In case of uncertainty about friction conditions in the threads and under the bearing surface, the lowest possible practical friction coefficient (e.g. initial assembly, maintenance, repair) μK = μG must be selected from table F.044.
Fasteners used are electro zinc plated Friction coefficient μK = μG = 0,14 – 0,24, lower friction coefficient μK = μG = 0,14
Maximum permissible torque, 90 % utilisation of yield point (ReL) respectively the 0.2 % yield strength (Rp0.2) can be found in the tables from page F.048. The values assume that one uses either precision torque wrenches or precision power drivers with a tool inaccuracy of maximum 5 %.
Hex cap screw per ISO 4017, M12 property class 8.8, zinc plated. In Table on page F.049 look for M12 in the thread column, in the friction column look for μK = μG = 0,14. Now move over to the right half of the table under «maximum tightening torque under property class 8.8» you will find the Maximum tightening MA max. = 93 Nm
The maximum resulting preload MA max from that torque FM max can be found in the same tables.
In the left half of the table in column «property class 8.8» and on line «M12/0,14», the resulting maximum installation preload FM max = 41,9 kN
The minimum preload can be calculated by dividing the maximum preload through the tightening factor αA – see table on page F.046.
For installations with commercial, modern torque wrenches, tightened in a uniform, uninterrupted fashion, with an estimated friction coefficient, a tightening factor αA = 1,6 to 2,0 must be applied. (see table at page F.046). For a signal type torque wrench, as used in the example, a tightening factor αA of 2,0 is adequate. We use a short screw M12x40, which only requires a small torque angle. This results in a relative stiff joint, therefore a lower tightening factor can be applied.
Assumed tightening factor αA = 1,8
Minimum expected preload (clamp load):
FM min = FM max/αA = 41,9 kN/1,8
FM min = 23,3 kN
checking using calculations in accordance with VDI 2230 is state of the art and is recommended for a safe design.
If one applies a tightening torque MA that is lower than the stated torque value in the table, the resulting maximum preload FM will be lower as well. The minimum possible preload FM min would be
affected as explained in step 4. Users (engineers) ought to verify parameters to assure an adequate clamp load in the bolted joint.
Possible reason for the torque to be different: