Multi-jackbolt tensioner

Multi-jackbolt tensioners (MJTs), registered under the trademark Superbolt or Supernut, are designed to decrease the torque required to tighten large bolted joints. One of the major problems associated with traditional bolt tightening methods is as the diameter of the bolt increases, the amount of torque required to tighten it increases in the third power of the diameter.[1] Because of this, the largest size bolt a person can typically tighten by hand is 1 in (25 mm).[2] Multi-jackbolt tensioners reduce the amount of torque required to clamp the joint by using multiple jackbolts threaded through the nut or bolt head.[3] The jackbolts, which are small enough to be tightened by simple hand tools, thrust against a hardened washer and generate clamping force on the joint. Loads up to 20 million pounds (9.1×10^⁶ kg) and greater are attainable using only hand tools. MJTs and related products are produced by Superbolt, Inc. in Carnegie, PA.

MJTs have a high mechanical advantage for tightening large nuts. To calculate mechanical advantage over standard hex nuts, the following formula is used:[1]

${\displaystyle MA={\frac {T}{T_{j}}}={\frac {nDA}{d}}}$

Where:

MA	= overall mechanical advantage
T	= main thread torque
Tj	= jackbolt torque
n	= number of jackbolts
D	= main thread pitch diameter
d	= jackbolt pitch diameter
A	= torque advantage of jackbolts over tension bolts (with graphite lube A = 1.37)

MJTs are a relatively safe bolting method because only small hand or air tools are required for tightening. Other methods, including thermal tightening, crane wrenching, hydraulic wrenches and sledgehammers, can cause serious worker injuries due to heat, electricity, highly pressurized hydraulic fluid and brute force. Back and other injuries can also be avoided because no heavy tightening equipment needs to be lifted onto the bolt. Safety is also increased because MJTs prevent parts from becoming loose.[4]

Due to dynamic loads, bolted joints have a tendency to loosen. Downtime is significantly reduced because MJTs are able to easily achieve the large clamping forces required to prevent bolted components from vibrating loose. Therefore, less time is spent tightening joints repeatedly and fixing problems caused by loose joints.[4] MJTs also reduce the likelihood the bolted joint will fail, which reduces downtime caused by needing to replace damaged components.

Even though more individual jackbolts must be tightened, installation is still fast in comparison to other methods. Because the torque required to tighten each small jackbolt is exponentially less than the torque required to tighten one large nut, it takes less time and effort overall to tighten the larger number of jackbolts. Also, no additional time is spent moving around heavy equipment to tighten each nut. Installation processes have been developed using air tools, which also greatly reduces installation time. Additionally, multiple workers can work on a joint simultaneously since the tooling required is inexpensive. Another common method, thermal tightening, can take a long time because the stud must be heated, and clamping force is not obtained and cannot be checked until it is cooled since it depends on shrinkage. If the stud is not at proper stretch, the heating process must be repeated.

MJTs are very versatile tools. They can be used in tight spaces because no bulky equipment is required to tighten the bolts. MJTs can also be engineered to specifications for almost any application. There are few size limitations, and they can be used with extremely large thread sizes (as high as 32 in (810 mm) thread diameter).[1] They can also be used effectively in high temperature applications, given proper materials are used. Overall, the unique properties of MJTs created by the use of jackbolts can simplify complicated applications, especially for large thread sizes.[2]

Thread galling and seizing is significantly reduced because mating bolts or studs are loaded in pure tension without twisting.[4]

In many applications, elasticity in a bolt significantly helps to keep a joint held together.[2] Tests have shown turbine-type MJTs increase the elasticity of the average bolting system by the equivalent of four stud diameters, and nut-type MJTs increase the elasticity by 4 to 12 diameter equivalents, which would triple or quadruple the elasticity of the bolting system.[1] In other words, MJTs increase elasticity by adding up to 2 to 4 equivalent bolt effective lengths. This is particularly advantageous in high-temperature bolting applications where bolts are subject to creep, because elastic bolting systems will take longer to reach a given relaxation stress than in a rigid system.[1] Elastic bolting systems can also improve the integrity of gasketed joints by compensating for temperature changes, joint movement and changes in internal pressure.[6]

Stress relief—Most bolts fail at the bottom of the nut in the first two or three threads. Hoop stresses on the nut in MJTs cause an increase in diameter at the bottom and decrease at the top, distributing thread stress more evenly and reducing the likelihood the stud or bolt will fail.[1]

MJTs may not be cost effective for bolts smaller than 1 in (25 mm) diameter, low-stress applications, and in plants where the number of studs/bolts to retrofit is difficult to justify versus prior investment in existing tightening equipment.