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Author Topic: Odd design - Will it work?  (Read 4576 times)
Alptraum
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« on: December 14, 2008, 02:12:13 pm »

I am in the middle of designing a sort of cat dumbwaiter with an auto return (this is because my cats can climb ladders but not get down them, which leads to incessant mewing and possible usage of my bed as a loo). The idea is to create something that is mostly self contained, can be fixed to the ceiling, and which will descend to around one meter above the floor if a cat steps onto it and then return itself to its old position when the cat gets off. I have a sheet of plans:
The point is that the counterweight will apply torque to the reel and cause it to wind up the rope again. I just have a feeling that it won't work. The counterweight weighs slightly more than the platform.
Thanks
BTW, does anyone have any better way to upload photos? linking to flickr can only last so long.
« Last Edit: December 14, 2008, 02:50:30 pm by Alptraum » Logged
tsjaikdus
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Netherlands Netherlands


« Reply #1 on: December 14, 2008, 03:09:43 pm »

first of all to get a stable platform that won't fly everywhere when the cat steps on it, you'll probably need linear guidance.

then you'll need a brake to reduce the dropping speed. First thing that comes into mind is an airbrake, like those used in old music boxes. I'm not sure what kind of gear train needs to be in between. Also depends on the size of the fan blades. My first guess would be using 90 and 18 teeth M1.5 gears. Twice. So the fan has 25 revolutions for every revolution of the wheel that drives the cord.

Just my guess based on stuff I've lying around here.
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Alptraum
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« Reply #2 on: December 14, 2008, 04:39:01 pm »

I've solved the platform stability problem by using 4 ropes instead of one. I like the airbrake idea, but I was planning on using an escapement.  I would like to use an airbrake, though - it would greatly simplify the  design and make it easier to draw on the CAD program I am using.
The problem is, will the counterweight cause the platform to go up again? As I said, I have a horrible feeling that this won't work. Also, won't the airbrake at some point gain enough momentum to move so fast as to become dangerous? I am under the impression that it will slowly speed up.
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H. MacHinery
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« Reply #3 on: December 14, 2008, 04:50:00 pm »

Might I suggest screen-door closers?   They are pneumatic, and return to the closed position - one (or more, depending on the adjustment) could be fastened to the top of the frame and to the platform, and when the cat is present, will slowly let out, and when the cat leaves, will slowly contract, raising the platform.

Might I also suggest perhaps a set of stair, made from cardboard?  A tight spiral might fit in the same space as your elevator, and would be cheap if you have recylced boxes as material.
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Alptraum
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« Reply #4 on: December 14, 2008, 05:00:02 pm »

My mum wants it to be mostly self contained, so a permanent fixture is out. that is why it is designed to hang from the ceiling.
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tsjaikdus
Snr. Officer
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Netherlands Netherlands


« Reply #5 on: December 14, 2008, 05:35:38 pm »

The stability that I think is not good is the moment that the cat steps onto the platform. The platform will simply be pushed away.

>> The problem is, will the counterweight cause the platform to go up again?

Why not? Depends on the fact if your escapement is bidirectional. However, the images are too small to exactly see what you're doing

As far as the air brake is concerned. The drag is proportional to the square of speed. For example if you drive twice as fast, your car will use 4 times as much gas. Thus it will cost more and more energy to speed up the blades at one point finding equilibrium at which they do not go any faster. This equilibrium is reached quickly as there's only little mass involved. So they would almost immediately be spinning at the final speed.

« Last Edit: December 14, 2008, 05:40:28 pm by tsjaikdus » Logged
HAC
Steam Theologian
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« Reply #6 on: December 14, 2008, 06:02:38 pm »

What kind of ceiling do you have? Make sure that you anchor into something solid like a joist. DO NOT TRUST drywall anchors, no matter what the box says. Kittehs will not forgive you being caught in a plummeting device...

