Cost Justification and Reliability Benefits of Multi-Jackbolt Tensioners by Allan Steinbock, Vice President Superbolt, Inc., Carnegie, PA Copyright 2005 by Superbolt, Inc, Carnegie, PA. All rights reserved. This document may not be reproduced in any form without permission from the copyright owner.
Cost Justification and Reliability Benefits of Multi-Jackbolt Tensioners 2 INTRODUCTION: Multi-Jackbolt Tensioners are bolt tightening devices that are widely used on critical equipment in most heavy industries. Although the concept is relatively simple, these tensioners have many benefits that are not commonly known. They have proven to be economical in terms of cost and in terms of equipment reliability, which will be the focus of this paper. The MJT system is composed of a round nut body with an internal thread identical to a standard fastener. In between the thread and the outside diameter is a series of drilled and tapped holes designed to accept hardened jackbolts that pass through the entire nut body. A hardened washer is always used under the jackbolts to protect the bearing area of the equipment (Fig. 2). DESCRIPTION OF SYSTEM: Multi-Jackbolt Tensioners (MJT s) come in many variations including nut style, thrust collars (unthreaded) or bolt style devices. Due to the wide variety of components available, the MJT system can be retrofitted into the same area as a standard OEM nut or bolt. This paper focuses on the nut style tensioner (Fig. 1) as this is the most commonly used. However, most of the benefits are applicable to the other designs. Fig. 2: Cutaway line drawing of typical nut style Multi- Jackbolt tensioner Fig. 1: Nut style Multi-Jackbolt Tensioner To apply the system, the hardened washer is placed over the existing stud, bolt, rod or shaft to be tightened. The nut body is then threaded onto the main thread of the standard fastener hand tight against the washer. The tightening torque is applied to the individual jackbolts with a standard hand held torque wrench or air tool. Turning the jackbolts creates a thrusting of the nut body away from the washer surface,
3 Cost Justification and Reliability Benefits of Multi-Jackbolt Tensioners Tensioners as an alternative bolting method is new to some people, cost justification and reliability issues come to the forefront when compared to alternative or existing methods. Although many of the equipment reliability issues tie into cost justification we will attempt to discuss them separately and prove them with actual case histories. COST JUSTIFICATION: Fig. 3: Cutaway line drawing of MJT installed on stud or bolt. creating bolt tension and imparting a stretch on the main thread (Fig 3 & 4). MJT s stay in place and remain on the equipment until removal for the next outage. An equivalent torque on a standard fastener can be achieved with a fraction of the torque input. For example, to pre-stress a 4-8tpi bolt to 45,000 psi (520,650 lb. of preload) you would need to torque a standard nut to 30,650 ft lb. With a standard MT series MJT, the same prestress can be achieved with only 190 ft lb on each jackbolt. Figures 5 & 6 show torque values that indicate the dramatic mechanical advantage of MJT s compared to standard hex nut torque values. Since applying Multi-Jackbolt INITIAL PURCHASE VS. ALTERNATIVES On new equipment and retrofits, Multi- Jackbolt Tensioners are generally more expensive than standard nuts/bolts. However, in many cases, the existing nuts and bolts are of special design or Fig. 4: 3D model of MJTs installed on a joint. Easy-turn jackbolts push nut body up MJT spun on hand tight Hardened washer protects joint surface Tremendous clamping force is created on joint Existing bolt/stud tightened in pure tension
