Page 34 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 1 Introduction to Mechanisms and Machines 15

3. Wheel and Axle
You have probably never thought of the steering wheel in a car as a machine, but
that’s exactly what it is. The large diameter of the steering wheel is fixed to an axle,
which acts on the steering system to turn the wheels. Let’s say the steering wheel has
a diameter of 15 in, and the axle it is fixed to has a diameter of 1 in. The ratio of input
to output size here is 15:1, and that’s our mechanical advantage. (For more on how
steering systems work, check this link: www.howstuffworks.com/steering.htm.)
Similarly, a screwdriver with a thick grip handle is much easier to use than one with
a handle the size of a pencil.

You can use a wheel and axle to magnify force, as in the steering wheel example, or
to magnify speed, as in the wheels of a bike. A bicycle’s rear cog is fixed to the rear
axle, so when you pedal, the chain turns the rear cog that turns the rear wheel. This is
the opposite setup as in a steering wheel. In a steering wheel, you turn a big thing
(steering wheel) to make it easier to turn a small thing (steering wheel axle). In a
bicycle, you turn a small thing (rear cog) in order to turn a big thing (rear wheel). You
don’t gain mechanical advantage in this setup, but you do gain speed.

4. Inclined Planes and Wedges
If you’ve ever done the move yourself from one home to another, you might have
used a ramp coming off the back of the moving truck to help you roll boxes on and
off the truck bed. This ramp, or inclined plane, is a simple machine.

Let’s say you have a 100 lb box of books you need to load into the truck. If you lift it
yourself, you obviously need to lift the whole 100 lbs to get the box into the truck.
However, if you use a 9 ft long ramp that meets the truck at 3 ft off the ground, you
can set the books on a dolly and roll them up the ramp. Since you are rolling 9 ft to
go up 3 ft, instead of just lifting the box 3 ft straight up, the ramp gives you a 3:1
mechanical advantage. So with the ramp, you can get the books into the truck with
only one-third of the force of lifting it directly. The mechanical advantage of a ramp is
the total distance of the effort exerted divided by the vertical distance the load is raised.

You’ve also probably used an inclined plane to prop open a door. A few horizontal
kicks to a triangular wooden stopper drive it under the door, and the vertical force
created by the inclined plane keeps the door propped up and open.