01D-4
MECHANICAL INTRODUCTION
Tightening torques: General information
01D
Creating tension (or preload) F0
The assembly is held together by the tension created in
the bolt when it is tightened.
A reliable assembly is only possible if the correct ten-
sion is used:
-insufficient tension: risk of loosening,
-too much tension: risk of deformation of the parts to
be assembled, or shearing of the bolt.Customer complaints resulting from incorrect tighten-
ing may be, following assembly, a safety issue (fire,
loss of control of the vehicle etc.), an immobilising fault
or a noise.
III - TIGHTENING PROCEDURES
The two controlled tightening procedures adapted to
automotive repairs because of their low cost and sim-
ple operation are torque tightening and angle tighten-
ing (also called torque and angle).
1 - Torque tightening
This is the most commonly used procedure. Is consists
of tightening until a given resisting torque is reached,
known as tightening torque.
The tightening torque is distributed in a large part as
friction torque (under the head and in the thread) and in
a small part as useful torque (to create the tension).
This practise spreads the tension significantly due to
the variation in the friction coefficients from one assem-
bly to another and the uncertainty of the tightening pro-
cedures and methods.
2 - Angle tightening
The principle consists of putting the parts of the assem-
bly in contact using a mating torque (approximately 25
to 30% of the final torque) then to tighten to a deter-
mined angle.
This method, which is not dependent on the friction of
the tightened assembly, gives more precise results
than torque tightening.
IV - OBSERVING THE TIGHTENING TORQUES AND
ANGLES
Bolted assemblies whose tightening torques and an-
gles are explicitly specified in the removal / refitting
procedures must be observed using the appropriate
tools (torque wrench, angle measuring disc). Failure to
observe this can lead to safety risks, immobilising
faults or unwanted noises.
For other bolted assemblies, non-measured tightening
(using standard spanners) is acceptable. Neverthe-
less, the corresponding tightening torque is indicated in
the table of standard tightening torques.
V - RECOMMENDED TIGHTENING TOOLS
For measured tightening, the repairer must have avail-
able torque wrenches to tighten from 4 to 400 N.m as
well as an angle measuring disc.
The torque wrenches used may be click type or elec-
tronic.
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(3) Bolt
(4) Assembled components
(5) Extension of the bolt
(6) Non-tightened assembly
(7) Tightened assembly
(X1) compression of the assembly
(Fo) tension
(C) tightening torque
