h a l f b a k e r y"Look on my works, ye Mighty, and despair!"
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This idea is for a toy which is deliberately designed to look
like the motor should just spin without moving the car
forward... but in fact, the toy moves quite well, even on
carpet.
The toy has eight wheels, four top and four bottom, and
can work either side up. None of the wheels is directly
motor-driven, instead each wheel merely has a ratchet and
pawl (or perhaps a sprag clutch), so it can spin forwards
but not backwards.
There are four parallelogram shaped linkages, two top and
two bottom, each linkage having two long segments, going
across the toy, and two short segments, parallel to the
sides of the toy.
Each short segment holds one wheel. Each long segment's
midpoint is attached to the chassis with a pivot. A spring
or rubber band pulls each linkage towards it's "neutral"
position, so that the wheels are less likely to hit the sides
of the chassis.
The two front linkages and the two rear linkages are
mirror images of one another, so I'll only describe the
front.
The front top and front bottom linkages are attached to
one another via a differential gear and a reversing gear.
When the middle of the differential is turned, the two
linkages move from being parallel to one another, to
forming an "X" shape.
(The reversing gear could be omitted, which would make
the toy simpler, but it would vibrate more, since turning
the differential would arrange the two linkages into a "/"
or "\" instead of an "X".)
The differential is not turned by a gear or a pulley, but
rather by a crank or lever.
Each of the toy's two levers are pushed and pulled by
connecting rods. The two connecting rods are attached to
the cranks of a single crankshaft. One end of the
crankshaft goes to the motor, the other protrudes from the
toy, and has a manually turn-able crank for playing with.
The two cranks that hold the connecting rods should be at
an angle to each other, so that when one lever is at the
forward-most or rear-most position, the other will be at
it's midpoint. Similarly, when the two front linkages form
an "X", the two rear linkages will be parallel to each other,
and vice versa.
When the toy is placed on a surface, and the hand-crank is
slowly turned, the bottom linkages won't move, but the top
ones will.
Children's logic might suggest that turning on the battery
powered motor will accomplish nothing but make the top
bits move faster...
In fact, as the top parts speed up, the bottom parts come
to life too!
The faster the top parts go, the more strongly the bottom
linkages twist.
And since the bottom linkages have ratcheting wheels on
them, the whole toy will move forwards.
Why does it work? It's a mystery!
I know why it works, and you, reading this, also might, but
what age child could identify the forces of physics
responsible?
If advertising jingle is needed, "The Marvelous Toy" comes
to mind.
[link]
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A diagram would be awesome. |
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sounds more like a desk top toy than something a child would like. |
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I too would relish a diagram. Because it sounds like this might be makeable with Lego Mindstorm or a similar build kit. |
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must... have... visual...
...hgn |
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It's definitely LEGO-able, though the only fancy bits are a
motor and two differentials, so one or two Lego Technic
kits would suffice. Since no computer is necessary, a
Lego Mindstorms kit is seriously overkill (though they do
come with powerful motors). |
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If you don't mind *building* the differentials, you could
probably do it with *one* Lego Technic kit, so long as it's
got a motor and enough gears. Since the differentials are
merely oscillated by a lever, and NOT rotated
continuously, you'll merely need three gears for each
one, and a "box" built of beams. |
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As for a diagram... my drawing-fu is too weak. |
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Then build it, and post video! Best of all! |
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So a centrifugal governor from the twistability of the bottom linkages? The twist would indicate the use of the 'goldbb' Lego part. Lego is all slide, and rotate, no twist. |
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wjt, by "twist," I meant "rotate in an oscillating manner."
Actual twisting of individual Lego pieces is usually a bad
idea. |
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bungston, A decade and some odd years ago, when I
moved out of my parents home, I felt I should give away
my childish things, including my Legos. Now I'm a man,
and am not afraid of childishness, but it wouldn't be right
to go and ask for my Legos back! |
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I *am* looking on ebay, though, to see if I can find a set
with enough pieces to build this, that won't blow my
budget. |
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Also, I'm in a *tiny* apartment, so I don't want to buy huge
quantities of legos... storage is an issue. |
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If I do build it, don't expect video though, since my
venerable cell
phone only takes still photos. |
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I've been thinking about how to build it while buying just
a single (reasonably priced) set, and while I haven't
managed that I've thought of a slightly simpler linkage
than a crankshaft and conventional differential. |
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Instead of a crankshaft, we have what's known as
rhombic drive. Two gears are one above another,
intermeshed. The motor will drive one of those gears.
Onto those two gears, we attach two opposite pivots of a
four arm linkage, with equal length arms. |
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Instead of a rotating differential, we can have a linear
one. |
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On each of the front and rear pivots of the rhombus, we
attach a free-spinning gear. Above each of these gears,
place a toothed rack, teeth downward. Below each of
these gears, place a toothed rack, teeth upwards. |
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As either gear moves forwards, it applies equal linear
forwards force to the two racks it's sandwiched
between... but, since the gear can turn, the racks don't
have to move at the same speed as one another. |
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Attach each rack to one of the assemblies holding the
wheels. |
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