Cost Justification and Reliability Benefits of Multi-Jackbolt Tensioners 4 materials. In larger size ranges, MJT s can be equal to or less than original nuts/ bolts. For example, on a recent job for a large coupling, (18) 6-13/16 tensioners were required. The initial cost of the tensioners was equivalent in cost to the machined standard nuts. However, in the comparison of MJT s to alternative tightening methods is where most cost justifications occur. For example, hydraulic wrenching is a commonly used method of torquing large diameter nut/bolts. The initial purchase price of hydraulic wrenches can be tens of thousands of dollars and there are costs of accessories such as adapters, hoses, hydraulic power units and special sockets. Depending on the manufacturer, in addition to that there can be heavy maintenance costs and reliability issues. For example, a recent job for a heat Fig. 5: Torque curve comparing hex nuts & MJT s. exchanger had (14) 3-1/4 A193-B7 studs to be tensioned using MJT s. The cost for these Multi-Jackbolt Tensioners was $5,628.00. This compares to a purchase price of around $17,000 for a dedicated hydraulic wrench. In some instances where there are hundreds of bolts to tighten, hydraulic wrenches may have less of a cost impact ignoring other factors to be discussed later. Another alternative method is hydraulic tensioning. Hydraulic tensioners are used as a tool to stretch a bolt but are removed once bolt tensioning has been accomplished. Hydraulic tensioners require longer studs that sit above the standard nut. If not set up for these, new studs must be purchased to utilize the system. The unit cost of hydraulic tensioners is generally much more expensive than the total cost of Multi-Jackbolt Tensioners. Accessories must be purchased, such as hoses, fittings and power units. The tensioner itself, however, can be moved from stud to stud if time is not a factor. Multiple hydraulic tensioners can be ganged together but there can be heavy initial cost depending on the number of units used. The tensioners rely on seals that can be prone to failure. Bolt heaters are another alternative bolting method. Depending on the design, these units can be very expensive to purchase. They Fig. 6: Hex nut torque and MJT torque comparison chart.
5 Cost Justification and Reliability Benefits of Multi-Jackbolt Tensioners are very slow, have limits in tensioning power and accuracy, and, if used carelessly, can be hazardous to handle. SAFETY Aside from the moral implications of providing for worker safety, organizations must be aware of the tremendous costs due to injuries on the job. Because Multijackbolt Tensioners require only small hand or air tools, they are arguably the safest bolting method for tightening large diameter bolts/studs. The sledgehammer is still probably the most dominant tool when attempting to bolt up a piece of equipment. The brute force method is prone to hand, arm, leg, face, and back injuries. For example, the piston rod to crosshead jamnut connection on a reciprocating compressor is such that tightening methods as described above cannot be utilized. The large nut is accessed through an inspection door and can be turned using a large, oversize wrench hooked to an overhead crane (Fig. 7), floor jack or most commonly, it is struck with a sledgehammer. In one real world example, a worker was using this method on a 2-1/2 piston rod when the wrench dislodged from the nut. The wrench then swung up, striking the worker in the face. The worker was put on disability for the good part of a year. Subsequently, (36) Multi-Jackbolt Tensioners, similar to Figure 8, were installed with no further injuries. Hydraulic wrenches also have safety Fig. 7: Example of using an overhead crane on a piston rod to crosshead jamnut connection. concerns, as they are high-energy tools. Specific instances of injuries are hand and arm injuries due to sockets exploding or reaction bars pinching or rotating under high pressure. Hydraulic pressure in these units is usually 10,000 psi or higher and instances have occurred where a hose has let go and injected a worker with poisonous hydraulic fluid causing severe injury. Figure 9 shows a photo of sockets that blew apart on a turbine deck, narrowly missing the personnel working in the area. Additionally, large wrenches can also be very heavy and they must be moved into place to function. This heavy lifting has contributed to back and other injuries. Fig. 8: Multi-Jackbolt Tensioner installed on a crosshead jamnut.
