Avanine-Commuter
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Need Ideas- Car Project!
Here is the rubric verbatim:
Joule Mobile Mass Transporter
The Objective is to build a device which will transport a 1 Kg mass a distance of 10.0 meters over a level surface (as level as possible) using the
energy released as the 1 Kg mass falls a distance of exactly 0.100 meters.
Rules:
1. A 1 Kg mass cyclinder (you will be provided with the dimensions in class) with a hook on top will be supplied for use in the competition. The mass
must be carried (not rolled or dragged across the floor) by the transporter.
2. The energy to run the transporter is to be stored by lifting the mass, vertically, 10.0 cm above the place (which must be parallel to the floor)
where it will rest on the transporter at the end of the run. The mass can not move or rest on a slant with relation to the floor. The force of the
falling mass will propel the transporter. Since the falling mass is to supply no more than one joule of energy to the transporter, care should be
taken to see that its center of mass does not fall greater than 10.0 cm and that it does not swing during the movement of the transporter.
3. All parts of the transporter, including the falling mass, must move with the transporter and cross the finish line.
4. The transporter must stop on its own using only the friction provided by the wheels and the floor surface. There will be no braking mechanism.
5. After the transporter begins to move, it must be self guided. If it does not move in a straight line, the component of its motion in the designated
direction will be measured as the official distance covered. If a part falls off the transporter, then the component in the designated direction to
that part will be measured.
6. If a rule clarification seems to be required, it should be discussed with your teacher before the due date. "Cute" solutions which meet the letter
of the rules but which violate the spirit of the contest will usually be judged unsatisfactory.
Grading:
120 points
40 points will be determined by workmanship/ design
80 points will be determined by its performance or functionality
Cute means smart-aleck btw. And also no motors or engines, but pullies are allowed as long as they don't add energy to the contraption.
Well basically I to drop a 1Kg mass 10 cm onto some sort of contraption I build my self, and that has to move the entire contraption including the
mass 10 meters. No limitations of size/materials. All I need are ideas, and since I have NO background on mechanics whatsoever (yea, the teacher is
teaching the section on mechanics AFTER this project!), I am clueless! Please, any ideas at all?
The only thing I thought of was to tie the weight to a pully so that when it is dropped onto a flat platform on the contraption, the string will pull
and spin the wheels?
Please, help me! My grade depends on this! 
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Ozone
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Cool!
Look up those friction car toys (the ones where you rev them up on the floor until a flywheel inside is moving at good RPM and then release--it goes
quite far). Then consider dropping the car with the weight attached along a string. This string could add your Joule to a flywheel, which then could
take off (via some clutching mechanism) when it hits the ground. If I have time, I'll do up a graphic; I must go to sleep now so...
Goodnight,
O3
Too bad you can't use a beer can (with the beer inside) as your mass...
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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12AX7
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So, er, is the goal 10.0 meters as close as possible, or anything greater than 10m?
The optimal solution is a massless cart powered by the energy of the weight. The weight must be moved, so at best, all its gravitational energy goes
into kinetic forward energy (which is quite easy to do with a quarter circle ramp or, if you prefer, a brachistochrone or some other neat curve).
From there on out, friction brings it to a stop. Clearly, frictional loss must also be minimized.
Practically, a light frame made of carbon fiber or other composites (or if this is too cheap a project, aluminum or light woods) is equipped with
wheels, several (say, an axle or two) driven by the weight. Wheel base should be sufficient for the height and mass carried. Drive could be
furnished by a cord wrapped around a drum fixed to the axle (possibly using the axle itself, if the right diameter), with the free end looped through
a pulley to the weight. A guide and stop is provided for the weight to ensure it remains with the machine per rule #3. A mechanism is provided to
release the string when the weight has delivered all its energy, allowing the machine to coast without rewinding like a yo-yo. A stiff end could be
fixed to the shaft end of the cord, so that it can be chinked into a slot in the shaft, held in place by tension. When the tension reverses
tangential direction, it flops out and for convienience, gets cought on a retainer.
