This formula was obtained by taking into account of the nut. The following theoretical formula allows the friction of the threads and the friction of the nut face estimating the residual tension load Fo in the bolt when against the flange. Tightening by mechanical Tightening elongation With this method, the tension load is directly applied to the with heater rod bolt Fig. This method consists of elongating the bolt by heating In general, the body of the nut is provided with a set of small it with a heater rod inserted down the bolt centre.
These screws apply - either directly or through a washer - is in contact with the flange. Upon cooling, the bolt will contract lengthwise, thereby They are turned one by one and step by step using a low tightening the nut. Simultaneous tightening of several bolts torque load until a suitable tension load for the bolt is reached.
The method is theoretically The bolt elongation is most often measured using one of accurate but in fact has several disadvantages: the previously mentioned methods. In spite of the fact that - A hole must be drilled down the centre of the bolt this method eliminates torsion stress in the bolt, it has to receive the heating rod. This is both tedious and time-consuming, - The method is exceedingly slow, due to the time required and the result is pseudo-simultaneous tightening.
As a result, when thermal elongation is described in the introduction. Changing from heater rods to hydraulic bolt tensioners As we have seen, the use of hydraulic bolt tensioners requires knowing the residual load applied to the bolt. The technical data known for heating rod tightening are as follows: the temperature increase, and the angle of rotation to apply to the nut once the temperature is reached. Cold extension is applied to the bolt by primarily to obtain the required accuracy of residual load in means of an annular hydraulic cylinder placed around it.
The bolt undergoes an axial traction load only. This double turn-down operation simply consists of repeating The stress-free nut is then turned down with very little effort stages 3, 4 and 5 illustrated in Fig.
When the fluid When performed in a workmanlike manner, this method pressure is released in the tensioner, the major part of the represents the best method of meeting the quality hydraulic load on the tensioner is transferred into the nut, requirements of correct tightening, as defined in the and tightening is completed Fig. The load does not The remote control feature allows operators to control depend on the various friction coefficients in the assembly.
This significantly reduces exposure of torque applied when driving down the nut. This ratio is described in detail on page Proper understanding of this ratio is important, for there are means available to obtain an accurate ratio for each assembly. Easy implementation This method is easy to perform and requires no physical effort, even for very large bolts. Occupational hazards and physical exertion are significantly reduced.
Material Variety Many different bolt materials, such as stainless steel, titanium, composite materials and others, can very easily be tightened with the hydraulic tensioning.
Suits a wide range of bolt diameters The method can be used for a very wide range of bolt diameters, from 5 to mm! No damage to components Internal stresses are controlled, and no friction is generated under heavy bearing pressure. Therefore this method protects the individual components of the assembly Fig. Easy untightening The untightening operation is extremely easy: generally the required hydraulic effort is approximately one percent greater than that required in the previous tightening phase.
The tightening load is now exerted through bolt tension. One washer every two, three, four or eight bolts etc. Once the bolt has been tightened by the initial cycle of may suffice, depending on the requirements. During to two percent by careful machining of the assembly this second pressurisation, the rising pressure value is components.
The resulting tension load of the bolt can be Note that no additional classic washers are required determined graphically using the change in the slope of the between the tightening nut and the sensor washer, graph. This method improves control over the final residual load. Elongation measurement This measurement can be performed with one of the previously described methods dial gauge or LVDT - see Fig.
This in turn offers three major advantages: - a reduction in overall dimensions, This point is very important for the quality of the joint. Controlling the stress in the bolt enables material choice and dimensioning to be optimised at the design stage.
In general, the bolt has a relatively low stiffness compared with that of the structural parts on which the compression stress is applied. Lines D1 and D2 can be drawn on the same graph, and the final tightening load - Fo - is shown by the intersection of the two lines Fig.
Naturally, the tension in the bolt has the same value but exactly the opposite sign as the compression of the structure Fig.
