Tornillo

Un tornillo y un perno (ver Diferenciación entre perno y tornillo a continuación) son tipos similares de sujetadores típicamente hechos de metal y caracterizados por una cresta helicoidal , conocida como rosca macho (rosca externa). Los tornillos y pernos se utilizan para sujetar materiales mediante el acoplamiento de la rosca del tornillo con una rosca hembra similar (rosca interna) en la parte correspondiente.

Una variedad de tornillos
Un perno (con una tuerca ) y un tornillo

Los tornillos a menudo son autorroscantes (también conocidos como autorroscantes) donde la rosca corta el material cuando se gira el tornillo, creando una rosca interna que ayuda a juntar los materiales sujetos y evitar que se salgan. Hay muchos tornillos para una variedad de materiales; los que se sujetan comúnmente con tornillos incluyen madera, láminas de metal y plástico.

Un tornillo es una combinación de máquinas simples ; es, en esencia, un plano inclinado envuelto alrededor de un eje central, pero el plano inclinado (rosca) también llega a un borde afilado alrededor del exterior, que actúa como una cuña cuando empuja hacia material sujetado, y el eje y la hélice también forman una cuña en forma de punta. Algunas roscas de tornillo están diseñadas para acoplarse con una rosca complementaria, conocida como rosca hembra ( rosca interna), a menudo en forma de tuerca u objeto que tiene la rosca interna formada en ella. Otras roscas de tornillo están diseñadas para cortar una ranura helicoidal en un material más suave a medida que se inserta el tornillo. Los usos más comunes de los tornillos son para sujetar objetos juntos y colocarlos.

Un tornillo para madera: a) cabeza; b) vástago sin rosca; c) vástago roscado; d) propina.

Un tornillo suele tener una cabeza en un extremo que permite girarlo con una herramienta. Las herramientas comunes para atornillar incluyen destornilladores y llaves . La cabeza suele ser más grande que el cuerpo del tornillo, lo que evita que el tornillo sea clavado a una profundidad mayor que la longitud del tornillo y proporciona una superficie de apoyo . Hay excepciones. Los pernos de carro tienen una cabeza abovedada que no está diseñada para ser accionada. Los tornillos de fijación a menudo tienen una cabeza más pequeña que el diámetro exterior del tornillo. Los tornillos de fijación sin cabeza también se denominan tornillos prisioneros. Los pernos en J tienen una cabeza en forma de J que no está diseñada para ser clavada, sino que generalmente está hundida en el concreto, lo que permite su uso como perno de anclaje . La parte cilíndrica del tornillo desde la parte inferior de la cabeza hasta la punta se conoce como vástago ; puede estar completamente roscado o parcialmente roscado. [1] La distancia entre cada hilo se llama "paso". [2]

La mayoría de los tornillos se aprietan mediante rotación en el sentido de las agujas del reloj , lo que se denomina rosca a la derecha ; [3] [4] un dispositivo mnemónico común para recordar esto cuando se trabaja con tornillos o pernos es "right-tighty, zurdo-flojo". Si los dedos de la mano derecha están enrollados alrededor de un hilo de la mano derecha, se moverá en la dirección del pulgar cuando se gire en la misma dirección en que se enrollan los dedos. Los tornillos con rosca a la izquierda se utilizan en casos excepcionales, donde las cargas tenderían a aflojar un sujetador a la derecha, o cuando se requiere la no intercambiabilidad con sujetadores a la derecha. Por ejemplo, cuando el tornillo estará sujeto a un par de torsión en sentido antihorario (que funcionaría para deshacer una rosca a la derecha), un tornillo con rosca a la izquierda sería una opción adecuada. El pedal del lado izquierdo de una bicicleta tiene una rosca a la izquierda.

De manera más general, un tornillo puede significar cualquier dispositivo helicoidal, como una abrazadera, un micrómetro , la hélice de un barco o una bomba de agua de tornillo de Arquímedes .

Un perno de carro con una tuerca cuadrada
Un perno estructural con tuerca hexagonal y arandela

No existe una distinción universalmente aceptada entre un tornillo y un perno. Una distinción simple que a menudo es cierta, aunque no siempre, es que un perno pasa a través de un sustrato y toma una tuerca en el otro lado, mientras que un tornillo no toma tuerca porque se enrosca directamente en el sustrato (un tornillo se atornilla en algo , una perno atornilla varias cosas juntas ). Por lo tanto, como regla general, cuando se compra un paquete de "tornillos", no se espera que se incluyan tuercas, pero los pernos a menudo se venden con tuercas a juego. Parte de la confusión sobre esto probablemente se deba a diferencias regionales o dialécticas. El Manual de maquinaria describe la distinción de la siguiente manera:

Un perno es un sujetador con rosca externa diseñado para la inserción a través de orificios en piezas ensambladas, y normalmente está diseñado para apretarse o soltarse apretando una tuerca. Un tornillo es un sujetador de rosca externa que puede insertarse en los orificios de las piezas ensambladas, acoplarse con una rosca interna preformada o formar su propia rosca, y apretarse o soltarse apretando la cabeza. Un sujetador con rosca externa que no se puede girar durante el ensamblaje y que se puede apretar o soltar solo apretando una tuerca es un perno. (Ejemplo: pernos de cabeza redonda, pernos de riel, pernos de arado). Un sujetador con rosca externa que tiene forma de rosca que prohíbe el ensamblaje con una tuerca que tiene una rosca recta de longitud de paso múltiple es un tornillo. (Ejemplo: tornillos para madera, tornillos roscadores) [5]

Esta distinción es consistente con ASME B18.2.1 y algunas definiciones de diccionario para tornillo [6] [7] y perno . [8] [9] [10]

Sin embargo, el problema de qué es un tornillo y qué es un perno no se resuelve por completo con la distinción del Manual de maquinaria debido a los términos confusos, la naturaleza ambigua de algunas partes de la distinción y las variaciones de uso. [11] [ verificación fallida ] Algunos de estos problemas se analizan a continuación:

Tornillos para madera

Los primeros tornillos para madera se fabricaban a mano, con una serie de limas, cinceles y otras herramientas de corte, y estos se pueden detectar fácilmente al observar el espaciado y la forma irregulares de las roscas, así como las marcas de las limas que quedan en la cabeza del tornillo. y en la zona entre hilos. Muchos de estos tornillos tenían un extremo romo, careciendo por completo de la punta afilada cónica en casi todos los tornillos para madera modernos. [12] Con el tiempo, los tornos se utilizaron para fabricar tornillos para madera, y la primera patente se registró en 1760 en Inglaterra. [12] Durante la década de 1850 , se desarrollaron herramientas de estampación para proporcionar un hilo más uniforme y consistente. Los tornillos hechos con estas herramientas tienen valles redondeados con roscas afiladas y rugosas. [13] [14] Algunos tornillos para madera se fabricaron con troqueles de corte ya a fines del siglo XVIII (posiblemente incluso antes de 1678, cuando el contenido del libro se publicó por primera vez en partes). [15]

Una vez que las máquinas de torneado de tornillos fueron de uso común, la mayoría de los tornillos para madera disponibles comercialmente se produjeron con este método. Estos tornillos para madera cortados son casi invariablemente ahusados, e incluso cuando el vástago ahusado no es obvio, se pueden distinguir porque las roscas no se extienden más allá del diámetro del vástago. Es mejor instalar estos tornillos después de perforar un orificio piloto con una broca cónica. La mayoría de los tornillos para madera modernos, excepto los de latón, se forman en máquinas laminadoras de roscas. Estos tornillos tienen un diámetro constante, roscas con un diámetro mayor que el vástago y son más fuertes porque el proceso de laminación no corta la veta del metal.

