Wikijunior:How Things Work/Screw

Screws are one kind of simple machines. They have a corkscrew-shaped ridge, known as a thread, wrapped around a cylinder. The head is specially shaped to allow a screwdriver or wrench to grip the screw when driving it in.

The most common uses of screws are to hold objects together &mdash; such as wood &mdash; and to position objects. Often screws have a head on one end of the screw that allows it to be turned. The head is usually larger than the body of the screw. The cylindrical portion of the screw from the underside of the head to the tip is called the shank.

Bolts are a type of screw that usually is designed to work with a nut or another threaded fastener.

Who invented it?
Historians do not know who invented the screw. Although it seems to have been invented only in the last few thousand years. The first known use of a screw was as part of the screw pump of Sennacherib, King of Assyria, for the water systems at the Hanging Gardens of Babylon and Nineveh in the 7th century BC.

Around 250 BC, the Greek inventor Archimedes made a screw pump. Archimedes' machine had a revolving screw-shaped blade inside a cylinder. The blade was turned by hand. This type of machine is called the Archimedes screw. It is still used today for pumping liquids and other materials like coal and grain. By the 1st century BC, wooden screws were commonly used throughout the Mediterranean world in devices such as oil and wine presses.

The metal screw did not become a common fastener until machines for mass production were developed at the end of the 18th century, because nails, which are straight, are much easier to make than screws. In the 1770s, English instrument maker Jesse Ramsden invented a machine that made metal screws. Handheld screwdrivers first appeared around 1800. These developments caused a great increase in the use of threaded fasteners.

Throughout the 19th century, the most common types of screw heads were round with simple slots that were turned using screwdrivers and square and hexagonal heads that were turned with wrenches. In the early 1930s, the Phillips-head screw was invented by Henry F. Phillips. This screw has a cross-shaped recess in the head for the driver tool.

What does it do?
Screws do one basic thing. They convert a force that goes around and around into a force that goes up and down. This force can be used to push against an object.

Machines that use screws to push against other objects are called presses. A press is used to make cider or wine by squashing fruit to extract the juice from it. Printing presses were also used to make books.

The up and down force generated by a screw can also be used to hold things together. Screws can thread into a metal nut and the up and down force holds the two together. One big advantage of screws used as fasteners is that they can be removed and reinserted many times without losing their effectiveness. They have greater holding power than nails and can be easily disassembled and reused.

Screws can also be used to lift things. A device called an auger is used for lifting or pumping water or another liquid. As the screw turns in the water, the water is lifted. This is also how a concrete mixer (truck) unloads its load of concrete.

How does it get power?
A screw is powered by the movement of the screw driver. It converts this rotational force (called torque) into up and down force. A screw's power depends on how close together the threads are and how far away from the center of the screw force is applied. You can get more power by making the threads closer together. If the threads are close together, then with each turn of the screw it travels a shorter distance but exerts more force. With tighter threads you have to turn the screw more times before it is tight. You can also get more power by using an object that allows you to apply force farther from the center of the screw. This is why it's easier to turn a screw or bolt with a long wrench than a shorter one. But there is a downside to using a longer wrench. With a longer wrench, you have to move the wrench farther in order to turn the screw the same distance.

How dangerous is it?
Screws can be dangerous around anyone especially children. Screws with a sharp point on the shank are designed to pierce material and they can puncture and really damage your skin or wherever you hurt yourself, if you are not being careful and cautious.

How does it vary?
Certain screws may have different shapes, sizes, etc. to suit different needs. Some screw threads are designed to mate with a complementary thread, known as an internal thread, often in the form of a nut or an object that has the internal thread formed into it. Other screw threads are designed to cut a helical groove in a softer material as the screw is inserted. Most screws are tightened by clockwise rotation, which is termed a right-hand thread. Screws with left-hand threads are used in exceptional cases. For example, when the screw will be subject to anticlockwise forces (which would work to undo a right-hand thread), a left-hand-threaded screw would be an appropriate choice. Threaded fasteners either have a tapered shank or a non-tapered shank. Fasteners with tapered shanks are designed to either be driven into a substrate directly or into a pilot hole in a substrate. Mating threads are formed in the substrate as these fasteners are driven in. Fasteners with a non-tapered shank are designed to mate with a nut or to be driven into a tapped hole.

How has it changed the world?
Inventions like the automobile would not be possible without the screw.

What idea(s) and/or inventions had to be developed before it could be created?
The screw is like a ramp or inclined plane wrapped around a pole so the ramp had to be invented first. Although some screws have been made from wood, screws made from metal are much more useful because they can be more precise. Smelters to purify ore and forges to work metal were needed to make screws that were effective fasteners. In order to mass produce screws, a special lathe had to be developed.