It can be seen on the graphs that not all of the external load is applied to the bolt, but only a fraction of it Figs. Although bolted assemblies at first appear very simple, they cause several problems for design engineers, assemblers, and maintenance departments. Rough-dimensioning methods are too often used at the design stage, leading to substantial oversizing of all the components of the assembly, which does not ensure assembly safety, quite the contrary. In reality, the design of a bolted assembly requires a methodical and rigorous approach, since mistakes can lead to failures with often costly and sometimes disastrous consequences.
Many surveys show that failures of bolted assemblies are mainly due to the fact that they were not properly designed analysis, drawing, calculation, choice of components or implemented tightening method, tooling, checking. The surveys also show that among the possible causes of assembly failure overloading, improper design, manufacturing defects etc.
Screw and nut Stud with nut on one end Stud with nuts on both ends Fig. To work well, a bolt must behave just like a spring. In operation, the tightening process exerts an axial pre-load tension on the bolt. This tension load is of course equal and opposite to the compression force applied on the assembled components. Depending on the application, the purpose of the tightening load is multiple: - ensure the rigidity of the whole assembly and make it capable of supporting external loads due to traction, compression, bending moments and shear; - prevent leakage at seals; - avoid shear stresses on the bolts; - resist spontaneous loosening effects; - reduce the influence of dynamic loads on the fatigue life of the bolts see Fig.
Furthermore, all components bolts and assembly parts must perform these tasks while remaining below the yield point of their respective materials. Bolt-tightening is optimal when the bolt is properly tightened: not too much, not too little! A bolt can fail just as often - and even more so - when it is not tightened enough, as when it is over-tightened. Controlling bolted assemblies It is fundamental to control the level of the tightening load, as well as the accuracy of the tightening value, to ensure that required performance of the bolted assembly will be achieved.
Uncontrolled tightening calls for oversized joints Controlled tightening allows optimised joint sizes Fig. The respective principles are quite different, as are the quality and accuracy levels achieved.
The following is a summary of the most commonly used methods. The torque wrench This is probably the most common tightening method. Its main advantage, especially when the bolt diameter does not exceed 30 mm, is that it is very simple and quick to use.
The resulting equivalent stress in the bolt Von Mises or Tresca criteria is greatly increased and can exceed the yield point of the material, whereas the tension stress itself remains within admissible limits see Fig. Furthermore, the residual torsion stress increases the risk of spontaneous loosening at a later stage. Furthermore, since the torque is most often applied in a non-symmetrical manner, there is also some bending stress, but because its value is comparatively small, it is often ignored.
However, in cases where the working conditions are near the limit, this bending stress should be taken into account. Characteristics of torque tightening High amount of uncertainty as to the final bolt tension load The final tightening load depends on the friction coefficients in the threads of the nut and the bolt, and on the bearingcontact surfaces between the nut and the flange.
In practical terms, it is impossible to know the value of these coefficients accurately and reliably. F0 min. At the next tightening operation, such damage will increase the friction forces, and the error in the final tightening load will increase accordingly Fig. Problematic tightening of large bolts When the required tightening torque exceeds Nm, various torque equipment must be used, such as impact wrenches, torque multipliers or hydraulic torque wrenches Figs.
This equipment provides the required tightening torque. However, with the impact wrench in particular, the accuracy is unreliable. Only the hydraulic torque wrench - on the condition that top-quality equipment be used by skilled operators following correct procedures - can provide some improvement in accuracy. Difficulties in untightening It is often much more difficult to unscrew a torqued bolt than it was to screw it on in the first place.
Damage to the contact surfaces, and corrosion problems, impose higher torque loads, which can cause damage to various parts of the assembly. Simultaneous tightening is rarely possible With the torque method, it is generally not possible to simultaneously tighten several or all of the bolts in an assembly.
When hydraulic torque wrenches are used, several bolts can theoretically be tightened at the same time. However, only a few bolts can actually be connected at one time because of space limitations and installation difficulties. Furthermore, this does not eliminate the inaccuracy problems decribed above. Successive assembly and disassembly increase this phenomenon. Checking by the angle of rotation of the nut There are two steps to this method. First, the nut is tightened to a torque value which is slightly lower than the required final torque.
