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Simple Machines: Wrenches and Wheels

Everyday Einstein continues his series on simple machines with an examination of wrenches, wheels, and axles.
By
Lee Falin, PhD,
May 24, 2013
Episode #055

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Today we’ll continue with our series on simple machines. We’ve already discussed pulleys, levers, and ramps, so please check out those episodes if you haven’t yet. And since we’re still in this mode, I started wondering: What’s the most common type of simple machine?

It’s difficult to calculate something like that because simple machines are virtually everywhere, and sometimes they’re in forms that you don’t recognize. Take the wheel and axle for example. When you think of a wheel and axle, your first thought is probably the wheels of your car or bike. However, wheels and axles show up in all sorts of places that you might not expect. Let’s talk about the wheel and axle in detail and look at some examples of this simple machine.

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Size Does Matter

If you grab a screwdriver out of your toolbox, chances are that the handle of the screwdriver is fatter than the shaft, or the part that you put into the screw. Screwdriver designers don’t just do this because they think it looks nice, the screwdriver is a type of wheel and axle and the mechanical advantage of a wheel and axle depends upon the size of the wheel compared with the size of the axle.

To calculate the mechanical advantage of a wheel and axle, you take the radius of the wheel, (which is the distance from the center of the wheel to the outside edge), and divide it by the radius of the axle.

So if your screwdriver’s handle has a 2 inch radius, and the shaft has a radius of 0.25 inches, the mechanical advantage of your screw driver is: 2 divided by 0.25, which equals 8. This means for every newton of force you apply to the screwdriver, you get 8 newtons of force applied to the screw.

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