Spinning to Wiggling: An Example of Rotary-to-Rectilinear Motion

Assignment: "Write a blog posting on a mechanism [to convert rotational motion to linear motion] you find compelling, explaining how it works and why you find it of interest."

My favorite model of motion was the bottom portion of "Model 042: Rotary into Rectilinear Motion", by Virginia Downward & William M. Clark, 1930.  I'm excited it seems like a woman contributed to this design, which is exciting for 1930.  It's a pretty neat way to change rotational motion into back-and-forth motion; the pattern is very regular, and I find the triangle design asethetically appealing.  Its motion was not immediately obvious to me--I did not anticipate the change of direction from the bottom arm of the inner piece pushing on the outer piece alternately with the topmost arm.

How it works:

Starting:

The outer piece is stationary.  The inside piece rotates couter-clockwise (blue curved arrow) so its top arm pushes the outside piece to the left (blue arrow).  The outer piece will quickly speed up as the middle piece continues to shove to the left, but the outer piece will slow down as soon as its bottom edge is hit by the lowest arm of the inner piece.

Going left:

The inside piece rotates couter-clockwise so its top arm pushes the outside piece to the left.  The outside piece was initially moving to the left (green arrow), but the combined forces of friction and the inside piece's rightward push soon slow, then momentarily stop the motion.

Turning around:

The inside piece has now exerted enough force to momentarily stop the previously-leftward motion of the outer piece; as the lowest arm of the inside piece continues to push, the outer piece will start to move to the right.

Going right:

The outer piece is now going right.  Its top edge has just been hit by the uppermost arm of the inner piece, which is starting to push the outer piece to the left; the outer piece will soon slow, then momentarily stop, then start to go left again (back in the position for "Going Left" above).

A movie and more information about the model:

http://kmoddl.library.cornell.edu/model.php?m=472&movie=show