Multijackbolt tensioners (MJT) are heavy-duty bolt-tightening devices that engineers use on critical equipment, especially those with bolts exceeding an inch in diameter. And though they seem complex with several components, they are less expensive and more reliable than the alternatives.
THE ABCs OF MJTs
Basically, an MJT consists of a round, toroidal nut body with threads on the inside that match the fastener being tightened. A series of holes drilled and tapped for hardened jackbolts circle the top of the nut body and go all the way through it, top to bottom. The preassembled device replaces existing hex nuts and bolts.
When installing an MJT, a hardened washer first goes over the bolt, rod, shaft, or stud being tightened. It gives the jackbolts a hard surface to press into and protects the bearing surface of the equipment being worked on. The nut body is screwed hand tight onto the fastener. Jackbolts are then tightened with a hand held torque wrench or air tool. This pushes the nut body away from the washer, generating tension on the bolt and stretching the main thread.
An MJT and its multiple jackbolts, in effect, replace a single, much larger nut. As a result, each jackbolt needs much less torque to tighten than the original nut. For example, to prestress a 4-in., 8-tpi bolt to 45,000 psi (520,650 lb of preload), a standard nut would need 30,650 lb-ft of torque. Generating the same prestress on an MT Series MJT needs only 190 lb-ft on each jackbolt.
EASE OF USE
Many of the advantages of MJTs can be summed up by the term "ease of use." Though more jackbolts must be used, they can be installed and removed by hand tools. So MJTs don't require large, heavy wrenches or exotic tightening methods. This also means MJTs can be located almost anywhere and still be quickly and easily serviced. For example, taking off the end nuts on pistons in reciprocating compressors generally calls for clamping the piston rod to torque and untorque the nut. But this can damage the rod surface, which must slide through packing material that is also susceptible to damage. Oftentimes, the nut must be machined away to remove it. In an extreme case, a gas company had to load the entire piston and rod onto a truck to ship it elsewhere when the piston needed work. Switching to MJTs let them do all their piston work on site at a workbench, saving shipping, downtime, and costs for outside maintenance.
And although MJTs can have more parts than the oversized nuts and bolts they replace, they save time and are quickly adjusted to new prestress levels in as little as a couple of minutes.
A common alternative is to use an extensiometer to check bolt tension, use a bolt heater to change the prestress in the bolt, then recheck the bolt with the extensiometer. If the value is off, workers must warm up the bolt heater and do it again. In one case, a turbine manufacture spent three days setting up extensiometers and heaters to check and adjust a bolt. After installing MJTs, the task was reduced to a single day.
And where large bolts need large tools, the small hand tools used on MJTs let several workers tighten jackbolts on the same MJT at the same time.
Another, less-obvious way in which MJTs save time is that they avoid the problem of studs seizing when trying to remove an oversized bolt from a blind hole. Threads on a stud can gall or rip under the high torque used to attach the bolt. It can take several shifts or even days to drill or machine the frozen stud. MJTs tighten studs in pure tension, so the risk of thread damage is reduced. Once workers unload the jackbolts, the nut body and stud are easily removed by hand.
MJTs also exert large preloads on bolts, which generate an equivalent clamping force on the joint, using only hand tools. In one example, an oil company increased the loads on the anchor bolts holding a compressor in place. The higher loads stopped the compressor from vibrating and trying to move across the floor. This saved the company hundreds of thousands of dollars it would have spent on regrouting the equipment.
In the oil industry, many large bolts that can be replaced with MJTs are located underneath equipment. Servicing them often means workers must lay on their backs and lift tools to do the job. MJTs make the work much easier.
SAFER THAN THE ALTERNATIVES
Engineers and workers have developed several ways to handle the difficulties of large-diameter bolting. And when used correctly, some are effective. But they also bring problems of their own.
Sledgehammers are probably the most often used tool to bolt up large pieces of equipment. For example, the piston rod-to-crosshead jamnut connections in reciprocating compressors are often torqued with a large, oversized jack hooked to an overhead crane, or a floor jack that can reach inside an inspection door. But smaller, "slugging" wrenches are more commonly used, and workers smack them with a sledgehammer to get jamnuts on or off. Heavy hammers and the brute method are prone to cause hand, arm, leg, face, and back injuries. In one instance, the hammer knocked the wrench off a 2.5-in.-diameter piston rod, striking the worker in the face and putting him on disability for the better part of a year.
A common way of removing large bolts is with a hydraulic wrench. But these are high-energy tools, often using lines pressurized to 10,000 psi or higher. Hand and arm injuries are likely when sockets fail or reaction bars pinch or rotate under pressure. There are also cases in which a hydraulic hose let go and injects a worker with hydraulic fluid. And hydraulic wrenches themselves can be heavy enough to cause back injuries if mishandled.
Another method of tightening bolts is to use carefully applied heat. But this takes high voltage and high temperatures, both of which can be dangerous. There have been cases in which heaters ignited nearby liquid and caused major fires.
THE BOTTOM LINE: COST
MJTs are generally more expensive than standard hex nuts and bolts. If the nut and bolt must be specially designed or made of exotic materials, however, it can rapidly become more economical to go with MJTs. And in cases of larger bolts, or covered and acorn nuts, MJTs cost about the same or less. Most often, it is less equipment downtime, not purchase price that tips the economic scale in MJTs favor. And tooling costs can also be lower.
For example, hydraulic wrenches are often used to torque large-diameter nuts and bolts. But they can cost tens of thousand of dollars, more if you include accessories such as adapters, hoses, hydraulic power units, and special sockets. The wrenches can also be a high-maintenance item with reliability issues of their own.
For comparison, a heat exchanger equipped with MJTs needed fourteen 31⁄4-in. studs tensioned. The MJTs cost $5,630. A dedicated hydraulic wrench to handle the job would have cost $17,000. In situations in which there are hundreds of bolts to tighten, the cost of installation tools becomes less important.
Hydraulic tensioners that stretch bolts during installation but are then removed, can also be used instead of MJTs. But they need extra-long studs, which must be purchased and installed if not already in place. A single tensioner usually costs much more than an entire MJT assembly. And if a company has only one tensioner, it must be set up, used, than broken down and moved to the next bolt. Several tensioners can be ganged together, but this pushes the price up even higher. Tensioners also rely on seals that are prone to fail.
MJTs offer several advantages for bolting large-diameter hex nuts and studs. And depending on the application, they may be an ideal alternative to solving several bolting problems.