Tornillos de máquina

Las normas ASME especifican una variedad de "tornillos de máquina" [16] en diámetros que van hasta 0,75 pulg. (19,05 mm). Estos sujetadores se usan a menudo como pernos con tuercas, pero también a menudo se introducen en orificios roscados (sin tuercas). Pueden considerarse un tornillo o un perno según la distinción del Manual de maquinaria . En la práctica, tienden a estar disponibles principalmente en tamaños más pequeños y los tamaños más pequeños se denominan tornillos o, de manera menos ambigua, tornillos de máquina, aunque algunos tipos de tornillos de máquina pueden denominarse pernos de estufa.

Tornillos de cabeza hexagonal

La norma ASME B18.2.1-1996 especifica tornillos de cabeza hexagonal cuyo rango de tamaño es de 0,25 a 3 pulg. (6,35 a 76,20 mm) de diámetro . Estos sujetadores son muy similares a los pernos hexagonales. Se diferencian principalmente en que se fabrican con tolerancias más estrictas que los pernos correspondientes. El Manual de maquinaria se refiere entre paréntesis a estos sujetadores como "Pernos hexagonales terminados". [17] Razonablemente, estos sujetadores pueden denominarse pernos, pero según el documento del gobierno de EE. UU. Distinguir pernos de tornillos , el gobierno de EE. UU. Podría clasificarlos como tornillos debido a la tolerancia más estricta. [18] En 1991, en respuesta a una afluencia de sujetadores falsificados, el Congreso aprobó la PL 101-592 [19] "Ley de calidad de sujetadores". Esto resultó en la reescritura de las especificaciones por parte del comité ASME B18. B18.2.1 [20] fue reescrito y, como resultado, eliminaron los "Pernos hexagonales terminados" y los renombró como "Tornillo hexagonal", un término que había existido en el uso común mucho antes, pero que ahora también se codificaba como un nombre oficial para el estándar ASME B18.

Pernos de orejeta y pernos de cabeza

Estos términos se refieren a sujetadores que están diseñados para enroscarse en un orificio roscado que forma parte del ensamblaje y, por lo tanto, según la distinción del Manual de máquinas, serían tornillos. Aquí los términos comunes están en desacuerdo con la distinción del Manual de Maquinaria . [21] [22]

Tornillo de tracción

Tornillo de retraso, también llamado tornillo de retraso

Los tirafondos (EE. UU.) O los tirafondos (Reino Unido, Australia y Nueva Zelanda) (también denominados tirafondos o tirafondos , aunque es un nombre inapropiado ) son tornillos grandes para madera. La cabeza es típicamente un hexágono externo. Los tirafondos métricos de cabeza hexagonal están cubiertos por DIN 571. Los tirafondos en pulgadas de cabeza cuadrada y hexagonal están cubiertos por ASME B18.2.1. Un tirafondo típico puede variar en diámetro de 4 a 20 mm o # 10 a 1,25 pulg. (4,83 a 31,75 mm), y longitudes de 16 a 200 mm o 1 ⁄ 4 a 6 pulg. (6,35 a 152,40 mm) o más, con las roscas gruesas de un tornillo para madera o un tornillo para chapa (pero más grande).

Los materiales suelen ser sustrato de acero al carbono con una capa de zinc galvanizado (para resistencia a la corrosión). El recubrimiento de zinc puede ser brillante (galvanizado), amarillo (galvanizado) o gris mate galvanizado por inmersión en caliente . Los tirafondos se utilizan para unir estructuras de madera, para colocar pies de maquinaria en pisos de madera y para otras aplicaciones de carpintería pesada. El retraso modificador atributivo provino de un uso principal temprano de tales sujetadores: la sujeción de retrasos como duelas de barril y otras partes similares. [23]

Estos sujetadores son "tornillos" de acuerdo con los criterios del Manual de maquinaria , y el término obsoleto "perno de retraso" ha sido reemplazado por "tornillo de retraso" en el Manual . [24] Sin embargo, para muchos comerciantes, son "tornillos", porque son grandes, con cabezas hexagonales o cuadradas.

Estándares del gobierno de Estados Unidos

El gobierno federal de los Estados Unidos hizo un esfuerzo por formalizar la diferencia entre un perno y un tornillo, porque se aplican aranceles diferentes a cada uno. [25] El documento no parece tener un efecto significativo en el uso común y no elimina la naturaleza ambigua de la distinción entre tornillos y pernos para algunos sujetadores roscados. El documento también refleja (aunque probablemente no se originó) una confusión significativa del uso de la terminología que difiere entre la comunidad legal / estatutaria / regulatoria y la industria de los sujetadores. La redacción legal / estatutaria / reglamentaria usa los términos "grueso" y "fino" para referirse al rigor del rango de tolerancia , refiriéndose básicamente a "alta calidad" o "baja calidad", pero esta es una mala elección de términos. , porque esos términos en la industria de los sujetadores tienen un significado diferente (refiriéndose a la inclinación del paso de la hélice ).

Problema histórico

Las antiguas normas USS y SAE definían los tornillos de casquete como sujetadores con vástagos roscados a la cabeza y los pernos como sujetadores con vástagos parcialmente sin roscar. [26] La relación de esta regla con la idea de que un perno por definición toma una tuerca es clara (porque se esperaba que la sección sin rosca del vástago, que se llama agarre , pasara a través del sustrato sin enroscarse en él). Esta es ahora una distinción obsoleta, aunque los pernos grandes a menudo tienen secciones de vástago sin rosca.

Aunque no hay razón para considerar obsoleta esta definición, porque está lejos de ser claro que "un perno por definición lleva una tuerca". Usando un "perno" de entrenador como ejemplo (y ha sido un "perno" durante mucho tiempo). Originalmente no estaba destinado a recibir una nuez, pero tenía un vástago. Su propósito no era atravesar todo el sustrato, sino solo una pieza, mientras que la parte roscada se mordía en la otra para dibujar y sujetar los materiales entre sí. El perno de 'carro' se derivó de esto y se empleó más para acelerar la fabricación que para lograr una función diferente. El perno de carro atraviesa ambas piezas de material y emplea una tuerca para proporcionar la fuerza de sujeción. Sin embargo, ambos siguen siendo tornillos.

Vocabulario controlado versus lenguaje natural

Las distinciones anteriores se aplican en el vocabulario controlado de las organizaciones de normalización . Sin embargo, a veces existen diferencias entre el vocabulario controlado y el uso del lenguaje natural de las palabras por parte de maquinistas, mecánicos de automóviles y otros. Estas diferencias reflejan la evolución lingüística moldeada por el cambio de tecnología a lo largo de los siglos . Las palabras perno y tornillo han existido desde antes de que existiera la combinación moderna de tipos de sujetadores, y el uso natural de esas palabras ha evolucionado de forma retroactiva en respuesta al cambio tecnológico. (Es decir, el uso de palabras como nombres de objetos cambia a medida que cambian los objetos). Los sujetadores sin rosca predominaron hasta el advenimiento del corte de tornillos práctico y económico a principios del siglo XIX. El significado básico de la palabra tornillo ha implicado durante mucho tiempo la idea de una rosca de tornillo helicoidal, pero el tornillo de Arquímedes y la barrena de tornillo (como un sacacorchos) precedieron al sujetador.

La palabra perno también es una palabra muy antigua, y se usó durante siglos para referirse a las varillas de metal que atravesaban el sustrato para sujetarse del otro lado, a menudo por medios no roscados (remachado, soldadura por forja, clavado, acuñamiento, etc. ). La conexión de este sentido con el sentido de un cerrojo de puerta o el cerrojo de ballesta es evidente. En el siglo XIX, los pernos sujetos a través de roscas de tornillo a menudo se llamaban pernos de tornillo en contraposición a los pernos de apriete .

En el uso común, la distinción (no rigurosa) es a menudo que los tornillos son más pequeños que los pernos y que los tornillos generalmente son cónicos, mientras que los pernos no. Por ejemplo, los pernos de culata se denominan "pernos" (al menos en el uso norteamericano) a pesar de que, según algunas definiciones, deberían llamarse "tornillos". Su tamaño y su similitud con un perno que tomaría una tuerca parecen anular lingüísticamente cualquier otro factor en esta proclividad natural a la elección de palabras.

