New Innovations to Solve Difficult Shaft Coupling Bolting Problems Conference Paper Stephen J. Busalacchi Product Manager Superbolt Division USA Nord-Lock Group Originally presented at Hydrovision Russia Moscow (Russia) 3-5 March 2015
Abstract Demanding bolting applications in the hydro industry, such as shaft couplings, have typically required the use of fitted or interference fit bolts for proper torque transfer. However, these bolts require large and expensive tools for tightening, precision machining and extreme tolerances/surface finishes. These extreme machining requirements also apply to the mating coupling bores. Assembly may require further mechanical adjustments and disassembly is often time consuming and cumbersome. Additional concerns with these methods include worker safety, stuck bolting, and failures as a result of fatigue. This paper will examine a possible means to overcome these bolting challenges and achieve a bolted connection that is pre-loaded safely, cost effectively, and with as little downtime as possible. One such system is a radial fit expansion bolt that utilizes split expanding sleeves and low input torque multi-jackbolt tensioners to achieve a true radial pre-load into the coupling bores as well as high axial clamping of the split line. Shaft coupling flanges are a critical component of hydro turbines all over the world. Utilizing the latest procedures and tools to achieve a secure bolted joint is essential to ensure many years of safe operations. New Innovations to Solve Difficult Shaft Coupling Bolting Problems Introduction 2
Content 1 Introduction 4 2 Coupling bolting elements 6 2.1 Through bolts 6 2.2 Fitted bolts 7 2.3 Conventional Type Expansion Bolts 7 2.3.1 MJT Type Expansion BoltsExternal 8 3 Tightening methods 10 3.1 Torqueing 10 3.2 Stretching 10 4 Case study 12 4.1 Background 12 4.1.1 Initial Findings 14 4.1.2 Test phase 14 4.1.3 Final results 16 4.2 FEM Study 16 4.2.1 Test background 16 4.2.2 Results 19 4.3 Conclusions 20 5 References 21 New Innovations to Solve Difficult Shaft Coupling Bolting Problems Introduction 3
1 Introduction Because of their sheer size, the turbine/generator shaft lines of hydro power plants consist of several sections which are bolted together. They are found within all types of hydroelectric facilities whether impoundment, diversion, or pumped storage and whether these facilities utilize impulse turbines like Peltons, or reaction turbines like Kaplan or Francis runners. In all these installations, high loads associated with the necessary torque transfer within the shaft connections (couplings) can be extremely high and therefore its bolting is highly critical. This means the coupling bolts are intricate and large, requiring high energy tools for manipulation. The bolts must also be closely fitted or interference fitted with their mating coupling bores in order to effect the proper torque transfer. Both the bolts and the mating bores require precision machining and extreme tolerances/surface finishes. Often the fitted bolts are machined to fit the bores or the bores are reworked to match the fitted bolts. Both methods are costly and time consuming because the dimensions must be measured accurately on site and then adapted. In the meantime, work progress is at a standstill. Furthermore fitted bolts never fit perfectly. There is always some minute sliding in the joint, which causes the fitted bolts to tilt inside the bores. Contact between the fitted bolt and the bore is limited to four points. Because of the extreme pressure at these contact points, the flange is plastically deformed which can lead to failures and make removal of the fitted bolts during maintenance impossible. When shaft line connections require disassembly because of equipment maintenance or necessary repair, the bolting must be manipulated, and all too often this process is problematic, dangerous, and time consuming. Even in operation, shaft line bolting is problematic and can be the source of unwanted equipment torsional vibrations, micro-movements, and even failures. This is especially true for pumped storage plants with ternary sets involving a single shaft line, where the generator is typically arranged with both a pump and the turbine. This kind of shaft line arrangement is a highly dynamic system. Large masses are concentrated in the pump runner, generator rotor and turbine runner, while the shaft sections connecting these masses act as torsional springs. This dynamic system is susceptible to torsional vibrations which are excited by the hydrodynamic and electrodynamic mechanisms of power transmission within pump, turbine and generator. Under unfavorable conditions of operation, torsional vibrations may occur which can damage the bolted connections of the shaft line. A new innovation in coupling bolting, the multi-jackbolt tensioner (MJT) type expansion bolt with split expanding