Complete Guide to Bolt Classification According to ISO
Over time, classifications and nomenclatures for bolts have been defined to regulate their mechanical and physical characteristics, both at national and international levels. Specifically, the main standards for fasteners are ISO, DIN, and UNI.
The creation of documents by national and international organizations that provide clear guidelines on technical aspects of the production of these items (such as units of measurement, dimensions, and manufacturing processes) allows for the introduction of products into the market that meet specific quality and safety standards worldwide.
Purchasing and using fasteners according to the aforementioned standards allows the end user to employ products that meet specific quality and reliability standards. Given the extensive use of these items in sectors such as construction, infrastructure and large works, the aerospace industry, and even furniture production, defining production standards, mechanical characteristics, and tolerances is the way to provide clear guidance to manufacturers worldwide.
But what do the acronyms ISO, DIN, and UNI mean? At what level are these classifications recognized? How can one distinguish a standardized product from a non-standardized one? We will answer all these questions throughout the article.
What Does the ISO Acronym in Screws Mean?
Articles produced according to ISO specifications derive their eponymous acronym from the International Organization for Standardization (ISO), which brings together standardization organizations from 164 countries worldwide. Over the years, the ISO entity has created specific, universally recognized standards for fasteners and bolts to facilitate their production and commercialization across various countries.
Some of the standards created define:
Characteristics of screws: dimensions, threads, tolerances, materials, mechanical properties, and strength requirements.
Quality of the materials used for screws: testing methodologies and conformity checks.
Adoption of sustainable production for bolt products, with a focus on environmental friendliness.
In general, ISO standards concern both the qualitative characteristics and the production aspects, including dimensional tolerances for fasteners and manufacturing tolerances.
In the fastener world, among the main ISO classifications for screws are:
ISO 4014: Specification for hexagon head screws with a partially threaded shank and coarse metric ISO thread.
ISO 4017: Refers to hexagon head screws with a fully threaded shank and coarse metric ISO thread.
Both of these classifications establish the dimensional characteristics (thread length, unthreaded length, head and bolt body), tolerances, and mechanical properties of the product.
What Does the DIN Acronym Mean?
The acronym DIN stands for "Deutsches Institut für Normung", which translates to "German Institute for Standardization." DIN is the German national standardization body that works on the publication of technical standards in various sectors, from mechanical engineering, the automotive industry, construction, electronics, and more. This national entity has its equivalent in many countries, including Italy, where the corresponding body is UNI (Ente Nazionale Italiano di Unificazione).
Regarding the world of fasteners, DIN standards for screws define the technical and dimensional criteria that products must meet to comply with German standards. Specifically, these standards refer to:
Dimensions and type of threading (e.g., ISO metric thread type M, threading for sheets).
Head shape (e.g., external hexagon, recessed hexagon, countersunk rounded head).
Recommended materials for production (e.g., carbon steel, alloy steel, stainless steel, brass, etc.).
Surface treatments to which the elements should be subjected to protect them from corrosion and improve their performance over time.
Strength class, meaning the mechanical strength of the products indicated by a class number (e.g., 4.8, 8.8, 10.9, etc.). These figures can be explained as follows: the number to the left of the decimal point, when multiplied by 100, indicates the tensile strength of the material. This value is expressed in N/mm² (Newton per square millimeter). The number to the right of the decimal point, when multiplied by 10 times the previous number, provides the yield strength value. This second result is also expressed in N/mm² or MPa (MegaPascal).
Markings that indicate the screws' type, dimensions, and material. Below are some examples of screw markings:
Additionally, there is no single and generic classification, but several have been established concerning different characteristics and dimensions of bolts. Among the main ones are:
DIN 931, a standard that pertains to hexagon head screws with a partially threaded shank.
DIN 933, referring to hexagon head screws that are fully threaded.
Given the comparison between the German body and those of other countries, it is useful to note that DIN 933/931 screws correspond to EN ISO 4017/4014 and UNI 5739/5737 standards. This latter classification will be described in the next paragraph.
