Screws and Bolts

Screws and Bolts (Fasteners and fixings)

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Overview of screws and bolts

Screws and bolts are cylindrical fasteners usually made of stainless steel that have a precisely machined helical ridge cut into the central steel pillar, known as the external thread. Typically, bolts are designed in such a way that they are fastened by means of a nut bearing an internal thread capable of precisely accepting the contours of the external thread as it is torqued into position with a spanner, wrench or Allen key.

Screws are usually of tapered design, with their external threads carving an internal thread as they are fastened into a softer material using a screwdriver (the distance between each ridge on the helical thread is constant throughout and is known as the “pitch”). Screws are commonly used to fix objects into place (such as shelving on walls) while bolts are commonly used to firmly fasten otherwise separate objects together (as in many self-assembly flat-pack furniture items).

Both screws and bolts have a larger diameter head; in the case of screws, the head will have a groove designed to fit snugly with a screwdriver bearing a complimentary protrusion at the driving end. The most common screw heads are the slot or flat groove (a single, straight groove usually spanning the diameter of the head), the Phillips groove and the Pozidriv (or Supadriv) groove, although other patterns also exist.

Bolt heads may, confusingly, have similar grooves on their heads so that they are tightened into place with screwdrivers. A particularly common design for a bolt head is the Hex cap, a hexagonal steel head that requires torqueing tightly into a non-tapered nut with a spanner. An alternative design is the “Hex socket”, a hexagonal socket into which an Allen key is inserted to drive the bolt into its nut. Generally, the practical function of the larger head is to prevent the screw or bolt from being driven deeper than its length.

But although both stainless steel screws and stainless steel bolts commonly have a head of a larger diameter than the central, helically threaded pillar, there are exceptions. Coach bolts, for example, have a smooth, domed head which is not intended to be driven (a square steel collar directly beneath the head functions in part to prevent rotation during tightening of the nut at the opposite end). Similarly, anchor or “J” bolts are not driven but plunged into concrete, whereupon their J-shaped tops provide anchorage.

The vast majority of screws and bolts have “right-hand threads”, which means that they are tightened by means of clockwise rotation. More rarely, some have left-hand threads and are tightened by means of anti-clockwise rotation. They tend to be used in situations in which they would be subjected to an anticlockwise torque (such as, for example the left peddle of a bicycle) which would loosen a right-hand thread version.

 

History

The man considered by many to be the father of mechanics, Archytas of Tarentum (428 BC - 350 BC), is thought to be the inventor of the screw thread in approximately 400BC. His invention was put to work in presses designed to extract juice from grapes and oil from olives, all of which were operated by a screw mechanism.

Archimedes (287BC – 212BC) is credited with developing the concept to build wooden “water screws” designed to lift water, which the Romans subsequently adapted for draining mines. In the first century AD, records reveal a description of the screw by Heron of Alexandria.

Major advances in screw production occurred in the eighteenth century. The first lathe permitting semi-automatic movement of the tool carriage was invented in 1750 by Antoine Thout, who used a screw drive to engineer the longitudinal movement. Lathes were then used in the manufacture of screws and bolts, permitting far greater precision and regularity in the helical threads than ever before. They were hugely popularised by the British inventor and toolmaker Henry Maudslay, who introduced screw-cutting lathes at the close of the eighteenth century.

By 1760, they were being mass produced in factories but there was no standardisation to speak of, an absence which caused enormous difficulties. By 1841, the problem was overcome by Joseph Whitworth who sampled a huge array of screws from British workshop and proposed two basic standards: there should be a prescribed number of threads per inch for different diameters and thread flanks should all be angled at 55 degrees. The American William Sellers proposed a 60-degree thread angle in 1864 that became the US Standard, which is also now the international metric thread angle.

The first square drive screw preceded the Phillips screw (which shares a similar design) by three decades, when the Canadian engineer P. L. Robertson invented it in 1908. In the early 1930s, Henry Phillips adapted the design and the first Phillips screw was born.

 

Technical aspects

The most common material used in the manufacture of screws and bolts is steel, although fasteners which are required to withstand a high degree of atmospheric weathering or where corrosion is a risk (such as with surgical screws and bolts which must be implanted in the body) may be made with resistant materials such as stainless steel, titanium, silicon bronze, brass or monel. Where the fastening does not require the robustness of metal screws, some forms of plastic (notably, PTFE and nylon) can be threaded, offering an exceptionally high level of resistance to corrosion.

Some bolts have numbers stamped into their heads (most commonly, 5.8, 8.8 and 10.9). These provide an ISO rating of the bolt’s strength, with the number in front of the decimal point representing the stress at which the bolt will fail (the ultimate tensile strength). The last number is more intricate, representing the ratio of tensile yield strength (the point at which the material will be permanently deformed by elongation) to ultimate tensile strength, multiplied by 10. High-strength bolts are rated at 8.8 of higher.

The ISO metric screw thread standard has now largely supplanted older standards in most countries.

 

How screws and bolts differ from each other

While a universal distinction that unambiguously distinguishes screws from bolts has yet to be devised, it is commonly accepted that a screw fastens by cutting its own internal thread as it is driven into a softer material, whereas a bolt fastens by being inserted through a pre-cut hole in assembled parts and then driven and tightened into a female, non-tapered socket or nut (usually with a spanner, wrench or Allen key rather than a screwdriver) whose internal thread is of exactly the same dimensions as the external thread on the male fastener.