Cost Justification and Reliability Benefits of Multi ... - Nord-Lock

Cost Justification 

and Reliability Benefits 

of Multi-Jackbolt Tensioners 

by Allan Steinbock, Vice President 

Superbolt, Inc., Carnegie, PA 

Copyright © 2005 by Superbolt ® , Inc, Carnegie, PA. All rights reserved. This document may 

not be reproduced in any form without permission from the copyright owner.

Cost Justification and Reliability Benefits of Multi-Jackbolt Tensioners 

INTRODUCTION: 

Multi-Jackbolt Tensioners are bolt 

tightening devices that are widely used 

on critical equipment in most heavy 

industries. Although the concept is 

relatively simple, these tensioners have 

many benefits that are not commonly 

known. They have proven to be 

economical in terms of cost and in 

terms of equipment reliability, which 

will be the focus of this paper. 

DESCRIPTION OF SYSTEM: 

Multi-Jackbolt Tensioners (MJT’s) come 

in many variations including nut style, 

thrust collars (unthreaded) or bolt style 

devices. Due to the wide variety of 

components available, the MJT system 

can be retrofitted into the same area 

as a standard OEM nut or bolt. This 

paper focuses on the nut style tensioner 

(Fig. 1) as this is the most commonly 

used. However, most of the benefits are 

applicable to the other designs. 

Fig. 1: Nut style Multi-Jackbolt Tensioner 

The MJT system is composed of a round 

nut body with an internal thread identical 

to a standard fastener. In between the 

thread and the outside diameter is a series 

of drilled and tapped holes designed 

to accept hardened jackbolts that pass 

through the entire nut body. A hardened 

washer is always used under the jackbolts 

to protect the bearing area of the 

equipment (Fig. 2). 

Fig. 2: Cutaway line drawing of typical nut style Multi- 

Jackbolt tensioner 

To apply the system, the hardened washer 

is placed over the existing stud, bolt, rod 

or shaft to be tightened. The nut body is 

then threaded onto the main thread of the 

standard fastener hand tight against the 

washer. The tightening torque is applied 

to the individual jackbolts with a standard 

hand held torque wrench or air tool. 

Turning the jackbolts creates a thrusting of 

the nut body away from the washer surface, 

2

3 

Fig. 3: Cutaway line drawing of MJT installed on stud 

or bolt. 

creating bolt tension and imparting a 

stretch on the main thread (Fig 3 & 4). 

MJT’s stay in place and remain on 

the equipment until removal for the 

next outage. An equivalent torque on a 

standard fastener can be achieved with a 

fraction of the torque 

input. For example, to 

pre-stress a 4”-8tpi bolt to 

45,000 psi (520,650 lb. of 

preload) you would need 

to torque a standard nut 

to 30,650 ft•lb. With a 

standard MT series MJT, 

the same prestress can be 

achieved with only 190 ft•lb 

on each jackbolt. Figures 

5 & 6 show torque values 

that indicate the dramatic 

mechanical advantage of 

MJT’s compared to standard 

hex nut torque values. 

Since applying Multi-Jackbolt 


Tensioners as an alternative bolting method 

is new to some people, cost justification 

and reliability issues come to the forefront 

when compared to alternative or existing 

methods. Although many of the equipment 

reliability issues tie into cost justification 

we will attempt to discuss them separately 

and prove them with actual case histories. 

COST JUSTIFICATION: 

INITIAL PURCHASE VS. 

ALTERNATIVES 

On new equipment and retrofits, Multi- 

Jackbolt Tensioners are generally more 

expensive than standard nuts/bolts. 

However, in many cases, the existing 

nuts and bolts are of special design or 

Fig. 4: 3D model of MJTs installed on a joint. 