sleeves, such as manufactured by Superbolt, (a division of the Nord-Lock Group) can be considered to improve not only the dynamic behavior of bolted shaft lines, but maintenance efficiency and equipment reliability. These expansion bolts are positive locking bolting elements which significantly enhance the rigidity of the bolted connections. Because of the increased rigidity, the load amplitudes on the shaft line are reduced. The expansion bolts also completely fill the bores through which they are inserted which increases the effective cross section available for power transmission in comparison to conventional bolts. Both effects - reduced loads acting on increased cross sections - combine to significantly lower the stresses and prolong the service life of the entire shaft line. New Innovations to Solve Difficult Shaft Coupling Bolting Problems Introduction 4
Since these expansion bolts utilize multi-jackbolt tensioners for manipulation, tightening requirements are low and only hand held tools are necessary. This means worker safety is greatly enhanced and the installation and removal of MJT fitted expansion bolts can be completed quicker and more accurately. Another alternative to consider for these challenging bolting applications is the hydraulic expansion bolt. These have also lessened some of the typical hassles of standard bolts. However, they still require precision machining and elaborate operating equipment. Also, pre-load (clamp load from the bolting) accuracy is questionable due to losses in load transfer and stretching. Perhaps most importantly, worker safety is highly compromised as dangerously high pressures are required to operate the hydraulic workings of the bolts. In order to evaluate the above assertions, this paper will detail a recent case study on the use of multijackbolt tensioner (MJT) type expansion bolts at a pumped storage facility in an attempt to solve on-going shaft line fractures. Also, a Finite Element Method (FEM) study to compare stresses on a shaft coupling utilizing MJT expansion bolts vs. one with standard close fitted bolts was performed. Because MJT expansion bolts are able to generate a radial pre-load, a small gap forms on the outside of the split line of the coupling flanges. However, the gap lies within manufacturing tolerances and more importantly does not change during operation. A gap also appears in the split line when using close fitted bolts, but only under operation and not in the pre-tightened condition. This means under rotation the torsional load causes higher strains in the coupling. MJT type expansion bolts, on the other hand, keep the coupling more rigid. The case study, FEM evaluation, and specifics on the MJT type expansion bolts, such as features, benefits, and advantages will be further explained in this paper. However, a brief discussion on typical bolting elements found within couplings and their associated challenges is essential. New Innovations to Solve Difficult Shaft Coupling Bolting Problems Introduction 5
2 Coupling bolting elements There are two main methods for transmitting torque, 1) Friction Connection, and 2) Positive Connection. In friction couplings, large axial pre-loads are generated with the bolting elements. These press the coupling halves together to create friction forces in the contact plane (split line) which enables torque transfer (Fig. 2). Since the tangential friction forces are small in relation to the axial pre-loads, the transmittable torques also tends to be small. Consequently, friction couplings are often equipped with additional positive elements, such as keys, to carry peak loads. In positive couplings, the torque is transmitted by touching contact planes. The contact planes of the coupling halves touch directly or, more often, indirectly via the bolting elements. Usually, fitted bolts are used as bolting elements (Fig. 4) and sometimes, especially larger sizes, hydraulic expansion bolts (Fig. 5). In both cases, the torque is transmitted by contact pressure between the bolting elements and the bores of the coupling flange bolt holes. The bolting elements themselves carry the torque in shear. The flange couplings discussed here differ significantly in their load carrying mechanisms. differences are primarily due to the type of bolting elements used. The 2.1 Through bolts Friction couplings with through bolts are commonly used and tend to have the lowest initial cost (Fig. 1). The through bolts (1) are inserted through holes (bores) in the coupling halves and are pre-loaded axially with nuts (2) on both ends to create the friction connection in the contact plane. Fig. 1 - Through bolts installed Fig. 2 - Load carrying mechanism of through bolts (Fig. 2) shows the load carrying mechanism of the flange couplings with through bolts. The black arrows indicate the circumferential loads in the coupling halves resulting from the