Contenido

El término tornillo se utiliza generalmente en forma genérica: son muchas las variedades de materiales, tipos y tamaños que existen. Una primera clasificación puede ser la siguiente:[3]​.

• - Tornillos tirafondos para madera.

• - Autorroscantes y autoperforantes para chapas metálicas y maderas duras.

• - Tornillos tirafondos para paredes y muros de edificios.

• - Tornillos de roscas cilíndricas.

• - Varillas roscadas de un metro de longitud.

wood screws

Wood screws are called wood lag screws. Their size and quality is regulated by the DIN-97 standard and they have a thread that occupies 3/4 of the length of the spike. They can be made of mild steel, stainless steel, brass, copper, bronze or aluminum, and can be galvanized, nickel-plated, dichromated, etc.

This type of screw tapers at the tip as a way of opening the way as it is inserted to facilitate self-tapping, because it is not necessary to make a previous hole, and the thread is sharp and cutting. They are normally screwed with an electric or manual screwdriver.

Their heads can be flat, oval or rounded; Each one will fulfill a specific function.

Flat head: used in carpentry, in general, where it is necessary to leave the head of the screw submerged or flush with the surface.

Pointed head: the lower portion of the head has a shape that allows it to sink into the surface and leave only the rounded upper part protruding. They are easier to remove and have a better presentation than the flat heads. They are used to fix metal elements, such as tools or doorknob plates.

Rounded head: used to fasten pieces that are too thin to allow the screw to sink into them; also for joining parts that will require washers. In general, they are used for similar functions to those with oval heads, but in unsinked holes. This type of screw is very easy to remove.

The heads can be of different kinds:

Milled head (straight slot): They have traditional straight slots.

Phillips head: They have cross-shaped slots to minimize the chance of the screwdriver slipping.

Allen type head: with a hexagonal hole, to fit an Allen wrench.

Torx head: with a star-shaped hole in the head of an exclusive Torx design.

The characteristics that define wood screws are: head type, constituent material, shaft diameter and length.

Lag screws for walls and wood DIN-571

There is a variety of screws that are thicker than the classic wooden ones, which are called lag screws and are widely used to screw the supports of heavy elements that are hung on the walls of buildings, such as awnings, air conditioners, etc. In these cases, the wall is drilled to the diameter of the chosen screw, and a plastic plug is inserted, then the screw that threads the plastic plug under pressure is screwed in and thus the support is firmly held. It is also used, for example, for screwing the wood of large packaging. These screws have a hexagonal head and range from M5 to M12.

Self-tapping and self-drilling for metal sheets and hard woods

Both types of screws can make their own way. They are manufactured in a wide variety of special shapes. The appropriate one is selected based on the type of work to be carried out and the material on which it will be used.

Self-tapping have most of their shaft cylindrical and the end conical. They can have a flat, oval, rounded or flat head. The thread is thin, with a flat bottom, so that the iron can stay in it. They are used in metal sheets or profiles, because they allow metal to wood, metal to metal, metal to plastic or other materials to be joined. These screws are fully machined (from tip to head) and have sharper edges than wood screws.

In self-drilling the tip is a drill bit, which avoids having to make guide holes to install them. They are used for heavier metals: they cut a thread in front of the main part of the screw.

The dimensions, head type and quality are regulated by DIN standards. These standards are technical requirements for quality and safety established by the German Institute for Standardization and are applied in industry, commerce, science, etc. They are especially considered when it comes to mechanisms and industrial parts, such as screws, nuts, washers, etc.

They are also used to be fixed to walls by means of plastic anchors called "Taco (construction)") or ramplugs.

Cylindrical thread screws for metal joints

To join metal parts, screws with triangular threads are used, which can be screwed into a blind hole or into a nut with a washer in a through hole.

This type of screw is the one normally used in machines and the most important thing that is required of them is that they withstand well the stresses to which they are subjected and that they do not loosen during the operation of the machine where they are inserted.

The notable thing about these screws is the thread system and the type of head they have since there are variations from one system to another. Due to the thread system, the most used are the following.

• - Metric thread with normal pitch or fine pitch.

• - "English" (British) Whitworth thread with normal or fine pitch.

• - "American" (American) SAE thread.

Depending on the type of head they have, the most used screws are the following:

• - Hexagon head: type DIN 933 and DIN 931.

• - Allen head: DIN 912 type.

• - Countersunk head.

• - Cylindrical head: DIN 84 type.

• - Torx head (shaped like a six-pointed star).

According to Iram 4520* (1999), in its item 2.1.2 “Conventional representation”, normally by convention, the representation of threads and threaded parts in all types of technical drawings are simplified as shown in the attached figure (graphic representation of a screw). In both screws and threaded holes, the crest that represents the crown of the thread is represented with a thick continuous line and the root with a thin line. In hidden views, both are drawn with a thin dashed line. In the sections, the striping extends to the crown. In a front view of a screw, the line representing the root (diameter at the bottom of a thread) will cover approximately 3/4 of the circumference to avoid errors of interpretation (preferably open in the upper right quadrant). In drawings of assembled threaded parts (assemblies), the lines that represent the external thread (screw, threaded bolt, tube) will always be shown covering the internal thread and will not be hidden by it.

Crowning is understood as the largest diameter in external threads (nominal diameter of the thread) and the smallest diameter in internal threads (hole diameter).

When we talk about root, it normally refers to the smallest diameter in external threads (diameter at the bottom of the thread) and the largest diameter in internal threads (nominal diameter of the thread).

El diseño de las cabezas de los tornillos responde, en general, a dos necesidades: por un lado, conseguir la superficie de apoyo adecuada para la herramienta de apriete de forma tal que se pueda alcanzar la fuerza necesaria sin que la cabeza se rompa o deforme. Por otro, necesidades de seguridad implican (incluso en reglamentos oficiales de obligado cumplimiento) que ciertos dispositivos requieran herramientas especiales para la apertura, lo que exige que el tornillo (si este es el medio elegido para asegurar el cierre) no pueda desenroscarse con un destornillador convencional, dificultando así que personal no autorizado acceda al interior.