Then, a further, specific angle of rotation is apllied. This slightly reduces the deviation in the final tension load. Monitoring the torque value This is the simplest method. However, as described above, even where the accuracy of the applied torque value is good, a great deal of uncertainty still remains as to the final tension load in the bolt. Bolt-elongation measurement methods The accuracy is significantly improved when direct bolt-elongation measurements are taken.
Several methods can be used: 10 Rod and knurled-wheel method A rod topped with a knurled wheel is screwed into a hole bored in the middle of the bolt. This method has certain disadvantages: - the extra cost of the additional parts and the drilling ; - the bolt is weakened; - need for preliminary calibration; - uncertain degree of accuracy, in particular because the operator must turn the knurled wheel a little to check the remaining clearance. Measurement by dial gauge or LVDT The entire length of the bolt is drilled through to house a measuring rod.
The variation in the distance between the top of the rod and the top of the bolt is measured with a dial gauge or an electronic sensor LVDT. This method is more accurate than the previous one, but it has similar disadvantages: - the extra cost of drilling the bolt, and the extra parts; - the bolt is weakened; - preliminary calibration is required. Ultrasonic measuring US method This consists in measuring the time it takes an ultrasound wave to travel down and back the longitudinal axis of the bolt.
The bolts are not drilled but they must be top-quality, and careful calibration is required. The method requires qualified, skilled operators. Constant improvements are making this method increasingly attractive, in particular for small-size screws diameters under 20 mm. Strain-gauge method Strain gauges are attached to the bolt and connected to a Weston bridge; the variation in the signal - which corresponds to the strain variation in the bolt - is measured.
Preliminary calibration is required. This is strictly a laboratory method and can not be used in industrial applications. In conclusion, the methods described above require specialised technicians and can be long to implement.
Furthermore their accuracy is directly proportional to their cost. In addition, they do not directly measure the tension tightening load, but rather the variation in the bolt elongation. It is highly recommended to install a common thin washer between the sensor washer and the nut, to avoid high friction on the sensor washer during tightening and untightening This washer acts as a load-cell sensor.
The accuracy is good and the method is easy to use. When a torque wrench is used, the friction forces may vary greatly from one bolt to another for the same assembly. If good accuracy is required, each assembly bolt should be fitled with a sensor washer.
Furthermore, this method provides easy periodic or permanent measurement and recording of the bolt tensile stress when the assembly is both on and off. Changing from torque wrenches to hydraulic bolt tensioners Using a hydraulic bolt tensioner entails knowing Fo the residual load applied to the bolt.
However, when tightening with the torque-wrench method, only the torque is recommended. It is usually expressed in Newton-metres Nm. The technical data known for heating rod tightening are as follows: the temperature increase, and the angle of rotation to apply to the nut once the temperature is reached.
In effect, the first turning-down operation compensates for clearances, compresses the roughness of the surfaces and sets the load balance, while the second operation serves primarily to obtain the required accuracy of residual load in the bolt. The bolt must have an end that protrudes above the tightening nut.
Cold extension is applied to the bolt by means of an annular hydraulic cylinder placed around it. The bolt undergoes an axial traction load only. The stress-free nut is then turned down with very little effort and does not transmit any torque to the bolt. When the fluid pressure is released in the tensioner, the major part of the hydraulic load on the tensioner is transferred into the nut, and tightening is completed Fig.
This double turn-down operation simply consists of repeating stages 3, 4 and 5 illustrated in Fig. When performed in a workmanlike manner, this method represents the best method of meeting the quality requirements of correct tightening, as defined in the introduction. Good accuracy It is accurate because the most important parameter, namely the traction load, is perfectly controlled through the hydraulic pressure in the tensioner.
The load does not depend on the various friction coefficients in the assembly. The only uncertainty in the method arises from the degree of torque applied when driving down the nut. Through simple precautions of good workmanship, good uniformity can be obtained in the driving-down operation. This ratio is described in detail on page Proper understanding of this ratio is important, for there are means available to obtain an accurate ratio for each assembly.
The remote control feature allows operators to control all phases of the tightening or untightening procedures, from a safe area.
