Power
Engineering / August 1997 / Pages 46-50
However, certain types of bolting systems do manage to stay tight without requiring close watch. Multi-jackbolt tensioners (MJT) have emerged as an effective deterrent against many loosening tendencies, keeping bolted joints safe and secure in the severest of equipment service environments. Performance preloading In any bolting system, the aim is to create an accurate and sustainable clamping load (preload) in the bolt that is higher than the working load. As long as the preload remains greater than the working load acting on it, the bolted joint will not fail. On the other hand, too much preload during tightening can damage the joint members and fastener. Depending on the application, preload should be one and a half to four times higher than the highest possible working load.
Normally, the largest bolt that can be tightened properly by hand is about 1 in. in diameter. In the past, larger bolts have required certain types of torque-amplifying equipment. There are several difficult, expensive and potentially hazardous methods of bolt tightening; the stud heater, for example, is used to apply heat to the bolt - using a hole drilled through the bolt's center - after which the nut is tightened. Proper tightening in this case depends on shrinkage, which occurs as the bolt cools - an extremely time-consuming procedure. More common techniques include using a sledge hammer or hooking up a wrench to an overhead crane. The hydraulic wrench is tried but untrue- while providing consistent torque, its resultant bolt tension has a 30 to 40 percent error rate. Also, these kinds of tools are heavy, awkward to place and move, and require expensive power units and cumbersome hoses. They also require torque reaction levers, which can lose their grip and make the tools jump off of the units, resulting in potentially unsafe operator conditions. And on large units, the tool's socket sometimes breaks into several pieces under load, resulting in heavy shrapnel flying long distances. The ideal solution is an easy-to-use bolting system that doesn't promote galling, relies on ordinary hand tools to easily generate the enormous clamping forces in the joint and bolt, while maintaining those forces through a wide range of working conditions. Just as important is the ease of disassembling a bolting system. The MJT is a relatively simple concept that provides a solution to the aforementioned problems. Conventional hexagonal nuts are replaced with a circular nut containing a series of hardened jackbolts. A tensioner is threaded onto a new or existing bolt, stud, threaded rod or shaft; this main thread positions the tensioner on the bolt (or stud) against a hardened washer and load-bearing surface. Once positioned, actual bolt tensioning requires only a simple hand tool to torque the jackbolts that encircle the main thread, creating the potential to obtain hundreds of thousands of pounds of preload. The MJT's most important property is its extremely high mechanical advantage; load capacities range from 10,000 to 20,000,000 lb. In steam turbines and chemical reactors, MJT's are capable of providing uninterrupted service for years. Introduced in 1984, the system now includes 40 different variations and is available in thread sizes ranging from 3/4 to 32 in. The secret of MJT's can be found in the jackbolts and tensioner design. Since high preload is directly proportional to the size and number of jackbolts used, turning the small-diameter jackbolts is accomplished with relatively little torque, while a strong thrust force is generated against the washer. An equally strong reaction force occurs in the main bolt head. Both forces result in a large clamping force on the flange. Because jackbolts are used in compression and heat-treated to as high as 300,000 psi compressive yield strength, a large number of the devices are not needed to achieve the desired clamping load. Figure 2 shows the tremendous mechanical advantage of MJT's over hexagonal nuts, particularly when large sizes are considered. Since bolt sizes on steam turbines are large, many power plant users are installing MJT's in order to take advantage of the ease of installation and removal of such tensioning systems. Material reminders Maintaining adequate clamping forces is just as important as establishing them. In high-temperature systems, only certain materials are able to sustain the forces generated by a tensioning system. Figure 3 (not included here) compares softness ratings of several metals at elevated temperatures. Some carbon steels lose their strength above 500º F, others at more than 800º F. Stainless steels are resistant to 1,000º F, while nickel superalloys are minimally affected until 1,300º F. In general, the lowest-cost, high temperature bolting material is ASTM A193-B7, which can be used up to 650º F. As temperatures increase, materials used in bolting systems become increasingly exotic and, unfortunately, increasingly costly. For example, at 1,000º F, Inconel 718 is about 20 times stronger than ASTM A193-B7; it is also about 20 times more expensive on a per pound basis. Inconel 718 is used in high-temperature MJT's extensively for jackbolts and thrust washers, which represent the critical part of the nut type MJT's. Elasticity extension Bolts and studs, when tightened, actually stretch and have elasticity - the property that keeps bolted assemblies together and assures leak-free seals in high-temperature joints. Elasticity also enables a material to return to its original form or condition after an applied force is removed. Thus, increasing bolting elasticity can compensate for flange and joint instability caused by temperature changes, changes in internal pressure and joint movement. Long, highly stressed bolts can provide high levels of elasticity, while short bolts require disc springs under the nut or bolt head to obtain sufficient elasticity. One alternative, albeit a somewhat clumsy one, is to replace the stubby bolt with a longer one and add spacers to take up the slack. A better option is to use a MJT system, which provides the elasticity needed to preserve bolted-joint integrity. Radial flexing of the nut reduces stress concentrations that would otherwise cause threads to break in the bolt or stud. Relieving such stress concentrations is especially important when the bolt is subject to alternating forces. Axial flexing is important in maintaining preload, induced when the bolted connection is tightened. An elastic system can maintain sufficient preload when length changes occur in the bolting system due to thermal expansion or settling of gaskets in their joints. Because of their elastic properties MJT's have helped seal many notoriously leaky joints. The expansion difference among various metals can reduce or destroy a bolting system's integrity. If a bolt has a larger heat-expansion coefficient than the flange it holds together, the joint can loosen completely at high temperatures. Conversely, if a bolt has a smaller heat-expansion coefficient than its flange, the bolt will be overloaded at high temperatures and permanently stretch. Then, during the next cooling cycle, the joint will leak and has to be re-tightened. Several such cycles fatigue and destroy the bolt. To fix bolting problems caused by differential expansion is difficult. Only a systems approach involving the housing, stud and tensioner can solve high-temperature bolting problems. High-temperature bolting systems, made from superalloys and intended for the hostile environments of inner turbine housings, are strong, highly elastic and corrosion-resistant alternatives to conventional bolting system problems. |
