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Screw

This article refers to the threaded fastener. For other meanings, see Screw (disambiguation)

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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.

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 or wrench 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 nails, 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.

Bolt

A bolt is a threaded fastener that passes through the workpiece and is held in place by a nut or a threaded hole on the other side. This is a very common way of holding together temporary and permanent constructions. An unthreaded hole is known as a clear hole.

A small diameter bolt is commonly 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 sometimes called a set screw.

A stud is similar to a bolt but without the head. Studs are threaded on both ends. In some cases the entire length of the stud is threaded, while in other cases there will be an unthreaded section in the middle. It may be anchored in concrete, for example, with only the threads on one end exposed.

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.

Critical applications of screws and bolts will specify a torque that must be applied when tightening. The main concept is to stretch the bolt, and compress the parts being held together, creating a spring like assembly. The stretch introduced to the bolt is called a pre-load. When external forces try to separate the parts, the bolt sees no strain unless the pre-load force is exceeded (this takes some effort to imagine).

As long as the pre-load is never exceeded, the bolt or nut will never come loose (assuming the full strength of the bolt is used). If the full strength of the bolt is not used (eg. a steel bolt into aluminum threads) then a thread locking adhesive may be used.

If the pre-load is exceeded during normal use the joint will eventually fail. The pre-load is calculated as a percentage of the bolt's yield strength, or the strength of the threads it goes into, whichever is less.

Tensile Strength

Screws and bolts are usually in tension when properly fitted. In most applications they are not designed to bear large shear forces. For example, when 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 (called property class) 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%. When elongating a fastener prior to reaching the yield point, the fastener is said to be operating in the elastic region. Whereas elongation beyond the yield point is referred to as operating in the plastic region, since the fastener has suffered permanent plastic deformation.

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

Types of Screws

• Cap screw has a convex head, usually hexagonal, designed to be driven by a spanner or wrench.
• Wood screw has a tapered shaft allowing it to penetrate undrilled wood.
• Machine screw has a cylindrical shaft and fits into a nut or a tapped hole, a small bolt.
• Self-tapping screw or thread cutting screw has a sharp thread and that cuts its own hole, often used in sheet metal or plastic. They usually have a notch at the tip, through several threads, that aids in chip removal during thread cutting.
• Thread rolling screw has a lobed crossection (instead of round). They form the threads (rather than cutting) during installation. They are usually used in steel holes.
• Drywall screw is a specialized self-tapping screw with a cylindrical shaft that has proved to have uses far beyond its original application.
• Set screw has no head, and is designed to be inserted flush with or below the surface of the workpiece.
• Dowel screw is a wood-screw with two pointed ends and no head, used for making hidden joints between two pieces of wood.

Shapes of Screw Head

• Pan head: disc with chamfered outer edge.
• Button or dome head: cylindrical with a rounded top.
• Round: dome-shaped, commonly used for machine screws.
• Truss: lower-profile dome designed to prevent tampering.
• Flat or Countersunk: conical, with flat outer face and tapering inner face allowing it to sink into the material, very common for wood screws.
• Oval: countersunk with a rounded top.
• Cheese head: disc with cylindrical outer edge.
• Mirror screw head: countersunk head with a tapped hole to receive a separate screw-in chrome-plated cover, used for attaching mirrors.

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.

• Slot head has a single slot, and is driven by a flat-bladed screwdriver. The slotted screw is common in woodworking applications, but is not often seen in applications where a power driver would be used, due to the tendency of a power driver to slip out of the head and potentially damage the surrounding material.
• Cross-head, or Phillips screw has a "+"-shaped slot and is driven by a cross-head screwdriver, designed originally for use with mechanical screwing machines. The Phillips screw drive has slightly rounded corners in the tool recess, and was designed so the driver will slip out, or cam out, under strain to prevent over-tightening. The Phillips Screw Company was founded in Oregon in 1933 by Henry F. Phillips, who bought the design from J. P. Thompson. Phillips was unable to manufacture the design, so he passed the patent to the American Screw Company, who were the first to manufacture it.
• Pozidriv is patented, similar to cross-head but designed not to slip, or cam out. It has four additional points of contact, and does not have the rounded corners that the Phillips screw drive has. Phillips screwdrivers will usually work in Pozidriv screws, but Pozidriv screwdrivers are likely to slip or tear out the screw head when used in Phillips screws. Pozidriv was jointly patented by the Phillips Screw Company and American Screw Company.
• Torx is a star-shaped or splined bit with six rounded points.
• Hexagonal or hex screw head has a hexagonal hole and is driven by a hexagonal wrench, sometimes called an Allen key, or by a power tool with a hexagonal bit.
• Robertson drive head has a square hole and is driven by a special power-tool bit or screwdriver. The screw is designed to maximize torque transferred from the driver, and will not slip, or cam out. It is possible to hold a Robertson screw on a driver bit horizontally, due to the close fit. Commonly found in Canada in carpentry and woodworking applications.
• Tri-Wing screws have a triangular slotted configuration. They are for instance used by Nintendo on its Gameboys to discourage home repair.
• Torq-Set is an uncommon screw drive that may be confused with Phillips; however, the four legs of the contact area are offset in this drive type.
• Spanner drive uses two round holes opposite each other, and is designed to prevent tampering.

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. See also: Unified Thread Standard

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, usually 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!