History of the Screw
We do not know who first designed the screw. There is historical evidence that by the first century it was used in a screw press. Screw presses used by the Romans to squeeze olives for oil and grapes for wine have been found. Archytas of Tarrentum is credited by the Greeks with the invention of the first screw. But the Greek philosopher Archimedes is also credited with inventing the screw around 234 BC. The Archimedes screw was in a water pump that employs a rotating helical chamber to cause water to move uphill. However, records show that screws were used as early as ancient Greek and Egyptian times.
Screw Cutting Lathes
As screws had to be laboriously cut by hand, they were used only in a few machines. Screw cutting lathes were not invented until the 15th century and therefore screws became more widely available. They were, for example, used in clocks and other items.
Classical Simple Machines
The screw is one of the six classical simple machines.1 It a mechanism that changes rotational motion to linear motion and rotational force or torque to a linear force. A screw can be regarded as a narrow incline plane wrapped around a cylinder when looked at geometrically. It 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. Galileo Galilei is credited with explaining the complete dynamic theory of all six simple machines including the screw in 1600. LE MECCANICHE or ON MECHANICS (1600) described his work on these subjects..
A screw is a helix. 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 handrail of spiral staircase. Helices are either right-handed or left-handed and these are defined by how the screw motion operates. A helix is right-handed if a clockwise screwing motion moves the helix away from an observer. However if it moves towards the observer, it is called a left-handed helix. Handedness is also known as chirality. Most screws are right-hand helices and the pitch of a screw is the height of one complete helix turn. It is measured parallel to the axis of the helix.
Helices come in several varieties. 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. 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.
Some curves in nature are constructed of multiple helices of different handedness joined together by transitions known as tendril perversions. Tendril perversions are exhibited by many climbing plants such as pea plants or morning glories. The cord of a land line phone is also a tendril perversion.
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. Mechanical advantage is described as the ratio of output to input force. But not all screws have threads. For example, a corkscrew is a helix-shaped rod with a sharp point but no threads
Craftech Industries offers a wide variety of screws ranging in size, style of head, driver point type, length, material, and characteristics of the thread. We also do custom drive styles. To learn more about our capabilities, please call us at 800-833-5130.
1The other five are the incline plane, wheel and axle, wedge, pulley and lever.