Easy-turn jackbolts push nut 

body “up” 

MJT spun on hand tight 

Hardened washer protects joint 

surface 

Tremendous clamping force is 

created on joint 

Existing bolt/stud tightened in 

pure tension


materials. In larger size ranges, MJT’s can 

be equal to or less than original nuts/ 

bolts. For example, on a recent job for a 

large coupling, (18) 6-13/16” tensioners 

were required. The initial cost of the 

tensioners was equivalent in cost to the 

machined standard nuts. However, in 

the comparison of MJT’s to alternative 

tightening methods is where most cost 

justifications occur. 

For example, hydraulic wrenching is a 

commonly used method of torquing 

large diameter nut/bolts. The initial 

purchase price of hydraulic wrenches can 

be tens of thousands of dollars and there 

are costs of accessories such as adapters, 

hoses, hydraulic power units and special 

sockets. Depending on the manufacturer, 

in addition to that there can be heavy 

maintenance costs and reliability issues. 

For example, a recent job for a heat 

Fig. 5: Torque curve comparing hex nuts & MJT’s. 

exchanger had (14) 3-1/4” A193-B7 studs 

to be tensioned using MJT’s. The cost 

for these Multi-Jackbolt Tensioners was 

$5,628.00. This compares to a purchase 

price of around $17,000 for a dedicated 

hydraulic wrench. In some instances where 

there are hundreds of bolts to tighten, 

hydraulic wrenches may have less of a 

cost impact ignoring other factors to be 

discussed later. 

Another alternative method is hydraulic 

tensioning. Hydraulic tensioners are 

used as a tool to stretch a bolt but are 

removed once bolt tensioning has been 

accomplished. Hydraulic tensioners require 

longer studs that sit above the standard 

nut. If not set up for these, new studs must 

be purchased to utilize the system. The unit 

cost of hydraulic tensioners is generally 

much more expensive than the total cost 

of Multi-Jackbolt Tensioners. Accessories 

must be purchased, such as hoses, fittings 

and power units. The tensioner itself, 

however, can be moved from stud to stud 

if time is not a factor. Multiple hydraulic 

tensioners can be ganged together but there 

can be heavy initial cost depending on the 

number of units used. The tensioners rely on 

seals that can be prone to failure. 

Bolt heaters are another alternative bolting 

method. Depending on the design, these 

units can be very expensive to purchase. They 

Fig. 6: Hex nut 

torque and 

MJT torque 

comparison 

chart. 

4

5 

are very slow, have limits in tensioning 

power and accuracy, and, if used carelessly, 

can be hazardous to handle. 

SAFETY 

Aside from the moral implications of 

providing for worker safety, organizations 

must be aware of the tremendous costs 

due to injuries on the job. Because Multijackbolt 

Tensioners require only small 

hand or air tools, they are arguably the 

safest bolting method for tightening large 

diameter bolts/studs. The sledgehammer 

is still probably the most dominant tool 

when attempting to bolt up a piece of 

equipment. The brute force method 

is prone to hand, arm, leg, face, and 

back injuries. For example, the piston 

rod to crosshead jamnut connection 

on a reciprocating compressor is such 

that tightening methods as described 

above cannot be utilized. The large nut 

is accessed through an inspection door 

and can be turned using a large, oversize 

wrench hooked to an overhead crane 

(Fig. 7), floor jack or most commonly, it 

is struck with a sledgehammer. 

In one real world example, a worker was 

using this method on a 2-1/2” piston 

rod when the wrench dislodged from the 

nut. The wrench then swung up, striking 

the worker in the face. The worker was 

put on disability for the good part of a 

year. Subsequently, (36) Multi-Jackbolt 

Tensioners, similar to Figure 8, were 

installed with no further injuries. 

Hydraulic wrenches also have safety 


Fig. 7: Example of using an overhead crane on a piston 

rod to crosshead jamnut connection. 

concerns, as they are high-energy tools. 

Specific instances of injuries are hand and 

arm injuries due to sockets exploding or 

reaction bars pinching or rotating under 

high pressure. Hydraulic pressure in these 

units is usually 10,000 psi or higher and 

instances have occurred where a hose 

has let go and injected a worker with 

poisonous hydraulic fluid causing severe 

injury. Figure 9 shows a photo of sockets 

that blew apart on a turbine deck, narrowly 

missing the personnel working in the area. 

