BOLTED

Foro sobre la optimización
del aseguramiento de uniones atornilladas

The Experts: Improving fatigue resistance

First published in Bolted #2 2015.

A: The fatigue capacity of a bolted joint is very small, as compared to its static capacity. To improve fatigue resistance, designers can increase the thread capacity and decrease the alternating stresses at the threads.

To increase the thread capacity, it is recommended to use a rolled thread instead of a cutting process. To increase the bolted joint capacity, utilize multiple smaller fasteners instead of a single larger fastener.

The capacity is also increased by using an improved connector, such as a Superbolt MJT (Multi-Jackbolt Fastener) or Flexnut, which improves the load distribution in the threads and adds elasticity to the bolted joint.

The best way to improve fatigue resistance is to reduce the alternating stresses at the threads. There are three main ways of doing this: Assembly design, assembly tightening, and assembly security.

The assembly design process provides an opportunity for improvement of the load distribution on bolted joints and to reduce the level of external stresses supported by each joint. To facilitate that, keep these principals in mind:

1. Use the highest possible preload
2. Minimize the bolt to load eccentricity
3. Use the largest possible contact surfaces
4. Use the largest possible clamping lengths
5. In most cases, use a preload higher than the working load

Other assembly design options include the use of necked-down studs or bolts, and the use of elastic washers, which counter the effects of relaxation, creeping, and thermal differential elongation.

With regard to assembly tightening, achieving the necessary preload is the main factor in reducing alternating stresses. It is recommended to use calibrated tools with high accuracy. It is also recommended to use a proper lubricant to achieve preload accuracy, and to reduce the risk of seizing. A suitable tightening sequence should be used to mitigate the risk of un-evenly loaded bolts and to ensure overall bolted joint integrity.

Regarding assembly security, it is recommended to secure the bolted joint against loss of preload. Further, secure the assembly against environmental effects, such as corrosion that could initiate a fatigue crack. This may be done through the selection of suitable materials and/or coatings for parts and fasteners.

 

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How do you choose the right size of Nord-Lock washer?

 

In this video we explain how you choose the right size of washer for your bolted joints.

► Read more: Introduction to Nord-Lock washers

► Video: Junker vibration test with Nord-Lock wedge-locking washers

How does the Nord-Lock washer work?

 

Nord-Lock washers secure bolted joints with tension instead of friction. Watch this video and let us explain how it works!

► Read more: Introduction to Nord-Lock washers

► Video: Junker vibration test with Nord-Lock wedge-locking washers


The history of the bolt

20 diciembre 2017
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Texto: Alannah Eames

foto: Illustration: Kent Zeiron

At first glance, a bolt may seem like a very simple item that holds things together. But dig a bit deeper and you’ll realise there’s more behind seemingly insignificant bolt and screws than first meets the eye. Without them, all our gadgets and machines would fall to pieces.

History of the bolt drawings

First published in Bolted #2 2012.

Bolts are one of the most common elements used in construction and machine design. They hold every­thing together – from screws in electric toothbrushes and door hinges to massive bolts that secure concrete pillars in buildings. Yet, have you ever stopped to wonder where they actually came from?

While the history of threads can be traced back to 400 BC, the most significant developments in the modern day bolt and screw processes were made during the last 150 years. Experts differ as to the origins of the humble nut and bolt. In his article “Nuts and Bolts”, Frederick E. Graves argues that a threaded bolt and a matching nut serving as a fastener only dates back to the 15th century. He bases this conclusion on the first printed record of screws appearing in a book in the early 15th century.

However, Graves also acknowledges that even though the threaded bolt dates back to the 15th century, the unthreaded bolt goes back to Roman times when it was used for “barring doors, as pivots for opening and closing doors and as wedge bolts: a bar or a rod with a slot in which a wedge was inserted so that the bolt could not be moved.” He also implies that the Romans developed the first screw, which was made out of bronze, or even silver. The threads were filed by hand or consisted of a wire wound around a rod and soldered on.

According to bolt expert Bill Eccles’ research, the history of the screw thread goes back much further. Archimedes (287 BC–212 BC) developed the screw principle and used it to construct devices to raise water. However, there are signs that the water screw may have originated in Egypt before the time of Archimedes. It was constructed from wood and was used to irrigate
land and remove bilge water from ships. “But many consider that the screw thread was invented around 400 BC by [Greek philosopher] Archytas of Tarentum, who has often been called the founder of mechanics and considered a contemporary of Plato,” Eccles writes on his website.

