Assembly

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.

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.

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.