The diameter of the drive drum should be small enough that energy is released as slow as optimal, as frictional loss is higher at higher speeds. It
also gets higher at very low speeds, due to static friction. Probably, walking pace will end up a reasonable rate. Mind to use good ball bearings
for the axles and the pulley, and use a reasonably sound cord such as fine steel cord or fishing line. A big fat rope that has to be flexed as it
unwinds and goes around the pulley isn't any good.
That was too easy. I'd rather design an induction heater, but, I already did. Dang...
Tim
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Rosco Bodine
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The mention earlier of something which would work like
a yoyo is right on track . But the two outer wheels
would probably be something like a foot diameter or
a bit larger disks , spaced perhaps four to six inches apart , on a fixed axle , and around the axle would
be an overrun clutch like a bicycle wheel , so that
when the descending weight had reached the lower
limit the entire thing would coast ....after the last
loop of cord around the hub had unwound . Something
like a small pin or retainer wire could be yanked out
by the last half loop to provide the clutch action ,
and the bearings for the overrun clutch hub could be a couple of skateboard ball bearings . The diameter of the
hub and the torque couple would have have to be worked out experimentally ....to match the mass of
the discs ...and their angular momentum derivable
from the unwinding counterweight . The wheels might
be made from styrofoam disks ....and then their mass
could be adjusted upwards by gluing washers as
weights on opposite sides ....but everything would have to be kept balanced and carefully calibrated for the
thing to work . Also the 10 meter roll would have to be across a surface that is dead level and glass smooth .
[Edited on 11-1-2007 by Rosco Bodine]
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Avanine-Commuter
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Wow thanks for all the ideas! Since you guys are all knowledgable in physics, I assume, I will try your ideas... if I know what you are talking about!
I don't understand exactly how to make the model with what's given right now. I sort of understand the concept but am lost on the construction.
It would be great if it can be simplified a little bit, please.
so option one= pully with string, created so wheels like a yo-yo that won't wind back up. This would be great.
option two= sort of like those toy cars that you roll back and let go. I like this one but I need to be certain that that doesn't add to the energy.
If it is, then this should be great and I can construct it.
If I get enough time I might make BOTH and see which one works better 
btw the goal is 10m, anything over is EXTRA CREDIT!!!!
Thanks so much for helping out, you guys are saving my life! 
[Edited on 11-1-2007 by Avanine-Commuter]
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Rosco Bodine
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You know those large spools like electrical wire
is wound on , the bulk reels which have a cylindrical
core with disks attached to the ends ?
If you roll one of those empty reels across the floor
it will roll in a pretty straight line .
What we are getting at is that if your 1Kg counter
weight was fastened to the end of your 10cm of
cord and wound around the hub of such a reel ,
when you set it down , the weight unwinding the cord
would cause the reel and weight to roll .....
but there must be a clutch mechanism that will
release when the string is fully unwound and the
thing is rolling along or else it will simply reel the
weight back up and come to a stop . So you see
it must have some sort of clutch mechanism for the hub
so the reel and weight can coast after the weight descends .
It is like the rear hub on a bicycle , when you stop
pedaling the bicycle coasts , and you hear a faint clicking noise that is the pawls in the overrun clutch in the
rear hub , they are slipping over the teeth inside
the hub . That is the overrun clutch we are talking about . You have to make the hub for the the reel
around which you wind the 10cm string attached
to the 1Kg weight so that it functions much the same way
as the rear hub of a bicycle ....but turned inside out
to what is the mechanism arrangement for the rear hub of a bicycle .
I will tell you that what your teacher is asking is absolutely
an impossible task , and you can tell him that a pilot asked
what sailplane ever built has a 100 to 1 glide ratio ,
and so what makes him think that he will beat that
with anything mechanical , with rolling friction ......
which is simply preposterous on its face , and politely tell him to go screw himself , just because I said so 
The only possibility here for a 100 to 1 glide ratio would be a maglev track of some variety .....it just can't be done with dumb wheels and dead
weight .
