Subject:
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Re: Mechanical tricks
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Newsgroups:
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lugnet.robotics
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Date:
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Fri, 2 Nov 2001 17:50:03 GMT
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Original-From:
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T. Alexander Popiel <popiel@&nospam&wolfskeep.com>
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Viewed:
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688 times
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In message: <GM61C1.FK1@lugnet.com>
"Rick Clark" <jrclark@nospam.aol.com> writes:
> Hello,
>
> I'm looking for a mechanical (non-robotic) solution to a technic problem
> I'm facing. (I crossposted to robotics, cause you guys think about gears
> all the time, too. Plus, you're really smart and stuff.)
>
> My input is a constantly rotating motor. What I'd like is to add a pause
> to a rotation at every 1/2 turn. That is, I'd like a wheel to turn halfway,
> then pause for a second or two, then turn the rest of the way, then pause
> again, then begin again.
>
> Alternately, it could turn halfway, pause, then turn backward halfway, then
> pause, etc.
>
> I've thought about cam gears, and about pushrods, and the like, but my
> mind does not visualize gears and such very well. Anyone got any bright
> ideas that could help me?
It sounds like you want either a linkage or an escapement, depending on
how strict you need to be with no motion during the pauses. I suggest
browsing around http://www.brockeng.com/mechanism/index.htm ; they have
many different linkages, line generators, cranks, and escapements
diagrammed there (and animated, if you have Java support in your
browser).
Also of interest may be http://www.keveney.com/Engines.html ; that site
diagrams many different engine types, which tend to include some of the
features that you are looking for.
To more directly answer your question, I'd suggest a Geneva Wheel
(diagrammed on the Brock site): a wheel with a quarter chunk taken out
of it with an arm holding a pin positioned outside the circumference of
the wheel in the center of the missing chunk, mated with a four-sided
involute wheel (looks like a square with bowed-in sides) with slots for
the pin on the diagonals.
It works as follows: most of the time, the pin wheel is nested in one
of the sides of the involute wheel, keeping the involute wheel from
turning. Then, as the pin wheel turns, the missing chunk fills the
involution, and the pin engages with one of the slots. With the pin
driving the involute wheel, the involute wheel makes a quarter turn,
before the pin disengages, and the mass of the pin wheel again keeps
the involute wheel from turning. (Yes, this is all much clearer with
the diagrams on the Brock site.)
The Geneva Wheel has a 1/4 duty cycle (it drives the involute wheel
for 1/4 turn, then pauses for 3/4 turn), and the driven wheel turns
1/4 turn each cycle, so you'll have a little gearing to do to get
your requested half-turns. Shouldn't be too hard, though.
Alternately, if you want a reciprocating motion (turn one way, then
turn back), you can use the intermittent linear motion from continuous
rotary motion also digrammed on the Brock site, with the linear motion
driving a shaft via a plate-gear. I won't even try to describe this
one in words. :-) It does have the disadvantage of not locking the
driven shaft during the off-duty times.
As yet another option if the pause doesn't have to be a complete stop,
you can use a drag-link mechanism; a good demonstration of this is the
second drag-link at http://www.brockeng.com/mechanism/DragLink2.htm .
Hope this helps.
- Alex
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Message has 1 Reply:
 | | Re: Mechanical tricks
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| There are many ways how to do this, but they aren't easy to describe in text. Make two "cranks" (i.e. wheels with a peg inserted off-center) and connect them with a rod. The first wheel should have the peg closer to the center than the second wheel. (...) (23 years ago, 2-Nov-01, to lugnet.robotics)
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