Screw manufacturing
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.
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
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].