Cost Justification and Reliability Benefits of Multi-Jackbolt Tensioners 6 Bolt heating has high-energy voltage requirements and also is a source of injuries mainly related to burns. There have been instances where the heaters have ignited flammable liquids nearby, causing major fires. This is especially a concern where hydrocarbons are involved. Fig. 10: Example of tighter stud spacing that is possible when using MJT s. Fig. 9: Damaged sockets from a hydraulic wrench. SPACE RESTRICTIONS With the wide range of materials available and the flexible nature of their design, MJT s have been used in many spacerestricted areas. Special MJT s have been designed for applications where severe costs would have occurred if the traditional methods were employed. For example, several cases have occurred where an end user received a pressure vessel or heat exchanger where it was made incorrectly and the flange bolt circle was too tight to use a socket on the existing nuts. In a recent case, a refinery had rigging crews on site to install a new heat exchanger. The maintenance crew had to make a decision whether to call in an on-site machining crew or come up with an alternative. Due to the small O.D. requirements on the nuts, turbine style MJT s with a 1.5x thread diameter O.D. were fabricated and on-site in two days. This saved the company tens of thousands of dollars in losses, as the rigging crews were able to keep working, not counting the savings in on-site machine costs. Some OEM s are looking to decrease the overall size of their machinery housings because the stud to stud spacing can be tighter with MJT s (Fig. 10). In some cases, fasteners are located inside or around a piece of equipment that is very difficult to access, especially with the large wrenches that are most commonly used. For instance, some of the frame bolting on reciprocating compressors is located inside the inspection door of the doghouse. The ease of using a hand torque wrench makes reaching these areas easy (Fig. 11).
7 Cost Justification and Reliability Benefits of Multi-Jackbolt Tensioners whole piston and rod onto a truck and shipped it to a facility to untorque and retorque the nuts. By utilizing the Multi- Jackbolt concept, they were able to do the job on a workbench, saving shipping cost and valuable downtime (Fig. 12b). Fig. 11: Superbolt MJT s installed on a reciprocating compressor doghouse. EASE OF USE Since only hand tools are required and since MJT s tighten studs in pure tension, there are cost savings vs. other methods. For example, the piston end nut on reciprocating compressors generally requires clamping of the piston rod to torque and untorque the nut if work on the piston is required (Fig. 12a). This can severely damage the rod surface, which must slide through packing material that could also be damaged. In many cases, the nut has to be machined away to get the piston off. In one extreme case, a liquid gas company had a remote plant site and every time they did piston work they loaded the Another time saver is the ability to adjust the bolt prestress in a short time. For instance, extensiometers are commonly used to check bolt tension. In the instance where stud heating is the method, one must apply heat, let the unit cool, check the stretch and if the value is off, the whole process must be repeated. With Multi-Jackbolt Tensioners one can dial in a new torque setting and bring up the value in a matter of minutes. As an example, it took one turbine manufacturer three days to bolt up a unit for hydro testing with stud heaters and checking with extensiometers. This was reduced to one day with MJT s and extensiometers. Large preloads are easily achievable with hand tools only and this is often a solution to big problems. For example, a large petrochemical producer was able to apply high loads to the anchorbolts on reciprocating compressors and in Fig. 12a: Example of jig to hold rod while torquing piston end nut. Note this is a small unit. Fig. 12b: Example of larger piston end nut for gas compressor.
Cost Justification and Reliability Benefits of Multi-Jackbolt Tensioners 8 this case was able to stop movement of the compressor avoiding the need to regrout them. According to the end user, the estimated cost savings were in the hundreds of thousands of dollars. Many bolting applications in the petrochemical industry wind up having the fasteners upside down on part of the equipment. It is very difficult work to lift heavy tools overhead and sometimes the work even has to be done while laying on ones back. Figure 13 shows a preheater being tightened with MJT s with two men. Small, light air tools were used except for the final pass which requires a light hand torque wrench. The job was performed in a tenth of the time and the workers were very happy to avoid the discomfort of using the old, heavy tools that were previously required. TIME SAVINGS A very common comment on MultiJackbolt Tensioners is that it must take a long time to torque up all of the Fig. 13: Multi-Jackbolt tensioners make bolting in awkward locations an easier task. FIG. 14 Fig. 14: Multi-Jackbolt tensioners installed on multiple heat exchangers. jackbolts, but, in fact, MJT s have reduced installation times compared to other bolting methods. As mentioned in a previous example, adjustments to the preload can be made quickly by dialing in a new torque value and torquing the