As a result, contemporary screws and screwdrivers are a hybrid of two basic machines: the inclined plane and the lever. The head of a screw is made up of several flat surfaces called lands, while the tip of the screwdriver engages only one of these lands at a time.
The inclination of each land on the head of the screw determines how much force is required to rotate it. If all the lands are flat, as in most old screws, moderate pressure with a small tool will be enough to turn them. However, if one or more lands are angled, as in most modern screws, much greater force is needed. The difference can be dramatic: studies have shown that it takes between 10 and 100 times more effort to drive an angle-headed screw than a straight one of equal size.
The evolution of the screw from an ancient tool used by stone cutters to its present-day form as a fastening device was not easy. Although hollow wooden pegs had been used for hanging clothes, they were easily broken, and plastic clips today are still based on the design of the peg. The first metal screws were probably made of bone because it is harder than other materials available in those days. Later, bronze, iron, and finally steel became available for screw manufacturing.
A screw with closely spaced threads facilitates work. It creates an inclined plane. The process is simple, but you will have to turn the screw several times. A screw with more widely spaced threads creates a shorter inclined plane. This means that many more acts of turning the screw are required to do the same job.
The thread determines how much work a screw does. If two screws have the same size head and length, the one with larger threads requires more turns to advance it the same distance as the other. That's because there is less surface area contact between the screw and the hole it goes in. As the number of threads on a screw increases, so too does its diameter, which means more area for greater holding power.
Screws are often used where space is limited or when large forces need to be applied. They are common-place, for example, when fixing panels to walls or floors. In this case, it doesn't matter how much work a single screw does. What matters is that many more screws are needed to fix something, since it becomes difficult to get to each one individually.
Also useful are screws that have their heads cut off, called countersunk screws. These come in very handy if you want to hide the point of attachment of something.
A machine screw is a flat-pointed screw or bolt. They are available in a number of drive types and heads, and they may be used in a wide range of applications. It is frequently inserted into tapped holes. They are used in conjunction with nuts and washers, which are sometimes known as "stove bolts" or "stovers."
A wood screw is a coarse thread screw used to secure wood panels and other materials together. They usually have a thick shaft and a very large head, designed for driving into solid wood. The term "wood screw" is also applied to screws that are used primarily for decorative purposes, such as the sewing pin found in many fabrics.
A bradawl is a small hand tool used for driving wooden fasteners called brads. The tool has a round, metal-tipped handle with a small hole through it, through which a threaded rod extends. A second rod protrudes from the opposite side of the handle. These rods fit into the head of each brad, holding the tool in place while you work.
A Bradawl can be used to make brad points for use with fabric pins or to drive wooden pegs into soft wood boards to serve as nailheads on furniture parts. It can also be used to push buttons on toys and instruments.
A brad point is a small, sharpened piece of steel attached to the end of a bradawl.
The screw was designated as one of the basic machines by Greek philosophers, who could calculate its (ideal) mechanical advantage. The first known analysis of the screw mechanism was done in 1600 by Johannes Kepler, who used it to explain why the moon causes tides in the ocean. He concluded that the ratio of diameter of the thread of the screw to the length of its shaft is equal to the ratio of diameter of the moon to the distance between Earth and moon. Today we know this is not quite true (the screw does not cause tides), but Kepler's analysis shows that it is a very efficient machine for turning energy into motion.
Screws are simple because they can be constructed from straight, rigid parts that do not require shaping or cutting, which are common features of more complex machinery at the time. Screws also require only two parts in contact with each other: a head and a shaft. This is in contrast to many other machines of the time that included three or more parts in contact with each other - for example, a four-bar linkage for producing linear movement or a water-powered mill for grinding grain.
In addition, screws can convert continuous energy input into continuous motion output, which means they are good tools for powering mechanisms or devices that need constant movement.
A wheel and axle machine is somewhat like a screwdriver. They both have three parts: one part for turning the wheel rim, another part for turning the hub, and a third part called a spindle that fits into the hubs and onto which you attach tools that turn when you push them down on the spindle.
However, a wheel and axle machine is much more complicated than a screwdriver. It has several different types of screws that go into various parts of the vehicle. A screwdriver by comparison uses only one type of screw: the flathead screw. It is used to fasten metal together, such as when you are attaching body panels to an automobile. The wheel and axle machine requires that you use a variety of sizes and shapes of screws to attach parts of the vehicle together.
For example, one part of the vehicle needs to be attached to another part with a large number of small screws. You can't use a single large screw because they don't fit between the parts being joined.
The person building the vehicle would not think to use a screwdriver for this job. They would need a tool that could hold several screws of different sizes and shapes.