Simple Machines – The Wheel and The Axle
Simple Machines – The Wheel and The Axle

This…is Engineering Marvels. In this episode, we finish our series on the
six simple machines by discussing the wheel and axle. It’s easy to see why the wheel was an early
human innovation. It’s much easier to pull a wagon than to push a box. One reason is
the classic wheel shape: the circle. Circles provide a single point of contact with the
ground, as opposed to squares or triangles. By minimizing contact with the ground, circles
minimize the amount of friction that needs to be overcome in order to turn the wheel. But the wheel by itself is not enough. If
all you did was sit a box on top of four wheels and push, your box would fall to the ground.
Try directly attaching the box to the wheels, and there’s no way for the wheels to rotate.
You end up shoving the box as it rests on the wheels. It’s a little easier, due to
lower friction on the wheels than on the box bottom, but not enough to make the wheel a
revolutionary force in human history. The wagon works best when attached to an axle,
which is in turn attached to the wheels. An axle, usually with a set of bearings, transfers
linear motion into rotational motion of the wheels with minimal friction. So, when you
pull forward on a wagon, the axle rotates, which causes the wheels to rotate and the
wagon to move forward with the smooth, easy motion you expect. But does the axle turn the wheel, or does
the wheel turn the axle? The answer is “yes.” In vehicles, we apply load to an axle in order
to turn a wheel. However, for many years water wheels were used to turn axles, and it was
the axles that performed the important jobs, such as lifting loads or grinding crops. This
same principle can be seen today in electricity-generating windmills, which just goes to show that good
ideas tend to keep coming around, and around, and around… Engineering Marvels is a joint production
of the Frank H. Dotterweich College of Engineering at Texas A&M University-Kingsville and South
Texas Public Radio. Past episodes are available online at tamuk ‘dot’ edu, forward slash
engineering. I’m Jason Marton.

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