torque to be transmitted. The friction forces in the contact plane are depicted as white arrows. However, the friction connection with the through bolts is not a rigid connection. Peak torques or nonuniform load distributions between the through bolts (i.e. due to differences in bolt pre-load) can lead to local micro-movements in the contact plane or under the nuts. These micro-movements can damage the contact surfaces, leading to fretting or even failure of the bolting elements. Bolt fatigue life, especially on bolts with short clamp lengths, is greatly reduced due to bolt bending stresses. This reduced bolt reliability becomes a significant concern. Although the initial costs are low, the costs resulting from a failure can easily surpass the savings compared to more expensive bolting elements. Unfortunately, through bolts are typically destroyed during removal because they cannot be loosened due to common fretting. New Innovations to Solve Difficult Shaft Coupling Bolting Problems Coupling bolting elements 6
2.2 Fitted bolts Fitted bolts (Fig. 3) are similar to through bolts with the difference being that the fitted bolt fills the bore except for a few hundredths of a millimeter. As a result, the accuracy of the parts and surface quality must be much higher. The fitted bolts are commonly tailored individually for every bore (match marked); thus the fitted bolts are initially more expensive than through bolts. Fig. 3 Fitted bolts installed Fig. 4 Load carrying mechanism of fitted bolts Theoretically, the load carrying mechanism of fitted bolts is greatly superior to that of through bolts due to the positive connection. Practically, they are very problematic. Since radial backlash is necessary for the function of the fitted bolts, non-uniform load distributions are common. At first, only one fitted bolt will carry the load. Only after it has been sufficiently deformed to compensate the radial backlash of the next fitted bolt will it to start to carry the load, and so forth. Under dynamic loads, this process repeats itself over and over. Again, undesirable micro-movements occur in the connection, which may lead to further unwanted problems. Worse yet, due to the radial backlash, the fitted bolt can position itself at an angle inside the bore (Fig. 4). On the edges, the contact pressure can become so high that they deform plastically. The fitted bolts are then stuck and can only be removed by destructive methods. Large monetary losses may be incurred due to unplanned downtime to remove stuck fitted bolts. 2.3 Conventional Type Expansion Bolts Expansion bolts can be viewed as a further development of the fitted bolt. Simply put, expansion bolts are fitted bolts with a variable outer diameter (Fig. 5). A bolt with a tapered shank (2) is pulled into a tapered sleeve (3) by a nut (4). The tapered sleeve expands radially creating an interference fit inside the bore. A second nut (1) is used to axially pre-load the coupling. The resulting clamping force creates an additional friction connection in the contact plane. Fig. 5 Hydraulic Expansion bolts installed Fig. 6 Load carrying mechanism of hydraulic expansion bolts With expansion bolts, it is possible to compensate for some of the major drawbacks of fitted bolts. The variable outer diameter eliminates radial backlash during installation and the load distribution is nearly uniform during operation. Also, the contact pressure on the edges is almost completely eliminated. Still, New Innovations to Solve Difficult Shaft Coupling Bolting Problems Coupling bolting elements 7
the contact pressure in the bore is unnecessarily high, because the expansion bolts lift off the bore on one side (Fig. 6). Unfortunately, there still are demands for extreme tolerances and surface finishes with the conventional expansion bolt. Since they are more elaborate than fitted bolts, the initial costs are higher. Further, known expansion bolts, such as hydraulic types, require sophisticated and cumbersome operating equipment. The accuracy of the pre-load is questionable, because of the unknown losses experienced due to stretching and relaxation of the main stud. Again, worker safety is a serious concern due to the extremely high pressures associated with the hydraulic equipment. 