Screw mechanism

Mechanism that converts motion, and forces, from rotational to linear

The screw is a mechanism that converts rotational motion to linear motion, and a torque (rotational force) to a linear force. It is one of the six classical simple machines. The most common form consists of a cylindrical shaft with helical grooves or ridges called threads around the outside. The screw passes through a hole in another object or medium, with threads on the inside of the hole that mesh with the screw's threads. When the shaft of the screw is rotated relative to the stationary threads, the screw moves along its axis relative to the medium surrounding it; for example rotating a wood screw forces it into wood. In screw mechanisms, either the screw shaft can rotate through a threaded hole in a stationary object, or a threaded collar such as a nut can rotate around a stationary screw shaft. Geometrically, a screw can be viewed as a narrow inclined plane wrapped around a cylinder. It first appeared in Mesopotamia during the Neo Assyrian period (911 609) BC, and then later appeared in Ancient Egypt and Ancient Greece. Records indicate that the water screw, or screw pump, was first used in Ancient Egypt, some time before the Greek philosopher Archimedes described the Archimedes screw water pump around 234 BC. Archimedes wrote the earliest theoretical study of the screw as a machine, and is considered to have introduced the screw in Ancient Greece. By the first century BC, the screw was used in the form of the screw press and the Archimedes' screw. Greek philosophers defined the screw as one of the simple machines and could calculate its (ideal) mechanical advantage. For example, Heron of Alexandria (52 AD) listed the screw as one of the five mechanisms that could "set a load in motion", defined it as an inclined plane wrapped around a cylinder, and described its fabrication and uses,
including describing a tap for cutting female screw threads. Because their complicated helical shape had to be laboriously cut by hand, screws were only used as linkages in a few machines in the ancient world. Screw fasteners only began to be used in the 15th century in clocks, after screw cutting lathes were developed. The screw was also apparently applied to drilling and moving materials (besides water) around this time, when images of augers and drills began to appear in European paintings.

The helix of a screw's thread can twist in two possible directions, which is known as handedness. Most screw threads are oriented so that when seen from above, the screw shaft moves away from the viewer (the screw is tightened) when turned in a clockwise direction. This is known as a right handed (RH) thread, because it follows the right hand grip rule: when the fingers of the right hand are curled around the shaft in the direction of rotation, the thumb will point in the direction of motion of the shaft. Threads oriented in the opposite direction are known as left handed (LH). By common convention, right handedness is the default handedness for screw threads. The left hand screw holding a circular saw blade or a bench grinder wheel on. In some devices that have threads on either end, like turnbuckles and removable pipe segments. These parts have one right handed and one left handed thread, so that turning the piece tightens or loosens both threads at the same time. In some gas supply connections to prevent dangerous misconnections. For example, in gas welding the flammable gas supply line is attached with left handed threads, so it will not be accidentally switched with the oxygen supply, which uses right handed threads. To make them useless to the public (thus discouraging theft), left handed light bulbs are used in some railway and subway stations.

Different shapes (profiles) of threads are used in screws employed for different purposes. Screw threads are standardized so that parts made by different manufacturers will mate correctly.

The thread angle is the included angle, measured at a section parallel to the axis, between the two bearing faces of the thread. The angle between the axial load force and the normal to the bearing surface is approximately equal to half the thread angle, so the thread angle has a great effect on the friction and efficiency of a screw, as well as the wear rate and the strength. The greater the thread angle, the greater the angle between the load vector and the surface normal, so the larger the normal force between the threads required to support a given load. Therefore, increasing the thread angle increases the friction and wear of a screw. The outward facing angled thread bearing surface, when acted on by the load force, also applies a radial (outward) force to the nut, causing tensile stress. This radial bursting force increases with increasing thread angle. If the tensile strength of the nut material is insufficient, an excessive load on a nut with a large thread angle can split the nut. The thread angle also has an effect on the strength of the threads; threads with a large angle have a wide root compared with their size and are stronger.

In threaded fasteners, large amounts of friction are acceptable and usually wanted, to prevent the fastener from unscrewing. This self locking property is one reason for the very large use of the screw in threaded fasteners such as wood screws, sheet metal screws, studs and bolts. Tightening the fastener by turning it puts compression force on the materials or parts being fastened together, but no amount of force from the parts will cause the screw to turn backwards and untighten. This property is also the basis for the use of screws in screw top container lids, vises, C clamps, and screw jacks. A heavy object can be raised by turning the jack shaft, but when the shaft is released it will stay at whatever height it is raised to. A screw will be self locking if and only if its efficiency is below 50%. Whether a screw is self locking ultimately depends on the pitch angle and the coefficient of friction of the threads; very well lubricated, low friction threads with a large enough pitch may "overhaul". Also considerations should be made to ensure that clamped components are clamped tight enough to prevent movement completely. If not, slipping in the threads or clamping surface can occur.