The screw is one of the six classical simple machines.1 It is a mechanism that changes rotational motion to linear motion and rotational force or torque to a linear force. If looked at geometrically, a screw can be regarded as a narrow incline plane wrapped around a cylinder. The screw is one of the most common simple machines and consists of a cylindrical shaft with helical grooves or ridges around the outside called threads. It was the last of the six simple machines to be developed.
The word helix comes from the Greek word for “twisted curve.” A helix is a curve in three-dimensional space such as coil springs or the handrails of spiral staircases. Famously a DNA molecule is described as two intertwined helices or a double helix. A double helix consists of two typically congruent helices with the same axis.
Helices are either right-handed or left-handed. These are defined by how the screw motion operates. If a clockwise screwing motion moves the helix away from an observer it is considered a right-handed helix. If it moves towards the observer it is called a left-hand helix. Handedness is also known as chirality. Most hardware screws are right-hand helices.
The pitch of a screw is the height of one complete helix turn measured parallel to the axis of the helix. It is not known who first developed the screw. We do know that by the first century it was used in a screw press. There are examples of wooden screw presses used by the Romans as olive or grape presses.
The Greeks claim that Archytas of Tarrentum invented the first screw. Although the Greek philosopher Archimedes is also credited by his countrymen with inventing the screw water pump around 234 BC.
Thus he is often cited as the inventor. Records however show that it was used in ancient Greek and Egyptian times.
Because the threads had to be laboriously cut by hand, screws were used only in a few machines. It was not until the 15th century with the invention of screw cutting lathes that screws began being more widely used, for example in clocks and other items. Galileo Galilei is credited with describing the complete dynamic theory of all six simple machines including the screw in 1600 in LE MECCANICHE or ON MECHANICS.
Like other simple machines, a screw can amplify force. The smaller the pitch or the distance between the screw threads the greater the mechanical advantage, which is described as the ratio of output to input force. Not all screws have threads. For example, a corkscrew is a helix-shaped rod with a sharp point. The Archimedes screw is a water pump that employs a rotating helical chamber to cause water to move uphill.
There are a variety of helices. The conic helix may be defined as a spiral on a conic surface. A circular helix has constant hand curvature and constant torsion. A general or cylindrical helix is a curve if the ratio of curvature to torsion is constant. And a curve is called a slant helix if its principal normal makes a constant angle with a fixed line in space. Some curves in nature are constructed of multiple helices of different handedness joined together by tendril perversions. Many climbing plants exhibit tendril perversions such as pea plants or morning glories. The cord of a land line phone is also a tendril peversion.