Cheers
Harold
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tsjaikdus
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Netherlands Netherlands


« Reply #7 on: December 14, 2008, 06:49:25 pm »

Oh, procrastination, why are thou so sweet?

http://rapidshare.de/files/41135402/domestic_feline_lowering_platform_002.png.html
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Alptraum
Guest
« Reply #8 on: December 14, 2008, 08:54:50 pm »

The anchors would be bolted into hardwood structural beams, not a drywall. These support the entire weight of the roof, so I think that around 5 kilos more won't do any harm.
Here are some more pics - enlarged and with better handwriting:
This is an exploded view. The mechanism is mounted inside the platform.
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Anders
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« Reply #9 on: December 14, 2008, 09:50:12 pm »

Fascinating project idea!  If guide rails are out due to the ban on "permanent" structures, I say the four ropes are fine.  Close to the ceiling, the platform won't sway too far out when the cat steps on it, but you might want to check how it will swing when the cat jumps off at the bottom.

If you don't want to go to the trouble of building your own ratchet system (for engaging the air brake only one direction), see if you could use the brake drum from an old kids' bike--the kind where pedaling backwards engages the brake.  Just a suggestion, anyway.  Oh, and just to state the obvious (this is the kind of thing I would forget), remember to take into account how close you are to the ceiling so the air brake can spin!
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tsjaikdus
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Netherlands Netherlands


« Reply #10 on: December 14, 2008, 10:18:05 pm »

I do not understand the ratchet. Why would you engage the airbrake in a single direction only?

Then, IMHO, I do not agree with the acceptable swing close to the ceiling. I think the stability is overestimated.
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Alptraum
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« Reply #11 on: December 14, 2008, 11:35:12 pm »

Oh, and just to state the obvious (this is the kind of thing I would forget), remember to take into account how close you are to the ceiling so the air brake can spin!
Speaking of which, could I have the propeller on its side, maybe mounted underneath?
I still need to know if the idea for the retraction of the platform will work.
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von Corax
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Prof. Darwin Prætorius von Corax


« Reply #12 on: December 15, 2008, 06:09:40 am »

I see no problem with that at all. You could either use a 90º bevel gear to connect the winch reel to the airbrake, or you could mount the winch on a vertical axis. You just need to make sure that the counterweight without its rope weighs more than the platform with its rope, and that (counterweight + rope) weighs less than (platform + kitten).

As far as stability, had you considered rope "guiderails?" I assume your bed is a typical bunk-bed style, from your description of the problem. You could attach a pair of parallel ropes between the upper and lower bedrails and thread them through a pair of screweyes on the edge of the platform, something like this:
Code:
   -|--+ anchor
bed |  |
   -|  |
       |
eyelet +-==== platform
       |
       |
       | rope
       |
       |
   -|  |
bed |  |
   -|--+ anchor

If Mum won't let you run bolts into the bedrails, you could attach the ropes with small C-clamps.

Also, it appears you have one winch for the platform, and a second winch for the counterweight. If you feel like experimenting, you could try replacing the dual winch with a windlass. You would need a short, wide cylinder with an hour-glass cross-section. You attach one end of the rope to the platform and the other to the counterweight, and loop two or three turns in the middle of the rope around the windlass so that the rope slides freely when it's slack, but grips when it's under tension.

I hope that makes sense.
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Alptraum
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« Reply #13 on: December 15, 2008, 07:40:21 pm »

The idea is that the reel for the counterweight is the same as the reel for the ropes which hang it from the ceiling, separated by a disc only in order to prevent tangling. The return works by having the counterweight apply torque to the reel, causing it to wind up again, but I have a funny feeling that the forces will balance out since the ceiling is a fixed object, and the lift will just get stuck. My mum doesn't want it to have any permanent guiding structures. Nonetheless, the only problem is if the platform will return up again. I suppose I could make a prototype...I will do that now.
BTW, I don't think that you have fully understood the problem... the purpose of the design is to create a device to lower a cat from the floor above with no further action on the part of the cat than stepping on the platform. The whole construction will be bolted into hardwood structural beams that hold the roof up. Thus mounting is not a problem. I only want the airbrake to engage in one direction because it makes for a much faster return, and also partly for the challenge. However, come to think of it, this could mean having a massive chunk of wood flying towards the ceiling, and thus causing lots  of falling bits of broken wood. Sorry, thinking as I write.
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tsjaikdus
Snr. Officer
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Netherlands Netherlands