2.3.1 MJT Type Expansion BoltsExternal Of all the typical bolting elements, the expansion bolt offers the best load carrying capability and appears to be the best solution from a technical standpoint. They offer a better load carrying capability and reduce costs at the same time. Cost reduction can be achieved by reducing down-time maintenance and lower initial costs vs. hydraulic systems. (Fig. 7) shows the assembly of a new design, multi-jackbolt tensioner (MJT) type expansion bolt with split expanding sleeves manufactured by Superbolt. Explained in more detail below, the MJT is a direct replacement to standard nuts/bolts and is used on the expansion bolt for easier manipulation. MJTs are further explained below. Similar to conventional expansion bolts, a tapered stud (1) is pulled into a tapered sleeve (2) by the right side MJT (4), thus expanding the tapered sleeve radially. The spacer (3) positions the tapered sleeve in the center of the contact plane. The left side MJT (4) is used to clamp the coupling halves together. Fig. 7 New MJT type expansion bolts installed Fig. 8 Load carrying mechanism of MJT expansion bolt In contrast to conventional expansion bolts, the tapered sleeve (Fig. 9) is split along its entire length. Compared to a closed tapered (or cylindrical) sleeve, it can expand farther. Thus, for manufacturing, wider tolerances can be allowed which helps save costs. More importantly, with the split tapered sleeve, much higher radial forces can be produced. The new design not only compensates for the radial backlash, but also affects a true radial pre-load. Thus, a radial load carrying capability matching axially pre-loaded bolting can be achieved. During operation, the split tapered sleeve will not lift off the bore as with conventional expansion bolts, but will remain in full contact along the entire surface. Thus, the contact pressures will also remain below the elastic limit. New Innovations to Solve Difficult Shaft Coupling Bolting Problems Coupling bolting elements 8
At the same time, the radial pre-load acts to effectively clamp both sides of the expansion bolt. The rigidity of the coupling is vastly increased and no damaging micro-movements can occur in either the contact plane or on the expansion bolt. The result is a superior load carrying capability combined with the highest possible reliability (Fig. 8). Another main benefit is easy removal. This is because the expansion bolts experiences only elastic deformation and return to their original state. Since no time-consuming, destructive loosening methods are necessary, and no new bolting elements are needed, great cost and time savings can be realized. Additionally they do not damage the holes of the coupling. Fig. 9 Split tapered sleeve New Innovations to Solve Difficult Shaft Coupling Bolting Problems Coupling bolting elements 9
3 Tightening methods Because the transmitted torques can be quite high in line shafting (especially in turbine/generator sets in power plants), large size couplings are required. In addition, the size of the bolting elements is large, which in-turn, presents a huge difficulty in getting them properly tightened with traditional bolting methods. There are numerous traditional bolt tightening methods currently being used. However, for simplicity, two common methods, torqueing and stretching, will be discussed. 3.1 Torqueing One of the major problems associated with traditional bolt tightening is as the diameter of the bolt increases, the amount of torque required to tighten it increases in the third power of the diameter. Because of this, the largest size bolt a person can typically tighten by hand is one inch. Therefore, installers have resorted to heavy duty devices such as pneumatic, hydraulic or electric torqueing tools. Others have used large and cumbersome torque multipliers. All of these are expensive, awkward, and difficult to handle. Because the nut is directly torqued, there is friction between the nut and bolt thread, friction in the nut to flange interface, thus affecting the pre-load accuracy. Direct torque methods also cause torsional stresses in the bolt which limits the level the bolt may be loaded. These reasons tend to cause designers to use higher strength bolt materials and/or larger size bolting, amounting to higher initial costs. 3.2 Stretching Stretching methods cause the bolt to elongate axially. Installed nuts keep the bolt in this tensioned state once the stretching mechanism is removed, and thus the bolted connection (joint) is pre-loaded. There are two main methods of stretching: Thermal Stretching is achieved by heating the bolts with heating elements. The bolt expands depending on the temperature. However, for larger bolts, high amounts of thermal energy are necessary. In some instances, the required temperature cannot be attained due to thermal energy losses. Sometimes the rate of heating is increased in order to compensate for the thermal losses. However, there is a real possibility that the bolt could be over heated, resulting in damage. There are numerous safety concerns with any of the commonly used thermal methods. However this method is not common in shaft connecting bolting. Hydraulic Stretching is achieved by elongating the bolt with a hydraulic torque tool or nut. Hydraulic tightening equipment is expensive and time consuming. Stretching by way of hydraulic torque tools still can cause thread galling. Further, pre-load accuracy is in question because there are losses in elastic energy during load transfer