When you look at a screw as you are putting together that piece of flat-pack furniture that you just bought from that huge box store, you might think that the screw is pretty simple, but it really isn't as simple as you might think.
Simple Machine
A screw is actually one of the classical machines that have been around for centuries. The screw itself is a force multiplier, which means that it takes the force that you put into it and increases it. A screw also takes the rotational force that you put into it and changes it. As you turn the screwdriver, the screw rotates and it turns into linear motion, which means that the screw goes forward. That's what makes the screw a simple machine, but that's not all that goes into the screw. There's also the thread.
Thread
The thread is the ridge that runs in a spiral around the body of the screw. There are all kinds of measurements and information that go into making that thread work for any application. The thread can run so that turning the screw to the right will tighten it up, or it can be reversed so that you have to turn left to tighten the screw. However. most screws turn to the right, which makes the mnemonic righty tighty, lefty loosey work so well. Most screws are also a single start screw. That means that there is only one thread that runs from the base to the top of the screw. Those are the most common type of screw out there. There are also double start screws. With these screws, there are two threads that run around the cylinder of the screw.
Screw Thread Measurements
When it comes to measuring the threads on the screws, there are two important measurements to know. Those are pitch and lead. The lead of a screw is the amount of distance that the thread travels around the screw through one whole rotation, so if you were to draw a straight line down the screw, the lead would be the distance the thread would travel from that line until that line comes back to the starting point again. The pitch of the screw is the distance between crests of the thread. In a double start screw, the lead would still be measured on only one thread at a time, but the pitch would be counted from crest to crest, no matter which start the crest is from.
That screw that you are hoping will hold your flat pack furniture together isn't as simple as it may look, but it will work, no matter what.