Así, se tienen cabezas de distintas formas:.

• - hexagonal (a).

• - redonda o alomada (b).

• - cilíndrica (d, g).

• - avellanada (c, e, f).

• - combinadas con distintos sistemas de apriete:

• hexagonal (a) o cuadrada para llave inglesa,

• ranura o entalla (b, c, d) y

• Phillips o estrella (f) para destornillador,

• agujero hexagonal (e) para llave Allen,

• moleteado (g) para apriete manual, etc.

Commercial Hex Head Screws

From certain diameters onwards, it is normal for the head of commercial screws to be hexagonal, mainly those that screw into metal parts or into their corresponding nut. There are several types of commercial hexagonal head screws manufactured according to DIN standards that differ from each other in the length of the thread that their shafts have.[4]​.

Commercial screws with Allen head

As with hexagonal heads, there are several models of screws with Allen heads, all of them standardized according to the corresponding DIN standards. Hex head screws are mainly used when smooth surfaces are desired and the tightening forces are not very high.[5]​.

Normally, this type of screw is used in tool making work since the head is embedded inside the molds. It is also used for automotive and motorcycling mechanics due to the ease of tightening when they are in places that are difficult to access. Depending on its thickness and length, it comes with an unthreaded neck.[6]​.

Screws for tightening with a screwdriver

With the modern electric and pneumatic screwdrivers that exist, the use of self-tapping screws is widely used in various types of carpentry, both wood and metal, since it is a quick screwing system. When screwing metal parts, it is used less because the tightening torque applied is low and is subject to loosening during the operation of the machine.

Los tornillos son elementos presentes en casi todos los campos de construcciones metálicas, de madera o de otras actividades, por eso hay muchos tipos, tamaños, y procesos de fabricación.

Desde el punto de vista de la utilización se pueden citar los siguientes tipos de tornillos.

• - Tornillos para usos generales.

• - Tornillos de miniatura.

• - Tornillos de alta resistencia.

• - Tornillos inviolables.

• - Tornillos de precisión.

• - Tornillos grandes o especiales.

• - Tornillos de titanio.

General purpose screws

Current fastener production is highly automated both with regard to head stamping and thread rolling. Therefore, it is easy to find the screw you need in specialized stores, as long as it is within the normal manufacturing range.

Normal screws differ in quality based on their mechanical resistance. The standard (EN ISO 898-1) establishes the following quality code 4.6, 5.6, 5.8, 6.8, 8.8, 10.9 and 12.9. Manufacturers are obliged to stamp the quality to which they belong on the heads of the screws.

The first number multiplied by 100 will indicate the breaking strength in Newtons/mm². Therefore, a 10.9 screw will have a resistance of 10*100=1,000 N/mm².

The second number indicates what percentage of the breaking limit is the elastic limit (it is the maximum stress that an "elastic" material can withstand without suffering permanent deformations). To translate it into something more understandable, it indicates how much we can tighten the screw without it deforming (and before breaking), which is why it is indicated as a percentage. Therefore, a 10.9 screw will have a yield strength of 900 N/mm².

As for dimensions, they are all standardized by DIN standards, and the available sizes, in metric threads for example with a hexagonal head, range between M3 and M68; The length of standard screws is variable in a step of 5 mm, from a minimum to a maximum depending on their diameter. However, if it is necessary to sporadically have longer screws, 1 m long threaded rods are manufactured, where it is possible to cut them to the desired length and with a fixation of two nuts at the ends to make the desired fixation.

miniature screws

With the development of increasingly smaller electronic components, it has been necessary to develop and manufacture especially small screws; This type of screw is characterized by being self-tapping in soft materials such as plastics, and its head is adapted to be driven by very small, precision screwdrivers; The material of these screws can be stainless steel, normal steel or brass.

High strength screws

High-strength screws are designated by the letters TR, followed by the shank diameter and shank length, separated by the x sign; will follow the type of steel they are built from

The nuts will be designated with the letters MR, the nominal diameter and the type of steel.

The characteristics of the steel used to manufacture the screws and nuts defined as high resistance are standardized.

The manufacturer of this type of screws is obliged to provide a guarantee certificate, so reception tests are not necessary, unless the Particular Technical Prescription Document requires them.

High-strength screws will have on their heads, marked in relief, the letters TR, the designation of the type of steel, and the name or trademark of the manufacturer.

On one of their bases, the high-strength nuts will bear, marked in relief, the letters MR, the designation of the type of steel, and the name of the manufacturer's registered trademark.[7]​.

Alternatively, with the advent of Eurocodes in recent years, the nomenclature for Unprestressed High Strength Screws has become metric + length + strength class, where the class is made up of two numbers separated by a period. The first of them indicates the nominal value of the breaking limit per 100 (fub) in N/mm², and the second the nominal value of the elastic limit (fyb) in N/mm², this value being the product of the breaking limit by this second number divided by 10.

For example, M18x120 10.9 indicates a metric 18 high strength screw, nominal length 120 mm, breaking limit 1000 N/mm² and elastic limit 900 N/mm². And M8x60 8.8 indicates a metric 8 screw, nominal length 60 mm, breaking limit 800 N/mm² and elastic limit 640 N/mm².

Other examples of normalized resistance classes are 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 10.9, 12.9.

Precision screws

Precision screws are installed when the pressures, efforts and speeds of the processes require stronger joints and more reliable screws that avoid failures that could trigger a breakdown in the machine or structure where they are installed.

These screws are characterized by having extra resistance to traction and fatigue efforts. The average resistance that these screws can have is 1300 N/mm² compared to 1220 N/mm² for those in the ordinary range.

This great resistance makes it possible to mount smaller screws or fewer screws, saving space, material and time.

