but allows the motor to stop whilst the blades release their inertia by spinning down slowly when the power is cut to the motor.
 
As mentioned by John above, I would try Arc Euro Trade as I'm sure they do similar bearings and roller clutches in "our" sizes.
 
Regards, Martin. 
David Colwill11/11/2009 18:54:03
782 forum posts
40 photos
You could try a magnetic clutch I found some in old photo copiers. Ebay may come through.
Ian Abbott11/11/2009 19:08:54
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279 forum posts
21 photos
How about a toothed belt and pulleys?
 
Ian 
keithmart11/11/2009 19:52:47
avatar
165 forum posts

Hi


What about a bicycle freewheel?


Keith


Leeds uk

Ian S C12/11/2009 10:18:03
avatar
7468 forum posts
230 photos
The bendix drive on a car starter motor.IAN S C
Circlip12/11/2009 10:24:18
1723 forum posts
If you see the principle Maff you could easily make a single rollered version.
 
   Regards  Ian.
Martin W12/11/2009 14:05:26
940 forum posts
30 photos
Martin
 
As an ex air whisker and thrasher I was thankful on more than one occasion that the free wheel unit was there. It was for me a very necessary part of the gear train especially when the engine stops unexpectedly!!  At least there is a chance of an auto-rotation of sorts. Never tried to auto-rotate deliberately though as was not to that skill level.
 
Got two airframes hanging from the ceiling but haven't flown for years. Last rebuild found me digging the gyro out of the ground after a cyclic servo failure!!
 
Still that's not for this thread. Take care and have lots of soft landings, any landing you can fly after is a good landing!
 
Cheers
 
Martin W
Maff12/11/2009 20:05:50
3 forum posts
Cheers for all the great advice people!
 
I'm still not sure though on how I could get one of these roller freewheel bearings to work... as in I'm not sure where it would go on the unit. Perhaps once buying it it will all become clear. You see the pulley is clamped onto the motor driveshaft (horiznotal) with a belt to a pulley clamped onto the vertical driveshaft with the disc clamped on top. Where would a freewheel clutch bearing fit into that?
John Haine12/11/2009 21:07:21
5563 forum posts
322 photos
Ideally, fit the roller clutch in the bore of the driven pulley so the vertical driveshaft runs inside it, putting the clutch the right way round so that the pulley drives the shaft but if the pulley stops the shaft will continue to rotate freely.  Alternatively, put the clutch between the motor shaft and its pulley.  Obviously you will need to provide some bushing to stop the pulleys moving along parallel to their shaft.
Billy Mills13/11/2009 00:54:43
377 forum posts
Hi All
Remove the grubscrew from the pulley, use a collar below the pulley to stop it dropping.
Fit a second collar above the pulley with a pivoting lever. Arrange for the lever to drop onto the top surface of the pulley at a shallow angle. When the pulley is driven the end of the lever will dig in and drive the shaft. When the pulley stops the lever will gently trail around the flange.
 
Improve the drive by cutting a few shallow slots in the flange if needed.
 
AND/
OR
Get an electronic friend to build a darlington emitter follower to power the motor. Base has 100uF to ground and a preset  47K resistor across  the capacitor. To run the motor the timer charges the capacitor to the supply voltage. When the timer times out the voltage to the motor is smoothly reduced to zero at a rate set by the preset.
 
AND/OR
Add a bit of friction to the disc to slow it more rapidly. Also  you could have a solenoid wired across the motor pulling off a brake when the motor was running but stopping the disc quickly when timed out.
 
If you are making a  coin operated machine then a good electronic coin mech would be a very good start. Go to your local coin-op company and shout abuse. They will then throw MS111 obsolete electronic mechs at you. Use enough abuse to also get the front plate mountings and the two orange plastic clips. You can set the mech to accept your wanted coin and reject ( return) all others.  Otherwise build an impressive collection of dud coins.
 
Alan.

Ian S C13/11/2009 10:15:42
avatar
7468 forum posts
230 photos
Hi maff,Just follow John Haines advice,have a look at a bicycle,thats exactly the same.I got some roller clutches out of some old printers,they are fitted inside the hub of a nylon gear and drive on a 5mm shaft.IAN S C

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