Conventional methods to apply prestress such as sledgehammers, overhead cranes, or stud heaters, can be inaccurate, ineffective, and time-consuming, even hazardous. And improper preloads can trigger fatigue fractures of press components.
Multijackbolt tensioners from Superbolt Inc., Carnegie, Pa. (superbolt.com), get around the problem. They replace a conventional hex nut with a threaded collar surrounded by small, threaded jackbolts. The heattreated jackbolts tighten against a hardened washer to distribute the aggregate load. Mechanical advantage scales with jackbolt size and count. MJTs need only ordinary hand torque wrenches to properly prestress any size press column.
A 3,250-ton Mesta forging press provides an example. Columns on the press are preloaded to 2,625,000 lb, which for a single hex nut, requires a torque = 695,756 lb-ft. The MJT jackbolts, in contrast, need just 275 lb-ft torque to reach the same preload. They let a single worker install a press in about 6 hr, a job that normally takes several workers more than two days. Unlike conventional nuts, the MJTs won't gall the main column thread during removal. Loosening the jackbolts relieves preload and lets the MJT easily spin off.