The thread profile of these screws is rounded eliminating the sharp V point that is the main cause of failure of many screws.[8]​.

Tamper-resistant screws

Tamper-proof screws are a type of special screws that, once screwed into the corresponding place, are impossible to remove unless they are forced and broken. This is thanks to the design of the head, which is inclined inside, so that if you try to loosen it, the key comes out without success. They are screws called anti-vandal and are widely used in locksmith work carried out in places with access to the streets or places where malicious people could act. Just as tamper-proof screws are manufactured, tamper-proof nuts are also manufactured. The standards for these metric thread screws correspond to ISO-7380 and ISO-7991 and are manufactured with an Allen head and a Torx head.[9]​.

Some are also used that have a seal attached to the head, preventing a wrench from being inserted to loosen it. These screws are sold with their corresponding cap, and are usually for Allen keys. As a temporary or improvised solution, you can hammer in some round fishing sinkers in the same place.

Large or special screws

With today's modern technologies it is possible to manufacture screws that, due to their dimensions, are outside standard production. In these cases, you must always act according to the technical specifications of the screw you want to manufacture, size, material, quality, etc.

titanium screws

• - Surgical titanium: one of the best properties that titanium has is that it is not toxic in contact with people's bodies, which, together with its mechanical qualities of hardness, light weight and mechanical resistance, has made possible a large number of very useful applications such as joint prostheses, dental implants, components for the manufacture of heart valves and pacemakers and osteosynthesis nails or plates for the recovery of bone fractures, in addition to many other products.

One of the essential elements for many of the surgical applications of titanium is to be able to have the entire range of screws that may be necessary according to the required application.

Since titanium began to be used in the treatment of fractures and in orthopedics, no case of incompatibility has been reported.

The titanium alloy most used in this field contains aluminum and vanadium depending on the composition: TiAlV"). Aluminum increases the temperature of the transformation between the alpha and beta phases. Vanadium decreases that temperature. The alloy can be well welded. It has high toughness.[10]​.

• - Titanium screws with high mechanical resistance: the aeronautical industry uses a large number of titanium screws and requires high quality and high mechanical resistance from them. The grade 5-CA -TiAlV- alloy is the one that meets such technical requirements. Some of these screws are manufactured with MoS (molybdenum disulfide) solid film lubricant coating.

• - Titanium screws for motorcycles, bicycles and hobby items: these screws improve the appearance and performance of steel screws and users appreciate the colors they have (gold, blue, black, etc.), obtained by anodizing processes.

• - Titanium screws for industrial use: in this case the property sought in the screw or piece requested is mainly its resistance to attack by all types of acids.[11]​.

In practice, most screws manufactured are made of steel or aluminum. Screws made of aluminum are common in joints of soft materials such as wood or plastic, for home applications or where their lightness is appreciated. Among the steel alloy screws, we must highlight the stainless steels for specific applications due to their durability, in the food industry or in corrosive conditions with adverse atmospheres. In steels, a low carbon content allows ductility to be maintained despite the hardness of the carbon; With the content of manganese and silicon, a low-cost heat treatment is achieved and with niobium, grain size control is maintained at high temperature. Furthermore, in stainless steels, chromium, together with nickel and especially molybdenum, determine the quality of the alloy.

The industrial process of manufacturing screws by stamping and rolling requires the use of highly ductile steel, that is, with low carbon content. This particularity means that the lower resistance screws, 4. 6, 5. 6, 5. 8 and 6. 8, do not receive a hardening heat treatment.

To manufacture more resistant screws of qualities 8. 8 and 10. 9, the steel producing company Sidenor,[12]​ for example, produces a steel created specifically for screws called DUCTILE 80 and DUCTILE 100, which is characterized by being pre-treated before the screw manufacturing process, thanks to its chemical composition that allows it to remain ductile even though it already has more mechanical resistance, making it possible to manufacture cold screws.

The chemical composition of so-called DUCTILE 80 is as follows:

C: (0, 06/0, 08), Mn: (1, 30/1, 80), Si: (0, 20/0, 40), Cr: (0, 20/0, 50), Ti: (0, 20/0, 40), Nb: (0, 03/0, 05).

This low C content allows ductility to be maintained despite its hardness, with the Mn and Si content hardenability is achieved at low cost and with Nb grain size control is maintained at high temperature.

The steel called DUCTILE 100 has a similar composition, although in this steel the C content becomes (0.05/0.20) to increase its mechanical resistance.

For the manufacture of high-strength screws, normal steels are usually used (and therefore cheaper than special steels) that allow greater hardening after a treatment by carburizing or nitriding. A drawback of some of these treatments is that the screw receives cyanidation that is harmless in the screw, but converts the waste into highly polluting due to the poisonous cyanide they contain.

Steel is the most used metal in the manufacture of screws. It satisfies most of the demands of the main industries in terms of technical and economic quality for certain uses. However, there are a number of limitations. For example, common steels are not very resistant to corrosion.

Generally, the function of screws is part of load support, so prolonged exposure can lead to damage to the integrity of the structure with the subsequent cost of repair and/or replacement. In addition, many screws work outdoors. For this reason, hot-dip galvanizing is used as one of the methods used to improve the corrosion resistance of screws by means of a small coating on the surface. Galvanizing allows the screws to be coated by immersion in a bath of molten zinc.

The electrolytic or mechanical zinc plating technique is the one most used for the anti-corrosion coating of screws. This technique consists of depositing a layer of zinc on the piece using direct current from a saline solution containing zinc. The process is used to protect smaller parts, when they require a more uniform finish than that provided by hot dip galvanizing. However, the thickness of the zinc layer is small and, therefore, its durability is lower.

Another anticorrosive protection process is the treatment called bluing.

Blueing is a black or bluish finish, shiny or matte, for steel parts, with great durability, decorative effect and resistance to corrosion.