Otras distinciones

Los pernos se han definido como sujetadores con cabeza que tienen roscas externas que cumplen con una especificación exacta y uniforme de roscas de pernos (como rosca métrica ISO M, MJ, estándar de rosca unificada UN, UNR y UNJ) de manera que pueden aceptar una tuerca no cónica . Luego, los tornillos se definen como sujetadores con cabeza, roscados externamente que no cumplen con la definición anterior de pernos. [ cita requerida ] Estas definiciones de tornillo y perno eliminan la ambigüedad de la distinción del manual de Maquinaria . Y es por eso, quizás, que algunas personas los favorecen. Sin embargo, no cumplen con el uso común de las dos palabras ni cumplen con las especificaciones formales.

Una posible distinción es que un tornillo está diseñado para cortar su propia rosca; no tiene necesidad de acceder o exponerse al lado opuesto del componente al que se sujeta. Esta definición de tornillo se refuerza aún más al considerar los desarrollos de sujetadores como los tornillos Tek, con cabezas redondas o hexagonales, para revestimiento de techos, tornillos autoperforantes y autorroscantes para diversas aplicaciones de sujeción de metal, tornillos de listón de techo para reforzar la conexión entre el listón del techo y la viga, tornillos de la plataforma, etc. Por otro lado, un perno es la parte macho de un sistema de fijación diseñado para ser aceptado por un dado (o tuerca) pre-equipado con exactamente el mismo diseño de rosca. [ cita requerida ]

Tipos de tornillos y pernos

Los sujetadores roscados tienen un vástago cónico o un vástago no cónico. Los sujetadores con vástagos cónicos están diseñados para introducirse en un sustrato directamente o en un orificio piloto en un sustrato. Se forman hilos de acoplamiento en el sustrato a medida que se introducen estos sujetadores. Los sujetadores con un vástago no ahusado generalmente están diseñados para acoplarse con una tuerca o para introducirse en un orificio roscado.

Sujetadores con vástago cónico

Nombre americanoNombre británicoDescripción
tornillo de aglomerado tornillo de tablero de
partículas		Es similar a un tornillo para paneles de yeso, excepto que tiene un vástago más delgado y proporciona una mejor resistencia a la extracción en el tablero de partículas, mientras que se compensa con una menor resistencia al cizallamiento. Las roscas de los tornillos para tableros de partículas son asimétricas.
Tornillo para hormigón
Tapcons Tornillo para
albañilería Tornillo
Confast Tornillo
multimaterial Tornillo
azul Tornillo
autorroscante para albañilería
Titen		Un tornillo de acero inoxidable o de acero al carbono para sujetar madera, metal u otros materiales al concreto o mampostería. Los tornillos para hormigón suelen ser de color azul, con o sin revestimiento anticorrosivo. Pueden tener una cabeza plana Phillips o una cabeza de arandela hexagonal ranurada. Los tamaños nominales (rosca) varían de 0,1875 a 0,375 pulg. (4,763 a 9,525 mm) y longitudes de 1,25 a 5 pulg. (32 a 127 mm). Por lo general, un instalador usa un taladro percutor para hacer un orificio piloto para cada tornillo de concreto y un destornillador de impacto eléctrico para impulsar el tornillo. El orificio de perforación debe ser 1/2 "más largo que la profundidad de penetración del tornillo. El tornillo en sí debe perforarse un mínimo de 1" en el concreto para sostener de manera efectiva y un máximo de 1-3 / 4 "o las roscas se desgastarán. y perderá fuerza de sujeción. Idealmente, de 1-1 / 4 "a 1-1 / 2" de rosca en el concreto. [27] Por ejemplo, si se atornilla una tabla de 1/2 "al concreto, una 1 Se deben utilizar tornillos para hormigón de -3/4 "a 2".
tornillo de cubiertaSimilar al tornillo para paneles de yeso, excepto que tiene una resistencia a la corrosión mejorada y generalmente se suministra en un calibre más grande. La mayoría de los tornillos para plataformas tienen una punta de corte de rosca tipo 17 (tipo barrena) para su instalación en materiales para plataformas. Tienen cabezas de corneta que permiten que el tornillo presione la superficie de la madera sin romperla.
Double screw dowel.pngperno de suspensión de tornillo de
pasador de tornillo de doble extremo
perno de pasamanosSimilar a un tornillo para madera pero con dos extremos puntiagudos y sin cabeza, se utiliza para realizar juntas ocultas entre dos piezas de madera.
Un perno de suspensión tiene roscas de madera en un extremo y roscas de máquina en el otro. Se utiliza un perno de suspensión cuando es necesario sujetar una pieza metálica a una superficie de madera.
tornillo de accionamiento
martillo tornillo de accionamiento		Se utiliza principalmente para colocar placas de datos de fabricantes en equipos. Cabeza redonda o en forma de hongo lisa con una rosca de arranque múltiple en el vástago, debajo del cual se encuentra el vástago de diámetro reducido que actúa como piloto. El tornillo se fija golpeando la cabeza con un martillo y no está diseñado para ser extraído. [28]
tornillo de yesoTornillo especializado con cabeza de corneta que está diseñado para unir paneles de yeso a postes de madera o metal, sin embargo, es un sujetador de construcción versátil con muchos usos. El diámetro de las roscas de los tornillos para paneles de yeso es mayor que el diámetro de la empuñadura.
ojo tornillo de
armella
vid ojo
tornillo loopheaded		ojo de tornilloAtornille con una cabeza en forma de bucle. Los más grandes a veces se denominan tornillos de ojo de retraso. Diseñado para usarse como punto de sujeción, especialmente para algo que se cuelga de él.
Un ojo de vid (al menos en el Reino Unido) es similar a un ojo de tornillo, excepto que tiene un vástago proporcionalmente más largo y una cabeza en forma de bucle más pequeña. Como sugiere el término, los ojos de enredadera se usan a menudo para unir líneas de alambre a través de la superficie de los edificios para que las plantas trepadoras puedan adherirse.
tirafondo del
tirafondo [29]		tornillo de entrenadorSimilar a un tornillo para madera, excepto que generalmente es mucho más grande y se extiende a longitudes de hasta 15 pulg. (381 mm) con diámetros de 0,25 a 0,5 pulg. (6,35 a 12,70 mm) en tamaños comúnmente disponibles (ferretería) (sin contar la minería más grande y retrasos y tirafondos de obra civil) y generalmente tiene un cabezal de accionamiento hexagonal. Los tirafondos están diseñados para sujetar de forma segura maderas pesadas ( postes y vigas , caballetes de madera y puentes) entre sí, o para sujetar madera a mampostería u hormigón. El estándar alemán es DIN 571, tornillos para madera de cabeza hexagonal.