Additionally, large wrenches can also be 

very heavy and they must be moved into 

place to function. This heavy lifting has 

contributed to back and other injuries. 

Fig. 8: Multi-Jackbolt Tensioner installed on a 

crosshead jamnut.


Bolt heating has high-energy voltage 

requirements and also is a source of 

injuries mainly related to burns. There 

have been instances where the heaters 

have ignited flammable liquids nearby, 

causing major fires. This is especially 

a concern where hydrocarbons are 

involved. 

Fig. 9: Damaged sockets from a hydraulic wrench. 

SPACE RESTRICTIONS 

With the wide range of materials available 

and the flexible nature of their design, 

MJT’s have been used in many spacerestricted 

areas. Special MJT’s have 

been designed for applications where 

severe costs would have occurred if the 

traditional methods were employed. For 

example, several cases have occurred 

where an end user received a pressure 

vessel or heat exchanger where it was 

made incorrectly and the flange bolt 

circle was too tight to use a socket on the 

existing nuts. In a recent case, a refinery 

had rigging crews on site to install a new 

heat exchanger. The maintenance crew 

had to make a decision whether to call 

Fig. 10: Example of tighter stud spacing that is possible 

when using MJT’s. 

in an on-site machining crew or come up 

with an alternative. Due to the small O.D. 

requirements on the nuts, ‘turbine’ style 

MJT’s with a 1.5x thread diameter O.D. 

were fabricated and on-site in two days. 

This saved the company tens of thousands 

of dollars in losses, as the rigging crews 

were able to keep working, not counting 

the savings in on-site machine costs. 

Some OEM’s are looking to decrease the 

overall size of their machinery housings 

because the stud to stud spacing can be 

tighter with MJT’s (Fig. 10). 

In some cases, fasteners are located inside 

or around a piece of equipment that is very 

difficult to access, especially with the large 

wrenches that are most commonly used. 

For instance, some of the frame bolting on 

reciprocating compressors is located inside 

the inspection door of the doghouse. The 

ease of using a hand torque wrench makes 

reaching these areas easy (Fig. 11). 

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7 

Fig. 11: Superbolt ® MJT’s installed on a reciprocating 

compressor doghouse. 

EASE OF USE 

Since only hand tools are required and 

since MJT’s tighten studs in pure tension, 

there are cost savings vs. other methods. 

For example, the piston end nut on 

reciprocating compressors generally 

requires clamping of the piston rod to 

torque and untorque the nut if work on 

the piston is required (Fig. 12a). This can 

severely damage the rod surface, which 

must slide through packing material that 

could also be damaged. In many cases, 

the nut has to be machined away to get the 

piston off. In one extreme case, a liquid gas 

company had a remote plant site and every 

time they did piston work they loaded the 

Fig. 12a: Example of jig to hold rod while torquing 

piston end nut. Note this is a small unit. 


whole piston and rod onto a truck and 

shipped it to a facility to untorque and 

retorque the nuts. By utilizing the Multi- 

Jackbolt concept, they were able to do the 

job on a workbench, saving shipping cost 

and valuable downtime (Fig. 12b). 

Another time saver is the ability to adjust 

the bolt prestress in a short time. For 

instance, extensiometers are commonly 

used to check bolt tension. In the instance 

where stud heating is the method, one 

must apply heat, let the unit cool, check 

the stretch and if the value is off, the 

whole process must be repeated. With 

Multi-Jackbolt Tensioners one can dial in a 

new torque setting and bring up the value in 

a matter of minutes. As an example, it took 

one turbine manufacturer three days to bolt 

up a unit for hydro testing with stud heaters 

and checking with extensiometers. This 

was reduced to one day with MJT’s and 

extensiometers. 

Large preloads are easily achievable 

with hand tools only and this is often a 

solution to big problems. For example, 

a large petrochemical producer was able 

to apply high loads to the anchorbolts 

on reciprocating compressors and in 

Fig. 12b: Example of larger piston end nut for gas 

compressor.


this case was able to stop movement of 

the compressor avoiding the need to 

regrout them. According to the end user, 

the estimated cost savings were in the 

hundreds of thousands of dollars. 

