Screw Screw Sedimentary rock Category="Fasteners"Category="Hardware (mechanical)"Category="Metalworking"Category="Woodworking"
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 or thread 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. It can also be defined as an Inclined plane wrapped around a shaft.

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 is essentially an inclined plane wrapped around a shaft. The thread mates with a complementary helix in the material. The material may be manufactured with the mating helix (tapped), or the screw may create it when first driven in (a Self-tapping screw). 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 reinserted without reducing their effectiveness. They have greater holding power than nails and permit disassembly and reuse.

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. Examples include rotating items such as the left hand grinding wheel on a Bench grinder or the left hand pedal on a Bicycle (both looking towards the equipment).

Threaded fasteners are traditionally made by a cutting action such as Taps and dies provide, however recent advances in tooling allows them to be made by rolling the blank (a section of rod) between two specially machined dies. The thread form and shape of the fastener are squeezed onto the blank. (Surprisingly, Shot pellets can be made in a similar fashion.) This method work hardens the threads and saves material, but there are those who believe the cut product was superior.

Bolt

[image] A bolt is a cylindrical (not conical) threaded fastener that passes through the work piece 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. See also Bolted joint.

The thread on a bolt sometimes occupies only part of the shaft, the remainder of the shaft being clear.

See also the article on the Bolt manufacturing process.

Other fastening methods

When screws and bolts cannot be used, nailing, riveting, [ roll pins], pinned shafts, Welding, soldering, Brazing, and gluing are all alternatives.

Materials and strength

Screws and bolts are made in a wide range of materials, with many varieties of Steel being perhaps the most common. Where great resistance to weather or corrosion is required, stainless steel, Titanium, or Bronze may be applied. Some types of plastic, such as Nylon or Teflon, can be threaded and used for fastening requiring moderate strength and great resistance to corrosion or electrical insulation. Even porcelain and glass can have molded screw threads that are used successfully in applications such as electrical line insulators.

The same type of screw or bolt can be made in many different grades of material. For critical high-tensile-strength applications, low-grade bolts may fail, resulting in damage or injury. On SAE-standard bolts, a distinctive pattern of marking is impressed on the heads to allow inspection and validation of the strength of the bolt. However, low-cost Counterfeit fasteners may be found with actual strength far less than indicated by the markings. Such inferior fasteners are a danger to life and property when used in aircraft, automobiles, heavy trucks, and similar critical applications.

Mechanical analysis

[image] A screw is a specialized application of the wedge or Inclined plane. It contains a wedge, wound around a cylinder or shaft, that either fits into a corresponding inclined plane in a nut, or forms a corresponding inclined 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 preload. When external forces try to separate the parts, the bolt sees no Strain unless the preload force is exceeded (this takes some effort to imagine).

As long as the preload 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 (e.g., a Steel bolt threaded into aluminum threads), then a thread-locking adhesive may be used.

If the preload is exceeded during normal use, the joint will eventually fail. The preload is calculated as a percentage of the bolt's yield Tensile strength, or the strength of the threads it goes into, or the compressive strength of the clamped layers (plates, washers, gaskets), 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 so 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 bolt and slipping bars may erode and weaken the bolt (called fretting). For this type of application, high-strength Steel bolts are used and these should be tightened with a torque wrench.

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High-strength bolts usually have a hexagonal head with an Iso strength rating (called property class) stamped on the head. The property classes most often used are 8.8 and 10.9. The number before the point is the tensile ultimate strength in MPa divided by 100. The number after the point is 10 times the ratio of tensile yield strength to tensile ultimate strength. For example, a property class 5.8 bolt has a nominal (minimum) tensile ultimate strength of 500 MPa, and a tensile yield strength of 0.8 times tensile ultimate strength or 0.8(500) = 400 MPa.

Tensile ultimate strength is the stress at which the bolt fails (breaks in half). Tensile 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 property class 4.6. High-strength bolts have property class 8.8 or above.

The size and property class of a bolt used for a specific application are dependent on a variety of factors, including availability, spatial constraints, and whether the load is static or dynamic. An M10, property class 8.8 bolt can very safely hold a static tensile load of about 15 kN.