To illustrate the mechanics of the problem which your teacher
proposes , consider that it is the same as if you had a very large disk shaped inflated rubber balloon something like a millstone balanced on edge as
if it were a wheel ready to roll
across a perfectly flat surface . The balloon is inflated with
a mixture of helium or other light gas to offset much of its
weight towards.... but not reaching neutral buoyancy ,
and the balloon is large many meters in diameter , sitting
on the flat floor of a draft free aircraft hangar . You place
your 1Kg weight inside the rim of the disk shaped balloon
at a point where the curvature of the rim places it 10cm above the floor .....so the rim weight creates an unbalanced wheel , and the diameter of the
disk is large enough so that
it will traverse the 10 meters as the rim weight causes the
"wheel" to roll to the position where the center of gravity
is nulled as the weight is nearest the floor .
With such an arrangement you could probably get the
100:1 horizontal travel to vertical displacement .....which is essentially like water flowing across a 1% grade .
But other than these sorts of fluid scenarios , or possibly a
pendulum travelling an arc , or possibly a ball or diamond shaped conical bearing traversing two sightly out of parallel divergent rails , you
probably won't find any mechanical system that will operate across a 100:1 slope .....because of friction .
Those hand lever operated railway buggies where two people work the treadle in a see-saw fashion to move the
buggy along the rails are something like what your teacher
is suggesting ......and getting a 100:1 translation from a
mechanical system is pretty much unobtainable , so
probably it is the realization of this that is the point of the assignment .......and everyone who builds a model is
simply investing a lot of time and effort in the futility
of what has no possibility of working , and has value only as a mental exercise and a learning experience .
Teachers are good at assigning impossible tasks ,
aren't they , and they should get paid extra just for
thinking up this stuff
[Edited on 11-1-2007 by Rosco Bodine]
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franklyn
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Try this
This sort of " contest " is usually held at engineering universities
to test mechanical aptitude. I take it you realize that getting a
ready made design defeats the purpose of the excercise. Still the
best ideas are old ones tried and tested so I don't see the need
to reinvent the wheel here.
Attached is a sketch for layout of parts for a tricycle held together
by a metal box in the shape of a prism. The rules do specifiy that the
weight must fall in a straight line , so a swinging counterweight
somewhat like a seesaw is used to pull two parallel lines attached to
the rear axle. The verticle distance will be to where the weight comes
to rest on a crossbar on the frame. Arrange the geometry so that the
weight hangs dangling from the swing arm.
If you have trouble visualizing this look at a picture of a Watt steam
engine to see how the verticle motions are coupled to the seesaw.
Wiki has one here _ http://en.wikipedia.org/wiki/Watt_steam_engine
This arrangement imparts forward momentum to the entire vehicle
which then will travel until this is dissipated and it rolls to a stop.
To disengage the driving pull line wrapped around the axle, attach
the end to the sleeve pawl arrangement shown in the inset on the
left of the sketch. This sleeve hooks a pin on the axle to pull , then
when all the line has been pulled and reached the end , the axle
continues to turn ( carried by forward momentum ) and kicks out
the sleeve pawl sideways on the axle.
IMPORTANT the weight must come to rest BEFORE
the line disengages, otherwise it will be held taught against the axle
acting as a friction break.
Rolling and frictional resistence must be reduced to almost nothing.
You do this by N O T using bearings for the axles. Reduce the contact
surface with the frame by countersinking the 4 holes for the 2 axles.
Initially this should be a tight fit which you will work into a slip fit.
Grind a pencil lead on sandpaper for graphite to lubricate this.
The axles are of course rigidly fixed to the wheels. The " tire " portion
of the wheels must be no more than a sixteen of an inch or so thick ,
should be solid metal or plastic , very very smooth and very round.
[Edited on 12-1-2007 by franklyn]
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franklyn
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Try this instead
UPDATE
Of course the minimalist version of this is shown in
a thumbnail sketch here below , and reduces
everything to only the essentials.
[Edited on 12-1-2007 by franklyn]
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Avanine-Commuter
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Okay so I've decide to do what everyone here pointed to:
the string on an axle with a clutch.