to pre-load the bolted joint. Also, in cases of instances where an intermediate solid piece is bridged in between the coupling halves, such as a spacer or turning gear, the sleeves can deform into the gap. Multi-Jackbolt Tensioning As mentioned above, the new expansion bolt design is equipped with mechanical Multi-Jackbolt Tensioners (MJT). MJTs (Fig. 10) provide an alternative to traditional bolt tightening methods. Rather than needing to tighten one large bolt, MJTs use several smaller bolts, called jackbolts, to drastically reduce the torque required to attain a certain pre-load. MJTs range in thread sizes from M16 to M1000 and can achieve up to 90 MN force and greater. MJTs only require hand-held tools, such as torque wrenches or air/electric impacts, for loading and unloading bolted joints. As shown here, a MJT consists of three main parts. A series of jackbolts are concentrically arranged in the nut body around the main thread. The jackbolts are supported by a hardened thrust plate. New Innovations to Solve Difficult Shaft Coupling Bolting Problems Tightening methods 10
For installation, the hardened thrust plate is placed over the bolt or stud, and the nut body is spun on by hand. The actual tensioning is achieved by tightening the small jackbolts. Each jackbolt produces a high compressive force, which amount to the desired pre-load. The hardened thrust plate protects the contact area from impressions by the jackbolts. Similar to stretching, the bolt or stud is only tensioned axially. The nut body is tensioned at the same time as the bolt, so the bolt cannot spring back. The bolt material can be utilized right up to its yield strength. This means that for transmitting the same torque, fewer, smaller bolting elements can be used. Manufacturing costs of the coupling as well as the initial costs for the bolting elements, are reduced substantially. Further advantages of MJTs are the high safety against loss of pre-load as well as the fast and simple installation and removal. Fig. 10 MJTs on standard bolted joint New Innovations to Solve Difficult Shaft Coupling Bolting Problems Tightening methods 11
4 Case study 4.1 Background In a pumped storage plant, fractures in a shaft line were discovered. The shaft line was extensively equipped with sensors to determine the cause of the fractures. A finite element (FE) simulation on the basis of the measured loads identified the weak point in the bolted connections. Nord-Lock AG suggested expansion bolts to remedy the weaknesses in the bolted connections. As the operator of the power plant was unfamiliar with these bolting elements, a second FE simulation was conducted to determine the effects and optimize the design. This FE simulation indicated significant improvements which convinced the operator to test the expansion bolts. With an agreed upon test duration of one year, the dynamic behavior of the installed expansion bolts would be monitored with the same sensor arrangement used previously. These follow-up measurements confirmed the predictions of the FE simulation. The success persuaded the operator to end the test phase after only three months and equip all four shaft lines with expansion bolts. With an agreed upon test duration of one year, the dynamic behavior of the installed expansion bolts would be monitored with the same sensor arrangement used previously. These follow-up measurements confirmed the predictions of the FE simulation. The success persuaded the operator to end the test phase after only three months and equip all four shaft lines with expansion bolts. Power Plant Details This particular power plant produces an average of 730 million KWh per year with four units with horizontal shaft lines. Each unit consists of two 30 MW Pelton turbines, a 75 MVA generator and a pump. Both Pelton runners are installed on the same side of the generator. Since the runners are identical and interchangeable with the other units, the runner to shaft connection is of a special design. The outer flange of the shaft and the runners are toothed so that the runner on the inner flange can pass through the outer flange (Error! Reference source not found., Error! Reference source not found. & Fig. 15). Issues Several fractures were found in the hub of the Pelton runners and the shaft flanges. The fractures initiated along the edges of the toothed shaft couplings. The fractures in the Pelton runner formed at the locations where the outer diameter of the shaft tooth touches the runner hub. The cracks propagated in tangential direction. Measurements The shaft line and runners were equipped with sensors to monitor torque and strains at the critical locations (Error! Reference source not found.). The start-up procedure and stationary operation at ominal load was investigated. New Innovations to Solve Difficult Shaft Coupling Bolting Problems Case study 12
