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TORQUE TIGHTENING
Torque tightening is the most common method of tightening. The assembly is made using a torque spanner with a tightening range appropriate to the assembly concerned. These spanners can be manual or automatic for large series. The method consists of the following two phases: - a first phase with a spanner set at 0.75% of the reference torque. All fasteners must be tightened in this way before proceeding to the second phase. An impact spanner may be used for this phase. - A second phase where the spanner should be set to a torque value of 110%. The torque should be applied smoothly and continuously Note: In the case of controlled tightening bolts (EN 14399), refer to the values based on the k-class declared by the manufacturer
TORQUE LIMITS
The result of tightening depends closely on the friction coefficients at the threads (μt) and under the head (μt) but in practice these parameters are very difficult to control. As a result, for the same reference torque, the dispersion in the final tension can be between +/- 20% at best, or even +/- 60%.... depending on the tightening method used. This wide dispersion is due to the combination of three phenomena: - inaccuracy of the applied torque - geometric defects and surface conditions of the assembled parts and the fastener - lubrication of the surfaces in contact.
Depending on the quality classes of the components of the connection, the friction coefficients and the desired tightening precision class, it is advisable to refer to standard NF E 25030. The phenomenon of parasitic torsional stresses The torque tightening method causes a parasitic torsional stress in the fastener, in addition to the desired axial stress, the level of which can reach 30% of the tensile stress. The resulting stress in the fastener is greatly increased and may exceed the elastic limit of the material, even if the tensile stress is within acceptable limits. In addition, this residual torsional stress may facilitate loosening during operation. Finally, as the torque is generally applied asymmetrically, there is a bending stress. The low level of bending stress generally allows it to be neglected, but it can also occur in borderline cases.
Deterioration of the surface finish
Friction of the workpieces under high loads causes damage to the surfaces in contact. This leads to frictional forces during subsequent tightening, which in turn leads to greater inaccuracy in the tightening tension. In addition, this deterioration can have consequences for the corrosion resistance of the assembled parts at the connection points.
Difficulty of loosening
The deterioration of the surface conditions, possibly increased by corrosion, requires the application of a very high loosening torque which can reach the elastic limit of the connection material and cause its ruin.
Limits in the case of large bolts
Above a reference torque of 1000 Nm, impact spanners are required but the accuracy of the preload decreases. A good quality hydraulic spanner may be a solution to consider.
Difficulty of simultaneous tightening
With the torque method, it is rarely possible to tighten all the fasteners in an assembly simultaneously.
CLAMPING AT AN ANGLE
Angular clamping is carried out in two phases: - Contacting by applying a docking torque as a threshold for angular measurement. The purpose of this operation is to position the parts and eliminate surface defects. It should be as low as possible. - screwing in at a specific angle. This method provides more accurate results than torque tightening because the preload obtained is almost proportional to the tightening angle and the friction coefficients are only moderately involved. A final torque check can detect any anomalies.
Limitations of angle clamping
The implementation of this method is therefore simple but the theoretical approach involves a large number of parameters, some of which are difficult to control. The determination of the angular set point requires prior development and verification tests. It can only be economically implemented for large production runs on a production line. The reliability of the preload achieved is improved by the rigidity of the parts and the consistency of the characteristics of the parts. For example, it is not advisable to clamp at an angle with joints. The inaccuracy of the tightening depends mainly on the inaccuracy of the engagement torque. The ratio between the engagement torque and the final torque depends on the rigidity of the parts (from 30% for very rigid parts to 50% if parts are easily deformed). The parasitic torsional stress can be significantly higher than that obtained by the torque method
COMBINED METHOD
As with the previous methods, tightening is carried out in two phases: - a first phase with a spanner set at 75% of the reference torque. The entire fastener must be tightened in this way before proceeding to the second phase. - A second phase in which a specified rotation is applied to the rotating part of the assembly. The position of the nut relative to the threads of the bolt should be marked (e.g. by painting) at the end of the first phase to determine the final rotation of the nut relative to the threads at the end of the second phase. Unless otherwise specified, the angle of rotation should be in accordance with the following values (for screws of quality class 8.8 or 10.9):
Note: Where the surface under the screw head (taking into account any skewed washers) is not perpendicular to the axis of the screw, the required angle of rotation should be determined by testing.
CLAMPING AT THE ELASTIC LIMIT
This method consists of tightening the screws until they reach their apparent elastic limit. It requires the torque and the tightening angle to be measured continuously and simultaneously and the operation to be stopped as soon as there is a loss of direct proportionality between these two parameters. The errors are due solely to the dispersion of the torsional stresses linked to the variation of the friction coefficient. This dispersion is small, a variation in the coefficient of friction of 25% resulting in an error of only 7%. A screwdriver equipped with a torque sensor and an angle sensor is used, sending their parameters to an electronic module which stops the screwing spindle as soon as proportionality is no longer achieved. Each screw is tightened to its own elastic limit, i.e. to the maximum possible, independently of any programming, which avoids the risk of breaking a screw that does not conform to the assembly. The pre-tension is obtained with an accuracy of +/- 8%. Limits of tightening to the elastic limit The use of a washer should be avoided as it can distort the measurement: any displacement will stop the screwing. If operating forces cause additional elongation of the screws, the elastic limit can be exceeded, causing a loss of preload, a change in the performance of the assembly, or even its ruin. Disassembly and reassembly of the same screw should be avoided as the deformation of the screw will be cumulative until it breaks. Replacement must be systematic.
CLAMPING BY HYDRAULIC TENSIONER
In this method, a tensile force is applied to the screw by means of a ring jack. Once tensioned, the nut is tightened manually or with a slight torque. The cylinder is then released and the hydraulic force is largely transferred to the assembly. In order to increase the accuracy, a double engagement is recommended: the first operation allows the compensation of the play and surface defects and establishes the balance of the forces within the assembly. The second operation allows the desired precision in the tensioning of the connection to be achieved. Torsional stresses in the screw are eliminated and friction coefficients are not involved in the tightening. This method is particularly suitable for the simultaneous tightening of several screws. Limitations of tightening with hydraulic tensioners The size of the device requires a larger distance between two screws than with other tightening tools. Tightening must be carried out on the nut side. A sufficient screw height must be provided to allow the cylinder to be hooked in. One can only install a part of the maximum permissible axial force of the screw as a preload. The main difficulty is the knowledge of the preload installed in the screw, which depends on the efficiency of the operation. This efficiency depends on the stiffness of the elements of the connection and varies between 50 and 90% depending on the geometry of the connection.
TORQUE CONTROL MEANS
Torque spanner check This is the easiest check to perform. However, there is still significant uncertainty about the final tension of the joint. The method consists of measuring the travel time of an ultrasonic wave along the axis of the screw. The implementation requires qualified personnel. It is particularly effective for small diameter screws (less than 20 mm). This is a laboratory method that cannot be used in industrial applications. Strain gauges are glued to the body of the bolt and connected to a Weston bridge; the variation in the signal obtained corresponds to the variation in elongation of the previously calibrated bolt. This method has the advantage of directly measuring the tightening force. The measuring washer is an instrumented washer placed under the tightening nut which acts as a force sensor. It is recommended that a "conventional" washer be placed between the nut and the measuring washer to avoid damage to the latter. This method allows the tension force in the bolt to be measured and recorded during the life of the connection, both at rest and in service.