The pics franklyn attached really helps, but I still need a little bit more help.
So the materials I need are:
1. fishing string
2. wood base?
3. four thin wheels (CDs?)
4. two poles for the axle
5. a pawl (where can I get this? Hardware store?)
6. Hot glue to peice things together
Now as for the procedures, how do I get the pawl on the pole, and what will stop the pawl? Is the pawl glued on or moveable? Where exactly does it
need to be?
Where is the string attached to, and how is it connected to the wheels?
Sorry if I'm asking for a lot, but I really need step by step help with this. Thanks for much to the people who have given their time to help already.
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franklyn
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Parts list :
2 , 16 inch bicycle wheels without tires
10 inches of 7/16 threaded rod
4 lock nuts for the above , 2 per wheel
5 inch sleeve metal tubing 7/16 inside diameter
4 inches of coat hanger wire
plastic sleeve tube to serve as pawl
22 inch long piece of 2 inch wide 1/32 gauge steel
You'll want the wheels to ride on their rims without tires. The threaded rod is
set first at one end to one wheel tightened so that it can only turn as one
piece.
Next place the metal sleeve tubing over the threaded rod so that it is
a tight fit , you may need to tape the rod first to do this.
Wrap the coat hanger wire one and a half turns then twist the ends together
with pliers. Cut off all excess more than 1/4 inch , and fold over the twisted
pair flat against the tube and only after everything else that follows is done
will you epoxy this in place.
The design of the pawl is in the first drawing of my first post above. It hooks
the wire you just bent , you make this yourself out of whatever hard plastic
tubing you can scrounge with a loose even wobbly fit.
Bend the 22 inch steel length so that you have two 9 inch arms separated by
a 4 inch platform to where the weight will land. Cut a slot sloping 45 degrees
to form a hook at the end of each arm. This has to fit over the tubing of the
axle. The rubbing surface you will need to grease before it is ready to use.
The length of line for suspending the weight , preferably nylon cord of the type
used on window blinds will be measured from the weight resting on the platform
up to the centerline of the axle N O T the bottom of the axle because you want
to have some slack. Tie off the string around the pawl using a clove hitch See_
http://www.42brghtn.mistral.co.uk/knots/42ktclov.html
Finally when the length is right use crazy glue to keep it in place.
You will now position the loop of coat hanger wire on the axle tube with the
pawl so that the string is verticle and the weight is resting in the center of the
platform. Epoxy the coat hanger wire in this horizontal location.
Before final assembly clean and polish the suface of the axle tube
Now you only need to attach the other wheel as you did the first.
A pedestal 10 centimeters tall can be stood on the platform to measure the
starting position of the weight.
Just one more thing , where do I send you my bill. .
[Edited on 16-1-2007 by franklyn]
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Avanine-Commuter
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Wow!! Thanks SO much for the detailed reply.
I will go out to buy these things at Home Depot tomorrow and begin assembly; If I run into trouble I'll ask.
THANKS!!
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Avanine-Commuter
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Okay... I knew it was too good to be true! My new problem is with the materials; who would've thunk that Home depot doesn't help you cut things!
They didn't have the correct sized locknuts; they didn't have the correct sized tubing, threaded rod and a 22x2 sheet metal.
The locknuts I have to go find it elsewhere, the threaded rod need to be cut with a hacksaw, and I have to find the tubing elsewhere as well since
they didn't carry the right size and the sheet metal needs to be cut and bent to specifications with a vice. That means I'll have to purchase a
hacksaw and a vice, which brings my cost up way more than I need.
Oh and my bicycle wheels are too large, and the store's bicycle wheels that fit the specifications are toooooo expensive!
I'm gonna try to "downsize" your great model and see what I can do...
I'm sorry I'm being so whiny. Thanks for all the help though.
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Avanine-Commuter
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Okay, so I changed some stuff.
I found two 18 in. rings of wood and I glued two flat styromfoam disks on them. I peirced and glued a 12 in wood pole through the styrofoam for my
axle. Then I made a pawl out of stacks of cut boba straws, used wire for the hook, and tied the string (and glued it) on the pawl. I put the weight on
the string and let go and.... nothing happens.
