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The purpose of this technical sheet is to communicate in a simple way the recommendations for using a
screwed assembly (support with the help of NFE25030) the.
1 - Loads to which screw connections are subjected
2 - Principle of an assembly
The resistance of an assembly is that of its weakest element. Except in special cases, this element must be the screw.
3 - inherent resistance of the screws
- Tensile breaking strength (Rm).
It is generally the same as that of the resistant section. However, the limit of use of the screws remains the elastic limit of the resistant section.
- Fracture strength in shear.
It is approximately 0.6 Rm applied to the sheared section.
In appendices 1 and 2 two calculation examples are proposed.
4 - thread lengths
In order to obtain an optimal distribution of the stresses, it is desirable to have at least 6 free pitches under the bearing surface of the nut. The end of the thread must generally have at least 2 free pitches. If there is a risk of oxidation (ease of disassembly), a zero length is permissible.
5 - Threading for screw or bolt
The mesh length of the nets must be at least the same:
1 x d → in steel and stainless steel.
1.5 x d → in cast iron and copper alloys.
2 x d → in aluminum and its alloys.
However, in the case of lack of space or insufficient resistance of the material, a thermal or thermochemical treatment can be carried out to improve the resistance of the thread. You can also use a subtle gradient to increase the number of threads in the mesh.
6 - Screw/nut connection
As indicated in Annex 1 of the NFE 25-400 standard, in an assembly tightened to failure, the bolt must generally yield before the nut.
A nut quality class is assigned to each bolt quality class:
Example:
8.8 for the screw → 8 for the nut (steel).
10.9 for the screw → 10 for the nut (steel).
A2-70 (Edelstahl).
A4-80 (stainless steel).
A bolt with a lower quality nut should not be used, on the other hand there is no technical disadvantage in using a nut that is too thick compared to the bolt, except for self-locking all-metal nuts (risk of deformation of the bolt thread).
7 - tightening procedure
There are a variety of tightening methods depending on the desired accuracy and the operating conditions. It should be noted that the accuracies of the preloads of the screws have an influence that can be important for the choice of the diameter or the quality class of the products.
The most common tightening methods are:
Tightening torque:
This is the most common method, but it does not protect the user from scattering of the tension achieved.
As an example, the following table shows the distribution of the machine's output torque depending on the material used.
These values apply to calibrated devices used under the conditions recommended by the manufacturer, taking into account parameters of use such as:
Variation of energy properties (e.g. compressed air).
Deviation by operator.
Wrong positioning of the key (angle key).
clamping speed.
By considering additional parameters such as:
Material handling,
Missing calibration.
usuryIt is
Shop information:
This method is done in two steps:
Tighten the assembly with a relatively low torque (corresponding to a theoretical tension of the order of 40% of the nominal value).
Rotation of the nut through a specific angle.
The tightening parameters are determined by preliminary tests on real assemblies.
Tightening with a wrench calibrated for tensionN :
The method uses a torque wrench that is regularly calibrated for tension, which integrates the coefficient scatter M and on the key.
Tightening by preloading the screw :
This method is similar to the angle tightening method. A device, generally hydraulic, puts the screw under axial tension, the nut is brought into contact without tightening, then the pressure is released (special application).
Clamping means “at the elasticity limit”:
This procedure performs the simultaneous control of the torque and the rotation, the ratio of these two quantities is constant until the beginning of the plasticization of the screw (the torque is proportional to the tension of the screw and the rotation of the extension).
The detection of the change in this ratio indicates that the elastic limit of the screw has been reached and compensated
outdated, but does not anticipate the excitement of assembly. This is namely influenced by the specific properties of the screw (yield point, friction scatter). This precise method requires the use of special equipment. It allows the detection of tightening anomalies in repetitive assemblies. The tightening parameters must then be determined by preliminary tests on real assemblies
Table with torque spread in industrial use
8 - Tables of tightening torques
When the screw is tensioned by torque tightening, the following tables give the values of the total average torque Cs (tightening torque under the head and torque in the thread), the torque is indicated for a Fomax value corresponding to a load of 85% the elastic limit and a value Fo min depending on the accuracy of the clamping device used. These details are given in the table above.
Each of these values is provided:
For diameters M3 to M39.
For quality classes 4.6, 5.6, 4.8, 5.8, 6.8, 8.8, 10.9, 12.9.
For average friction values"M" 0.10, 0.15 and 0.20, the most common values.
As a first approximation, these values correspond to the following coating and lubricating properties:
M medium = 0.10 → Phosphated or zinc-plated screws with suitable quality lubrication.
M medium = 0.15 → Black or galvanized screws with basic lubrication (delivery condition).
M Average = 0.20 → screws coated or not, dry assembly.
Use :
The following tables can only be used if not all calculations have been carried out and if no optimization is sought, since the selected values for M disregard scattering
Annex 1
Calculation of the theoretical resistance
M8 screw class 12.9
For information :
1 N/mm² = 0,1 kg/mm² = 1 MPa
N = Newtons = weight
Kg = Kilogram = Mass
Fracture strength = maximum allowable force before fracture (the part stretches before fracture, this is the plastic range).
Elastic limit = maximum allowable force with which the screw regains its original length.
Data :
Tensile strength of class 12.9 steel: Rm = 1200 N/mm²
Yield strength of grade 12.9 steel: Re = 1080 N/mm²
Cross-section of the core of an M8 screw: s = 36.6 mm² (see Appendix 3)
Calculation:
The maximum breaking load under tension : 36,6 x 1200 =43920 N or 4392 kg
Minimum elastic limit of the screw: Re = 36,6 x 1080 =39528 N or 3952 kg
Important: The screw must be used at a maximum value, called the proof load, which corresponds to 90% of the elastic limit.
The test load: 39528 x 0,9 =35575N or 3557kg
sheer strength :
A screw in an assembly must not have a shear stress whose value is approximately 60% of the tensile strength value; In the case of shear stress, a pin must be installed.
sheer strength: Rc = 35575 x 60 % =21345 N or 2134 kg
These are only theoretical values, a screw never works strictly in tension or shear. Visserie-Service cannot be held responsible for calculating the resistance of its deliveries.
Annex 2
Theoretical resistance calculation
M6 screw class 8.8
For information :
1 N/mm² = 0,1 kg/mm² = 1 MPa
N = Newtons = weight
Kg = Kilogram = Mass
Fracture strength = maximum allowable force before fracture (the part stretches before fracture, this is the plastic range).
Elastic limit = maximum allowable force with which the screw regains its original length.
Data :
Tensile strength of class 8.8 steel: Rm = 800 N/mm²
Yield strength of grade 8.8 steel: Re = 640 N/mm²
Cross-section of the core of an M6 screw: s = 20.1 mm² (see Appendix 3)
Calculation:
The maximum breaking load under tension : 20,1 x 800 =16080 N or 1608 kg
Minimum elastic limit of the screw: Re = 20,1 x 640 =12864 N or 1286 kg
Important: The screw must be used at a maximum value, called the proof load, which corresponds to 90% of the elastic limit.
The test load: 12864 x 0,9 =11577 N or 1157 kg
sheer strength :
A screw in an assembly must not have a shear stress whose value is approximately 60% of the tensile strength value; In the case of shear stress, a pin must be installed.
sheer strength: Rc = 11577 x 60 % =6946N or 694 kg
These are only theoretical values, a screw never works strictly in tension or shear. Visserie-Service cannot be held responsible for calculating the resistance of its deliveries.