This significantly reduces exposure to poor or dangerous working conditions, such as radiation, hazardous media, high temperatures, high noise levels, risk of component failure. Easy implementation This method is easy to perform and requires no physical effort, even for very large bolts. Occupational hazards and physical exertion are significantly reduced.
Material Variety Many different bolt materials, such as stainless steel, titanium, composite materials and others, can very easily be tightened with the hydraulic tensioning. Suits a wide range of bolt diameters The method can be used for a very wide range of bolt diameters, from 5 to mm! No damage to components Internal stresses are controlled, and no friction is generated under heavy bearing pressure. Therefore this method protects the individual components of the assembly Fig.
Easy untightening The untightening operation is extremely easy: generally the required hydraulic effort is approximately one percent greater than that required in the previous tightening phase. The tightening load is now exerted through bolt tension. During this second pressurisation, the rising pressure value is recorded and plotted versus the displacement of the top part of the bolt. The resulting tension load of the bolt can be determined graphically using the change in the slope of the graph.
This method improves control over the final residual load. Elongation measurement This measurement can be performed with one of the previously described methods dial gauge or LVDT - see Fig. Since this sensor washer remains in the assembly, it is particularly useful for periodical or continuous monitoring of load variations over time, where required.
Unlike torque tightening, hydraulic tensioning, which is reproducible, does not require one sensor washer for every bolt. One washer every two, three, four or eight bolts etc. With sensor washers, the measurement precision as to the bolt load is about five percent, but it can be improved to two percent by careful machining of the assembly components.
Note that no additional classic washers are required between the tightening nut and the sensor washer, since hydraulic tensioning generates no surface friction. If the bolt is long enough, the final tension stress can be safely brought very close to the yield point without any risk of exceeding the latter.
This in turn offers three major advantages: - a reduction in overall dimensions, This point is very important for the quality of the joint. Contrary to what has been commonly believed, case studies show that the closer the bolt stress to the yield point: - a reduction in weight, - a reduction in cost. Controlling the stress in the bolt enables material choice and dimensioning to be optimised at the design stage.
In general, the bolt has a relatively low stiffness compared with that of the structural parts on which the compression stress is applied. Lines D1 and D2 can be drawn on the same graph, and the final tightening load - Fo - is shown by the intersection of the two lines Fig. Naturally, the tension in the bolt has the same value but exactly the opposite sign as the compression of the structure Fig.
It can be seen on the graphs that not all of the external load is applied to the bolt, but only a fraction of it Figs. For a compressive external load FE the diagram in Fig.
The F1 part of the load which is taken up by the bolt can be be calculated as follows and depends on the bolt stiffness - RB and the RS stiffness structure. In the case of cyclic external loads, the diagrams in Figs. This is very important since we know that the alternating fraction of the load has a strong influence on the fatigue of the material figure 17, p.
Lower limit: narrow pitch thread. Tightening an existing assembly First consider two flanges with an outer diameter of mm. The bolted joint comprises sixteen M20x2. The pitch of the threads is 2. Therefore, we take 1. Uncertainty in the hydraulic tension load Uncertainty as to the hydraulic tension load first depends on the tensioner itself and on the accuracy on the hydraulic pressure measurement.
The General Catalogue describes simple precautions to limit turn-down deviation. For this purpose, the stiffnesses of the bolts and of the assembly are required.
There is great homogeneity. However, for higher levels of external load, there is the risk of exceeding the safety limit set for the bolts. Tightening with a torque wrench Let us now tighten the same set of bolts with a torque wrench. It is decided to use a manual calibrated torque wrench. Uncertainty factors with torque wrench tightening Let us examine these factors and the values they can reach. Uncertainty due to bolt and assembly tolerances These deviations are primarily due to dimensional tolerances, geometrical defects or variations in material characteristics.
It is also seen that even with an accurate tightening tool, the tolerance range on the final tightening tension load is, in our example, almost three times greater with a torque wrench than with a hydraulic bolt-tensioner.
In our example, there is a very high risk that bolt stricture will occur during the tightening operation, and that the bolt will break under working conditions! International Login. Back to top. Home Industries Pulp and paper Handbook. What is it?
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