Energy-producing machinery can test a bolting system's endurance to the breaking point. Conventionally bolted joints often fail and loosen under the large and constant forces of the equipment's vibration, heat, pressure, and corrosion. The goal is to achieve an accurate and sustainable clamping load (preload) in the bolt that is higher than the working load, and to do so using a bolting method that is fast and safe for workers.
In the past, larger bolts have required certain types of torque-amplifying equipment. The stud heater, for example, has been 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. This is an extremely time-consuming procedure.
More common techniques include using a sledgehammer or hooking up a wrench to an overhead crane (Figure 1: Using a crane attached to an oversize wrench to loosen bolts can be hazardous). Besides being an inaccurate method of bolting, sledgehammers are also a common cause of worker injuries. Using an overhead crane is also unsafe for workers, and is inefficient as valuable crane time is being used. Hydraulic wrenches are also used with some success; however, these kinds of tools are heavy, awkward to place and move, and require expensive power units and cumbersome hoses. They are also time consuming and inaccurate.
Another method being used today is the multi-jackbolt tensioner (MJT). MJT's can be retrofitted into the same area as a standard OEM nut or bolt. They break down the torque requirement by threading several jackbolts through the main body of the nut (Figure 2: A cutaway view of MJT. The MJT design was guided with this basic bolting principle in mind: on a properly designed joint, if the preload remains greater than the working load acting on it, the bolted components should not fail). The MJT design allows the user to achieve high preloads with a small torque input. Only hand tools are required for installation or removal. Let's take a closer look at the bolting challenges facing large equipment, and how MJT's address them.
When compared with other bolting methods, MJTs are a safe alternative and have many benefits including reduced installation times (Figure 3: Workers tighten MJTs on the underside of a preheater with air tools in one-tenth the time it took using conventional tools). To install a MJT, you first place a hardened washer over the existing stud or bolt, then thread the nut body hand tight against the washer. Each jackbolt is torqued against the hardened washer using a hand held torque wrench or air impact. Turning the jackbolts pushes the nut body away from the washer surface and creates clamping force on the joint by stretching the bolt or stud. As a rule, the larger the main stud/bolt diameter, the greater the MJT's mechanical advantage (Figure 4 - Below: Table (a) and graph (b) depict torque values for standard bolts vs. MJTs, with ratio (mechanical advantage) between them.).
Worker safety concerns associated with other bolting methods are eliminated when using the MJT system. Besides the moral implications of providing worker safety, high costs associated with worker injury has been reason enough for some companies to switch to MJTs. Furthermore, because only hand tools are needed, expensive hydraulic tool breakage (and the resulting downtime) is also eliminated.