The history can be broken down into two parts: the threads themselves that date back to around 400 BC when they were used for items such as a spiral for lifting water, presses for grapes to make wine, and the fasteners themselves, which have been in use for around 400 years.

Moving forward to the 15th century, Johann Gutenberg used screws in the fastenings on his printing presses. The tendency to use screws gained momentum with their use being extended to items such as clocks and armour. According to Graves, Leonardo da Vinci’s notebooks from the late 15th and early 16th centuries include several designs for screw-cutting machines.

What the majority of researchers on this topic do agree on, though, is that it was the Industrial Revolution that sped up the development of the nut and bolt and put them firmly on the map as an important component in the engineering and construction world.

The “History of the Nut and Bolt Industry in America” by W.R. Wilbur in 1905 acknowledges that the first machine for making bolts and screws was made by Besson in France in 1568, who later introduced a screw-cutting gauge or plate to be used on lathes. In 1641, the English firm, Hindley of York, improved this device and it became widely used.

Across the Atlantic in the USA, some of the documented history of the bolt may be found in the Carriage Museum of America. Nuts on vehicles built in the early 1800s were flatter and squarer than later vehicles, which had chamfered corners on the nuts and the flush was trimmed off the bolts. Making bolts at this time was a cumbersome and painstaking process.

Initially, screw threads for fasteners were made by hand but soon, due to a significant increase in demand, it was necessary to speed up the production process. In Britain in 1760, J and W Wyatt introduced a factory process for the mass production of screw threads. However, this milestone led to another challenge: each company manufactured its own threads, nuts and bolts so there was a huge range of different sized screw threads on the market, causing problems for machinery manufacturers.

It wasn’t until 1841 that Joseph Whitworth managed to find a solution. After years of research collecting sample screws from many British workshops, he suggested standardising the size of the screw threads in Britain so that, for example, someone could make a bolt in England and someone in Glasgow could make the nut and they would both fit together. His proposal was that the angle of the thread flanks was standardised at 55 degrees, and the number of threads per inch, should be defined for various diameters.
While this issue was being addressed in Britain, the Americans were trying to do likewise and initially started using the Whitworth thread.

In 1864, William Sellers proposed a 60 degree thread form and various thread pitches for different diameters. This developed into the American Standard Coarse Series and the Fine Series. One advantage the Americans had over the British was that their thread form had flat roots and crests. This made it easier to manufacture than the Whitworth standard, which had rounded roots and crests. It was found, however, that the Whitworth thread performed better in dynamic applications and the rounded root of the Whitworth thread improved fatigue performance.

During World War I, the lack of consistency between screw threads in different countries became a huge obstacle to the war effort; during World War II it became an even bigger problem for the Allied forces. In 1948, Britain, the USA and Canada agreed on the Unified thread as the standard for all countries that used imperial measurements. It uses a similar profile as the DIN metric thread previously developed in Germany in 1919. This was a combination of the best of the Whitworth thread form (the rounded root to improve fatigue performance) and the Sellers thread (60 degree flank angle and flat crests). However, the larger root radius of the Unified thread proved to be advantageous over the DIN metric profile. This led to the ISO metric thread which is used in all industrialised countries today.

Those working in the industry have witnessed much fine-tuning of bolts during recent decades. “When I started in the industry 35 years ago the strength of the bolts was not as fully defined as it is today,” recalls Eccles. “With the introduction of the modern metric property classes and the recent updates to the relevant ISO standards, the description of a bolt’s strength and the test methods used to establish their properties is now far better defined.”

As the raw materials industry has become more sophisticated, the DNA of bolts has changed from steel to other more exotic materials to meet changing industry needs.

Over the last 20 years there have been developments in nickel-based alloys that can work in high temperature environments such as turbochargers and engines in which steel doesn’t perform as well. Recent research focuses on light metal bolts such as aluminum, magnesium and titanium.