Many bolting applications in the 

petrochemical industry wind up having 

the fasteners upside down on part of the 

equipment. It is very difficult work to lift 

heavy tools overhead and sometimes the 

work even has to be done while laying on 

ones back. Figure 13 shows a preheater 

being tightened with MJT’s with two men. 

Small, light air tools were used except for 

the final pass which requires a light hand 

torque wrench. The job was performed 

in a tenth of the time and the workers 

were very happy to avoid the discomfort 

of using the old, heavy tools that were 

previously required. 

TIME SAVINGS 

A very common comment on Multi- 

Jackbolt Tensioners is that it must take 

a long time to torque up all of the 

Fig. 13: Multi-Jackbolt tensioners make bolting in 

awkward locations an easier task. 

FIG. 14 

Fig. 14: Multi-Jackbolt tensioners installed on multiple 

heat exchangers. 

jackbolts, but, in fact, MJT’s have reduced 

installation times compared to other 

bolting methods. As mentioned in a 

previous example, adjustments to the 

preload can be made quickly by dialing 

in a new torque value and torquing the 

jackbolts to the new value. 

The use of air tools also greatly speeds up 

the tightening process. Although special 

air tools can be purchased to apply a very 

accurate torque, it has been found that 

with proper guidelines, standard air impact 

tools can do the job without the higher 

expense. The standard procedure calls for 

the use of air tools to speed up the rounds 

of tightening and taking a final pass with a 

calibrated hand held torque wrench. 

Figure 14 shows a bank of heat 

exchangers in a refinery. The job required 

(96) 3-1/4” studs to be tensioned to 45,000 

psi bolt stress. This job previously took 3 

days to perform using the previous method 

of hydraulic wrenches. Using the Multi- 

Jackbolt system, the only special tooling 

required were low-cost in-line regulators 

combined with standard air impact tools. All 

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Fig. 15: Multiple workers can be used to further speed 

up installation. 

four exchangers were bolted up in a total of 

8 hours using two workers. 

On some flanges with concealed gaskets 

the speed of the system can be increased 

by selecting four or eight tensioners 180º 

apart to bring metal to metal contact of the 

flanges. The remaining tensioners therefore 

do not require additional passes just to 

create gasket crush. However, it should be 

noted that manufacturer procedures should 

not be superseded without consultation. 

Another major time savings can be 

accomplished by using multiple workers 

to tighten several MJT’s simultaneously. 

Since only hand tools are used, the tooling 

required is easily obtainable and economical, 

especially in emergency situations. Figure 15 

shows a job where (18) 6-3/16” tensioners 

were tensioned using four workers 90º apart 

from each other in a very restricted space. 

Extensiometers were used to check stud 

stretch and, including doing calculations 

and taking breaks, the entire procedure was 

performed in 2-1/2 hours. The previous 

method using a combination of fabricated 

components and hydraulic rams was 

estimated to have taken 150-200 man hours. 


In a drastic example, for instance on a turbine 

casing, as many workers as can fit next to 

each other can be working on the unit. 

Other time factors that can be encountered 

are not that obvious at first glance. For 

instance, stud seizure into a blind hole such 

as in a turbine housing is a fairly common 

occurrence. It can sometimes take several 

shifts or even days to remove frozen studs 

that often have to be drilled or machined 

out on-site. Since MJT’s tighten a stud in 

pure tension, there is no galling or ripping 

of thread surfaces which can occur when 

applying a high torque to achieve stud 

tension. Therefore, once the jackbolts are 

unloaded in a MJT, the nut body can be 

removed leaving the stud as if it had been 

hand tight. The stud is then easily removed 

with nominal torque applied. 

In some instances time is saved because 

the preparation, such as with the piston rod 

example used earlier, is so time consuming 

(i.e. clamping the rod in a special jig and 

rigging up special tools to apply a large 

turning force. Additionally, valuable crane 

time is not wasted on time consuming 

bolting.). 