Los tirafondos se utilizan generalmente con un inserto expansivo llamado retraso en paredes de mampostería o concreto, el retraso fabricado con una chaqueta de metal duro que muerde los lados del orificio perforado, y el metal interior en el retraso es una aleación de plomo más suave. o zinc aleado con hierro dulce. La rosca gruesa de un perno de retraso y una malla de retraso se deforman ligeramente, haciendo una sujeción segura, casi hermética, anticorrosiva y mecánicamente fuerte.

tornillo de espejoEste es un tornillo para madera de cabeza plana con un orificio roscado en la cabeza, que recibe una cubierta cromada atornillada. Suele utilizarse para montar un espejo.
tornillo de chapaTiene hilos afilados que cortan un material como láminas de metal, plástico o madera. A veces tienen muescas en la punta para ayudar a eliminar las virutas durante el corte de roscas. El vástago generalmente se enrosca hasta la cabeza. Los tornillos para láminas de metal son excelentes sujetadores para unir herrajes de metal a la madera porque el vástago completamente roscado proporciona una buena retención en la madera.
Tornillo TwinfastUn tornillo Twinfast es un tipo de tornillo con dos roscas (es decir, un tornillo de inicio doble ), de modo que se puede accionar dos veces más rápido que un tornillo normal (es decir, de inicio único) con el mismo paso. [30] Los tornillos para paredes de yeso designados como finos son los tornillos más comunes para usar el estilo de roscas twinfast. [31]
tornillo de maderaUn tornillo de metal con una punta afilada diseñado para unir dos piezas de madera. Los tornillos para madera están comúnmente disponibles con cabezas planas, planas u ovaladas. Un tornillo para madera generalmente tiene un vástago parcialmente sin roscar debajo de la cabeza. La parte sin rosca del vástago está diseñada para deslizarse a través de la tabla superior (la más cercana a la cabeza del tornillo) de modo que pueda ajustarse firmemente a la tabla a la que se está uniendo. Los tornillos para madera del tamaño de una pulgada en los EE. UU. Están definidos por ANSI-B18.6.1-1981 (R2003), mientras que en Alemania están definidos por DIN 95 (tornillos para madera de cabeza avellanada (ovalada) levantada ranurada), DIN 96 (madera de cabeza redonda ranurada tornillos) y DIN 97 (tornillos para madera de cabeza avellanada (plana) ranurada).
Security head screwThese screws are use for security purposes and where vandalism and/or theft is likely. The head of this type of screw is impossible to reverse. It requires special tools or mechanisms like spanners, tri-wings, torxes, square drivers, etc. In some screws, the head can be removed by breaking it after installing the screw.

Fasteners with a non-tapered shank

American nameBritish nameDescription
anchor boltA special type of bolt that is set into concrete, with screw threads protruding above the concrete surface to accept a nut.
breakaway boltA breakaway bolt is a bolt with a hollow threaded shank, which is designed to break away upon impact. Typically used to fasten fire hydrants, so they will break away when hit by a car. Also used in aircraft to reduce weight.
Tornillo (Tipos de cabeza).pngcap screwThe term cap screw refers to many different things at different times and places. Currently, it most narrowly refers to a style of head (see the gallery below). More broadly, and more commonly, it refers to the group of screws: shoulder screws, hex heads, counter-sunk heads, button heads, and fillister heads. In the United States, cap screws are defined by ASME B18.6.2 and ASME B18.3.[32][33] In the past, the term cap screw, in general, referred to screws that were supposed to be used in applications where a nut was not used; however, the characteristics that differentiate it from a bolt vary over time. In 1910, Anthony defined it as screw with a hex head that was thicker than a bolt head, but the distance across the flats was less than a bolt's.[34] In 1913, Woolley and Meredith defined them like Anthony, but gave the following dimensions: hex head cap screws up to and including 7⁄16 inch (11.1125 mm) have a head that is 3⁄16 inch (4.7625 mm) larger than the shank diameter; screws greater than 1⁄2 inch (12.7 mm) in diameter have a head that is 1⁄4 inch (6.35 mm) larger than the shank. Square head cap screws up to and including 3⁄4 inch (19.05 mm) have a head 1⁄8 inch (3.175 mm) larger than the shank; screws larger than 3⁄4 inch (19.05 mm) have a head 1⁄4 inch (6.35 mm) larger than the shank.[35] In 1919, Dyke defined them as screws that are threaded all the way to the head.[26]

Button head bolt.tif		socket screwA socket cap screw, also known as a socket head capscrew, socket screw, or Allen bolt, is a type of cap screw with a cylindrical head and hexagonal drive hole. The term socket head capscrew typically refers to a type of threaded fastener whose head diameter is nominally 1.5 times that of the screw shank (major) diameter, with a head height equal to the shank diameter (1960 series design). Forged heat-treated alloy examples are high strength fasteners intended for the most demanding mechanical applications, with special alloy formulations available that are capable of maintaining strength at temperatures in excess of 1000 degrees F (587 degrees C).

In addition to the 1960 series design, other head designs include low head, button head and flat head, the latter designed to be seated into countersunk holes. A hex key (sometimes referred to as an Allen wrench or Allen key) or hex driver is required to tighten or loosen a socket screw. Socket head capscrews are commonly used in assemblies that do not provide sufficient clearance for a conventional wrench or socket.

carriage boltcup head bolt, coach boltA carriage bolt, also known as a coach bolt, has a domed or countersunk head, and the shank is topped by a short square section under the head. The square section grips into the part being fixed (typically wood), preventing the bolt from turning when the nut is tightened. Carriage bolts are used to provide a smooth finish on automobile metal bumper exteriors, the square section aligning with a square hole in the bumper to provide anti-rotation. A rib neck carriage bolt has several longitudinal ribs instead of the square section, to grip into a metal part being fixed.
elevator boltAn elevator bolt is a similar to a carriage bolt, except the head (or foot, depending on the application) is thin and flat. There are many variations. [36] Elevator bolts are designed to be used for leveling appliances or furniture.
eye boltAn eye bolt is a bolt with a looped head.
hex cap screw
hex bolt		A hex cap screw is a cap screw with a hexagonal head, designed to be driven by a wrench (spanner). An ASME B18.2.1 compliant cap screw has somewhat tighter tolerances than a hex bolt for the head height and the shank length. The nature of the tolerance difference allows an ASME B18.2.1 hex cap screw to always fit where a hex bolt is installed but a hex bolt could be slightly too large to be used where a hex cap screw is designed in.
Fine adjustment screwThe term fine adjustment screw typically refers to screws with threads from 40–100 TPI (Threads Per Inch) (0.5 mm to 0.2 mm pitch) and ultra fine adjustment screw has been used to refer to 100–254 TPI (0.2 mm to 0.1 mm pitch). These screws are most frequently used in applications where the screw is used to control fine motion of an object.
machine screwA machine screw is generally a smaller fastener (less than 1⁄4 inch (6.35 mm) in diameter) threaded the entire length of its shank that usually has a recessed drive type (slotted, Phillips, etc.). Machine screws are also made with socket heads (see above), in which case they may be referred to as socket head machine screws.
plow boltplough boltA plow bolt is bolt similar to a carriage bolt, except the head is flat or concave, and the underside of the head is a cone designed to fit in a countersunk recess. Plow bolts provide a smooth surface for attaching a plow moldboard to its beam, where a raised head would suffer from soil abrasion. There are many variations, with some not using a square base, but rather a key, a locking slot, or other means. The recess in the mating part must be designed to accept the particular plow bolt. ASME B18.9 standard recommends a No. 3 head (round countersunk head square neck) plow bolts and No. 7 head (round countersunk reverse key head) plow bolts for new designs. The necessary dimensions for the head styles can be found in the standard.[37][38][39]
Vis-auto-foreuse.jpegself-drilling screw
Tek screw		Similar to a sheet metal screw, but it has a drill-shaped point to cut through the substrate to eliminate the need for drilling a pilot hole. Designed for use in soft steel or other metals. The points are numbered from 1 through 5; the larger the number, the thicker metal it can go through without a pilot hole. A 5-point can drill through 0.5 in (12.7 mm) of steel, for example.
Vis-auto-taraudeuse.jpegself-tapping machine screwA self-tapping machine screw is similar to a machine screw except the lower part of the shank is designed to cut threads as the screw is driven into an untapped hole. The advantage of this screw type over a self-drilling screw is that, if the screw is reinstalled, new threads are not cut as the screw is driven.
set bolttap bolt, setscrewA bolt that is threaded all the way to the head. An ASME B18.2.1 compliant set/tap bolt has the same tolerances as an ASME B18.2.1 compliant hex cap screw.
Setscrews (PSF).pngset screwgrub screwA set screw is generally a headless screw but can be any screw used to fix a rotating part to a shaft, such as a line shaft or countershaft. The set screw is driven through a threaded hole in the rotating part until it is tight against the shaft. The most often used type is the socket set screw, which is tightened or loosened with a hex key.
shoulder bolt
shoulder screw		stripper boltA shoulder screw differs from machine screws in that the shank is held to a precise diameter, known as the shoulder, and the threaded portion is smaller in diameter than the shoulder. Shoulder screw specifications call out the shoulder diameter, shoulder length, and threaded diameter; the threaded length is fixed, based on the threaded diameter, and usually quite short. Shoulder screws can be manufactured in many materials such as alloy heat-treated steel for maximum strength and wear resistance and stainless steel for its corrosion-resistance and non-magnetic properties. Common applications for shoulder screws include rotating mechanism joints, linkage pivots, and guides for the stripper plate of a metal forming die set. In the latter application, the term stripper bolt is often substituted. Stainless steel shoulder screws are used with linear motion devices such as bearings, as guides and as pivots in electronic and other critical mechanical applications.
stove boltgutter boltA stove bolt is a type of machine screw that has a round or flat head and is threaded to the head. They are usually made of low grade steel, have a slot or Phillips drive, and are used to join sheet metal parts using a hex or square nut.[40]
tension control boltA tension control bolt (TC bolt) is a heavy duty bolt used in steel frame construction. The head is usually domed and is not designed to be driven. The end of the shank has a spline on it which is engaged by a special power wrench which prevents the bolt from turning while the nut is tightened. When the appropriate torque is reached the spline shears off.
thread rolling screwsThese have a lobed (usually triangular) cross-section. They form threads in a pre-existing hole in the mating workpiece by pushing the material outward during installation. In some cases the properly prepared hole in sheetmetal uses an extruded hole. The extrusion forms a lead-in and extra thread length for improved retention. Thread rolling screws are often used where loose chips formed by a thread cutting operation cannot be tolerated.