« Reply #14 on: December 15, 2008, 08:18:02 pm »

if you want a ratchet then I would mount it on the blades. Engaging big blades for lowering and a small pair of blades for quick return. Nevertheless because of the difference in load due to the presence or absence of the cat some quick return action is already part of the design
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von Corax
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Prof. Darwin Prætorius von Corax


« Reply #15 on: December 16, 2008, 04:14:04 am »

The idea is that the reel for the counterweight is the same as the reel for the ropes which hang it from the ceiling, separated by a disc only in order to prevent tangling. The return works by having the counterweight apply torque to the reel, causing it to wind up again, but I have a funny feeling that the forces will balance out since the ceiling is a fixed object, and the lift will just get stuck.

Ah – you want to have the reel moving up and down with the platform? Won't work. You're getting very close to overunity/perpetual motion territory with that concept. The mathematics of it simply becomes unworkable.

It would be much simpler, both in the mathematics and in the implementation, if the reel is stationary relative to the ceiling, and the counterweight and platform see-saw back and forth on either side of it.

BTW, I don't think that you have fully understood the problem... the purpose of the design is to create a device to lower a cat from the floor above...

Something like a loft? (It was the ladder that confused me. 'Round these parts, the "floor above" is usually reached by staircase...)

The reason I (and others) keep dwelling on the guiderail thing is that you need some way to prevent the platform from flipping out from under the cat when the cat steps on it, which would cause the cat's descent to be rather more precipitous than one would wish. Cat'll only do that once, and it will never set foot on the platform again no matter what you do.

OOH— Sudden brainstorm! I was about to suggest ballasting the platform for stability, but what about a basket? If the basket and handle are a single rigid structure, the cat's own weight will stabilize it. A nice cushioned lining, and your cat will be riding in luxury!
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tsjaikdus
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Netherlands Netherlands


« Reply #16 on: December 16, 2008, 09:49:29 am »

Yes, it will work. As long as the net movement of the weight is downwards, it will provide the energy to lift the platform. It will greatly complicate the design, but it will work.

Then I think the shape of the platform will not improve the tability issue. If you don't have linear guidance, then you'll at least need a docking station of some sort at the top to stabilize the system during boarding.
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Alptraum
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« Reply #17 on: December 16, 2008, 06:54:45 pm »

Again, the platform will be suspended by four ropes, in order to prevent rocking motion. the thing moving away from the floor is not too much of a problem, I think, because the swing will be two small to be a problem, especially with a cat. At the bottom, stability is not a problem anymore as there is nothing to keep stable - the only remaining function is for it to return to the ceiling.
Yes, it will work. As long as the net movement of the weight is downwards, it will provide the energy to lift the platform. It will greatly complicate the design, but it will work.
vs.
Ah – you want to have the reel moving up and down with the platform? Won't work. You're getting very close to overunity/perpetual motion territory with that concept. The mathematics of it simply becomes unworkable.
Which of you do I believe?
The thing that I am not sure of here is whether the reel, moving WITH the platform, which has a counterweight that is larger than the weight of the platform and the rope, but smaller than that of the platform, rope and cat, will have torque applied to it and wind up the rope again. I have this feeling that (and please do correct me if I  am wrong) this is like when you apply a force to the ground - at my level of study I know that whatever force is applied to the ground is equalled out, thus the ground stays stationary - I think. Since the rope is anchored to the ceiling, which rests its weight on the ground, I think  that the rope would simply constrict the reel instead of moving it, no matter what the weight.
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tsjaikdus
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Netherlands Netherlands


« Reply #18 on: December 16, 2008, 08:00:54 pm »

Maybe I do not understand the mechanism. The thing is that you'll need work to lift the platform. Work is force (gravity) x distance. So a falling weight will give you what you need. If the weight doesn't fall, then there's no work and the platform can not be lifted.

