You'll notice that in the animation of the grasshopper escapement (http://en.wikipedia.org/wiki/Grasshopper_escapement) that the main wheel does a small "backwards" rotation once per cycle; most escapements which actually _catch_ to prevent backward movement (some actually use friction, and don't have to _catch_) must have a small retrograde motion to lock the finger in the teeth. http://en.wikipedia.org/wiki/File:Graham_Escapement.png shows a deadbeat escapement. Notice that if the teeth are slightly more angled, or if the pallets are off-center, that the teeth must be pushed backward for the next cycle to start.
Maybe this will get your student started :)
/************************************
Down with categorical imperative!
flutzpah@yahoo.com
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________________________________
From: Dwight K. Souder <crvhs_dks@crestview-richland.k12.oh.us>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Sent: Monday, March 16, 2009 1:44:57 PM
Subject: [Phys-l] Interesting complex machines
Greetings everyone. In my physics classes, we've been going over simple
and complex machines. I've given the assignment for the students to
find a "simple" complex machine, identify the simple machines within the
complex machine, and build a working model of the device. Examples of
this include transferring rotational motion into linear motion as seen
here: http://www.youtube.com/watch?v=wpO8_RRp_Z8 or http://www.youtube.com/watch?v=8dR38uQN74c, or transferring rotational
motion into a "walking motion" as seen here: http://www.youtube.com/watch?v=CufN43By79s.
We've also taken apart several mechanical clocks, door mechanisms,
locks, etc. to study the mechanisms at work. The students love it. I
even have one group of students who are working on a pendulum driven
clock that is primarily made of old CDs.
One of my students came to me and she's trying to find the mechanism
that allows for a small rotation in one direction, but then a larger
rotation in the opposite direction. She's presented several theories of
what they may be, but none of them have worked. It has both of us
stumped. She found a video that shows it at: http://www.youtube.com/watch?v=XI9Z0svRisU . If you watch the wheel in
the upper right, as the spiral wheel begins to rotate in the opposite
direction, something is triggered to allow the upper right wheel to back
up a little and then make a larger rotation in the opposite direction.
These kinetic sculptures are powered only by constant force springs and
are entirely mechanical. They are amazing to watch.
Does anyone know how such a mechanism may be put together to prevent
rotation in one direction (ie. clockwise), until a small amount of
opposite rotation is generated (ie. counterclockwise), that would allow
for the primary rotation to occur (ie. clockwise)? I love it when a
student has me stumped. :-) Any help would be greatly
appreciated...from both of us. (hahaha)
Mr. Souder
Science Teacher
Crestview High School
1575 State Route 96
Ashland, OH 44805
419.895.1700 (x-3501)
www.crestview-richland.k12.oh.us
<http://www.crestview-richland.k12.oh.us>
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