Today’s bolt technology has come a long way since the days when bolts and screws were made by hand and customers could only choose between basic steel nuts and bolts. These days, companies like Nord-Lock have invented significant improvements in bolting technology, including wedge-locking systems. Customers can select pre-assembled zinc flake coated or stainless steel washers, wheel nuts designed for flat-faced steel rims, or combi bolts, which are customised for different applications. The acquisition of US company Superbolt Inc. and Swiss company P&S Vorspannsysteme AG (today Nord-Lock AG) has added bolting products used in heavy industry, such as offshore, energy, and mining, to Nord-Lock’s portfolio, taking a huge step in becoming a world leader in bolt securing.

There is also much more emphasis now on analysing joints. “In the past, people used to decide upon a certain size of fastener based on their experience alone. And, fingers crossed, it would work,” Eccles explains. “Nowadays, people focus more on analysis and making sure things work before products are built and sent out into the market.”

 

Video: Comparison of common bolt locking methods

Video: Tightening large bolts with only hand tools

Installing a Bolt Tensioner from Boltight

Designed with the benefit of more than 30 years experience in the field, Boltight has created a range of tools to meet the challenge of today’s bolt tensioning requirements.

Just watch how easy it is to install these tensioners!

► Click here for more information about Boltight

No más tornillos sueltos en grúas gigantes

4 diciembre 2017
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Texto: Roxana Ortiz

foto: Paceco

First published in Bolted #2 2017.

CLIENTE: PACECO ESPAÑA S.A.
PRODUCTOS: GRÚAS DE MUELLE Y ASTILLEROS Y SERVICIOS Y SISTEMAS PARA LA MANIPULACIÓN DE CONTENEDORES
AÑO DE FUNDACIÓN: 1967
ACCIONISTAS: GRUPO MITSUI Y URSSA, S. COOP.
PRODUCTO DE NORD-LOCK: NORD-LOCK 20 / NL20

La empresa de ingeniería Paceco España debe adaptarse a las necesidades de sus clientes en su oferta de grúas, servicios y sistemas dentro del sector de la manipulación de contenedores. El incremento en el tamaño de las embarcaciones obliga a aumentar tanto la altura y alcance de las grúas de muelle y astilleros como la eficiencia de estas. Actualmente, Paceco España es capaz de cargar y descargar buques de 25 líneas de contenedores. La empresa produce a día de hoy una de las grúas más grandes y eficientes del mercado, la Portainer Malaccamax, que proporciona un alcance máximo de 72,5 metros, una separación de 52,5 metros bajo bastidor y un tramo de riel de 30,48 metros.

El primer contacto de Paceco España con Nord-Lock tuvo lugar en 2009, al toparse la compañía con problemas en una de sus grúas de muelle. Dicha grúa, con una capacidad de carga de 65 toneladas, mostraba problemas en la fijación de los reductores de pórtico, es decir, las cajas de engranajes que la desplazan a lo largo de la dársena. Al operarla, los tornillos de fijación se soltaban a causa de la vibración.

En su análisis del problema, los ingenieros de Paceco España se comunicaron con Nord-Lock y, al presentarles esta última una solución, la compañía española se mostró gratamente sorprendida. “Llevamos usando sus arandelas desde 2009 y no hemos tenido desde entonces ningún problema con las conexiones atornilladas sometidas a vibraciones”, declara el ingeniero Pelayo Bobes. “Las arandelas Nord-Lock nos han permitido garantizar la plena satisfacción de los clientes y han supuesto también un ahorro en dinero y tiempo”.

¿Qué significa la marcación de tuercas y pernos?

27 noviembre 2017
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Texto: Damien Thomas

First published in Bolted #2 2017.

P: ¿Qué significa el marcado de tornillería?

R:  La tornillería suele ir marcada con números, letras, barras inclinadas, puntos o una combinación de otros símbolos. Las fijaciones presentan por lo general dos marcaciones distintas: un símbolo identificativo único de fabricante (como, por ejemplo, letras o un distintivo) e información relativa a la resistencia del dispositivo de fijación. Dicha señalización variará en función del método de fabricación de los elementos. En la tabla de la derecha, podrá encontrar la descripción y ajuste a normas ISO. Los dispositivos de fijación roscada UNC cumplen por lo general con las normas ASTM.

Las marcas pueden no incluirse en los tamaños más pequeños por falta de espacio, tales como los diámetros inferiores a M5 según ISO 898-1. Ahora bien, la clase de perno debe figurar en la cabeza por encima de este tamaño.

