Your cart
There are no more items in your cart
For screwed joints, the relationship between the applied torque and the resulting tension in the screw is dependent on friction. This friction is located : - between the threads of the screw and the nut or the thread, - but also between the bearing face and the fastener driven in rotation (screw or nut, depending on the tightening process). Optimised assemblies require good control of friction and the use of precise assembly tools in order to respect the targeted tension range. Torque/tension tests are carried out - either to determine the actual assembly characteristics (provided that the parts and fasteners to be assembled are available), - or to check the characteristics of a fastener under standardised reference conditions: -> ISO 16047 + NF E 25-039 in the general case, -> EN 14399-2 for steel construction bolts, -> Internal standards of users, car manufacturers, aeronautics ... These tests require the use of a tightening bench and specialised calculation algorithms.
The coefficient of friction μ is generally used in mechanical engineering, automotive, aerospace, railways ...). μ, a dimensionless number calculated from μ, a dimensionless number calculated from measured physical quantities, depends on the nature and geometry of the surfaces in contact. In the case of an ISO metric thread, the relationship between torque and tension in the in the connection is written : C = F x (0.16 x P + μ x (0.577 x d2 + Rm)) where C: is the tightening torque, F: the tension in the connection, P: the thread pitch, d2: the thread flank diameter, Rm: the average bearing radius under the rotating part (screw head or nut), and μ: the coefficient of friction. The coefficient of friction, extracted from the previous formula, makes it possible to summarise the relationship The coefficient of friction, extracted from the previous formula, makes it possible to summarise the torque/tension relationship simply and independently of the geometric characteristics of the connection.
Note: If the characteristics of the test equipment allow it, it will be possible to distribute the friction dissipated in the thread and under the rotating part of the connection (head or nut). Steel construction for pretensioned bolts (HR, HRC, HV) prefers to use the torque efficiency coefficient K. This dimensionless number is calculated from the torque and the measured tension by the following relationship: C = K x d x F where : C: is the measured torque, K: the torque efficiency coefficient, d: the diameter of the fastener, and : F: the measured tension in the connection. Note: In addition to the K coefficient, criteria related to rotation or elongation can be checked, but require suitable test equipment. During the manufacture of fasteners, control of the values and dispersion of μ or K is achieved by the use of lubricants, which may be incorporated in the case of some coatings.