The bluing attracts and retains lubricating oils. The coating does not increase or decrease the dimensions of the treated metals, so tolerances for fitting parts are not affected. Additionally, treated surfaces can be welded, waxed, varnished or painted. A matte coating is obtained when applied to a surface treated with sandblasting or a chemical mordant, and a glossy coating is obtained on a polished or smooth surface. The colors that can be obtained vary from black to bluish, depending on the type of alloy treated.

For situations with greater anti-corrosion protection, screws made of stainless steel are used, which are logically more expensive, and even for more specific cases, titanium screws are manufactured whose anti-corrosion resistance is almost total.

There are two different means to measure or verify the thread of the screws: those that are direct measurement and those that are indirect measurement.

For direct measurement, micrometers "Micrometer (instrument)") are generally used, the tips of which are adapted to enter the flank of the threads. Another method of direct measurement is to do it with the micrometer and a set of rods that are inserted into the flanks of the threads and allows the average diameters on the flanks to be directly measured according to the diameter of the rods.

Several methods are used to indirectly measure threads, the most common being gauges. With these gauges composed of two parts in which one of them is called PASS and the other DOES NOT PASS.

There is also a very common gauge that is a set of templates for the different thread pitches of each system, which easily allows you to identify which pitch a screw or nut has. Profile projectors are also used in metrology laboratories, ideal for verifying precision threads.[13]​.

The regulated torque is normally established as the preload that must be applied when screwing a screw using the appropriate tool.

• - The tightenings are regulated for blued or zinc-plated screws, with adequate lubrication (µ = Dynamic viscosity coefficient) and quality of screw used. Regulated tightening provides essential improvements to the assembly because it will prevent the anchor from being loose with the risk of loosening or applying a preload that is too strong, with the risk of deformation of the assembled parts, or breakage of the screw.

• - Preload is a function of the tightening torque applied to the screw and the coefficient of friction. Preload is the force with which the screw presses on the joined parts once tightened.

• - The tightening torque is the product of a force (For example: manual or pneumatic) applied at the end of a lever arm constituted by the tools (wrenches, screwdriver, etc.).

The recommended tightening torques vary depending on the elastic limit, the breaking limit and the dimensions and qualities of the screw. The materials of the pieces to be joined must also be taken into account, since strong tightening could deform the pieces and/or lead them to a state of plasticity in which the pieces will be unable to exert the reaction force to keep the screw taut. Sometimes the screws are tightened with a tension greater than their elastic limit to deform them and thus prevent them from loosening, calculating that in no case will the breaking limit be exceeded. It will also depend on whether we use flat washers, grower tensioners, Nordlock type cams, etc. But despite the variations, as general information there are tables that regulate the recommended tightening torques for each case.[14]​.

It is crucial that attention be paid to tightening torques and installation instructions where determined by mounting specifications. Vehicle engines are especially sensitive to improper tightening torque. Modern engines react particularly sensitively to assembly errors.

The tool used to tighten a screw with the regulated torque is called a torque wrench.

Screws in general are an important part of the rigidity and good functioning that can be expected and desired from the assembled elements. That is why failures or defects that a screw may have can cause an unwanted failure or breakdown.

The first defect that a screw can present is a design or calculation defect because its dimensions or quality are not adequate. In this case, the failure that can be caused is a premature breakage of the screw due to not being able to withstand the tensions and efforts to which it is subjected.

The second most important defect that a screw can have is a manufacturing defect where the quality of the constituent material is not as provided for in the design, or a dimensional defect with regard mainly to the tolerances that its threading must have. In this case, the screw may break or the thread may deteriorate.

The third defect may be poor assembly due to not applying the appropriate tightening torque, according to its quality and dimensions. In this case, if it is an excess of tightening, the screw may break or deteriorate the thread, and if it is a defect in the tightening, the assembly remains loose and if it is a moving object, unwanted vibrations appear that cause a breakdown in the assembled mechanism.

The fourth defect occurs due to deterioration of the screw if it is attacked by oxidation and corrosion if it has not been properly protected. In this case and during routine preventive maintenance operations on the mechanism, all damaged screws must be replaced with new ones and adequately protected from corrosion and oxidation.

The last serious defect that a screw can have is when an assembly is dismantled and if, due to oxidation and corrosion, the screw loses its head when trying to loosen it. In these cases of damaged screws, lubricating products must be used that allow loosening without breaking the screw.

The first antecedents of the use of threads date back to the Archimedean screw, developed by the Greek wise man around 300 BC. C., already being used extensively in the Nile Valley to raise water at that time.

During the Renaissance, threads began to be used as fastening elements in watches, war machines and other mechanical constructions. Leonardo da Vinci developed methods for thread cutting at that time; However, these will continue to be manufactured by hand and without any kind of standardization until well into the Industrial Revolution.

In 1841, the British engineer Joseph Whitworth defined the thread that bears his name.

In 1864, William Sellers did the same in the United States.

This situation lasted until 1946, when the International Organization for Standardization (ISO) defined the metric thread system, currently adopted in practically all countries. In the United States, however, the SAE standard is still used (Society of Automotive Engineers).

The metric thread has a triangular section that forms a 60° angle and the head is slightly truncated to facilitate greasing.

• - Washer.

• - Bolt.

• - Screwdriver.

• - Escarpia.

• - Spike&action=edit&redlink=1 "Spike (part of a tool or bladed weapon) (not yet drafted)".

• - Threaded insert.

• - Wrench (tool) "Wrench (tool)").

• - Bolt.

• - Rivet or rivet.

• - Threading.

• - Lathe for carving screws.

• - Nut.

• - Wiktionary has definitions and other information about screw.

• - Wikimedia Commons hosts a multimedia category on screws.

• - High Strength Screws since 1920.

• - Screws according to DIN standards.

• - International Thread Standards.

• - screw locking machine.

• - The content of this article incorporates material from an entry of the Free Universal Encyclopedia, published in Spanish under the Creative Commons Share-Alike 3.0 license.