UNI Standards for Fasteners: UNI 5739
A UNI 5739 screw is characterized by a hexagonal head, with a fully threaded shank and coarse pitch. In general, this standard describes the following characteristics of screws:
Threading along the entire length of the screw;
Hexagonal-shaped head;
Production materials such as carbon steel, alloy steel, stainless steel, brass, etc.;
Another relevant standard is UNI 5737, specific to bolts with a hexagonal head and partially threaded shank. In detail, it describes:
Hexagonal head shape;
Partial threading;
Materials that can be used, such as carbon steel, alloy steel, and stainless steel;
Dimensions and tolerances;
Applications in sectors such as construction and civil engineering, the automotive industry, machinery production, etc.
Comparison Table: ISO, DIN, and UNI
When choosing an item, it is important to consider the reference standard as production tolerances and the dimensions in which the product can be supplied vary, along with the construction materials. Below is a comparison table of ISO, DIN, and UNI standards for some of the most commonly used products.
To give a concrete example in the sale of items such as washers, the specification of the steel strength class, such as 100HV, 140HV, 200HV, or 300HV, implies that the item is supplied according to one specific standard versus another.
When choosing to purchase a specific item, it is always advisable to consult the technical data sheet of the item.
What are the UNI Standard Certifications?
The UNI (Italian National Unification Body) exercises regulatory activities in Italy, similar to the DIN entity in Germany. The purpose of the association is to enhance the characteristics of the Italian production system by translating them into technical documents.
Among the main UNI certifications that regulate production and commercial activities across a wide range of sectors, we can find:
UNI EN 15048: A standard on structural fasteners that governs preloaded assemblies, i.e., bolts and nuts known as Structural Bolts.
UNI EN 14399: This standard specifies preloaded assemblies, including bolts, nuts, and washers, with HV, HR, and HRC systems.
UNI EN 1090: An obligatory certification introduced in 2014, defining the criteria for placing construction products and structural components made of steel and aluminum on the market.
UNI EN ISO 898-1: A standard referring to hexagon bolts made of carbon steel and steel alloys with specific mechanical properties. This standard distinguishes between two different classes: high-strength fasteners and medium/low-strength fasteners (4.6, 4.8, 5.6 low strength – 6.8 medium strength – 8.8, 10.9, 12.9 high strength).
The aforementioned are just some of the main UNI standards concerning screws and bolts.
Another important document to mention is the 3.1 certification, which is included within the EN 10204 standard and, in accordance with the UNI EN 1090 standard, is prepared by the manufacturer and issued to the purchaser to certify material conformity. In detail, this certification specifies the tests performed on the cast and the material of the used batch.
What Does EN ISO Mean?
The acronym EN ISO, which has been mentioned in some standards described in the previous paragraph, indicates an integration between European and international standards. EN stands for "Europaische Norm" in German, meaning "European Standard" while ISO is the globally recognized standard.
Therefore, when a standard bears this acronym, it is adopted as a standard both internationally and in Europe. Consequently, this implies that:
The standard is approved by ISO.
Subsequently, the standard is adopted as a European norm by CEN (European Committee for Standardization).
CEN member states are required to adopt the standard as a national norm. This ensures uniformity in specifications and practices.
Among the most important EN ISO standards are:
EN ISO 9001, related to the quality management system.
EN ISO 14001, related to the environmental management system.
In general, the standard that encompasses the various standards for the production of fasteners is ISO 8992. Drafted by the ISO/TC 2 Technical Committee, it is a reference standard for fasteners (covering specifics such as terminology, sizing, and tolerances). Within this regulatory framework, there are also documents concerning production tolerances, mechanical properties of the product, threading characteristics based on the type of production, specifications on heat treatments such as electroplating or hot-dip galvanizing, etc.
Consult the Screw Tables: Guide to the UNI Table
To identify the correct screw specifications and choose the right product, it is useful to refer to summary tables of the reference standards and required specifications. Below is a generic excerpt related to the production and tolerances of items according to UNI standards.
The world of fasteners is therefore subject to specific standards that, over time, may undergo changes or be annulled by the decision of the standardization bodies.
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