Fasteners with built in washers

A fastener with a built in washer is called a SEM or SEMS, short for pre-asSEMbled.[41][42] It could be fitted on either a tapered or non-tapered shank.

Other threaded fasteners

Superbolt, or multi-jackbolt tensioner

A superbolt, or multi-jackbolt tensioner is an alternative type of fastener that retrofits or replaces existing nuts, bolts, or studs. Tension in the bolt is developed by torquing individual jackbolts, which are threaded through the body of the nut and push against a hardened washer. Because of this, the amount of torque required to achieve a given preload is reduced. Installation and removal of any size tensioner is achieved with hand tools, which can be advantageous when dealing with large diameter bolting applications.

Bone screws

The field of screws and other hardware for internal fixation within the body is huge and diverse. Like prosthetics, it integrates the industrial and medicosurgical fields, causing manufacturing technologies (such as machining, CAD/CAM, and 3D printing) to intersect with the art and science of medicine. Like aerospace and nuclear power, this field involves some of the highest technology for fasteners, as well as some of the highest prices, for the simple reason that performance, longevity, and quality have to be excellent in such applications. Bone screws tend to be made of stainless steel or titanium, and they often have high-end features such as conical threads, multistart threads, cannulation (hollow core), and proprietary screw drive types (some not seen outside of these applications).

List of abbreviations for types of screws

These abbreviations have jargon currency among fastener specialists (who, working with many screw types all day long, have need to abbreviate repetitive mentions). The smaller basic ones can be built up into the longer ones; for example, knowing that "FH" means "flat head", it may be possible to parse the rest of a longer abbreviation containing "FH".

These abbreviations are not universally standardized across corporations; each corporation can coin their own. The more obscure ones may not be listed here.

The extra spacing between linked terms below helps the reader to see the correct parsing at a glance.

Abbreviation Expansion Comment
BHbutton head
BHCSbutton head cap screw
BHMSbutton head machine screw
CScap screw
FHflat head
FHCSflat head cap screw
FHPflat head Phillips
FHSCSflat head socket cap screw
FHPMSflat head Phillips machine screw
FTfull thread
HHCShex head cap screw
HSHCSHexalobular socket head cap screws
MSmachine screw
OHoval head
PHPhillips head
RHround head
RHMSround head machine screw
RHPround head Phillips
RHPMSround head Phillips machine screw
SBHCSsocket button head cap screw
SBHMSsocket button head machine screw
SHsocket headAlthough "socket head" could logically refer to almost any female drive, it refers by convention to hex socket head unless further specified.
SHCSsocket head cap screw
SHSSsocket head set screwSometimes Socket Head Shoulder Screw.
SSset screwThe abbreviation "SS" more often means stainless steel. Therefore, "SS cap screw" means "stainless steel cap screw" but "SHSS" means "socket head set screw". As with many abbreviations, users rely on context to diminish the ambiguity, although this reliance does not eliminate it.
STSself-tapping screw

Screws and bolts are usually made of steel. Where great resistance to weather or corrosion is required, like in very small screws or medical implants, materials such as stainless steel, brass, titanium, bronze, silicon bronze or monel may be used.

Galvanic corrosion of dissimilar metals can be prevented (using aluminum screws for double-glazing tracks for example) by a careful choice of material. Some types of plastic, such as nylon or polytetrafluoroethylene (PTFE), can be threaded and used for fastenings requiring moderate strength and great resistance to corrosion or for the purpose of electrical insulation.

Often a surface coating is used to protect the fastener from corrosion (e.g. bright zinc plating for steel screws), to impart a decorative finish (e.g. japanning) or otherwise alter the surface properties of the base material.

Selection criteria of the screw materials include: size, required strength, resistance to corrosion, joint material, cost and temperature.

The numbers stamped on the head of the bolt are referred to the grade of the bolt used in certain application with the strength of a bolt. High-strength steel bolts usually have a hexagonal head with an ISO strength rating (called property class) stamped on the head. And the absence of marking/number indicates a lower grade bolt with low strength. The property classes most often used are 5.8, 8.8, and 10.9. The number before the point is the ultimate tensile strength in MPa divided by 100. The number after the point is the multiplier ratio of yield strength to ultimate tensile strength. For example, a property class 5.8 bolt has a nominal (minimum) ultimate tensile strength of 500 MPa, and a tensile yield strength of 0.8 times ultimate tensile strength or 0.8 (500) = 400 MPa.

Ultimate tensile strength is the tensile stress at which the bolt fails. Tensile yield strength is the stress at which the bolt will yield in tension across the entire section of the bolt and receive a permanent set (an elongation from which it will not recover when the force is removed) of 0.2% offset strain. Proof strength is the usable strength of the fastener. Tension testing of a bolt up to the proof load should not cause permanent set of the bolt and should be conducted on actual fasteners rather than calculated.[43] If a bolt is tensioned beyond the proof load, it may behave in plastic manner due to yielding in the threads and the tension preload may be lost due to the permanent plastic deformations. 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 of the bolt material. If a bolt is loaded in tension beyond its proof strength, the yielding at the net root section of the bolt will continue until the entire section is begins to yield and it has exceeded its yield strength. If tension increases, the bolt fractures at its ultimate strength.

Mild steel bolts have property class 4.6, which is 400 MPa ultimate strength and 0.6*400=240 MPa yield strength. High-strength steel bolts have property class 8.8, which is 800 MPa ultimate strength and 0.8*800=640 MPa yield strength or above.

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.[44] 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.[45]

Metric

The international standards for metric externally threaded fasteners are ISO 898-1 for property classes produced from carbon steels and ISO 3506-1 for property classes produced from corrosion resistant steels.