Contenido

El término tornillo se utiliza generalmente en forma genérica: son muchas las variedades de materiales, tipos y tamaños que existen. Una primera clasificación puede ser la siguiente:[3]​.

• - Tornillos tirafondos para madera.

• - Autorroscantes y autoperforantes para chapas metálicas y maderas duras.

• - Tornillos tirafondos para paredes y muros de edificios.

• - Tornillos de roscas cilíndricas.

• - Varillas roscadas de un metro de longitud.

wood screws

Wood screws are called wood lag screws. Their size and quality is regulated by the DIN-97 standard and they have a thread that occupies 3/4 of the length of the spike. They can be made of mild steel, stainless steel, brass, copper, bronze or aluminum, and can be galvanized, nickel-plated, dichromated, etc.

This type of screw tapers at the tip as a way of opening the way as it is inserted to facilitate self-tapping, because it is not necessary to make a previous hole, and the thread is sharp and cutting. They are normally screwed with an electric or manual screwdriver.

Their heads can be flat, oval or rounded; Each one will fulfill a specific function.

Flat head: used in carpentry, in general, where it is necessary to leave the head of the screw submerged or flush with the surface.

Pointed head: the lower portion of the head has a shape that allows it to sink into the surface and leave only the rounded upper part protruding. They are easier to remove and have a better presentation than the flat heads. They are used to fix metal elements, such as tools or doorknob plates.

Rounded head: used to fasten pieces that are too thin to allow the screw to sink into them; also for joining parts that will require washers. In general, they are used for similar functions to those with oval heads, but in unsinked holes. This type of screw is very easy to remove.

The heads can be of different kinds:

Milled head (straight slot): They have traditional straight slots.

Phillips head: They have cross-shaped slots to minimize the chance of the screwdriver slipping.

Allen type head: with a hexagonal hole, to fit an Allen wrench.

Torx head: with a star-shaped hole in the head of an exclusive Torx design.

The characteristics that define wood screws are: head type, constituent material, shaft diameter and length.

Lag screws for walls and wood DIN-571

There is a variety of screws that are thicker than the classic wooden ones, which are called lag screws and are widely used to screw the supports of heavy elements that are hung on the walls of buildings, such as awnings, air conditioners, etc. In these cases, the wall is drilled to the diameter of the chosen screw, and a plastic plug is inserted, then the screw that threads the plastic plug under pressure is screwed in and thus the support is firmly held. It is also used, for example, for screwing the wood of large packaging. These screws have a hexagonal head and range from M5 to M12.

Self-tapping and self-drilling for metal sheets and hard woods

Both types of screws can make their own way. They are manufactured in a wide variety of special shapes. The appropriate one is selected based on the type of work to be carried out and the material on which it will be used.

Self-tapping have most of their shaft cylindrical and the end conical. They can have a flat, oval, rounded or flat head. The thread is thin, with a flat bottom, so that the iron can stay in it. They are used in metal sheets or profiles, because they allow metal to wood, metal to metal, metal to plastic or other materials to be joined. These screws are fully machined (from tip to head) and have sharper edges than wood screws.

In self-drilling the tip is a drill bit, which avoids having to make guide holes to install them. They are used for heavier metals: they cut a thread in front of the main part of the screw.

The dimensions, head type and quality are regulated by DIN standards. These standards are technical requirements for quality and safety established by the German Institute for Standardization and are applied in industry, commerce, science, etc. They are especially considered when it comes to mechanisms and industrial parts, such as screws, nuts, washers, etc.

They are also used to be fixed to walls by means of plastic anchors called "Taco (construction)") or ramplugs.

Cylindrical thread screws for metal joints

To join metal parts, screws with triangular threads are used, which can be screwed into a blind hole or into a nut with a washer in a through hole.

This type of screw is the one normally used in machines and the most important thing that is required of them is that they withstand well the stresses to which they are subjected and that they do not loosen during the operation of the machine where they are inserted.

The notable thing about these screws is the thread system and the type of head they have since there are variations from one system to another. Due to the thread system, the most used are the following.

• - Metric thread with normal pitch or fine pitch.

• - "English" (British) Whitworth thread with normal or fine pitch.

• - "American" (American) SAE thread.

Depending on the type of head they have, the most used screws are the following:

• - Hexagon head: type DIN 933 and DIN 931.

• - Allen head: DIN 912 type.

• - Countersunk head.

• - Cylindrical head: DIN 84 type.

• - Torx head (shaped like a six-pointed star).

According to Iram 4520* (1999), in its item 2.1.2 “Conventional representation”, normally by convention, the representation of threads and threaded parts in all types of technical drawings are simplified as shown in the attached figure (graphic representation of a screw). In both screws and threaded holes, the crest that represents the crown of the thread is represented with a thick continuous line and the root with a thin line. In hidden views, both are drawn with a thin dashed line. In the sections, the striping extends to the crown. In a front view of a screw, the line representing the root (diameter at the bottom of a thread) will cover approximately 3/4 of the circumference to avoid errors of interpretation (preferably open in the upper right quadrant). In drawings of assembled threaded parts (assemblies), the lines that represent the external thread (screw, threaded bolt, tube) will always be shown covering the internal thread and will not be hidden by it.

Crowning is understood as the largest diameter in external threads (nominal diameter of the thread) and the smallest diameter in internal threads (hole diameter).

When we talk about root, it normally refers to the smallest diameter in external threads (diameter at the bottom of the thread) and the largest diameter in internal threads (nominal diameter of the thread).

El diseño de las cabezas de los tornillos responde, en general, a dos necesidades: por un lado, conseguir la superficie de apoyo adecuada para la herramienta de apriete de forma tal que se pueda alcanzar la fuerza necesaria sin que la cabeza se rompa o deforme. Por otro, necesidades de seguridad implican (incluso en reglamentos oficiales de obligado cumplimiento) que ciertos dispositivos requieran herramientas especiales para la apertura, lo que exige que el tornillo (si este es el medio elegido para asegurar el cierre) no pueda desenroscarse con un destornillador convencional, dificultando así que personal no autorizado acceda al interior.

