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various screws

A screw is a shaft with a helical groove formed on its surface. Its main uses are as a threaded fastener used to hold objects together, and as a simple machine used to translate torque into linear force.

Table of contents
1 Threaded Fastener
2 Bolt
3 Mechanical Analysis
4 Tensile Strength
5 Types of Screw
6 Shapes of Screw Head
7 Types of Screw Drive
8 Tools Used
9 Screw Measurements
10 External Links

Threaded Fastener

A screw used as a threaded fastener consists of a shaft, which may be cylindrical or conical, and a head. The shaft has a helical ridge or thread formed on it. The thread mates with a complementary helix in the material. The material may be manufactured with the mating helix, or the screw may create it when first "driven" in. The head is specially shaped to allow a screwdriver to grip the screw when driving it in. It also stops the screw from passing right through the material being fastened and provides compression.

Screws can normally be removed and re-inserted without reducing their effectiveness. This may make them preferable in some applications to nailss, which are frequently unusable after being removed.

A screw that is tightened by turning it clockwise is said to have a right-hand thread. Screws with left-hand threads are used in exceptional cases, when the screw is subject to anticlockwise forces that might undo a right-hand thread.


A bolt is a screw that passes right through the workpiece and is fastened by a nut on the other side. This is a very common way of holding together temporary and permanent constructions. It avoids the need for making a threaded hole to fit the screw. An unthreaded hole is known as a clear hole.

A small bolt is called a machine screw.

The thread on a bolt often occupies only part of the shaft, the remainder of the shaft being clear. A bolt whose shaft is threaded along its whole length is called a set screw.

Other Fastening Methods

When screws and bolts cannot be used, riveting, welding, soldering, brazing and gluing are all alternatives.

Mechanical Analysis

A screw is a specialized application of the wedge or inclined plane. It contains a wedge, wound around an interior cylinder or shaft, that either fits into a corresponding plane in a nut, or forms a corresponding plane in the wood or metal as it is inserted. The technical analysis (see also statics, dynamics) to determine the pitch, thread shape or cross section, coefficient of friction (static and dynamic), and holding power of the screw is very similar to that performed to predict wedge behavior. Wedges are discussed in the article on simple machines.

Tensile Strength

Screws and bolts are usually in tension when properly fitted. In most applications they are not designed to bear large shear forcess. When, for example, two overlapping metal bars joined by a bolt are likely to be pulled apart longitudinally, the bolt must be tight enough that the friction between the two bars can overcome the longitudinal force. If the bars slip then the bolt may be sheared in half, or friction between the bars (called fretting) may weaken them. For this type of application, high-tensile steel bolts are used and these should be tightened with a torque wrench.

High-tensile bolts are usually in the form of hexagonal cap screws with an ISO strength rating stamped on the head. The strength ratings most often used are 8.8 and 12.9. The number before the point is the ultimate tensile strength in N/mm2 (or MPa) divided by 100. This is the stress at which the bolt will fail, i.e. break in half.

The number after the point is the yield strength as a percentage of the ultimate tensile strength, divided by 10. Yield strength is the stress at which the bolt will receive a permanent set (an elongation from which it will not recover when the force is removed) of 0.2%.

Mild steel bolts have a 4.6 rating. High-tensile bolts have an 8.8 rating or above.

Types of Screw

Shapes of Screw Head

(a) Pan, (b) Button, (c) Round, (d) Truss, (e) Flat, (f) Oval

Types of Screw Drive

Modern screws employ a wide variety of drive designs, each requiring a different kind of tool to drive in or extract them. The most common screw drives are the slotted and Phillips; hex, Robertson, and torx are also common in some applications. More exotic screw drive types may be used in situations where tampering is undesirable, such as in electronic appliances that should not be serviced by the home repairperson.

(a) Slotted, (b) Phillips, (c) Pozidriv, (d) Torx, (e) Hex, (f) Robertson, (g) Tri-Wing, (h) Torq-Set, (i) Spanner

Many screw drives, including Phillips, Torx, and Hexagonal, are also manufactured in tamper-resistant form. These typically have a pin protruding in the center of the bit, necessitating a special tool for extraction. The slotted screw drive also comes in a tamper-resistant one-way design with sloped edges; the screw can be driven in, but the bit slips out in the reverse direction.

Tools Used

The hand tool used to drive in most screws is called a screwdriver. A power tool that does the same job is a power screwdriver; power drills may also be used with screw-driving attachments. The hand tool for driving cap screws and other types is called a spanner (UK usage) or wrench (US usage).

Screw Measurements

There are many systems for specifying the dimensions of screws, but in Europe the ISO metric system has displaced the many older systems.

Metric Screws

The diameter of a metric screw is usually specified in millimetres (mm) prefixed by the capital letter M, as in "M5" for a 5mm diameter screw. The diameter of a screw is the outer diameter of the thread, which is approximately equal to the diameter of the shaft before a thread was cut in it.

ISO Metric screw threads are available in coarse and fine versions. The coarse thread is by far the more common. Fine metric threads are sometimes found in electronic equipment made in the Far East.

Non-metric Screws

Before the metric system was common, many engineering companies had their own standard screw sizes. The first person to create a standard (in about 1841) was the English engineer Sir Joseph Whitworth. Whitworth screw sizes are still used, both for repairing old machinery and where a coarser thread than the metric fastener thread is required. This system had two thread sizes: coarse (BSW) and fine (BSF). The thread angle was 55°.

A later standard in the UK was the BA system, named after the British Association for Advancement of Science. Screws were described as "2BA", "4BA" etc., the odd numbers being rarely used. While not related to ISO metric screws, the sizes were actually defined in metric terms, a 0BA thread having a 1mm pitch. These are still the most common threads in some niche applications. Certain types of fine machinery, such as moving-coil meters, tend to have BA threads wherever they are manufactured.

The USA has its own system, usally called SAE, for Society of Automotive Engineers. Screws are described as 4-40, 6-32, 8-32, 10-32, 10-24, etc. (for numeric sizes, odd numbers are rare), or 1/4"-20, 1/4"-28, etc. (for inch unit sizes), with the first number giving shaft diameter (numeric or inches) and the second number being threads per inch. These screws are sometimes found outside the USA in personal computers based on the IBM PC specification. There must be millions of PCs outside the USA that have metric screws jammed into non-metric holes!

External Links

Modern-day powered ships and boats are nearly always driven by screw propellers, often referred to as the "ship's screws". In the early days of steam power for ships, when both paddle wheels and screws were in use, ships were often characterized by their type of propellers, leading to terms like screw steamer or screw sloop.

Also see Archimedes' screw for a type of pump containing a screw as its impeller.