jackbolts to the new value. The use of air tools also greatly speeds up the tightening process. Although special air tools can be purchased to apply a very accurate torque, it has been found that with proper guidelines, standard air impact tools can do the job without the higher expense. The standard procedure calls for the use of air tools to speed up the rounds of tightening and taking a final pass with a calibrated hand held torque wrench. Figure 14 shows a bank of heat exchangers in a refinery. The job required (96) 3-1/4 studs to be tensioned to 45,000 psi bolt stress. This job previously took 3 days to perform using the previous method of hydraulic wrenches. Using the MultiJackbolt system, the only special tooling required were low-cost in-line regulators combined with standard air impact tools. All
9 Cost Justification and Reliability Benefits of Multi-Jackbolt Tensioners In a drastic example, for instance on a turbine casing, as many workers as can fit next to each other can be working on the unit. Fig. 15: Multiple workers can be used to further speed up installation. four exchangers were bolted up in a total of 8 hours using two workers. On some flanges with concealed gaskets the speed of the system can be increased by selecting four or eight tensioners 180º apart to bring metal to metal contact of the flanges. The remaining tensioners therefore do not require additional passes just to create gasket crush. However, it should be noted that manufacturer procedures should not be superseded without consultation. Another major time savings can be accomplished by using multiple workers to tighten several MJT s simultaneously. Since only hand tools are used, the tooling required is easily obtainable and economical, especially in emergency situations. Figure 15 shows a job where (18) 6-3/16 tensioners were tensioned using four workers 90º apart from each other in a very restricted space. Extensiometers were used to check stud stretch and, including doing calculations and taking breaks, the entire procedure was performed in 2-1/2 hours. The previous method using a combination of fabricated components and hydraulic rams was estimated to have taken 150-200 man hours. Other time factors that can be encountered are not that obvious at first glance. For instance, stud seizure into a blind hole such as in a turbine housing is a fairly common occurrence. It can sometimes take several shifts or even days to remove frozen studs that often have to be drilled or machined out on-site. Since MJT s tighten a stud in pure tension, there is no galling or ripping of thread surfaces which can occur when applying a high torque to achieve stud tension. Therefore, once the jackbolts are unloaded in a MJT, the nut body can be removed leaving the stud as if it had been hand tight. The stud is then easily removed with nominal torque applied. In some instances time is saved because the preparation, such as with the piston rod example used earlier, is so time consuming (i.e. clamping the rod in a special jig and rigging up special tools to apply a large turning force. Additionally, valuable crane time is not wasted on time consuming bolting.). RELIABILITY As some of the examples have indicated, the reliability of various equipment can be improved with the proper utilization of Multi-Jackbolt Tensioners. First and foremost is the concept of preload versus working load, which is a widely misunderstood subject. In most bolted joints which have a through bolt (or stud), clamping two members together such as
Cost Justification and Reliability Benefits of Multi-Jackbolt Tensioners 10 a flange connection, the preload (or clamping force) must exceed the working load (or separating force) for the joint to have integrity (See below). GENERAL BACKGROUND OF PRELOAD: Preload: although a fully detailed study into bolting principals is above the scope of this paper, some major points need to be discussed. The most important point is the concept of preload vs. working load on a bolted joint. Following is a commonly used example of how preload works: 150 lb 100 lb A) A fish scale is loaded with a weight of 150 lbs. The spring represents a bolt tightened to a tensile preload of 150 lbs. B) A block is forced into the indicated position and the weight is removed. C) A 100 lb. weight is applied to the system. The assembly now behaves like a bolt preloaded to 150 lbs and an external load of 100 lbs. Adding the weight does not increase the tension in the spring (which represents the bolt). If the tension were greater than 150 lbs, the scale would read more than 150 lbs and the block would fall out. Preload: Preload and working load are not additive as often believed. If a bolt is preloaded to 100,000 lbs, it induces an equivalent clamping load of 100,000 lbs on the bolted members. Unless the working load or separating force on the joint approaches the preload, the bolt does not feel additional load. In other words, if the separating force of the machine or working load is 50,000 lbs, the bolt will not be loaded to 150,000 lbs but will still only have the initial preload of 100,000 lbs. However, when the working load exceeds the preload, the joint separates and the full working load is carried by the bolt. It is this situation where the working load cycles against the heads of the bolts or on the faces of the nuts. This is when loosening and bolt breakage occurs.
