How do windshield wipers move back and forth?

1. Rain covers the windshield, and the wipers sweep it away, back and forth, again and again. But the motor that drives them can only spin in a circle. How does that work?
2. Hidden under the windshield is a small electric motor. It spins very fast. A screw-shaped gear called a worm turns a toothed wheel, which goes around much more slowly but pushes much harder.
3. A short arm called a crank is fixed to the slow wheel. Its end travels in a circle. A long rod is hooked onto that end, so the rod is pushed and then pulled, once every turn.
4. The other end of the rod is joined to a lever on the wiper's pivot. The lever is longer than the crank, so it cannot go all the way around. It can only rock back and forth, and the wiper arm rocks with it.
5. A second rod links the first lever to a matching lever on the other wiper. Now one motor moves both wipers, and they always stay in step.
6. A spring in the wiper arm presses a soft rubber blade onto the glass. The thin rubber edge tips over and pushes the water ahead of it like a squeegee, leaving clear glass behind.

In the garage, the panel under the windshield has been taken off. Tap each part of the wiper mechanism.

• Electric motor. The electric motor spins very fast when the wiper switch is on. It always spins the same way.
• Worm gear. A worm is a gear shaped like a screw. Each turn of the worm moves the big wheel by just one tooth, so the wheel turns slowly and pushes hard.
• Crank. The crank is a short arm fixed to the slow gear wheel. Its end goes around in a circle and carries one end of the rod with it.
• Connecting rod. The connecting rod joins the crank to the lever. As the crank goes around, the rod moves mostly back and forth, pushing and pulling the lever.
• Rocker lever. The rocker is a lever fixed to the wiper's pivot. The rod pushes its end one way and then the other, so it rocks, and the wiper arm on the same pivot rocks with it.
• Tie rod. The tie rod joins the two rocker levers. When one lever moves, it drags the other along, so the two wipers always sweep together.
• Wiper arm. The wiper arm is fixed to the pivot that pokes out below the glass. A spring inside the arm pulls it down so the blade presses on the windshield.
• Rubber blade. The blade is a strip of soft rubber with a thin edge. It works like a squeegee, pushing the water off the glass and leaving it clear.

Drag the crank around by hand, or switch the motor on. Then make the crank shorter or longer. How does the sweep of the wipers change?

Experiment: What happens to the wipers when you make the crank longer?

1. Switch the motor on with a short crank. How much glass do the wipers clear?
2. Make the crank longer, a little at a time.
3. Find where the blades start to bump the window frame.

What we found: A longer crank travels around a bigger circle, so it pushes and pulls the rod farther. That rocks the lever through a bigger angle and the wipers sweep a wider arc. If the crank is too long, the blades bump the frame.

Turn on JavaScript to try it yourself.

It is pouring! Choose a crank length and a motor speed so the wipers clear most of the glass, keep up with the rain and do not bump the frame. Then press Test.

Electric motors are good at one thing: spinning. A wiper has to do something different. It must swing one way, stop, swing back, stop, and do it again and again. The parts between the motor and the wiper change one kind of motion into the other.

First the motor is slowed down. Its shaft has a screw-shaped gear called a worm, which turns a toothed wheel. The wheel goes around about once a second, slowly but with a lot of push. A short arm called a crank is fixed to this wheel, so the end of the crank travels around in a circle.

A rod joins the end of the crank to a lever on the wiper's pivot. When the crank's end moves away, the rod pushes the lever. When it comes back around, the rod pulls the lever. The lever is longer than the crank, so it can never go all the way around. It just rocks, and the wiper arm rocks with it. One more rod passes the rocking on to the second wiper.

At the end of the arm, a spring presses a soft rubber blade onto the glass, and the blade pushes the water away. Waving your hand is a good picture of the wiper's motion, but your arm uses muscles that pull both ways. The wiper needs none of that: one small motor, always turning the same way, does it all.

Key words

Crank

A short arm that turns in full circles around one end.

Linkage

A set of rods and levers joined by pins that passes motion from one place to another.

Rocker

A lever that swings back and forth through part of a circle instead of going all the way around.

Worm gear

A screw-shaped gear that turns a toothed wheel slowly and strongly.

Oscillate

To move back and forth again and again, like a swing or a wiper.

Sweep angle

The angle a wiper arm turns through from one end of its stroke to the other.

Check your thinking

1. What kind of motion does the wiper motor make?

   Show the answer

   It spins around and around, always the same way. The motor only spins. The crank, rod and lever turn that into the back-and-forth sweep.

2. Why does the lever rock instead of going all the way around?

   Show the answer

   It is longer than the crank, so the rod can only push it part of the way. The crank's small circle can only move the end of the long lever a short distance each way, so the lever rocks.

3. You fit a longer crank. What happens to the wipers?

   Show the answer

   They sweep through a wider angle. A longer crank makes a bigger circle, so the rod is pushed and pulled farther and the lever rocks more.

4. A wiper motor spins at 3,000 rpm and drives the crank through a 50 to 1 worm gear. How many wipes a minute?

   Show the answer

   60. The crank turns at 3,000 / 50 = 60 turns a minute, and each turn of the crank is one complete wipe, there and back.

5. Why is the motor's fast spin slowed down by the worm gear?

   Show the answer

   So the crank turns slowly but pushes hard enough to move the wipers. Gearing down trades speed for push. The slow wheel is strong enough to drag two blades across wet glass.

6. How do both wipers stay exactly in step?

   Show the answer

   A tie rod joins their two levers. The tie rod makes the second lever copy the first one, so one motor drives both wipers together.