According to national and international standards, fasteners have to follow specific naming schemes with the purpose of identifying and making manufacturing processes homogeneous.
Both bolts and nuts generally have notations made, in relief or engraved by the manufacturer that define their basic characteristics.
Knowing and understanding these notations is very important for their correct use.
Among the most important characteristics on which particular attention must be paid is undoubtedly the property class, which is determined by specific properties of the bolt or nut, based on its yield strength, tensile strength, etc.
The property class is defined by precise national and international standards.
First of all, the UNI EN ISO 898-1 norm defines two different categories of screws, hi-tensile strength screws and med / low strength screws.
Furthermore, several property classes are detailed and strictly identified by means of a numerical code, that consists of a series of numbers separated by a dot: 4.6, 4.8, 5.6 (low tensile strength screws and bolts), 6.8 (medium tensile strength screws and bolts), 8.8, 10.9, 12.9 (hi-tensile strength screws and bolts).
The first digits, on the left side of the dot, when multiplied by 100, define the ultimate tensile strength of the material, expressed in N/mm² (Newtons per millimetre squared) or in MPa (Mega Pascals) whereas the second one, after the dot, when multiplied by 10 times the preceding number gives the Yield Strength in N/mm² or MPa.
However, only hi-tensile strength fasteners (i.e. property class 8.8 or more) have to be marked with the appropriate code.
If the choice of the suitable product makes a huge difference, in particular when special tasks have to be accomplished, paying attention to the different grades of resistance helps to evaluate what a bolt, o a nut is able to be used for.
Generally, grade 8.8 (medium carbon steel) is defined as the structural grade for bolts and considered the most common form of high tensile material. Usually, grade 8.8 fasteners are zinc plated and enable a controlled tightening by means of a torque wrench. They have an ultimate tensile strength of 800 N/mm² circa.
Grade 10.9 (low carbon steel or alloy steel) bolts and nuts, are largely used in automotive applications and are often made using carbon steel and boron steel. Socket and Countersunk buttons are often manufactured in this grade. They have an ultimate tensile strength of 1000 N/mm² circa.
High tensile bots of grade 12.9 (alloy steel) are the highest grade with an ultimate tensile strength of 1200 N/mm². They are employed for the heaviest tasks, like securing engines.
Regarding nuts their property class indicates the maximum property class of bolt they can be coupled with. Therefore, nut property classes are specified by the initial number of the bolt class designation.
For example, a nut of grade 6 can be assembled with a bolt of grade 6.8 at the most.
Instead, nuts of a higher property class can normally be used in the place of nuts of a lower property class.