11 Cost Justification and Reliability Benefits of Multi-Jackbolt Tensioners IMPORTANCE OF PROPER PRELOAD High preloads are required in so many applications that it would be difficult to name them all, but a good example is the pressure vessel. With the institution of ASME Sect 8 Div 3 pressure vessel code, extremely high pressures (over 10,000 psi system pressure) are allowed for reactors in various processes. This also means that the size of the reactors and the size of the studs that seal them have dramatically increased in size. For instance, pressure vessels with 8 to 10 diameter studs are becoming more common. 50,000 psi stress in a 10 stud requires a preload of 3,700,000 lbs. It is almost impossible to torque a hex nut to this load. There are very few methods that can achieve high stud stress in large diameter studs without the time, safety, and economic consequences described earlier. MJT s can be used in all pressure vessel codes and conform to ASME and ASTM design and material requirements. VIBRATION ISSUES If proper preload is applied above the working load, the fasteners will not vibrate loose. Many Band-Aid methods have been tried to stop fasteners from vibrating loose on vibrating and rotating equipment, such as double nutting, welding a fastener in place, locking adhesives, locking with cotter pins, and many more. This is fine when the fastener does not carry much load, but the trouble with this approach is that the main bolt/stud can still be going through full loading cycles. Even if the nuts on the ends do not come loose, stud/ bolt breakage can occur due to the fatigue cycles felt by the main tension member. A good example of this was on 4 reciprocating compressor rod piston end nuts being used in a large pumping station. The maintenance procedure was to use a chisel and a hammer to turn against the slots on the top of the nut. The piston nut and the rod were then drilled and tapped and fitted with a locking set screw. The customer experienced loosening of the locking screw, which would dislodge and run in the compressor until the rattle was noticed. Several rod failures also occurred due to the fatigue exposure when the nut backed off. After discussing the situation it was explained that the customer was only applying roughly 500 ft lb to the nut. This translated to only 8,500 lb of preload, which was significantly under the separating force of the rod load. To achieve the necessary rod stress of 25,000 psi (289,250 lb), a torque of 17,000 ft lb would need to be applied to the standard nut. Using the MJT system, the user was able to achieve the required rod stress with only 175 ft lb on each jackbolt. Due to past practices the end user wanted to install the locking set screw but was later convinced that the preload would hold the system tight. Fifty MJT style Piston end nuts of various sizes have been retrofitted with this user over a seven year period with no loosening of the pistons or nuts. Many bolted joints in reciprocating machinery are subject to fatigue failures. The constant reversal of stresses from tension to compression weakens the metal structures, which leads to cracks and eventually breakage. The time to failure depends on the metal used and its treatment, the design of the structure (stress risers), the size of stress reversals, and the number of cycles. The amount of stress reversals
Cost Justification and Reliability Benefits of Multi-Jackbolt Tensioners 12 is more important than the level of stress at which the reversals occur. For example, fatigue occurs more often if the reversals are between 0 and 30,000 psi than if they are between 40,000 and 45,000 psi. In most joints, the amount of stress reversal can be substantially reduced, if not eliminated, by preloading the joint to a level where the bolt does not see the load change. This is shown in Fig 16. Fig. 16: Chart showing joint fatigue life. TIGHTENING STUDS IN PURE TENSION MJT s tighten studs/bolts in pure tension created by the axial thrusting of the jackbolts on the nut body. As mentioned, this prevents thread galling picking up of material on the surfaces of the threads that can eliminate the need to drill/ machine out frozen studs. Aside from this, tightening in tension eliminates galling of the spot faces or machined faces of the equipment where a nut usually slides under load. It also eliminates situations such as where a hex nut is not machined perfectly perpendicular to the main thread, which can cause the nut to dig in to the bearing surface. This can add a large friction that will affect tension output due to torque. Up to 5º stud misalignment can be compensated on applications such as anchorbolts, which can be slightly off line. Tightening in tension can also be helpful when it is critical not to have a turning moment due to tightening a standard nut. There have been instances where great time and expense have been spent laser aligning a machine only to throw it out when banging on the standard nuts that anchor the machine down. Also, there are applications such as on the ends of shafts where it is difficult to hold the shaft steady while torquing the nut on the end. Tightening in pure tension eliminates these kind of scenarios. Tightening in tension also eliminates unwanted torsion in the bolt or stud that can create crack starters, weakening the fatigue capacity of the bolt. Fatigue breakage causes unwanted and expensive downtime not to mention the cost of expensive studs. EVEN TENSION AROUND THE FLANGE MJT s are very repeatable with regard to bolt load (+-10% error or 5% with calibration). Commonly available bolting manuals have stated that standard nuts/ bolts have a 30-40% error in preload even with a very accurate torque wrench (+- 3%). Bolting organizations have been