Así, se tienen cabezas de distintas formas:.

• - hexagonal (a).

• - redonda o alomada (b).

• - cilíndrica (d, g).

• - avellanada (c, e, f).

• - combinadas con distintos sistemas de apriete:

• hexagonal (a) o cuadrada para llave inglesa,

• ranura o entalla (b, c, d) y

• Phillips o estrella (f) para destornillador,

• agujero hexagonal (e) para llave Allen,

• moleteado (g) para apriete manual, etc.

Commercial Hex Head Screws

From certain diameters onwards, it is normal for the head of commercial screws to be hexagonal, mainly those that screw into metal parts or into their corresponding nut. There are several types of commercial hexagonal head screws manufactured according to DIN standards that differ from each other in the length of the thread that their shafts have.[4]​.

Commercial screws with Allen head

As with hexagonal heads, there are several models of screws with Allen heads, all of them standardized according to the corresponding DIN standards. Hex head screws are mainly used when smooth surfaces are desired and the tightening forces are not very high.[5]​.

Normally, this type of screw is used in tool making work since the head is embedded inside the molds. It is also used for automotive and motorcycling mechanics due to the ease of tightening when they are in places that are difficult to access. Depending on its thickness and length, it comes with an unthreaded neck.[6]​.

Screws for tightening with a screwdriver

With the modern electric and pneumatic screwdrivers that exist, the use of self-tapping screws is widely used in various types of carpentry, both wood and metal, since it is a quick screwing system. When screwing metal parts, it is used less because the tightening torque applied is low and is subject to loosening during the operation of the machine.

Los tornillos son elementos presentes en casi todos los campos de construcciones metálicas, de madera o de otras actividades, por eso hay muchos tipos, tamaños, y procesos de fabricación.

Desde el punto de vista de la utilización se pueden citar los siguientes tipos de tornillos.

• - Tornillos para usos generales.

• - Tornillos de miniatura.

• - Tornillos de alta resistencia.

• - Tornillos inviolables.

• - Tornillos de precisión.

• - Tornillos grandes o especiales.

• - Tornillos de titanio.

General purpose screws

Current fastener production is highly automated both with regard to head stamping and thread rolling. Therefore, it is easy to find the screw you need in specialized stores, as long as it is within the normal manufacturing range.

Normal screws differ in quality based on their mechanical resistance. The standard (EN ISO 898-1) establishes the following quality code 4.6, 5.6, 5.8, 6.8, 8.8, 10.9 and 12.9. Manufacturers are obliged to stamp the quality to which they belong on the heads of the screws.

The first number multiplied by 100 will indicate the breaking strength in Newtons/mm². Therefore, a 10.9 screw will have a resistance of 10*100=1,000 N/mm².

The second number indicates what percentage of the breaking limit is the elastic limit (it is the maximum stress that an "elastic" material can withstand without suffering permanent deformations). To translate it into something more understandable, it indicates how much we can tighten the screw without it deforming (and before breaking), which is why it is indicated as a percentage. Therefore, a 10.9 screw will have a yield strength of 900 N/mm².

As for dimensions, they are all standardized by DIN standards, and the available sizes, in metric threads for example with a hexagonal head, range between M3 and M68; The length of standard screws is variable in a step of 5 mm, from a minimum to a maximum depending on their diameter. However, if it is necessary to sporadically have longer screws, 1 m long threaded rods are manufactured, where it is possible to cut them to the desired length and with a fixation of two nuts at the ends to make the desired fixation.

miniature screws

With the development of increasingly smaller electronic components, it has been necessary to develop and manufacture especially small screws; This type of screw is characterized by being self-tapping in soft materials such as plastics, and its head is adapted to be driven by very small, precision screwdrivers; The material of these screws can be stainless steel, normal steel or brass.

High strength screws

High-strength screws are designated by the letters TR, followed by the shank diameter and shank length, separated by the x sign; will follow the type of steel they are built from

The nuts will be designated with the letters MR, the nominal diameter and the type of steel.

The characteristics of the steel used to manufacture the screws and nuts defined as high resistance are standardized.

The manufacturer of this type of screws is obliged to provide a guarantee certificate, so reception tests are not necessary, unless the Particular Technical Prescription Document requires them.

High-strength screws will have on their heads, marked in relief, the letters TR, the designation of the type of steel, and the name or trademark of the manufacturer.

On one of their bases, the high-strength nuts will bear, marked in relief, the letters MR, the designation of the type of steel, and the name of the manufacturer's registered trademark.[7]​.

Alternatively, with the advent of Eurocodes in recent years, the nomenclature for Unprestressed High Strength Screws has become metric + length + strength class, where the class is made up of two numbers separated by a period. The first of them indicates the nominal value of the breaking limit per 100 (fub) in N/mm², and the second the nominal value of the elastic limit (fyb) in N/mm², this value being the product of the breaking limit by this second number divided by 10.

For example, M18x120 10.9 indicates a metric 18 high strength screw, nominal length 120 mm, breaking limit 1000 N/mm² and elastic limit 900 N/mm². And M8x60 8.8 indicates a metric 8 screw, nominal length 60 mm, breaking limit 800 N/mm² and elastic limit 640 N/mm².

Other examples of normalized resistance classes are 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 10.9, 12.9.

Precision screws

Precision screws are installed when the pressures, efforts and speeds of the processes require stronger joints and more reliable screws that avoid failures that could trigger a breakdown in the machine or structure where they are installed.

These screws are characterized by having extra resistance to traction and fatigue efforts. The average resistance that these screws can have is 1300 N/mm² compared to 1220 N/mm² for those in the ordinary range.

This great resistance makes it possible to mount smaller screws or fewer screws, saving space, material and time.