Head markings and properties for metric hex-head cap screws[46]
Head marking Grade, material and condition Nominal size range (mm) Proof strength Yield strength, min. Tensile strength, min. Core hardness (Rockwell)
MPaksi MPaksi MPaksi
Hex cap screw-class 3.6.svg Class 3.6[47]1.6–36 180 26 190 28 330 48 B52–95
Hex cap screw-class 4.6.svg Class 4.6
Low or medium carbon steel 		5–100 225 32.6 240 35 400 58 B67–95
Hex cap screw-class 4.8.svg Class 4.8
Low or medium carbon steel; fully or partially annealed 		1.6–16 310 45 340 49 420 61 B71–95
Hex cap screw-class 5.8.svg Class 5.8
Low or medium carbon steel; cold worked 		5–24 380 55 420 61 520 75 B82–95
Hex cap screw-class 8.8.svg Class 8.8[48]
Medium carbon steel; quench and tempered 		Under 16 (inc.) 580 84 640 93 800 120
17–72 600 87 660 96 830 120 C23–34
Hex cap screw-class 8.8 line.svg Class 8.8 low carbon
Low carbon boron steel; quench and tempered
Hex cap screw-class 8.8.3.svg Class 8.8.3[49]
Atmospheric corrosion resistant steel; quench and tempered
Hex cap screw-ASTM A325M 8S.svg ASTM A325M - Type 1[50][51]
Medium carbon steel; quench and tempered 		12–36
Hex cap screw-ASTM A325M 8S3.svg ASTM A325M - Type 3[50][51]
Atmospheric corrosion resistant steel; quench and tempered
Hex cap screw-class 9.8.svg Class 9.8
Medium carbon steel; quench and tempered 		1.6–16 650 94 720 104 900 130 C27–36
Hex cap screw-class 9.8 line.svg Class 9.8 low carbon
Low carbon boron steel; quench and tempered
Hex cap screw-class 10.9.svg Class 10.9
Alloy steel; quench and tempered 		5–100 830 120 940 136 1,040 151 C33–39
Hex cap screw-class 10.9 line.svg Class 10.9 low carbon
Low carbon boron steel; quench and tempered
Hex cap screw-class 10.9.3.svg Class 10.9.3[49]
Atmospheric corrosion resistant steel; quench and tempered
Hex cap screw-ASTM A490M 10S.svgASTM A490M - Type 1[50][52]
Alloy steel; quench and tempered 		12–36
Hex cap screw-ASTM A490M 10S3.svg ASTM A490M - Type 3[50][52]
Atmospheric corrosion resistant steel; quench and tempered
Hex cap screw-class 12.9.svg Class 12.9
Alloy steel; quench and tempered 		1.6–100 970 141 1,100 160 1,220 177 C38–44
Hex cap screw-A2.svg A2[48]
Stainless steel with 17–19% chromium and 8–13% nickel 		up to 20 210 minimum
450 typical 		30 minimum
65 typical 		500 minimum
700 typical 		73 minimum
100 typical
ISO 3506-1 A2-50[citation needed]
304 stainless steel-class 50 (annealed) 		210 30 500 73
ISO 3506-1 A2-70[citation needed]
304 stainless steel-class 70 (cold worked) 		450 65 700 100
ISO 3506-1 A2-80[citation needed]
304 stainless steel-class 80 		600 87 800 120

Inch

There are many standards governing the material and mechanical properties of imperial sized externally threaded fasteners. Some of the most common consensus standards for grades produced from carbon steels are ASTM A193, ASTM A307, ASTM A354, ASTM F3125, and SAE J429. Some of the most common consensus standards for grades produced from corrosion resistant steels are ASTM F593 & ASTM A193.

Head markings and properties for inch-system hex-head cap screws[48]
Head marking Grade, material and condition Nominal size range (in) Proof strength Yield strength, min. Tensile strength, min. Core hardness (Rockwell)
ksiMPa ksiMPa ksiMPa
Hex cap screw-no markings.svg SAE Grade 0[53]Strength and hardness is not specified
SAE grade 1
ASTM A307[54]
Low carbon steel 		1⁄4– 1+1⁄2 33 230 60 410 B70–100
ASTM A307 - Grade B[54]
Low or medium carbon steel 		1⁄4–4 60 minimum
100 maximum 		410 minimum
690 maximum 		B69–95
SAE grade 2
Low or medium carbon steel 		1⁄4– 3⁄4 55 380 57 390 74 510 B80–100[55]
Greater than 3⁄4 33 230 36 250 60 410 B70–100[55]
SAE grade 4[56]
Medium carbon steel; cold worked 		1⁄4– 1+1⁄2 100 690 115 790
Hex cap screw-grade 3.svgSAE grade 3[54]
Medium carbon steel; cold worked		1⁄4–1 85 590 100 690 B70–100
Hex cap screw-grade 5.svgSAE grade 5
Medium carbon steel; quench and tempered 		1⁄4–1 (inc.) 85 590 92 630 120 830 C25–34[55]
1– 1+1⁄2 74 510 81 560 105 720 C19–30[55]
ASTM A449 - Type 1[54]
Medium carbon steel; quench and tempered		1– 1+1⁄2 (inc.) 74 510 105 720 C19–30
1+1⁄2–3 55 380 90 620 Brinell 183–235
Hex cap screw-grade 5.1.svgSAE grade 5.1[57]
Low or medium carbon steel; quench and tempered		No. 6– 1⁄2 85 590 120 830 C25–40
Hex cap screw-grade 5.2.svgSAE grade 5.2[57]
Low carbon martensitic steel; quench and tempered		1⁄4–1 85 590 120 830 C26–36
ASTM A449 - Type 2[57]
Low carbon martensitic steel; quench and tempered		C25–34
Hex cap screw-ASTM A325.svgor Hex cap screw-ASTM A325 lines.svgASTM A325 - Type 1[54]
Medium carbon steel; quench and tempered		1⁄2–1 (inc.) 85 590 92 630[56]120 830 C24–35
1– 1+1⁄2 74 510 82 570[56]105 720 C19–31
Hex cap screw-ASTM A325 type 3.svg[58]ASTM A325 - Type 3[54]
Atmospheric corrosion resistant steel; quench and tempered		1⁄2–1 85 590 92 630[56]120 830 C24–35
1– 1+1⁄2 74 510 82 570[56]105 720 C19–31
Hex cap screw-ASTM A354 grade BC.svgASTM A354 - Grade BC[54]
Medium carbon alloy steel; quench and tempered		1⁄4– 2+1⁄2 (inc.) 105 720 109 750[56]125 860 C26–36
2+1⁄2–4 95 660 99 680[56]115 790 C22–33
Hex cap screw-grade 7.svgSAE grade 7
Medium carbon alloy steel; quench and tempered		1⁄4– 1+1⁄2 105 720 115 790 133 920
Hex cap screw-grade 8.svgSAE grade 8
Medium carbon alloy steel; quench and tempered		1⁄4– 1+1⁄2 120 830 130 900 150 1,000 C32–38[55]
Hex cap screw-ASTM A354 grade BD lines.svgASTM A354 - Grade BD[59]1⁄4– 2+1⁄2 (inc.) 120 830 130 900[59]150 1,000 C33–39
Hex cap screw-ASTM A354 grade BD.svg2+1⁄2–4 105 720 115 790[59]140 970 C31–39
Hex cap screw-grade 8.2.svgSAE grade 8.2[55]
Medium carbon boron martensitic steel; fully kilned, fine grain, quench and tempered		1⁄4–1 120 830 150 1,000 C33–39
Hex cap screw-ASTM A490.svg ASTM A490 - Type 1[54]
Medium carbon alloy steel; quench and tempered 		1⁄2– 1+1⁄2 120 830 130[56]900 150 minimum
170 maximum 		1,000 minimum
1,200 maximum 		C33–38
Hex cap screw-ASTM A490 type 3.svg[58]ASTM A490 - Type 3[54]
Atmospheric corrosion resistant steel; quench and tempered
18/8 Stainless
Stainless steel with 17–19% chromium and 8–13% nickel 		1⁄4– 5⁄8 (inc.) 40 minimum
80–90 typical 		280 minimum
550–620 typical 		100–125 typical 690–860 typical
5⁄8–1 (inc.) 40 minimum
45–70 typical 		280 minimum
310–480 typical 		100 typical 690 typical
over 1 80–90 typical 550–620 typical