13 Cost Justification and Reliability Benefits of Multi-Jackbolt Tensioners investigating how many variables affect preload. There are dozens that mainly influence the friction factor, such as surface condition, materials, and lubricants... etc. The scope of this subject is beyond this paper but more information is available if desired. (Some studies have shown flange bolts from 0-200% tight due to multiplication of the error from multiple passes with a wrench.) Other methods also are prone to error and loss of preload when applying shrinkage methods such as thermal tightening and hydraulic tensioning. Even tension from bolt to bolt is especially important when sealing up a gasketed flange or pressure vessel joint. This helps eliminate leakage problems associated with one end of a flange having low load comparatively by creating even pressure around the flange interface. ELASTICITY It is desirable for most bolting systems to have as much elasticity as possible. This is also true for high temperature bolting. Due to creep, joints become loose after prolonged service and start to leak. Repeated cooling and reheating of the system often aggravates this. Tests show that turbine-type MJT s will add the approximate equivalent of four stud diameters to the elasticity of bolting systems; this is an increase of 50-100% in elasticity for an average bolting system. Since creep continues at a uniform rate at a given temperature, this extra elasticity can significantly prolong the service life of high temperature joints. ELASTICITY OF MJT S IN GASKETED JOINTS In conventional gasketed joints, a gasket is placed between the raised faces of two flanges and then clamped by a number of bolts. The clamping forces have to be sufficient to resist the blowout of the gasket by the pressure acting on the flange. The gasket is more or less compressed by the clamping forces and will stabilize at a certain thickness. If conditions in the joint remained stable, the joint would probably last forever. In an actual working joint, however, conditions are not stable. The gasket can be attacked by its environment and shrink or expand. Internal pressures can fluctuate, changing load conditions on the gasket and thus fatiguing it. Heat can act on the gasket in several ways. It can change the chemical state of the gasket, it can change the gasket s elasticity and strength, and it can cause the load on the gasket to fluctuate due to thermal expansions and contractions of the various joint components. If during the operation of a gasketed joint the gasket gets thinner for any of these reasons, the joint may lose its integrity, and the gasket may blow out from the pressure acting on the joint. Whether a gasketed joint loses its integrity or not depends partly on the gasket material, but the bolting system employed is just as important to the integrity of the joint. For example, a gasket may be 0.0625 inches thick when new, but after tightening, it may be compressed to 0.050 inches. While the gasket is being compressed by
Cost Justification and Reliability Benefits of Multi-Jackbolt Tensioners 14 0.0125 in., the bolts will be stretched. The elongation of the bolt depends on its original length and the stress in the bolt after tightening. Example 1: If the grip length of a bolt is 4 in and the bolt is tightened to a stress of 45,000 psi as shown in Fig. 17, it will stretch 0.006 in. Now assume that the gasket only shrinks 0.003 in. The remaining elongation in the bolt will only be 0.003, which means that the stress in the bolt will be only 22,500 psi. The joint has lost 50% of its tension. What we have in this example is a stubby rigid bolting system. Example 2: If a bolt is three times as long as the one in Example 1, it will be 12 long initially. The elongation = 3 x 0.006 (which is 0.018) at 45,000 psi stress as shown in Fig 18. If the gasket now shrinks the same 0.003 as in Example 1, the remaining elongation in the bolt is still 0.015. The stress in the bolt is still 37,500 psi, and the joint has lost only 16.7% of its original tension. This is an example of a relatively elastic bolting system. Tests have shown that nut-type MJT s can add elasticity to an average bolting system. Standard nut-type MJT s will add as much as 4 to 12 diameter equivalents to the elasticity of bolting systems. If one considers that the average clamping length of a bolt is around four diameters, it can be concluded that nut-type MJT s will triple or quadruple the elasticity of the average bolting system. It is rather easy to establish the relative elasticity in an MJT bolting system. The procedure is illustrated in Fig 19. STRESS RELIEF THROUGH MJT s The interaction of the jackbolt force and the opposing force of the main bolt creates a bending moment. Since the bending moment is created in a circular manner, the resulting stress in nut-type MJT s is a hoop stress. The hoop stress causes an increase of the tensioner diameter at the bottom and a decrease of the diameter at the top as shown in Figure 20. Fig. 17: Application example 1. Fig. 18: Application example 2 - a more elastic system.