 

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Confiando en Superbolt desde hace tres décadas

1 noviembre 2017
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Texto: Chad Henderson

foto: John Kelly

Si hay un usuario veterano de Superbolt, ese es el norteamericano Mike ­Bruno. Hace más de tres décadas participó en una de las primeras instalaciones de tensores Superbolt en una turbina hidroeléctrica. Hoy continúa alabando el rendimiento de estos tensores. Aquí comparte con nosotros algunas ideas inspiradoras.

First published in Bolted #2 2017.

Comenzó a trabajar con tensores Superbolt en la represa “Diablo” en 1984. ¿Cómo fue eso?

“Trabajaba de operador en Seattle City Light, la compañía eléctrica de dicha localidad del estado de Washington. Salíamos de trabajar del taller de maquinaria de Seattle y luego subíamos a la represa “Diablo” como personal de apoyo. En 1984 procedieron a la inspección del estator-rotor de la turbina, así que tuvieron que desmontar el rotor. Ello implica el desmontaje del rodamiento de empuje, que va montado sobre el eje de turbina. Resulta fundamental situar el bloque de empuje en perpendicular respecto al eje con un margen inferior a una milésima de pulgada. De lo contrario, quedará descentrado y temblará”.

¿Cómo ayudaron los tensores Superbolt en este empeño?
“Por aquel entonces, si deseabas obtener una tensión adecuada en los pernos tenías que calentarlos para que se elongaran, realizar la instalación y luego esperar a que se enfriaran a lo largo de la noche. Si el rodamiento de empuje no quedaba bien colocado en la parte superior del eje, tenías que repetir toda la operación”.

“Los ingenieros de la presa de Diablo habían estado en contacto con Superbolt y modificado los pernos para no tener que efectuar este engorroso proceso. Ahora bastaba con apretar esos pequeños pernos. Si el rodamiento no estaba perfectamente perpendicular, simplemente manipulabas los pernos del lado opuesto. Fue un cambio que permitió ahorrar mucho tiempo”.


Actualmente trabaja en la represa Wells. ¿A qué se dedica?

“Llevo unos 17 años gestionando y supervisando el Proyecto Hidroeléctrico de Wells. Una cosa que siempre me ha gustado de mi trabajo es que todos los días afrontas nuevos retos o algo que arreglar. Contamos con sistemas de aire, eléctricos, mecánicos e hidráulicos… todos los diferentes sistemas auxiliares que alimentan las turbinas que operan las 24 horas del día”.

¿Cómo se ha modernizado la represa con el paso de los años?
“Una de las maneras de actualizarla ha sido con la instalación de controladores lógicos programables, o PLC, en la mayoría de nuestros sistemas de alarma. Hoy disponemos de más de 2.500 puntos de alarma en distintos sistemas. Esto nos permite establecer más parámetros en los puntos de alarma, registrar tendencias a lo largo del tiempo y realizar comparaciones entre diferentes máquinas. Si algo comienza a fallar, puedes definir un parámetro para lanzar una alarma y examinarlo antes de que se produzca el fallo propiamente dicho”.

“También estamos empleando tensores Superbolt en el reacondicionamiento de nuestras turbinas. Se aplican en los tornillos de carga que sujetan las zapatas del rodamiento de turbina, así como en la cubierta del cabezal exterior de la misma, donde no puedes acceder a los pernos con una llave de gran tamaño por lo angosto del espacio. Son extraordinariamente fiables”.

FICHA: MIKE BRUNO
PUESTO: Superintendente de proyecto, Proyecto Hidroeléctrico de Wells, Distrito de Servicios Públicos del Condado de Douglas.
EDAD: 60 años.
LUGAR DE RESIDENCIA: Chelan, estado de Washington.
ANTECEDENTES: Graduado en Tecnología Industrial por la Escuela Superior de Shoreline. También realizó estudios en la Escuela Superior de Cogswell. Trabajó en Seattle City Light como operario de energía hidroeléctrica y capataz hasta 1990. Luego se trabajó hasta 2000 como supervisor mecánico del Proyecto Hidroeléctrico del Río Skagit. Desde entonces hasta ahora trabaja en el Proyecto Hidroeléctrico de Wells.
PASIONES: Casado. Tiene tres hijas mayores y dos nietas. Le gusta la caza con arco y el golf.