Published in May 2006 American Longwall Magazine

Designers have been looking for ways to simplify bolted joints practically since the first machine was built. Because of the high working loads that large machinery generate, bolting becomes especially problematic.

To see why, let us take a closer look at the bolted joint. The clamping force on a joint is equivalent to the bolt load (preload) generated by your chosen bolt-tightening method. The preload on the bolts must be above the working load (separating force) in order for the joint to stay tight.

A major obstacle to achieving this is that a bolt fastener’s strength increases with the square of its diameter but the torque required for pre-stressing increases in the third power. Because of this reality, bolts larger than 1-inch in diameter cannot be torqued effectively with hand tools such as wrenches, torque wrenches and air impact wrenches.

There are several methods available to apply clamping force on the joint and overcome this problem. Some are better than others, depending on the specific challenges presented by each application.

Worker safety, speed of installation and removal, pre-load accuracy and durability are key factors to consider when choosing the best method for your equipment.

A successful bolting system is defined by its ability to connect parts cost efficiently and generate proper clamping force in any environment.

For typical longwall mining applications, common bolting obstacles include high vibration and high working loads in tight working conditions. Patented Multi-Jackbolt Tensioners (MJTs) have been successfully utilized on many of these type of applications, including cutter-head motors, conveyers, crushers, shakers, holding machine segments together and many more.

Since its introduction, more than 30 variations of MJTs have been developed to accommodate a multitude of applications. The various configurations are available in thread sizes from 3/4in to 32in and in load capacities ranging from 5 tons to 10,000t.

This high total load tensioning can be achieved with a handheld wrench or air impact tool, permitting high-integrity joints even in locations with limited working clearances.

To install (see figure 1), a hardened washer is positioned over an existing stud, and the tensioner body is threaded onto the main thread of the stud, hand-tight, against the washer.

The MJT’s most important property is its high mechanical advantage where the clamping load is directly resultant from a relatively small torque input on the jackbolts. For example, the torque required to achieve 1,209,600lb of preload on a 6in hex nut is 106,868 lb-ft. The torque required on an MJT is 189 lb-ft, which means that MJT has a mechanical advantage of 565:1. The worker applies the appropriate torque to each jackbolt using a standard hand-held torque wrench or air tool.

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. MJTs stay in place and remain on equipment until removal for the next outage, assuming the joint was properly designed in the first place. The high preload (and subsequent clamping load) above the working load does the job of keeping the joint tight.

Recently, patented tapered jackbolt threads have reduced the friction factor by distributing the load more evenly over the threads, which also relieves stress concentrations on the first few threads. Patented rounded jackbolt tips reduce friction and leave a small marking of the washer, also resulting in reduced torque requirements and easier removal.

Because only hand tools are required, they are considered one of the safest bolting methods for tightening large diameter bolts and studs and have been recommended by insurance companies to their industrial accounts. Special MJTs have been designed for applications where traditional methods would not fit the prescribed space.