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Assembly

Assembly method

Torque controlled tightening

Torque-controlled tightening can be performed with indicating or signaling torque wrenches or motorized torque wrenches. In addition to the control variable torque, the angle of rotation is often also measured from a threshold torque in order to monitor the tightening process. Torque-controlled tightening is most widely used due to its ease of use and low-cost tightening devices. All impact wrenches and torque wrenches should only be adjusted in tightening tests on the original part. This can be done either by means of the breakaway torque, the continued torque or the extension measurement on the screw. The breakaway torque is the torque required to continue turning the screw after the tightening process has been completed. It differs from the nominal tightening torque for torque tightening by the retightening factor, which can vary between 0.85 and 1.30, depending on the type of screw, the friction and the compliance conditions. The retightening torque can only be measured with special tightening tools, which measure the angle of rotation and torque during retightening and calculate the torque required for retightening in the assembly state at a tightening angle of 0° after the static friction has been overcome (due to the higher breakaway torque). Using ultrasound or mechanical means, the extension of the screw can be measured and the preload force achieved can be determined via the screw compliance. Rotary impact wrenches transmit energy by impulse. As with rotary impact wrenches, the adjustment of rotary impact wrenches must be carried out on the original component. The tightening factors in the elastic range are so high that this tightening method cannot be recommended for highly stressed bolted joints. With each impulse, the peak torque acting for a short time and the angle of further rotation can be measured. Newer impulse wrenches with impulse monitoring thus allow stretch limit-controlled tightening.

Angle controlled tightening

Tightening controlled by the angle of rotation is an indirect method of length measurement, since the change in length of the screw via the pitch of the thread is (theoretically) directly proportional to the angle of rotation covered. Both the compressive deformations within the clamped parts and the elastic and plastic deformations occurring in the parting surfaces up to full contact are also measured. Since the deformations of the parting surfaces cannot usually be determined in advance and are irregular, in the practical implementation of this principle - as with yield point-controlled tightening - a joining torque is first applied until full-surface contact of all parting surfaces occurs. The angle of rotation is then counted only after the threshold torque has been exceeded. In addition to the control variable of the angle of rotation, the torque is often also measured in order to monitor the tightening process. Practice has shown that this method achieves its greatest accuracy only when the screw is tightened into the overelastic range, because angular errors then have little effect due to the approximately horizontal course of the deformation characteristic in the overelastic range. Here, the coefficient of friction in the support has no influence on the assembly preload force achieved. In the elastic range, on the other hand, angular errors fall into the steep curve of the elastic part of the deformation curve. In this case, too, however, there is a reduced influence of friction on the preload dispersion compared with torque-controlled tightening. If possible, the angle of rotation should be determined in tests on the original component in order to correctly record the compliance of the design. If the angle of rotation is suitable, breakage of the bolt or overstressing due to exceeding the tensile strength can be safely ruled out. However, because the yield strength of the bolt material is exceeded, the reusability of the bolts is limited. The process can only be used if the deformation capacity of the bolts is sufficient (free loaded thread or expansion shaft length). Angle-controlled tightening is state of the art in the automotive industry

Yield point controlled tightening

In the yield point-controlled tightening process, the yield point of the screw serves as the control variable for the assembly preload force. Irrespective of the friction in the support, the bolt is tightened until the yield point or yield strength of the bolt is approximately reached as a result of the total stress from tensile and torsional stress. As with torsion-controlled tightening, the joint must first be pretensioned with a joining torque. In yield point-controlled tightening, the start of yielding of the bolt is detected by measuring the torque and angle of rotation during tightening and forming their difference quotient dMA/dJ, equivalent to the slope of a tangent in the torque/angle of rotation curve. As soon as plastic deformations occur, the difference quotient drops. This drop to a certain fraction of the previously determined maximum value in the linear part of the torque/angle of rotation curve triggers the switch-off signal. If the assembly preload force is increased as a result of lower thread friction, the torsional component is reduced accordingly. A separate design of the screw for the maximum possible assembly preload force FMmax is therefore not necessary here. The tightening factor aA > 1, which is always present, is therefore not taken into account in the design of the screw. The plastic elongation that the screw undergoes is very small, so that the reusability of yield strength-controlled tightened screws is hardly affected. The bolt drop hardness, the threshold torque and the cut-off criterion should be adapted to the connection under consideration.

Friction coefficients

The friction values µtotal, µthread, µcontact area show scattering, as they depend on many factors, such as the material pairing, the surface finish (peak-to-valley height Rz, Ra), the surface treatment (bright, blackened, electroplated) and the type of lubrication (without/with oil, molybdenum disulfide, Molykote® paste, sliding coatings, etc.).