(a) pan, (b) dome (button), (c) round, (d) truss (mushroom), (e) flat (countersunk), (f) oval (raised head)
Combination flanged-hex/Phillips-head screw used in computers
Pan head
A low disc with a rounded, high outer edge with large surface area.
Button or dome head
Cylindrical with a rounded top.
Round head
A dome-shaped head used for decoration. [60]
Mushroom or Truss head
Lower-profile dome designed to prevent tampering.
Countersunk or flat head
Conical, with flat outer face and tapering inner face allowing it to sink into the material. The angle of the screw is measured as the aperture of the cone.
Oval or raised head
A decorative screw head with a countersunk bottom and rounded top. [60] Also known as "raised countersunk" in the UK.
Bugle head
Similar to countersunk, but there is a smooth progression from the shank to the angle of the head, similar to the bell of a bugle.
Cheese head
Disc with cylindrical outer edge, height approximately half the head diameter.
Fillister head
Cylindrical, but with a slightly convex top surface. Height to diameter ratio is larger than cheese head.
Flanged head
A flanged head can be any of the above head styles (except the countersunk styles) with the addition of an integrated flange at the base of the head. This eliminates the need for a flat washer.

Some varieties of screw are manufactured with a break-away head, which snaps off when adequate torque is applied. This prevents tampering and also provides an easily inspectable joint to guarantee proper assembly. An example of this is the shear bolts used on vehicle steering columns, to secure the ignition switch.

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 in the US; hex, Robertson, and Torx are also common in some applications, and Pozidriv has almost completely replaced Phillips in Europe. 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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An electric driver screws a self-tapping phillips head screw into wood

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 but not excessive force is developed by the screw. The hand tool for driving hex head threaded fasteners is a spanner (UK usage) or wrench (US usage), while a nut setter is used with a power screw driver.

There are many systems for specifying the dimensions of screws, but in much of the world the ISO metric screw thread preferred series has displaced the many older systems. Other relatively common systems include the British Standard Whitworth, BA system (British Association), and the Unified Thread Standard.

ISO metric screw thread

The basic principles of the ISO metric screw thread are defined in international standard ISO 68-1 and preferred combinations of diameter and pitch are listed in ISO 261. The smaller subset of diameter and pitch combinations commonly used in screws, nuts and bolts is given in ISO 262. The most commonly used pitch value for each diameter is the coarse pitch. For some diameters, one or two additional fine pitch variants are also specified, for special applications such as threads in thin-walled pipes. ISO metric screw threads are designated by the letter M followed by the major diameter of the thread in millimetres (e.g. M8). If the thread does not use the normal coarse pitch (e.g. 1.25 mm in the case of M8), then the pitch in millimeters is also appended with a multiplication sign (e.g. "M8×1" if the screw thread has an outer diameter of 8 mm and advances by 1 mm per 360° rotation).

The nominal 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 shank, and the nut or threaded hole is made by tapping threads into a hole of 5 mm diameter (6 mm - 1 mm).

Metric hexagon bolts, screws and nuts are specified, for example, in International Standards ISO 4014, ISO 4017, and ISO 4032. The following table lists the relationship given in these standards between the thread size and the maximum width across the hexagonal flats (wrench size):

ISO metric thread M1.6M2M2.5M3M4M5M6M8M10M12M16M20M24M30M36M42M48M56M64
Wrench size (mm) 3.2455.578101316 or 1719243036465565758595

In addition, the following non-preferred intermediate sizes are specified:

ISO metric thread M3.5M14M18M22M27M33M39M45M52M60
Wrench size (mm) 6212734415060708090

Bear in mind that these are just examples and the width across flats is different for structural bolts, flanged bolts, and also varies by standards organization.

Whitworth

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. Whitworth became British Standard Whitworth, abbreviated to BSW (BS 84:1956) and the British Standard Fine (BSF) thread was introduced in 1908 because the Whitworth thread was too coarse for some applications. The thread angle was 55°, and the depth and pitch varied with the diameter of the thread (i.e., the bigger the bolt, the coarser the thread). Spanners for Whitworth bolts are marked with the size of the bolt, not the distance across the flats of the screw head.

The most common use of a Whitworth pitch nowadays is in all UK scaffolding. Additionally, the standard photographic tripod thread, which for small cameras is 1/4" Whitworth (20 tpi) and for medium/large format cameras is 3/8" Whitworth (16 tpi). It is also used for microphone stands and their appropriate clips, again in both sizes, along with "thread adapters" to allow the smaller size to attach to items requiring the larger thread. Note that while 1/4" UNC bolts fit 1/4" BSW camera tripod bushes, yield strength is reduced by the different thread angles of 60° and 55° respectively.

British Association screw thread

British Association (BA) screw threads, named after the British Association for Advancement of Science, were devised in 1884 and standardised in 1903. Screws were described as "2BA", "4BA" etc., the odd numbers being rarely used, except in equipment made prior to the 1970s for telephone exchanges in the UK. This equipment made extensive use of odd-numbered BA screws, in order—it may be suspected—to reduce theft. BA threads are specified by British Standard BS 93:1951 "Specification for British Association (B.A.) screw threads with tolerances for sizes 0 B.A. to 16 B.A."

While not related to ISO metric screws, the sizes were actually defined in metric terms, a 0BA thread having a 6 mm diameter and 1 mm pitch. Other threads in the BA series are related to 0BA in a geometric series with the common factors 0.9 and 1.2. For example, a 4BA thread has pitch  mm (0.65 mm) and diameter  mm (3.62 mm). Although 0BA has the same diameter and pitch as ISO M6, the threads have different forms and are not compatible.

BA threads are still common in some niche applications. Certain types of fine machinery, such as moving-coil meters and clocks, tend to have BA threads wherever they are manufactured. BA sizes were also used extensively in aircraft, especially those manufactured in the United Kingdom. BA sizing is still used in railway signalling, mainly for the termination of electrical equipment and cabling.

BA threads are extensively used in Model Engineering where the smaller hex head sizes make scale fastenings easier to represent. As a result, many UK Model Engineering suppliers still carry stocks of BA fasteners up to typically 8BA and 10BA. 5BA is also commonly used as it can be threaded onto 1/8 rod.[61]

Unified Thread Standard

The Unified Thread Standard (UTS) is most commonly used in the United States, but is also extensively used in Canada and occasionally in other countries. The size of a UTS screw is described using the following format: X-Y, where X is the nominal size (the hole or slot size in standard manufacturing practice through which the shank of the screw can easily be pushed) and Y is the threads per inch (TPI). For sizes 1⁄4 inch and larger the size is given as a fraction; for sizes less than this an integer is used, ranging from 0 to 16. The integer sizes can be converted to the actual diameter by using the formula 0.060 + (0.013 × number). For example, a #4 screw is 0.060 + (0.013 × 4) = 0.060 + 0.052 = 0.112 inches in diameter. There are also screw sizes smaller than "0" (zero or ought). The sizes are 00, 000, 0000 which are usually referred to as two ought, three ought, and four ought. Most eyeglasses have the bows screwed to the frame with 00-72 (pronounced double ought – seventy two) size screws. To calculate the major diameter of "ought" size screws count the number of 0's and multiply this number by 0.013 and subtract from 0.060. For example, the major diameter of a 000-72 screw thread is .060 – (3 x .013) = 0.060 - 0.039 = .021 inches. For most size screws there are multiple TPI available, with the most common being designated a Unified Coarse Thread (UNC or UN) and Unified Fine Thread (UNF or UF). Note: In countries other than the United States and Canada, the ISO Metric Screw Thread System is primarily used today. Unlike most other countries the United States and Canada still use the Unified (Inch) Thread System. However, both are moving over to the ISO Metric System. It is estimated that approximately 60% of screw threads in use in the United States are still inch based.[62]

Bolt Forming.svg
Screw (bolt) 13-n.PNG

There are three steps in manufacturing a screw: heading, thread rolling, and coating. Screws are normally made from wire, which is supplied in large coils, or round bar stock for larger screws. The wire or rod is then cut to the proper length for the type of screw being made; this workpiece is known as a blank. It is then cold headed, which is a cold working process. Heading produces the head of the screw. The shape of the die in the machine dictates what features are pressed into the screw head; for example a flat head screw uses a flat die. For more complicated shapes two heading processes are required to get all of the features into the screw head. This production method is used because heading has a very high production rate, and produces virtually no waste material. Slotted head screws require an extra step to cut the slot in the head; this is done on a slotting machine. These machines are essentially stripped down milling machines designed to process as many blanks as possible.