The thread profile of these screws is rounded eliminating the sharp V point that is the main cause of failure of many screws.[8]​.

Tamper-resistant screws

Tamper-proof screws are a type of special screws that, once screwed into the corresponding place, are impossible to remove unless they are forced and broken. This is thanks to the design of the head, which is inclined inside, so that if you try to loosen it, the key comes out without success. They are screws called anti-vandal and are widely used in locksmith work carried out in places with access to the streets or places where malicious people could act. Just as tamper-proof screws are manufactured, tamper-proof nuts are also manufactured. The standards for these metric thread screws correspond to ISO-7380 and ISO-7991 and are manufactured with an Allen head and a Torx head.[9]​.

Some are also used that have a seal attached to the head, preventing a wrench from being inserted to loosen it. These screws are sold with their corresponding cap, and are usually for Allen keys. As a temporary or improvised solution, you can hammer in some round fishing sinkers in the same place.

Large or special screws

With today's modern technologies it is possible to manufacture screws that, due to their dimensions, are outside standard production. In these cases, you must always act according to the technical specifications of the screw you want to manufacture, size, material, quality, etc.

titanium screws

• - Surgical titanium: one of the best properties that titanium has is that it is not toxic in contact with people's bodies, which, together with its mechanical qualities of hardness, light weight and mechanical resistance, has made possible a large number of very useful applications such as joint prostheses, dental implants, components for the manufacture of heart valves and pacemakers and osteosynthesis nails or plates for the recovery of bone fractures, in addition to many other products.

One of the essential elements for many of the surgical applications of titanium is to be able to have the entire range of screws that may be necessary according to the required application.

Since titanium began to be used in the treatment of fractures and in orthopedics, no case of incompatibility has been reported.

The titanium alloy most used in this field contains aluminum and vanadium depending on the composition: TiAlV"). Aluminum increases the temperature of the transformation between the alpha and beta phases. Vanadium decreases that temperature. The alloy can be well welded. It has high toughness.[10]​.

• - Titanium screws with high mechanical resistance: the aeronautical industry uses a large number of titanium screws and requires high quality and high mechanical resistance from them. The grade 5-CA -TiAlV- alloy is the one that meets such technical requirements. Some of these screws are manufactured with MoS (molybdenum disulfide) solid film lubricant coating.

• - Titanium screws for motorcycles, bicycles and hobby items: these screws improve the appearance and performance of steel screws and users appreciate the colors they have (gold, blue, black, etc.), obtained by anodizing processes.

• - Titanium screws for industrial use: in this case the property sought in the screw or piece requested is mainly its resistance to attack by all types of acids.[11]​.

In practice, most screws manufactured are made of steel or aluminum. Screws made of aluminum are common in joints of soft materials such as wood or plastic, for home applications or where their lightness is appreciated. Among the steel alloy screws, we must highlight the stainless steels for specific applications due to their durability, in the food industry or in corrosive conditions with adverse atmospheres. In steels, a low carbon content allows ductility to be maintained despite the hardness of the carbon; With the content of manganese and silicon, a low-cost heat treatment is achieved and with niobium, grain size control is maintained at high temperature. Furthermore, in stainless steels, chromium, together with nickel and especially molybdenum, determine the quality of the alloy.

The industrial process of manufacturing screws by stamping and rolling requires the use of highly ductile steel, that is, with low carbon content. This particularity means that the lower resistance screws, 4. 6, 5. 6, 5. 8 and 6. 8, do not receive a hardening heat treatment.

To manufacture more resistant screws of qualities 8. 8 and 10. 9, the steel producing company Sidenor,[12]​ for example, produces a steel created specifically for screws called DUCTILE 80 and DUCTILE 100, which is characterized by being pre-treated before the screw manufacturing process, thanks to its chemical composition that allows it to remain ductile even though it already has more mechanical resistance, making it possible to manufacture cold screws.

The chemical composition of so-called DUCTILE 80 is as follows:

C: (0, 06/0, 08), Mn: (1, 30/1, 80), Si: (0, 20/0, 40), Cr: (0, 20/0, 50), Ti: (0, 20/0, 40), Nb: (0, 03/0, 05).

This low C content allows ductility to be maintained despite its hardness, with the Mn and Si content hardenability is achieved at low cost and with Nb grain size control is maintained at high temperature.

The steel called DUCTILE 100 has a similar composition, although in this steel the C content becomes (0.05/0.20) to increase its mechanical resistance.

For the manufacture of high-strength screws, normal steels are usually used (and therefore cheaper than special steels) that allow greater hardening after a treatment by carburizing or nitriding. A drawback of some of these treatments is that the screw receives cyanidation that is harmless in the screw, but converts the waste into highly polluting due to the poisonous cyanide they contain.

Steel is the most used metal in the manufacture of screws. It satisfies most of the demands of the main industries in terms of technical and economic quality for certain uses. However, there are a number of limitations. For example, common steels are not very resistant to corrosion.

Generally, the function of screws is part of load support, so prolonged exposure can lead to damage to the integrity of the structure with the subsequent cost of repair and/or replacement. In addition, many screws work outdoors. For this reason, hot-dip galvanizing is used as one of the methods used to improve the corrosion resistance of screws by means of a small coating on the surface. Galvanizing allows the screws to be coated by immersion in a bath of molten zinc.

The electrolytic or mechanical zinc plating technique is the one most used for the anti-corrosion coating of screws. This technique consists of depositing a layer of zinc on the piece using direct current from a saline solution containing zinc. The process is used to protect smaller parts, when they require a more uniform finish than that provided by hot dip galvanizing. However, the thickness of the zinc layer is small and, therefore, its durability is lower.

Another anticorrosive protection process is the treatment called bluing.

Blueing is a black or bluish finish, shiny or matte, for steel parts, with great durability, decorative effect and resistance to corrosion.