15 Cost Justification and Reliability Benefits of Multi-Jackbolt Tensioners It is well known that most bolts fail at the bottom of the nut in the first two or three threads because of the stress concentrations that occur in that region of a rigid nut. Tests have shown that the flex action of a nut-type MJT can sufficiently Fig. 19: The elasticity of an MJT can be established by measuring the bolt elongation after loading. This indicates the actual tension in the bolt. Measuring the gap between the nut body and the hardened washer measures the elasticity of the bolt plus the nut body. There is more elasticity in the pintle of the jackbolts. The elasticity in the pintle cannot be measured, but it can be calculated using Hooke s Law. relieve the concentration of stresses to increase the static bearing capacity of high strength bolts by as much as 15%. (Low strength, ductile type bolts are not much affected by the stress-relieving action of MJT s). MJT s also increase the fatigue resistance of dynamically stressed bolts, but it is difficult to put a numerical value to that. The amount of stress relief depends primarily on the detail design of the MJT. Basically it can be stated that the higher the hoop stress in the MJT, the more stress relief one can expect. Most MJT s are designed to have hoop stress equal to twothirds to three-fourths of the main bolt stress. This ensures that the bolt will always fail before the MJT does; it also establishes hoop stresses large enough to substantially relieve stress concentrations in the critical bolt thread areas. The stress relieving action of nut-type MJT s is simulated in a flexible flange nut as shown in Fig 21. The flanged nut (or Flexnut) bears on the outside of the flange, thus creating the desired bending (a) (b) Fig. 20: Schematic of stress-relieving action by torquing a nut-type MJT. (a) Before torquing; (b) after torquing. Flexing as shown is highly exaggerated.
Cost Justification and Reliability Benefits of Multi-Jackbolt Tensioners 16 moment. Flexnuts are used extensively in combination with bolt-type MJT s. However, they are not very suitable for replacing standard hex nuts because they require excessive torque if tightened directly. Flexnuts are designed to flex out at the bottom and flex in toward the top. This distributes the bolt along more threads, adds elasticity, and prevents stress concentrations in the first few threads. Fig. 21: Schematic of stress-relieving action of flanged nuts in combination with bolt-type MJT s. CONCLUSION Multi-jackbolt tensioners offer the benefits of properly loading studs and bolts with the use of small hand and air tools. Cost justification is accomplished on many levels; the most significant of which is related to reducing expensive downtime. The speed of applying the system along with the reliability and fatigue resistance gets equipment on line faster and keeps it together once it is installed. About the author: Allan Steinbock is Vice President of Superbolt, Inc. and has been with the company since 1984. He has a Bachelor of Science Degree in Mechanical Engineering from the University of Pittsburgh and is a member of SME and other industry specific organizations. Allan has consulted on thousands of bolting projects and has designed products for a broad range of industries. In most cases, the system is more economical than other bolting methods to purchase and it provides a means for personnel to do their job safely. Reliability is enhanced with the added elasticity and the uniform preload capabilities. Safety, economic and environmental concerns created by leakage problems are eliminated.