Fig. 11 - Sensor arrangement (original design); courtesy of Sensoplan GmbH Fig. 12 - Torque vibrations during start- up (original design); courtesy of Sensoplan GmbH The measurements during start-up revealed that the Pelton runner passed through a torque resonance near 200 rpm causing torque amplitudes corresponding to 170% of the nominal torque of 520 knm (0). At nominal speed, normal vibration levels were found. FE simulation of the original design A FE simulation of the toothed runner to shaft connection (0) revealed that the maximum stress occurred at the same location where the cracks were found. When the measured torque resonance amplitude was applied to the model, a maximum stress amplitude of 237 MPa was determined (0). Fig. 13 - FE model; courtesy of Sensoplan GmbH In addition to the large cyclic stress, the analysis also showed that relative micromovements of several hundreths of a millimeter occurred at the edge of the coupling tooth (0). At the same location, the contact pressure resulting from the bolt pre-load reached values of up to 870 MPa. Under the simultaneous action of sliding and high contact pressure, the fatigue limit of alloy steels may be reduced to 20-30% of its normal value. Thus, for the runner material with a tensile strength of 800 Mpa, the fretting fatigue limit can be as low as 80-110 MPa. New Innovations to Solve Difficult Shaft Coupling Bolting Problems Case study 13
Fig. 14 - FE results (left: stresses, right: relative micro-movement); courtesy of Sensoplan GmbH 4.1.1 Initial Findings The measurement showed for each start-up 36 stress cycles above 110 MPa and 56 stress cycles above 80 MPa. Based on previous operating data, the average number of start-ups per year is 1600. This results in 58,000 to 90,000 stress cycles per year above the fretting fatigue limit. During a five year maintenance period, significant fractures can occur. Measurements and FE simulation coincide with the occurrence of the fractures in the runners. Essentially, the original design of the bolted connection is not durable. Further investigations revealed that there were no simple solutions, and an extensive and costly re-design of the bolted connection was necessary. At this point Nord-Lock AG was invited by the operator to install and test their pre-loaded expansion bolts. These replaced the existing fitted bolts. Neither shaft flanges nor runners were modified. FE Simulation of Enhanced Design A FE simulation was conducted to check the effectiveness of the pre-loaded expansion bolt and determine the pre-load level. The radial pre-load was set to a mean contact pressure of 60 MPa on the bore. The axial clamping force was 2 500 kn. With these pre-loads, the bolted connection was substantially stiffened. At the resonance torque of 170% of nominal torque, the maximum stress amplitude was reduced by 42% in comparison with the original design. The micro-movements were completely eliminated. With no sliding under high contact pressure, the full fatigue limit of the runner material at 315 MPa could be used for evaluating the durability. The maximum stress amplitude remained under 50% of the fatigue limit. Thus, the bolted connection is predicted to be durable with a safety factor above 2. 4.1.2 Test phase The company has established a quality policy to communicate their overall philosophy concerning the role of quality and its importance in their business process, products and services. It is communicated to employee and displayed within the facility, to ensure that everybody understands his role towards achieving quality objectives, product quality and customer satisfaction. The encouraging results of the FE simulation convinced the operator to install a set of six pre-loaded expansions bolts per runner on one shaft line. The torque vibrations were to be monitored during full operation for one year. Installation New Innovations to Solve Difficult Shaft Coupling Bolting Problems Case study 14
The expansion bolts were installed by the operator s maintenance personnel with the assistance of Nord- Lock AG (Fig. 15). The radial expansion was controlled by measuring the axial movement of the conical bolt relative to the conical sleeve. The axial clamping force of 2 500 kn was exerted by torque controlled tightening of the jackbolts on the multi jackbolt tensioners. The installation time was well inside the projected time frame for the original fitted bolts. Fig. 15 - Installation of pre-loaded expansion bolts (left: outer toothed flange, right: inner solid flange) Measurements The measurements showed the same torque vibration pattern with the same critical resonance speeds during start-up. However, the torque amplitude was substantially reduced to 118% nominal torque in comparison to 170% for the original design (Fig. 16). In addition to the greatly improved stresses determined by the FE simulation at the same load level, the pre-loaded expansion bolts also reduced the load level by more than 30%. In terms of durability, this corresponds to an extension of fatigue life by factors. As there was no measureable change in the torque vibrations under continued operation, the operator decided to equip all four shaft lines with pre-loaded expansion bolts after only three months. New Innovations to Solve Difficult Shaft Coupling Bolting Problems Case study 15