The bluing attracts and retains lubricating oils. The coating does not increase or decrease the dimensions of the treated metals, so tolerances for fitting parts are not affected. Additionally, treated surfaces can be welded, waxed, varnished or painted. A matte coating is obtained when applied to a surface treated with sandblasting or a chemical mordant, and a glossy coating is obtained on a polished or smooth surface. The colors that can be obtained vary from black to bluish, depending on the type of alloy treated.

For situations with greater anti-corrosion protection, screws made of stainless steel are used, which are logically more expensive, and even for more specific cases, titanium screws are manufactured whose anti-corrosion resistance is almost total.

There are two different means to measure or verify the thread of the screws: those that are direct measurement and those that are indirect measurement.

For direct measurement, micrometers "Micrometer (instrument)") are generally used, the tips of which are adapted to enter the flank of the threads. Another method of direct measurement is to do it with the micrometer and a set of rods that are inserted into the flanks of the threads and allows the average diameters on the flanks to be directly measured according to the diameter of the rods.

Several methods are used to indirectly measure threads, the most common being gauges. With these gauges composed of two parts in which one of them is called PASS and the other DOES NOT PASS.

There is also a very common gauge that is a set of templates for the different thread pitches of each system, which easily allows you to identify which pitch a screw or nut has. Profile projectors are also used in metrology laboratories, ideal for verifying precision threads.[13]​.

The regulated torque is normally established as the preload that must be applied when screwing a screw using the appropriate tool.

• - The tightenings are regulated for blued or zinc-plated screws, with adequate lubrication (µ = Dynamic viscosity coefficient) and quality of screw used. Regulated tightening provides essential improvements to the assembly because it will prevent the anchor from being loose with the risk of loosening or applying a preload that is too strong, with the risk of deformation of the assembled parts, or breakage of the screw.

• - Preload is a function of the tightening torque applied to the screw and the coefficient of friction. Preload is the force with which the screw presses on the joined parts once tightened.

• - The tightening torque is the product of a force (For example: manual or pneumatic) applied at the end of a lever arm constituted by the tools (wrenches, screwdriver, etc.).

The recommended tightening torques vary depending on the elastic limit, the breaking limit and the dimensions and qualities of the screw. The materials of the pieces to be joined must also be taken into account, since strong tightening could deform the pieces and/or lead them to a state of plasticity in which the pieces will be unable to exert the reaction force to keep the screw taut. Sometimes the screws are tightened with a tension greater than their elastic limit to deform them and thus prevent them from loosening, calculating that in no case will the breaking limit be exceeded. It will also depend on whether we use flat washers, grower tensioners, Nordlock type cams, etc. But despite the variations, as general information there are tables that regulate the recommended tightening torques for each case.[14]​.

It is crucial that attention be paid to tightening torques and installation instructions where determined by mounting specifications. Vehicle engines are especially sensitive to improper tightening torque. Modern engines react particularly sensitively to assembly errors.

The tool used to tighten a screw with the regulated torque is called a torque wrench.

Screws in general are an important part of the rigidity and good functioning that can be expected and desired from the assembled elements. That is why failures or defects that a screw may have can cause an unwanted failure or breakdown.

The first defect that a screw can present is a design or calculation defect because its dimensions or quality are not adequate. In this case, the failure that can be caused is a premature breakage of the screw due to not being able to withstand the tensions and efforts to which it is subjected.

The second most important defect that a screw can have is a manufacturing defect where the quality of the constituent material is not as provided for in the design, or a dimensional defect with regard mainly to the tolerances that its threading must have. In this case, the screw may break or the thread may deteriorate.

The third defect may be poor assembly due to not applying the appropriate tightening torque, according to its quality and dimensions. In this case, if it is an excess of tightening, the screw may break or deteriorate the thread, and if it is a defect in the tightening, the assembly remains loose and if it is a moving object, unwanted vibrations appear that cause a breakdown in the assembled mechanism.

The fourth defect occurs due to deterioration of the screw if it is attacked by oxidation and corrosion if it has not been properly protected. In this case and during routine preventive maintenance operations on the mechanism, all damaged screws must be replaced with new ones and adequately protected from corrosion and oxidation.

The last serious defect that a screw can have is when an assembly is dismantled and if, due to oxidation and corrosion, the screw loses its head when trying to loosen it. In these cases of damaged screws, lubricating products must be used that allow loosening without breaking the screw.

The first antecedents of the use of threads date back to the Archimedean screw, developed by the Greek wise man around 300 BC. C., already being used extensively in the Nile Valley to raise water at that time.

During the Renaissance, threads began to be used as fastening elements in watches, war machines and other mechanical constructions. Leonardo da Vinci developed methods for thread cutting at that time; However, these will continue to be manufactured by hand and without any kind of standardization until well into the Industrial Revolution.

In 1841, the British engineer Joseph Whitworth defined the thread that bears his name.

In 1864, William Sellers did the same in the United States.

This situation lasted until 1946, when the International Organization for Standardization (ISO) defined the metric thread system, currently adopted in practically all countries. In the United States, however, the SAE standard is still used (Society of Automotive Engineers).

The metric thread has a triangular section that forms a 60° angle and the head is slightly truncated to facilitate greasing.

• - Washer.

• - Bolt.

• - Screwdriver.

• - Escarpia.

• - Spike&action=edit&redlink=1 "Spike (part of a tool or bladed weapon) (not yet drafted)".

• - Threaded insert.

• - Wrench (tool) "Wrench (tool)").

• - Bolt.

• - Rivet or rivet.

• - Threading.

• - Lathe for carving screws.

• - Nut.

• - Wiktionary has definitions and other information about screw.

• - Wikimedia Commons hosts a multimedia category on screws.

• - High Strength Screws since 1920.

• - Screws according to DIN standards.

• - International Thread Standards.

• - screw locking machine.

• - The content of this article incorporates material from an entry of the Free Universal Encyclopedia, published in Spanish under the Creative Commons Share-Alike 3.0 license.