The car doesn't move!
I'm thinking it has something to do with the positioning of the hook and the pawl/string? How does it have to be, and HOW will I drop the weight so it
can get the car to move?
More specifically, the weight I'm using is 10cm from tip of hook to flat bottom. How long does my string have to be, when do I attach it to the pawl,
and where does it have to be positioned to the hook??
PLEASE help me one last time, due date is in two days!
[Edited on 21-1-2007 by Avanine-Commuter]
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chickentown
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now i'm facing the same problem???? I don't know how to start the project... Anyone have any idea on this joule mobile thing but with a simpler
construction with materials that I can easy to obtain ). My goal is 8 meter, that's all....
Thanks
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Rosco Bodine
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Precision is absolutely required for a scheme like this
to ever have a chance of working .
Several things would be involved in that precision ,
including the roundness and balance matching of the wheels .
The diameter of the wheels and the breakaway
torque required to start their rolling will define
the diameter of the hub around which the string
is wound . And the unwinding of that cord must
cause the wheel to traverse more than half the total distance to be travelled , probably at least two thirds
of the distance to be travelled so a minimum wheel diameter is going to be established by these factors .
A: the hub diameter must be large enough so that
the torque provided by the weight on the unwinding string is sufficient for breakaway into rotation ....AND
B: the number of rotations of the wheel while being
accellerated by the unwinding weight is sufficient to
carry the vehicle more than half the target distance to
be travelled , and probably closer to two thirds the
target distance . It could even be greater , as this will
be dependant upon the frictional drag of the over run
clutch .....the more easily it slips when free wheeling ,
the better .
You see these relationships hold because the vehicle
must have sufficient drive torque to be self starting .
And the distance through which the vehicle will coast
in decelleration will be no more than the distance it
travels while being accellerated .....rolling friction of the weels being the governing factor
during both accelleration and coasting . However the efficiency of the rolling must overcome additional friction of
the cord itself uncoiling during the accelleration phase ,
and then the friction of the freewheeling clutch hub
when the vehicle is coasting .
[Edited on 21-1-2007 by Rosco Bodine]
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franklyn
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My apologies to you Avanine-Commuter. I assumed you at least had the tools
and could scrounge from available junk to fabricate this gizmo. You should
complain loudly to your task master about not providing you with a workshop.
Your description leaves me wanting for details as to what exactly you have
improvised. Your mention of wood and styrofoam makes me shudder , If this is
the wheel you're in trouble now. You have just discovered the purpose of this
exercise which is to acquaint you with Rolling resistence. The contact surface
of the wheels must be as NARROW as possible and the thickness of an
ice skate blade , the rim must be very hard , very round , and very SMOOTH.
Do this test: on a table top
push it by a light touch , it should roll effortlessly for a couple of inches at least.
Otherwise they are out of round more than enough to keep from rolling.
You may try spinning the rims against a block with sandpaper to smooth them.
I would also try painting the rim with something like lacquer to harden them.
Roscoe is quite correct _
| Quote: | Originally posted by Rosco BodineThe diameter of the wheels and the breakaway torque required to start their
rolling will define the diameter of the hub around which the string is wound
-- the hub diameter must be large enough so that the torque provided by the
weight on the unwinding string is sufficient for breakaway into rotation |
The design I gave you would allow the string to wrap a bit under 2 turns around
the axle , unwinding to produce about 30 centimeters forward movement
for every 1 centimeter drop of the weight , or about 3 meters powered distance
whatever coasting results would be gravy. ( the reason for a pawl to disconnect
the weight from the axle's rotation. )
Now with your design you will have to
wrap tape around the axle to build up the diameter where the string will wrap
around. If you need a lot of torque , sleeve the axle with some tube. Understand
that this is independent of your pawl arrangement and can even be much larger
and wider than those parts. Once in motion as the string unwinds it will just carry
over to the smaller diameter without a problem
| Quote: | | Originally posted by Avanine-CommuterI'm thinking it has something to do with the positioning of the hook and the pawl/string?
