It’s All About Trade‑offs
Remember building Ollie’s doghouse? The screws were the same length. The threads were different. The screws with more threads were easier to turn. But, you had to turn them more often. There are always trade‑offs. The more threads a screw has, the less force you need. But, you have to use that force over a greater distance.
A screw with 30 threads means 30 turns of the screw. Stretch out those turns. Add them up. You’ve used force over a distance of several feet.
To cut down on that distance, use more force. It’s like moving a stack of books from one desk to another. Take a couple books at a time. You use less force, but you make more trips.
If you move the whole stack at once. You use more force, but it’s only one trip.
It’s the same thing with the screw. Fewer threads mean more effort. Also, fewer threads provide less friction to hold onto the screw.
A jar and its lid have threads that fit together.
Hold on Tight
Screws come in many shapes and sizes. How do you take the lid off a jar? You unscrew it! A lid is a screw. Take a look at the rim of a jar. See the threads that spiral around? Now look inside the lid. It has threads, too. They fit between the threads of the rim. The threads match up and seal the jar.
Screws keep more than lids tight. The stem of a lightbulb is a screw. The threads of the stem turn between the threads in the socket. Friction between these threads keeps the bulb in place. A good thing, too! Lightbulbs in the ceiling hang upside down. If they slip out, they might fall onto your head!
The threads on a lightbulb stem screw into the socket of a light.
thread
A drill bit is shaped like a screw! Can you see the thread?
Dig In
When you drive a screw into wood, it’s easier if a narrow hole is there. That’s where a drill helps. And guess what? A drill bit is a screw.
A motor spins the drill bit quickly. The tip of the bit digs through the wood. In a few seconds, you have a narrow hole.
Some larger drills make larger holes in the ground. They dig holes to make wells for water or oil. They also make holes to set fence posts and telephone poles.