It can be difficult to properly fasten bolts larger than 1 in. in diameter because strength increases with the square of the diameter while the torque needed to attach them increases with the third power. So these large bolts cannot be torqued with hand tools. But torquing them with slugging, crane, or hydraulic wrenches can be expensive, time consuming, and dangerous. Multi-jackbolt tensioners (MJTs) from Superbolt, Carnegie, Pa., offer an alternative.
MJTs can be installed using standard torque wrenches or air tools, and several technicians can work on the same MJT to speed assembly. The tensisoners won’t loosen when properly installed, despite vibrations, pulsations, and reciprocating loads. They can also be installed in cramped, awkward spaces, such as on top of heavy equipment. The MJTs apply predictable loads, which can be critical for sensitive application such as gasketed flanges. The elasticity of the MJTs lets them be used on joints subject to thermal and dynamic cycling. And because the bolt is held in pure tension, there is no thread galling or stud seizure.
An MJT consists of a round, toroidal nut body with threads on the inside that match the stud/bolt being tightened. A series of holes drilled and tapped for hardened jackbolts circle the top of the nut body and pass all the way through it, top to bottom. When installing an MJT, a hardened washer 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 tensioner body is screwed hand-tight onto the stud/bolt. Jackbolts are then tightened with a handheld torque wrench or air tool. This pushes the nut body away from the washer, generating tension on the bolt and stretching the main thread.