Fig. 16 - Torque vibrations during start-up (enhanced design); courtesy of Sensoplan AG 4.1.3 Final results Through the use of pre-loaded expansion bolts, the detrimental torque vibrations during start-up could be reduced to an uncritical level without modifying runners or shaft lines. Further, the micro-movements in the joint were completely eliminated. The runner to shaft connection is now durable. The first installation was in 2001. With approximately a two year interval, all four shaft lines have been equipped with pre-loaded expansion bolts with full interchangeability of the runners. Since then, no cracks were discovered and no unscheduled shutdowns attributed to the runner to shaft connections occurred. All of the original pre-loaded expansion bolts are still in use. Inspections when exchanging the runners revealed no signs of wear. Only the protective coating against corrosion has been renewed. Otherwise, no maintenance on the pre-loaded expansion bolts was required. FE simulation, measurements on the shaft line and a combined service life of more than 15 years on this application confirm, that pre-loaded expansion bolts are durable and an efficient means of upgrading troublesome bolted connections. 4.2 FEM Study 4.2.1 Test background Due to their advantages, the new MJT type expansion bolts have quickly generated a lot of interest. Because fitted bolts are often used, it is interesting to see how the strain on the flange with these bolts installed compares with a flange utilizing MJT expansion bolts. Example: Application: rigid coupling on a line shaft Performance: 10 MW Torque: 1123 KNm No. of Bolts: 10 per coupling Size of Bolts: ø65 x 195 clamping length As previously mentioned, the load carrying ability of both kinds of bolts is fundamentally different: The close tolerance fitted bolt, such as a hydraulic bolt with zero gap, does not affect a true radial pre-load. Therefore, the circumferential forces act as reaction forces. Every load change results in a pulsating New Innovations to Solve Difficult Shaft Coupling Bolting Problems Case study 16
stress with full amplitude from zero to the maximum onto the contact surface between sleeve and bore of the coupling hole (Fig. 7). The MJT expansion bolt system operates differently because they provide a radial pre-load. Further, the radial pre-load acts along the entire outer circumference of the split sleeve, hence also in circumferential direction (Fig. 9). As with a pre-loaded bolted connection, only that part of the work load exceeding the pre-load acts as alternating load with small amplitude at load changes. This effect is shown below. The close tolerance fitted shear bolt in (Fig. 17) experiences the circumferential force as full pulsating load. (Fig. 18) shows the radially pre-loaded MJT expansion bolt, which simply experiences the added load. Fig. 17 Stress amplitudes of close tolerance fitted shear bolt Fig. 18 Stress amplitudes of pretensioned expansion bolt To verify this, a recent pilot study was performed on a cylindrical model using FEM. The radial pre-load of the MJT expansion bolt in reference to the axial tightening travel between zero and five millimeters is determined. This results in a maximum radial expansion of 0.125mm (with stud/sleeve taper cone of 1:20). For this study, an axial tightening travel of 2mm (0.05mm radial expansion) was chosen. The axial tightening travel of zero compares to the close tolerance fitted shear bolt. The obtained values were then applied to the shaft coupling (Fig. 19). This permits a relatively simple way to obtain the load distribution on the flange and the level of the reached stresses (Fig. 20). Fig. 19 FEM grid structure for shear bolt and expansion bolt Fig. 20 Maximum values of combined stresses according to Von Mises for 0 to 5mm tightening travel New Innovations to Solve Difficult Shaft Coupling Bolting Problems Case study 17
The numerical calculations were carried out without considering the friction on the contact surfaces. Due to this loss of friction, the pre-load is reduced by 10%. For the shear bolts, the friction ratio is dependent on the fitting accuracy. For simplicity and for better comparison, the same value is being used for the axial pre-load. The values are based on realistic conditions of the chosen example: Table 1 Comparison shear bolt to expansion prior to load Table 2 Comparison shear bolt & expansion bolt under load New Innovations to Solve Difficult Shaft Coupling Bolting Problems Case study 18