The blanks are then polished[citation needed] again prior to threading. The threads are usually produced via thread rolling; however, some are cut. The workpiece is then tumble finished with wood and leather media to do final cleaning and polishing.[citation needed] For most screws, a coating, such as electroplating with zinc (galvanizing) or applying black oxide, is applied to prevent corrosion.

A lathe of 1871, equipped with leadscrew and change gears for single-point screw-cutting.
A Brown & Sharpe single- spindle screw machine.

While a recent hypothesis attributes the Archimedes' screw to Sennacherib, King of Assyria, archaeological finds and pictorial evidence only appear in the Hellenistic period and the standard view holds the device to be a Greek invention, most probably by the 3rd century BC polymath Archimedes.[63][dubious – discuss] Though resembling a screw, this is not a screw in the usual sense of the word.

Earlier, the screw had been described by the Greek mathematician Archytas of Tarentum (428–350 BC). By the 1st century BC, wooden screws were commonly used throughout the Mediterranean world in screw presses for pressing olive oil from olives and pressing juice from grapes in winemaking. Metal screws used as fasteners were rare in Europe before the 15th century, if known at all.[64]

Rybczynski has shown[65] that handheld screwdrivers (formerly called "turnscrews" in English, in more direct parallel to their original French name, tournevis[66]) have existed since medieval times (the 1580s at the latest), although they probably did not become truly widespread until after 1800, once threaded fasteners had become commodified, as detailed below.

There were many forms of fastening in use before threaded fasteners became widespread. They tended to involve carpentry and smithing rather than machining, and they involved concepts such as dowels and pins, wedging, mortises and tenons, dovetails, nailing (with or without clenching the nail ends), forge welding, and many kinds of binding with cord made of leather or fiber, using many kinds of knots. Prior to the mid-19th century, cotter pins or pin bolts, and "clinch bolts" (now called rivets), were used in shipbuilding. Glues also existed, although not in the profusion seen today.

The metal screw did not become a common fastener until machine tools for their mass production were developed toward the end of the 18th century. This development blossomed in the 1760s and 1770s[67] along two separate paths that soon converged:[68] the mass production of wood screws (meaning screws made of metal to be used in wood) in a specialized, single-purpose, high-volume-production machine tool; and the low-count, toolroom-style production of machine screws (V-thread) with easy selection among various pitches (whatever the machinist happened to need on any given day).

The first path was pioneered by brothers Job and William Wyatt of Staffordshire, UK,[69] who patented in 1760 a machine that we might today best call a screw machine of an early and prescient sort. It made use of a leadscrew to guide the cutter to produce the desired pitch,[69] and the slot was cut with a rotary file while the main spindle held still (presaging live tools on lathes 250 years later). Not until 1776 did the Wyatt brothers have a wood-screw factory up and running.[69] Their enterprise failed, but new owners soon made it prosper, and in the 1780s they were producing 16,000 screws a day with only 30 employees[70]—the kind of industrial productivity and output volume that would later be characteristic of modern industry but was revolutionary at the time.

Meanwhile, English instrument maker Jesse Ramsden (1735–1800) was working on the toolmaking and instrument-making end of the screw-cutting problem, and in 1777 he invented the first satisfactory screw-cutting lathe.[62] The British engineer Henry Maudslay (1771–1831) gained fame by popularizing such lathes with his screw-cutting lathes of 1797 and 1800, containing the trifecta of leadscrew, slide rest, and change-gear gear train, all in the right proportions for industrial machining. In a sense he unified the paths of the Wyatts and Ramsden and did for machine screws what had already been done for wood screws, i.e., significant easing of production spurring commodification. His firm would remain a leader in machine tools for decades afterward. A misquoting of James Nasmyth popularized the notion that Maudslay had invented the slide rest, but this was incorrect; however, his lathes helped to popularize it.

These developments of the 1760–1800 era, with the Wyatts and Maudslay being arguably the most important drivers, caused great increase in the use of threaded fasteners. Standardization of threadforms began almost immediately, but it was not quickly completed; it has been an evolving process ever since. Further improvements to the mass production of screws continued to push unit prices lower and lower for decades to come, throughout the 19th century.[71]

In 1821, the first screw factory in the United States was built by Hardman Philips on Moshannon Creek, near Philipsburg for the manufacture of blunt metal screws. An expert in screw manufacture, Thomas Lever was brought over from England to run the factory. The mill was run by steam and water power, and the fuel used was hardwood charcoal. The screws were made from wire prepared by “rolling and wire drawing apparatus” from iron manufactured at a nearby forge. The screw mill was not a commercial success. It eventually failed due to competition from the lower cost, gimlet-pointed screw and ceased operations in 1836. [72]

The American development of the turret lathe (1840s) and of automatic screw machines derived from it (1870s) drastically reduced the unit cost of threaded fasteners by increasingly automating the machine tool control. This cost reduction spurred ever greater use of screws.

Throughout the 19th century, the most commonly used forms of screw head (that is, drive types) were simple internal-wrenching straight slots and external-wrenching squares and hexagons. These were easy to machine and served most applications adequately. Rybczynski describes a flurry of patents for alternative drive types in the 1860s through 1890s,[73] but explains that these were patented but not manufactured due to the difficulties and expense of doing so at the time. In 1908, Canadian P. L. Robertson was the first to make the internal-wrenching square socket drive a practical reality by developing just the right design (slight taper angles and overall proportions) to allow the head to be stamped easily but successfully, with the metal cold forming as desired rather than being sheared or displaced in unwanted ways.[73] Practical manufacture of the internal-wrenching hexagon drive (hex socket) shortly followed in 1911.[74][75]

In the early 1930s, the Phillips-head screw was popularized by American Henry F. Phillips.[76]

Threadform standardization further improved in the late 1940s, when the ISO metric screw thread and the Unified Thread Standard were defined.

Precision screws, for controlling motion rather than fastening, developed around the turn of the 19th century, were one of the central technical advances, along with flat surfaces, that enabled the industrial revolution.[77] They are key components of micrometers and lathes.

Alternative fastening methods are:

  • nails
  • rivets
  • pins (dowel pins, taper pins, roll pins, spring pins, cotter pins)
  • pinned shafts (keyed shafts, woodruff keys, gibb-headed key)
  • screw bolt, pin bolt or cotter bolt, and clench bolt- as used in clinker boat building
  • welding
  • soldering
  • brazing
  • joinery (mortise & tenon, dovetailing, box joints, lap joints)
  • gluing
  • taping
  • clinch fastening

  • Bolted joint
  • Dowel
  • Fastener
  • Gender of connectors and fasteners
  • Syndesmotic screw
  • Tap and die
    • Die head
  • Thread angle
  • Threaded fastener
  • Threaded insert
  • Threaded rod (e.g. studs, allthread)
  • Threading
  • Thread-locking compound
  • Thread pitch gauge
  • Wall plug

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  • How the World Got Screwed
  • NASA-RP-1228 Fastener Design Manual
  • Imperial/Metric fastening sizes comparison
  • "Hold Everything", February 1946, Popular Science" article section on screws and screw fastener technology developed during World War Two
  • How to feed screws and dowels