Mike R-F
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| posted on 1/2/06 at 12:20 AM |
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Engineering genius required!
I realise this is completely off topic but I need some help in creating something that may be impossible, but, hey, that's never stopped anyone
on this site before!
As part of an ongoing project, I need to produce a giant set of 'working the wrong way round' scales. In other words, scales that go down
on the lightest side and up on the heavier side. Something a bit like the balance in Justice's hand on top of the Old Bailey. The height of this
would be about 1.5-2M high at the point of pivot & about 4M across the balance arm. The mechanics for making the scales work in reverse can be
hidden within the central pivot section.
This problem has been driving me up the wall for some time now because I think it
should be possible, I just don't know how!
If anyone comes up with an idea that works, you'll get my vote for engineer of the year!
Mike
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k33ts
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| posted on 1/2/06 at 12:32 AM |
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surely the only way is moving the pivot point then that would only work depending on the weight difference
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Liam
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| posted on 1/2/06 at 12:50 AM |
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Trouble is you want to lift a heavy object with a light object without any mechanical advantage (leverage). You have to put energy into the system
somehow to achieve this. I can't see how it can work without some kind of power source therefore.
You could do it without power by having the pivot very close to the heavy object and very far from the light object, but it wouldn't really look
like scales and would only work one way round.
Liam
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iank
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| posted on 1/2/06 at 01:16 AM |
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What you need there is an anti-gravity machine. Where do I claim my prize.  
If you actually want it to work like scales i.e. look like scales and move 'realistically' i.e. exactly the same as real scales but upside
down, you will need to actively power the thing. The control system will also need to know which is heaver (and by how much if you want to control
the speed for realism purposes).
Measuring the weight should be possible with strain gauges on the arms, and getting the movement could be electric, hydraulic or pneumatic depending
on what kind of weight you want to move around (from the size of the thing you could get some big stuff on there), and how you want it to
look/sound/cost etc etc.
The control system is the key to it's realism, and might not be trivial to get right.
Oh and if people are going to be putting stuff on with their hands you might want to consider safety - it's a big chunk of metal/wood to have
swinging around close to heads. Especially as it's designed to be completely counter-intuitive 
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stevebubs
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| posted on 1/2/06 at 01:31 AM |
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Everything is relative... stand on your head...
I'll get my coat....
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stevebubs
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| posted on 1/2/06 at 01:35 AM |
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More seriously, and given that an anti-grav machine is against us so it must be powered, how about...
Fit a set of scales under the pad you're placing objects into. This will give you the mass of the object placed. Feed this input into a control
system
Drive the whole assembly on a giant stepper motor or whatever so all up/down movement is controlled electronically based upon the mass input...
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stevebubs
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| posted on 1/2/06 at 01:36 AM |
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BTW If you need to measure the difference between the two sides, you'll obviously need a set of scales / pressure pads etc on both sides...
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iank
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| posted on 1/2/06 at 01:58 AM |
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quote:
Originally posted by stevebubs
...
Drive the whole assembly on a giant stepper motor or whatever so all up/down movement is controlled electronically based upon the mass input...
A stepper motor that could move a 4 meter beam would be an impressive beast indeed 
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Mix
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| posted on 1/2/06 at 07:28 AM |
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Hydraulics are one solution
The weight acting on the pivot bar would operate a pilot valve to control a servo piston which would move the beam, with a bit more thought I could
probably come up with a system to make the movement proportional to the load.
Mick
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Mike R-F
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| posted on 1/2/06 at 07:59 AM |
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Thanks for all the input, even the anti-grav suggestion! I should have mentioned that the site this device is going onto has no power so the mechanism
has to be purly mechanical. The suggestion of using hydraulics appeals. Any mileage in this?
A simple way of achieving the desired result would be to have one half of the balance arm constructed out of a heavier material than the other &
'locking off' the scales in equilibrium. Then, once a 'heavy' & 'light' object had been placed on the scales,
a pin could be removed allowing the balance to work in the opposite way to convention. But I still feel sure that inter-connected pistons or springs
would create the same effect without the need to lock the balance arm off or resort to such a crude solution.
Keep them comming, please!
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Mike R-F
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| posted on 1/2/06 at 08:09 AM |
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Mick, you have U2U.
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caber
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| posted on 1/2/06 at 08:10 AM |
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What you have is probably the best suggestion however I am not sure how the scales will reset themselves with no load or input from the user. Before
you said no power I was thinking about adapting a power steering rack to provide force. There has to be some power input somewhere, any number of
pulleys and levers will absorb energy by frictionand you still need to work out the amount of force you will be applying to determine the multiplying
factor between light and heavy sides.
Caber
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cossey
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| posted on 1/2/06 at 08:31 AM |
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a way round it would be to have a normal set of scales made of aluminium but with iron/steel pans. then place the whole scale setup on a pedestal and
put electromagnets in that then with the items removed and the magnets off they arm will balance straight but turn on the magnets as you place the
items and you can raise the heavy side and drop the lightside.
