As part of the U.S. contribution for the CERN Hadron Super Collider, Superbolt® Inc. was chosen to supply more than 1,600 high-strength mechanical tensioners, expansion bolts and Torquebolts®. The mechanical tensioners, or multi-jackbolt tensioners (MJTs), were used to assemble the end cap disks, which were comprised of numerous steel plates (see Figure 1). To be able to resist the large force of the 4 Tesla magnetic field, the bolts needed to be very strong and clamped at a very high preload. A preload of 560,000 lb (2.5 mN) on M72-6 bolts was required with an accuracy of ±5 percent. Space was also extremely limited, which made tightening the bolts with traditional methods nearly impossible (see Figure 2). Superbolt bolt-type MJTs proved to be the best possible solution for this very sensitive and difficult application.
The MJTs were able to clamp the plates despite the space restrictions because they only require hand tools for tightening and loosening. Traditional methods often require relatively easy access to the bolt, as well as suitable anchor points to brace the torquing equipment; however, hand tools are more likely to reach into limited-space areas. In comparison, MJTs use a number of relatively small-diameter jackbolts to generate clamping force on the joint. The MJTs were also strong enough to prevent the steel from deflecting, while achieving the high clamping force required with little effort from the workers. The M72 bolts used 14 jackbolts torqued to 130 ft-lb each to achieve the 560,000 lb preload. In comparison, a standard hex would require 23,375 ft-lb torque to achieve the same preload.
The bolts were tested in Japan and at the University of Wisconsin, and met or exceeded all CMS requirements. They were made from a heat-treated, high-grade alloy steel, with threads rolled on a high-capacity thread rolling machine. Because no additional heat treatment was required, thread dimensions were extremely accurate. Superbolt® also pre-strained each bolt to 110 percent of the target preload value to work-harden the bolt and ensure each bolt could meet the preload requirement.
To apply this system (see Figure 3), a hardened washer is placed onto the bolt, which is then threaded into the tapped hole. A number of jackbolts are threaded through the bolt head, and each jackbolt is tightened to a pre-determined torque value using a hand-held torque wrench or air tool. This pushes the bolt head away from the washer surface, tightening the joint in pure tension. Because the torque is divided among a number of jackbolts, the preload is very accurate and consistent, which was critical for this application.