Problem
You and a friend are helping to build a doghouse. The wooden boards are cut. The holes are drilled. There is just one thing left to do. You have to connect the boards to the frame. You grab a screwdriver, some screws and go to work.
The first turn of the screw is easy. But then it gets harder to turn. You hold the screwdriver tightly. You strain to turn the screw. Uhhhh! Grrrr! After struggling, the screw is still sticking out. Your dog Ollie tilts his head. He must be wondering what’s going on. You are wondering, too!
Solution
Guess what? Your friend is having a much easier time. She is putting in her third screw. You ask her. “Why are your screws so easy to turn?” She looks at you and smiles. “I guess I’m just stronger than you.”
“No, really. Our screwdrivers are the same size. The holes are the same size. Even the screws are the same size. So what’s the difference?”
Then you notice it. “Hey, look at this. The threads are different.” The ridges, or threads, are farther apart than on your friend’s screws. Your friend’s screws have a lot more threads. And they are really close together. That must be it!
You try one of her screws. You have to turn it a lot more times. But, the screw goes in fast. “That’s more like it!” you say. “Now I have the right tools. We’ll have this built in no time.” “And Ollie is going to love his new house,” your friend adds.
Woof! Woof! Ollie thinks so, too.
It's Simple
A screw is a simple machine. Simple machines help us do jobs. They help move things faster, farther, or more easily than we can without them.
You probably don’t think of a screw as a machine . .. Instead, you might picture a car or a power tool. These machines have lots of parts.
Simple machines have only a few parts. Some are made of only one part. Nearly every machine has one or more simple machines. Think about garden shears. The sharp edges of its blades are wedges. The blades are also levers. So is the handle. The pivot point on which the blades move is a screw.
A Closer Look
A screw with fewer threads will require more force but take less time to move.
easier to turn, takes more time
harder to turn, takes less time
threads
When you turn the screwdriver, the screw redirects the force forward so it goes into the wood.
The Turn of the Screw
A screw is a neat little machine. Those ridges, or threads, that wrap around are all connected. They form a single ramp like a tiny spiral slide. That’s what makes a screw do its job. Try twisting a screw into a piece of wood. Place the tip of a screwdriver into the slit in the top of the screw. Turn the handle. That takes some effort. This effort provides the force that drives the screw into the wood.
Turning the screwdriver, directs the force in a circle. It also transfers the force to the screw. The screw moves in a circle. But then it changes the direction of the force! Instead of going around it goes straight forward.
So, a screw redirects force. With each turn, another thread moves forward and cuts into the wood. Friction between the threads and the wood hold the screw with a grip that’s hard to loosen.
How do you remove a screw? Turn the screwdriver in the opposite direction. Then the screw will redirect the force backward instead of forward.