Types of screws and bolts

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Shapes of screw head

[image] (a) pan, (b) button, (c) round, (d) truss, (e) flat, (f) oval

Some varieties of screw are manufactured with a break-away head, which snaps off when adequate torque is applied. This prevents tampering and disassembly and also provides an easily-inspectable joint to guarantee proper assembly.

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. Some types of drive are intended for automatic assembly in mass-production of such items as automobiles. 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 repair person.

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

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Lara Specialty Tools has a more extensive guide to 24 screw head types.

Tamper resistant screws

[image] 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 screw head, necessitating a special tool for extraction.

[image] 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. Where the holding power of the screwed joint is critical, torque-measuring and torque-limiting screwdrivers are used to ensure sufficient and not excess force is developed by the screw. 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 much of the world the ISO preferred series metric has displaced the many older systems.

Metric screws

The diameter of a ISO preferred series screw is specified in millimetres (mm) prefixed by the capital letter M, as in "M6" for a 6 mm diameter screw.

The pitch of metric threads varies according to the diameter, but not absolutely regularly. Some examples: a M3 thread has a 0.5 mm pitch, M4: 0.7 mm, M6: 1 mm, M10-12: 1.5 mm, M14-16: 2 mm, M18-22: 2.5 mm.

The diameter of a metric screw is the outer diameter of the thread. The tapped hole (or nut) into which the screw fits, has an internal diameter which is the size of the screw minus the pitch of the thread. Thus, an M6 screw, which has a pitch of 1 mm, is made by threading a 6 mm shaft, and the nut or threaded hole is made by tapping threads in a 5 mm hole.

Metric screw threads are also available in "fine pitch" versions, sometimes several pitches for one diameter (example: M18/fine in 1, 1.5 and 2 mm pitches). The fine thread series is deprecated and not recommended for use in new designs. The fine metric threads were once found in equipment made in the Far East, but that has changed with the standardisation of the ISO preferred thread series.

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 1 mm 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. Curiously enough, even though every other fastener on a camera may use metric threads, the tripod socket will likely be a 1/4"-20 thread.

The USA has its own system, usually called the Unified Thread Standard. A version of this standard, called SAE for the Society of Automotive Engineers, was used in the American automobile industry. The SAE is still associated with inch based fasteners by the public even though the U.S. auto industry (and other heavy industries relying on SAE) switched to ISO preferred series fasteners back in the 1970s and afterwards.

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 older model personal computers based on the IBM PC specification. Since the computer industry is now based in Asia (mostly Taiwan), the industry now uses metric fasteners. As more and more products are made outside the U.S. and then imported into the U.S., the use of inch based fasteners is declining, and it is only a matter of time before the inch fastener series is gone forever.

Other thread systems include BSP (British Steam Pipe; used for other purposes as well) and CEI (Cycle Engineers Institute, used on bicycles in Britain and possibly elsewhere), NPT and NPTF (coarse and fine pipe threads), and PG (German: "Panzer-Gewinde"), used in thin plate metal, such as for switches and nipples in electrical equipment housings.

History

In antiquity, the Greek mathematician Archytas of Tarentum (428 - 350 BC) was credited with the invention of the screw. By the 1st century BC, wooden screws were commonly used throughout the Mediterranean world in devices such as Oil and wine presses. Metal screws did not appear in Europe until the 1400s.

The metal screw did not become a common woodworking fastener until machine tools for mass producing it were developed at the end of the 18th century. The British engineer Henry Maudslay patented a screw-cutting Lathe in 1797; a similar device was patented by David Wilkinson in the United States the next year.

Standardization of screw thread forms accelerated during WWII so that interchangeable parts could be produced by any of the Allied countries.

Though not screws, perhaps also see cotter or pin bolts, and "clinch bolts" (now called rivets), used in ship building prior to the mid 19th century.

See also

References

Henry H. Ryffel (ed.), Machinery's Handbook 23rd Edition, Industrial Press Inc. New York, 1988 ISBN 083111200X

External links