So the question is: does the distance of the weight to the ground decreases while the platform is lifted. Or better: does the center of mass of the total system move towards earth.

It is also possible to use a constant torque spring motor instead of a weigth. However they are difficult to get and they are dangerous to work with. But it would make a nice solution.
« Last Edit: December 16, 2008, 09:11:56 pm by tsjaikdus » Logged
Alptraum
Guest
« Reply #19 on: December 16, 2008, 10:07:42 pm »

O.K, I just made a very hasty prototype of the retraction mechanism, and it says no.
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tsjaikdus
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Netherlands Netherlands


« Reply #20 on: December 16, 2008, 10:39:29 pm »

Then, I think I do not understand what you are building.
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Otto Von Pifka
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« Reply #21 on: December 16, 2008, 11:23:32 pm »

ever notice how slow the handle goes on a bicycle pump when you have one of those little needle inflator tips on it? some sort of piston damper might work. even a bicycle pump might work if you use pulleys to shorten the travel of your counterweight, they could be attached directly to the pump handle.
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von Corax
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Prof. Darwin Prætorius von Corax


« Reply #22 on: December 17, 2008, 03:07:53 am »

Then, I think I do not understand what you are building.

As I understood Alptraum's description, he* would have had two winch reels connected by a common axle which was fixed to the moving platform. From one reel a rope would extend up to an anchor on the ceiling, and from the other a rope would extend downward to the counterweight. If one were to attempt to raise the platform by pulling down on the counterweight, one would find the counterweight pulling downward on the platform, counteracting any upward pull it might exert through the winch.

In theory it could be made to work if the winches were geared so that several rotations of the counterweight winch produced a single rotation of the elevating winch. However, this would require that the counterweight drop through several times the distance the platform rises, and that entire distance would have to be below the lowest position of the platform. In other words, Alptraum would need a whacking deep hole in the floor.

Then I think the shape of the platform will not improve the tability issue. If you don't have linear guidance, then you'll at least need a docking station of some sort at the top to stabilize the system during boarding.

I beg to differ. Try this experiment (or gedankenexperiment, if you prefer not to make a mess): suspend a square platform by the corners from a single rope. (Aside to Alptraum: I know you said four ropes, but if all four ropes go to the ceiling, you'll need four head-pulleys, additional guide pulleys and probably a four-partition winch to keep them untangled. A single rope will vastly simplify things.) Now gently toss a cat (or an approximately-cat-shaped object, like a beanbag) onto the platform, at a low angle from a short distance. I expect you'll find that if the "cat" lands close to the near edge, the platform will flip up and swing away, dropping the "cat" onto the floor; more often, the "cat" will simply slide off the far edge.

Now replace the platform with a shallow basket or box. (Earlier, I said "with a rigid handle," but now I think that is unnecessary.) Again, toss your "cat" into the basket, and I think you'll find it nearly impossible to do it in such a way that the "cat" falls out. (If you're using a real cat, the top edge of the basket should be between chest-height and eye-level when the cat is standing in the bottom.)

I do hope that has been, if long-winded, at least somewhat understandable.

Regards,

Prof. Darwin Prætorius von Corax
Senior Researcher in Theoretical Kinetofelinology
The Leverkusen Institute of Paleocybernetics

*I surmise "he," lacking any immediate evidence to the contrary. Do forgive me if I have guessed wrong.
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tsjaikdus
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Netherlands Netherlands


« Reply #23 on: December 17, 2008, 07:29:42 am »

just did an experiment with a toiletrol (how is of called?), 2 meter of cord an a small weigth. Having attached one end to the ceiling the weight was able to pull the cylinder upwards by unwinding his end of the cord
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Alptraum
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« Reply #24 on: December 17, 2008, 02:42:19 pm »

K. old  design scrapped. see this. keep in mind that the reel is attached to the platform.
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