4.2.2 Results The pre-loaded MJT expansion bolts create an all-over frictional connection and form closure on the shaft coupling. With an axial travel of two millimeters, this form closure is also maintained under torsional load when in operation. However, a small gap of 0.003 mm forms on the outside of the split line because of the radial pre-load. This gap is within allowable tolerance. Even more important is that this gap does not change under operation. Hence the shaft coupling is tightened safely against loss of pre-load. Also, with close tolerance shear bolts, a gap of similar size forms in the split line, but only when in operation and not already in the pre-loaded state. Furthermore, higher strains occur in the shaft coupling under torsional load. Thus, the MJT expansion bolts prove to be better tensioning elements. This is shown graphically in the following figures. Particularly notice (Fig. 24), which gives a good comparison for each bolted element corresponding stress levels within the flange coupling bolt holes, for both the preloaded state and under operation. Fig. 21 Expansion bolt, axial displacement under pre-load (radial and axial) Fig. 22 Expansion bolt, axial displacement under pre-load (radial and axial) and torsion Fig. 23 - Shear bolt, axial displacement under pre-load (only axial) and torsion The calculated results fully confirm the difference in load carrying ability. Under load, the shear bolt experiences a pulsating stress of over 200 N/mm 2 on the contact surface of sleeve to flange, whereas the stress oscillation for the pre-load of MJT expansion bolts is only 50 N/mm 2. The radially pre-loaded MJT expansion bolt can therefore be justified as a great advancement in the field of bolted connections. Thanks to the pre-load, the load is uniformly distributed, and susceptibility to maintenance and risk of breakdowns are therefore reduced. The clearly defined flow of forces enables the design engineer to do a stress analysis of all the involved components without major calculation effort. New Innovations to Solve Difficult Shaft Coupling Bolting Problems Case study 19
It is worth mentioning, that the above-described comparison refers to hydraulic shear bolts with the radial clearance adjusted to zero. Common fitted bolts, which cannot be easily installed without clearance, actually end up being strained considerably more due to clearance. Edge pressure occurs at the stud ends which leads to superimposed bending moments. Local deformations on such bolts are not uncommon and the expenditure for maintenance therefore is relatively high. Fig. 24 Stress patterns within coupling bolt hole for both expansion bolt and fitted bolt under loads 4.3 Conclusions MJT type expansion bolts have been shown by proven use and testing to be superior to other bolting elements, such as fitted bolts. This enables connections, such as couplings, to be of much higher quality. The dynamic behavior of bolt shaft lines is greatly improved. MJT type expansion bolts are purely mechanical. They facilitate quick and easy assembly / disassembly, and provide substantial time and cost savings. They also increase productivity and long term maintainability for both the hydro OEM and plant/utility owner. New Innovations to Solve Difficult Shaft Coupling Bolting Problems Case study 20
5 References ALLEMANN H., & SCHNEIDER, N. Improving the dynamic behavior of bolted shaft lines with expansion bolts, 2008 KÜTTEL, M. Stress Report on a Shaft Coupling, SITEC Institute for Systems and Safety, Engineering Department of the Rapperswil College of Technology (HSR), Rapperswil, Switzerland, on Behalf of P&S Tensioning Systems Ltd., 2003 MANN, C. Follow-up measurement with modified Pelton turbine coupling, September 2002 NN, Testing, FEA and failure analysis of a Pelton turbine coupling, October 2000 NN, FEA of Pelton turbine coupling new design with Superbolt joint, Rev. 1, February 2001 PLOKE, G. Enhancing the Bolting Quality on Couplings of High Power Throughput with Expansion Bolts, VDI-Bericht Nr. 1786, Düsseldorf, Germany, 2003 SCHNEIDER, N. Expansion Bolts A Well-known Bolting Element Re-discovered, HYDRO Congress, Kiris, Antalya, Turkey, 2002 STEINBOCK, R. The Multi-Jackbolt Tensioning System. Handbook of Bolts and Bolted Joints. Ed. John H. Bickford and Sayed Nassar. New York: Marcel Dekker, Inc., 1998, 507-533. New Innovations to Solve Difficult Shaft Coupling Bolting Problems References 21
Nord-Lock Group Tel: +46 40 602 46 00 Email: info@nord-lock.com www.nord-lock.com Copyright 2016 Nord-Lock Group. NORD-LOCK, SUPERBOLT and BOLTIGHT are trademarks owned by different companies within the Nord-Lock Group. All other logos, trademarks, registered trademarks or service marks used herein are the property of the Nord-Lock Group. All rights reserved. New Innovations to Solve Difficult Shaft Coupling Bolting Problems References 22