Exploring Mechanisms

Multiple Straight Line Drive
(http://kmoddl.library.cornell.edu/model.php?m=468&movie=hide)

In preparation for the next project, I looked at many different mechanisms that converts circular motion to liner motion (or something similar to a linear motion).

There were many interesting mechanisms listed on the Kinematic Models for Design Digital Models website (http://kmoddl.library.cornell.edu/model.php?m=reuleaux), but one that struck out to me was the multiple straight line drive. 

When I first saw this mechanism, my initial reaction was that it looked very similar to a mechanism for a car engine. However, while the car engine uses the energy from the gasoline combustion to convert the linear motion of a piston to a rotational motion of the crankshaft, the multiple straight line drive is converting a circular motion of the gears to a linear motion of the piston.

The multiple straight line drive was particularly interesting to me because it converts circular motion into a simple harmonic motion, and the piston moves in a perfectly linear path rather than veering off to the side. 

As seen in the above picture, the axis of the smaller gear is secured onto the brown circular board and as the board moves in a circle (possibly with the help of a motor) the small gear meshes with the external gear. The external gear spans the outer rim of the circle but is not part of the rotating board.

As the small gear moves in a circle around the external gear, the joint that attaches the piston to the small gear moves up as the small gear reaches the top of the circle and moves down as the small gear reaches the bottom of the circle.

As seen in the left picture, the piston moves up a distance that is equal to the diameter of the external gear.
 


I also noticed that there were particular conditions that need to be met in order for this mechanism to work. The diameter of the external gear must be exactly half of that of the external gear in order for the piston to move up and down in a straight line. 

Since the diameter of the small gear is exactly half of that of the external gear, the joint that is attached to the small gear always lines up with the center-line of the external gear (indicated by a green line in the right figure) This allows the piston to move along the center-line in a straight motion.






If the diameter of the small gear was greater than what it is now, the piston would not move up and down in a straight motion. I have made a simplified diagram depicting this:

As seen in the above figure, the piston is slanted if the diameter of the small gear is not exactly half of that of the external gear.

For devices powered by motors (or maybe wheels), this mechanism can be used to convert the circular motion into linear motion. For example, devices like the sewing machine is powered by a motor but a linear motion is needed for the needle to move up and down. This mechanism also reminds me of a water mill, where the circular motion of the wheel is converted to linear motion in order to process flour.