A doorknob is a fine example of a wheel and axle.

Made from fitting together a flattened circular shape and a cylinder shaft, a wheel and axle is a simple machine that most of us use every day without considering how it works. The wheel and axle share a common axis; therefore, a force applied to either the wheel or the axle will automatically move the other in the same direction. Take a look around. Can you identify a wheel and axle, other than ones you see on cars and bicycles? Let’s take a look at the doorknob – the handle is the wheel and the shaft is the axle.

When you grasp a door knob and give it a twist – you’re applying an input force to the wheel. Because the knob (wheel) is larger than the shaft (axle), the turning force of the axle is multiplied. If the door knob were no larger than the shaft, it would be very difficult to operate.