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Mike R-F
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| posted on 1/2/06 at 08:52 AM |
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Caber, re-setting the scales is not a problem. This is intended for an educational facility where groups of school children will be guided round &
a numer of the activities will be 'set up' by the guide prior to the tour. All we are trying to achieve is a little bit of
'magic' where things don't work the way kids would expect them to.
Electromagnets wold be great if I had the means to power them which, unfortunatly, I don't.
Thanks again for all the suggestions, please keep them comming.
Mike
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cossey
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| posted on 1/2/06 at 08:57 AM |
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if it is completely power free then it is impossible. however if you mean you cant use electricity then you are going to have to use springs or
pneumatics with a small cylinder of compressed gas hidden somewhere.
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nick205
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| posted on 1/2/06 at 09:02 AM |
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Can we ask what you need to do this for?
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DaveFJ
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| posted on 1/2/06 at 09:04 AM |
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how about making the arm to the smal weight hollow and having a weight inside that can move. maybe you could connect it to the pan so that a small
object would move the weigth along the arm... something like this....
 
Description
Dave
"In Support of Help the Heroes" - Always
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skint scotsman
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| posted on 1/2/06 at 09:19 AM |
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as its for school kids couldn't you just make one side of the scales heavier than the other but look the same lock them of at the
'balance' point then make abit of a show and release the mechanism.
you would have to get your sums right so as there was no way the little blighters could upset the balance by overloading the supposed heavy side.
just my 2 euros worth hope it makes sense
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DarrenW
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| posted on 1/2/06 at 09:39 AM |
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Why not make the scales in a conventional manner but play about with the weights.
Make the smaller weight out of lead and Paint on the front 10Kg
Make the larger weight out of something hollow, fill it with sand so it is just lighter than the smaller one and paint on the front 20Kg.
Net effect is the larger and assumed heavier weight will move up whereas the smaller and assumed lighter weight will go down.
Iam making the assumption here that the scales are huge, kids are not and you wont be allowing anyone to touch or feel the weights first.
You could also have a small set in front for the kids to play with that work normally.
[Edited on 1/2/06 by DarrenW]
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jos
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| posted on 1/2/06 at 11:41 AM |
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I just had a thought, coul dyou use a gyroscope to create an opposite force. I remember being sat on a chair (at school) with castors holding the
axle of a spinning bicycle wheel and making myself turn by using opposing lean angles on the axle.
Just a thought, but probably not possible due to lack of power for rotating or powering the gryo
.: Motorsport / motor racing circuit / track wall art Apex Traxs :.
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MikeRJ
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| posted on 1/2/06 at 12:23 PM |
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If the heavy weight has to move up as much as the light weight moves down, then without resorting to trickery and illusions this is physicaly
impossible to achieve. If you do manage this, then congratulations, you have the beginnings of a perpetual energy machine
Pre-loading the scales with hidden weights and being carefull never to show the balanced "unloaded" is probably the easiest method.
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Mike R-F
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| posted on 1/2/06 at 12:45 PM |
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Hi all, thanks for so much input on this. I like DaveFJs idea of a concealed moveable weight.
In answer to the questions about the purpose of this, I should perhaps explain a little. The scales form part of an outdoor educational facility being
built by a local authority to motivate schoolchildren to think more positivly about the environment and, in particular, the way we dispose of waste.
Without getting into the graphic message, the scales are intended to form part of an area about 'Restoring the Balance' in which children
would look into the problems of landfill/incineration/recycling, hence the scales. E.g., one bag full of plastic milk bottles (light) put into
landfill is worse for the environment than a months worth of organic waste (heavy) put into composting, which is good. I appreciate that you can argue
that this would work with the scales in a conventional manner but the way in which the message is put across works best with the scales in reverse.
The scales would be operated by the guide, not the children, so use of hidden weights/levers could work well.
Hope this helps!
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Spyderman
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| posted on 1/2/06 at 01:39 PM |
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One way I can see it working would involve mirrors.
If the scales were in a cabinet behind a selection of mirrors then you could make left seem right and right seem left. As long as both weights looked
similar in shape and size then the audience would not know they were seeing the opposite side.
Again using mirrors the scales could be built upside down, but would look right way up from front then the different size and weights would appear to
go against gravity. Both weights would need to be magnetic to stick to upside down pan.
Another method would be to use magnets. With a powerful magnet under one side of the scales (or both) it would affect which way the scales tipped,
dependent on which way the poles of the magnetic weights were placed onto scales.
Probably not very clear, but I know what I mean!
Spyderman
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jambojeef
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| posted on 1/2/06 at 02:31 PM |
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How about this?
Its based on Dave's idea but the balance should go the other way when the weight is placed on the scales...
If the balance of weights is correct and stuff.....?
Geoff
 
Rescued attachment balance.JPG
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cossey
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| posted on 1/2/06 at 03:32 PM |
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that would work if you had a spring going from the left of the weight to the arm that way the weight will only move by the right amount if carefully
setup.
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