The following table contains friction coefficients for thread and contact or bearing surface. For safe assembly, it is important to define the friction conditions precisely and to keep their scatter as low as possible. If the scatter is large, the preload force achieved will vary greatly. The usual tolerance of the tightening torque, on the other hand, has only a small influence on the result. An exact determination of the friction values is specified in ISO 16047.


Friction coefficient class

µG / µK

Typical examples
Material / Surface Lubricant


A


0,04 - 0,10

metallic plain
tempered black
phosphated
galvanic coatings
zinc flake coatings

Solid lubricants such as:
MoS2, graphite, PTFE, PA, PE, PI
in bonded coatings, as top coats
or in pastes
Wax melts;
Wax dispersions



B



0,08 - 0,16

metallic plain
tempered black
phosphated
galvanic coatings
zinc flake coatings
Al and Mg alloys
hot-dip galvanized
organic coatings
austenitic steel

Solid lubricants such as
MoS2, graphite, PTFE, PA, PE, PI
in bonded coatings, as top coats
or in pastes
Wax melts;
Wax dispersions, greases, oils
MoS2; graphite
Wax dispersions
with integrated solid lubricant
or wax dispersion
Solid lubricants or waxes;
Pastes


C


0,14 - 0,24

austenitic steel
metallic bright
phosphated
galvanic coatings
zinc flake coatings
adhesive

Wax dispersions, pastes
Delivery condition (lightly oiled)
without

D

0,20 - 0,35

austenitic steel
galvanic coatings
hot-dip galvanized

Oil
without

E

≥ 0,30

galvanic coatings
austenitic steel
Al & Mg alloys

without

Sources:
VDI 2230 Part 1 - Systematic calculation of highly stressed bolted joints - Cylindrical screw-in joints
DIN EN ISO 16047 - Fasteners - Torque/preload force test

Tightening torques

The following table lists tightening torques for bolted connections with expansion and solid shanks. However, the calculated torques MA cannot be converted exactly into preload force, since the actual friction in particular can deviate from the assumed values. In order to limit such scatter as far as possible, the calculations are based on the most common friction conditions:

a) oiled or greased contact surfaces (coefficient of friction µ=0.12)

b) Contact surfaces covered with MoS2 or similar substances (coefficient of friction µ=0.10)

(Contact surfaces = thread flanks and head or nut support)

With repeated tightening, mainly in case a), the friction values can decrease for hard materials due to smoothing of the surfaces, and increase for soft materials due to roughening or lubrication. Lubricants containing molybdenum or graphite cause a smaller increase in friction. Despite a certain spread of the preload achieved, controlled tightening with torque wrenches leads closer to the theoretical values than using conventional wrenches.

In the area of medium and large bolt dimensions, problems of friction at the contact points of a bolted joint - thread flanks and head or nut support - and the resulting torsional stress in the bolt can be eliminated by purely axial pretensioning through the use of hydraulic tensioning devices. This results in a uniaxial stress state in the bolt, which generates higher clamping forces due to better utilization of the material yield point.

Temperature-stressed bolts are usually tightened to 70% of the minimum yield strength of the bolt material.

Note

Knowledge of the existing coefficient of friction is a prerequisite for the exact determination of the preload force or the tightening torque. The data given are only guide values. These values cannot replace a detailed bolt calculation. This applies in particular to parts that are relevant to safety, are subject to official regulations or fulfill sealing tasks. No guarantee can be given for these values.

Overview of preload forces / tightening torques

Thread Screw with full shank Screw with expansion shaft
 
Preload force
Tightening torque
Preload force
Tightening torque
µ = 0,1 µ = 0,12 µ = 0,1 µ = 0,12
FM [N] MA [Nm] MA [Nm] FM [N] MA [Nm] MA [Nm]
M10 40.600 58,7 69 26.000 38 44,5
M12 58.800 100,8 118 39.200 70 80,5
M14 80.500 160 187 54.600 108 125
M16 109.900 242 283 79.100 178 208
M18 134.400 336 393 92.400 241 280
M20 171.500 467 548 123.200 342 399
M22 212.100 623 733 158.200 465 545
M24 247.100 806 947 177.800 580 675
M27 321.300 1.175 1.385 230.300 842 985
M30 392.700 1.606 1.891 290.500 1.190 1.395
M33 486.000 2.155 2.540 357.000 1.690 1.860
M36 571.900 2.790 3.273 415.100 2.020 2.365
M39 683.200 3.575 4.220 511.000 2.670 3.150
M42 784.000 4.400 5.230 580.300 3.285 3.850
M45 910.000 5.500 6.480 692.300 4.170 4.900
M48 1.029.000 6.680 7.880 773.000 5.015 5.900
M52 1.232.000 8.560 10.100 924.000 6.420 7.550
M56 1.421.000 10.680 12.600 1.063.000 7.980 9.400

Values apply to a material with a tensile strength of 1,000 MPa. Values for other materials can be determined using the rule of three.