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This bears no relation exept it should disconnect as I have described before when in motion.
| Quote: | | Originally posted by Avanine-CommuterHow does it have to be, and HOW will I drop the weight so it can get the car to move?
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You will physically restrain the motion by holding it or blocking it. If its not turning on its
own , I don't even want to think about it.
| Quote: | Originally posted by Avanine-CommuterMore specifically, the weight I'm using is 10cm from tip of hook to flat bottom.
How long does my string have to be, |
The design calls for a 10 centimeter drop , so this is all you have , the 10 cm of verticle
descent of the weight , so the string can only be 10 cm. plus perhaps 1/2 centimeter
of extra slack. ( This added length is required as I had already stated to allow the pawl to
run loosely after it disconnects from the rotation of the axle. ) You also have the 10 cm
height of the weight , the 9 inch radius of your wheel will provide enough clearance.
| Quote: | Originally posted by Avanine-Commuterwhen do I attach it [string] to the pawl, and where does it have to be
positioned - to the hook?? |
Re-read the proceedure I outlined above in my previous post , it's all there.
Rules:
1. The mass must be carried ( not rolled or dragged across the floor ) by the transporter.
2. The energy to run the transporter is to be stored by lifting the mass, vertically, 10.0 cm
above the place (which must be parallel to the floor) where it will rest on the transporter
at the end of the run.
Re- read what I stated about measuring the height /drop of the weight by placing it on
a pedestal 10 centimeters high resting on the platform to determine the starting position
of the weight.
This brings me to the glaring observation that you have not described a platform on which
the weight comes to rest after the 10 centimeters of string unwinds. Given your axle is
wood you will need to sleeve it with 2 short rings of metal tube onto which to place the
hooked platform I descibed. It need not be sheet metal , you may improvise something
using coat hangers perhaps. Just grease it lightly where it rubs on the 2 tube ring sleeves.
GOOD LUCK
.
[Edited on 22-1-2007 by franklyn]
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Rosco Bodine
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For a 26" diameter bicycle wheel pair , the outside
diameter of the hub sleeve upon which the cord is wound
should be 3/8" . The winding of 10cm length of cord will be 3.33 turns which while unwinding
will transit the vehicle a distance of 3.33 X pi X 26" = 272" which is
69% of the required 393.7" ( 10 meters ) .....hoping
that the vehicle will coast the rest of the way .
You could *possibly* fudge a bit on overcoming the breakaway adhesion
by putting a slight out of balance point in the rims , not enough to cause rollaway by itself ,
but to ease the torque requirement slightly for
the coiled weight . The vehicle would lope slightly
from the slight imbalance , but the weight would end up
in just about exactly its optimum and starting height position at the end of the ten meters .....
so this could not be construed as " cheating " by adding any " extra "
energy ....as it would all null out across the ten meters ,
but it would ease the starting friction . You could set the
out of balance point slightly lower on the forward side
of the wheel at its starting point to what its height will be
on the trailing side on the rise at the completion of the 4.82 revolutions which traverse the distance ....so the null point
at the distance of 10 meters will be such that it is clear that no energy was "secretly added" to the system .
Knife edging one rim on each of the bicycle wheels ,
and machining down the parallel rim edge so it doesn't even touch , and running the contraption across a
10 meter surface of float glass perfectly polished and level would be good .
Also , if you could find some trapezoidal form magnetic
thrust bearings for the clutch hub and an electromagnetic
clutch instead of a mechanical pawl , let your battery and
control sensor be the 1 Kg weight and the power wires
be the 10 cm of unwinding cord ......
do these things and you just *might* be approaching
a level of mechanical finesse which has a chance of
making the 100:1 " glide " across the glass
Simple huh , and the whole thing shouldn't cost more
than a thousand dollars or so
[Edited on 22-1-2007 by Rosco Bodine]
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franklyn
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| Quote: | Originally posted by Avanine-Commuter
The car doesn't move! |
It may be of academic interest to you to understand why it won't move.
