How to use reference values

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) μ_K = μ_G must be selected from table F.044.

Example:
Fasteners used are electro zinc plated Friction coefficient μ_K = μ_G = 0,14 – 0,24, lower friction coefficient μ_K = μ_G = 0,14

Maximum permissible torque, 90 % utilisation of yield point (R_eL) respectively the 0.2 % yield strength (R_p0.2) 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 %.

Example:
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 μ_K = μ_G = 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 M_{A max.} = 93 Nm

The maximum resulting preload M_{A max} from that torque F_{M max} can be found in the same tables.

Example:
In the left half of the table in column «property class 8.8» and on line «M12/0,14», the resulting maximum installation preload F_{M max} = 41,9 kN

The minimum preload can be calculated by dividing the maximum preload through the tightening factor α_A – see table on page F.046.

Example:
For installations with commercial, modern torque wrenches, tightened in a uniform, uninterrupted fashion, with an estimated friction coefficient, a tightening factor α_A = 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 α_A 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 α_A = 1,8

Minimum expected preload (clamp load):
F_{M min} = F_{M max}/α_A = 41,9 kN/1,8
F_{M min} = 23,3 kN

checking using calculations in accordance with VDI 2230 is state of the art and is recommended for a safe design.

• Is the minimum preload F_{M min} adequate for the intended application?
• Are surface pressures in the bearing areas brought in line with strength of clamped parts?
• How high is the residual clamp force when work forces are applied?
• Will the bolted joint be used in a manner not to exceed the fatigue limit?

If one applies a tightening torque M_A that is lower than the stated torque value in the table, the resulting maximum preload F_M will be lower as well. The minimum possible preload F_{M 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:

• Friction is lower than anticipated, possibly leading to a bolt fracture during assembly
• Tightening tools are not as accurate as they should be, again leading to a premature bolt fracture either during assembly or in use.
• Clamped parts are deformed unexpectedly (head embeds into material)
• Inadequate knowledge of assembly personnel