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How to use reference values of Preload and
preload and tightening torques tightening torques

(Table T.042)

1)
This procedure neither replaces the calculation as defined in VDI 2230 nor meets the current state of technology. However, it will allow one to approximate a
torque that does not cause a bolt fracture during assembly. The main reason for such fractures is that the actual friction is lower than anticipated.
Step 1: Friction coefficient μtotal 2)
The exact conditions of surface roughness, finish, and lubrication in the thread and in the under-head bearing are often not known. To make
sure the bolted joint is not over-tightened, one should use the lower friction coefficient. Also, if fasteners are re-used (retightened), the friction
is likely to be different than when the joint was initially tightened (VDI 2230 friction table on page T.039)
Example: Fasteners used are electro zinc plated
zinc plated Friction coefficient μtotal = 0,14 – 0,24, lower friction coefficient μtotal = 0,14
Step 2: Tightening torque MA max.
Maximum permissible torque, utilizing 90% of the specified yield strength (0.2 limit),
is found in torque and preload tables starting at page T.042. The values assume that F M [kN] 90% of specified 1
one uses either precision torque wrenches or precision power drivers with a tool inac- yield strength μ min.
curacy of maximum 5%. F M max. 3
Example: Hex cap screw per ISO 4017 M12x40 property class 8.8, zinc plated.
A Preload scatter
In Table on page T.042 look for M12 in the thread column, in the friction column look
for 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 μ max.
Maximum tightening MA max. = 93 Nm.
Step 3: Maximum Preload FM max.
the maximum resulting preload MA max. from that torque FM max. can be found in the same
tables.
Example: To find the maximum preload, start again in the thread column, look for M12, then the
F M min.
friction coefficient 0.14, move over into the left half of the table, the preload value can
4
be found under property class 8.8
2
Maximum preload F M max. = 41,9 kN
Tightening torque M A [Nm]
Step 4: Minimum preload FM min.
The minimum preload can be calculated by dividing the maximum preload through the 5 Control
tightening factor A – see table on page T.040.
Example: For installations with commercial, modern torque wrenches, tightened in a uniform, uninterrupted fashion, with an estimated friction coef-
ficient, a tightening factor A = 1.6 – 2.0 must be applied. (see table at page T.040).
For a signal type torque wrench, as used in the example, a tightening factor 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: = 1,8
A

Minimum expected preload (clamp load)
FM min. = FM max. / = 41,9 kN / 1,8 FM min. = 23,3 kN
A
Step 5: Double checking values, using calculations as stated in VDI 2230 is highly recommended. The calculations in VDI 2230 are state of the art
Is the minimum preload FM 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 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:
– 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)
Bossard engineering recommends using torque / tension equipment to verify specific parameters such as friction, tightening
torque, clamp load, etc. To calculate the friction coefficient μtotal one can use DIN 946 / ISO 16047.

1)
Bolt calculation guideline prepared by: The German Engineering association available in English and German
2) Under head and thread friction combined, assuming same friction underneath the head and in the thread.