The reason is that the point where the wheel rims are in contact with the
ground surface is not strictly speaking a point but rather a very small flat
portion or perhaps even a depression acting like a minute base pedestal.
The harder the rims and the ground surface are , the smaller this will be.
At rest , if an imaginary line passing through the center of mass of the
entire device traces to a point inside of this flat portion then the device
is in stable equilibrium with respect to gravity and will remain at rest. The
reason is that for it to move , it must pivot on the forward most point of
contact of the flat contact area , and to do so the center of mass of the
entire thing would need to lift.
You can make the transporter light as a cotton ball and suspend a ton of
weight to drive it , but if the center of mass must go up instead of down
because it is weighing down over the flat , it cannot move by gravity alone.
For the device to move , the suspended weight must situate the center
of mass of the whole device ahead of this forward most point of contact.
Because it is now out of balance and in unstable equilibrium the wheel tips
forward over this point of contact as the center of mass drops lower down.
Since the wheel only moves horizontally it is the suspended weight instead
which falls.
.
[Edited on 22-1-2007 by franklyn]
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Rosco Bodine
Banned
Posts: 6370
Registered: 29-9-2004
Member Is Offline
Mood: analytical
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Actually I didn't want to say it till now ,
but the chances of this working at all would
involve the whole mechanism being made the
hub of *one* bicycle wheel ......
like a unicycle sort of arrangement ,
and knife edge grinding the two
edges to a high precision .
The added wind resistance and mass
for two wheels would probably be too much ,
even with the magnetic bearings and magnetic clutch .
This assignment is really about like something
that should be thrown at an MIT engineering class ,
with a machine shop and money to burn .
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Rosco Bodine
Banned
Posts: 6370
Registered: 29-9-2004
Member Is Offline
Mood: analytical
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Another approach to the whole clutch hub thing
would be just to have a fixed hub with no mechanism
at all , but the hub would be conical and have a spiral
groove in which the cord was wound , like the
string is wound upon an old fashioned spinning top ,
the " gear ratio " as it were for each turn of the coiled string being progressive ,
so that the last turn to unwind
was a very small loop and each turn before was gradually
larger ...so that the unwinding of the first coil would have
greater " leverage " and overcome the breakaway torque
requirement easily , but the applied torque would steadily then decrease
with each revolution out to
the 4.82 revolutions where the 10 meters was covered .
The hub would have the same grooved profile as the
tapered point section of a wood screw ....if you follow what I am describing ,
with the cord wound in the spiral groove .
It would provide a tapering mechanical advantage across a distance profile
something like reverse of what the cams do on a compound archery bow .
A similar effect might easiest be achieved by instead of using a cord for suspending the weight ,
to use a flat strap of some
braided strap or even perhaps rubber like large rubber bands
are made , and winding this around a small diameter solid hub so that the thickness
of each successive layer increases
the diameter of the hub ....the strap being wound upon it like
a roll of tape . With a carefully selected thickness
of strap
properly matched to the hub diameter .....the tapering torque
could be well matched to the task .
A specially manufactured strap having a tapered thickness
along its length would be the ideal , since the torque change
would be occurring smoothly even during rotation through
a part of one rotation , the torque moment would be steadily
decreasing , and not dependent upon the thickness of any
layers beneath producing a stepwise change in the torque moment
as each revolution uncovered another layer of the strap .
[Edited on 22-1-2007 by Rosco Bodine]
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Avanine-Commuter
Harmless
Posts: 15
Registered: 4-5-2006
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Mood: No Mood
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Rosco and Franklyn-- Thanks so much for all the help and the guidance!
So I fixed the wheels so they are more smooth, I couldn't fit metal tubing over the axle without ripping the wheel off so I used some nylon string and
hot glued them to a peice of driftwood for the platform. The only problem is the wheels (styrofoam sucks, it crumbles and hot glue doesn't mix well
with it) but it's the best I can do right now. It is at LEAST able to make 5 meters which is decent enough.
THANKS a lot for the help, I really appreciate it.
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