Mechanisms

For our next project, we will be using a direct current (DC) motor to build a lego car. However, before we start putting together, we must understand the mechanisms first. As our motor produces a rotary motion, but our car will be moving in a linear motion, we want to understand different ways of converting rotational motion to linear motion.

First I would like to talk about the importance of this conversion. Even beyond our project, DC motors are used commonly. Most DC motors produce rotary motions. Typically, DC motor works like this: when the current goes through the coil around the rotor, it creates a magnetic field the same as the permanent magnet stator around it. As a result, the two magnetic fields repels each other, causing the rotor to turn to the other side. The rotor will keep turning either due to inertia, or due to the fact the the electromagnetic field has changed its direction with respect to the motor and thus repels the magnet stator, causing the rotor to keep turning.

"Electric motor cycle 2". Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Electric_motor_cycle_2.png#mediaviewer/File:Electric_motor_cycle_2.png

While many DC motors produce rotary motion, that is not always what we need. For example, in our scenario, we would like a linear motion as our final output. Therefore, it is clear that we must study different ways of conversion from rotational motion to linear motion.

In particular, I was intrigued by Model 037 Straight Line Drive from the Kinematic Models for Design Digital Models from Cornell.

http://kmoddl.library.cornell.edu/model.php?m=467

This structure consists a flywheel merged with a small gear, two bigger gears (one of which connects with the small gear), rank arms, and a piston. The flywheel is used here since it has a great moment of inertia and is more reluctant to change its rotational velocity. It is particularly useful when the source of energy is not continuous. However, if the center small gear is directly connected to a sustainable energy source, the fly wheel can be eliminated as it actually takes up energy to move.

The connection between the small gear and the bigger gear has a great gear reduction. Although it is not clear what the number is, we can tell by our own eyes that the big gear has much more teeth than the small one. This connection reduces the rotational velocity of the big gear, but increase the torque significantly.

The two bigger gears have the same rotational velocity and are attached to the piston with two crank arms. The tangent force applied to the crank arms at the connection on the perimeter of the gear can be split into forces in the horizontal direction and in the vertical direction. The force in the horizontal direction causes and crank arms to move from the left to the right and vice versa, but the vertical force is transferred to the piston. Since there are two crank arms connected to two gears, this puts double amount of force to the piston, making the mechanism very powerful. 

In general, this model transforms high velocity to greater torque.