RELIABILITY 

As some of the examples have indicated, 

the reliability of various equipment can 

be improved with the proper utilization 

of Multi-Jackbolt Tensioners. First and 

foremost is the concept of preload 

versus working load, which is a widely 

misunderstood subject. In most bolted 

joints which have a through bolt (or stud), 

clamping two members together such as


a flange connection, the preload (or clamping force) must exceed the working load (or 

separating force) for the joint to have integrity (See below). 

GENERAL BACKGROUND OF PRELOAD: 

Preload: although a fully detailed study into bolting principals is above the scope of this paper, 

some major points need to be discussed. The most important point is the concept of preload vs. 

working load on a bolted joint. Following is a commonly used example of how preload works: 

150 lb 

A) A fish scale is loaded 

with a weight of 150 lbs. 

The spring represents a 

bolt tightened to a tensile 

preload of 150 lbs. 

Preload: Preload and working load are 

not additive as often believed. If a bolt 

is preloaded to 100,000 lbs, it induces an 

equivalent clamping load of 100,000 lbs on 

the bolted members. Unless the working load 

or separating force on the joint approaches 

the preload, the bolt does not feel additional 

load. In other words, if the separating force 

of the machine or working load is 50,000 lbs, 

B) A block is forced into the 

indicated position and the 

weight is removed. 

100 lb 

C) A 100 lb. weight is applied to the 

system. The assembly now behaves 

like a bolt preloaded to 150 lbs and 

an external load of 100 lbs. Adding 

the weight does not increase the tension 

in the spring (which represents 

the bolt). If the tension were greater 

than 150 lbs, the scale would read 

more than 150 lbs and the block 

would fall out. 

the bolt will not be loaded to 150,000 lbs 

but will still only have the initial preload of 

100,000 lbs. However, when the working load 

exceeds the preload, the joint separates and 

the full working load is carried by the bolt. It 

is this situation where the working load cycles 

against the heads of the bolts or on the faces 

of the nuts. This is when loosening and bolt 

breakage occurs. 

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IMPORTANCE OF PROPER 

PRELOAD 

High preloads are required in so many 

applications that it would be difficult to 

name them all, but a good example is the 

pressure vessel. With the institution of 

ASME Sect 8 Div 3 pressure vessel code, 

extremely high pressures (over 10,000 psi 

system pressure) are allowed for reactors 

in various processes. This also means that 

the size of the reactors and the size of 

the studs that seal them have dramatically 

increased in size. For instance, pressure 

vessels with 8” to 10” diameter studs 

are becoming more common. 50,000 psi 

stress in a 10” stud requires a preload of 

3,700,000 lbs. It is almost impossible to 

torque a hex nut to this load. There are very 

few methods that can achieve high stud 

stress in large diameter studs without the 

time, safety, and economic consequences 

described earlier. MJT’s can be used in all 

pressure vessel codes and conform to ASME 

and ASTM design and material requirements. 

VIBRATION ISSUES 

If proper preload is applied above the working 

load, the fasteners will not vibrate loose. Many 

“Band-Aid” methods have been tried to stop 

fasteners from vibrating loose on vibrating 

and rotating equipment, such as double 

nutting, welding a fastener in place, locking 

adhesives, locking with cotter pins, and many 

more. This is fine when the fastener does not 

carry much load, but the trouble with this 

approach is that the main bolt/stud can still 

be going through full loading cycles. Even if 

the nuts on the ends do not come loose, stud/ 

bolt breakage can occur due to the fatigue 


cycles felt by the main tension member. 

A good example of this was on 4” 

reciprocating compressor rod piston end nuts 

being used in a large pumping station. The 

maintenance procedure was to use a chisel 

and a hammer to turn against the slots on the 

top of the nut. The piston nut and the rod 

were then drilled and tapped and fitted with a 

locking set screw. The customer experienced 

loosening of the locking screw, which would 

dislodge and run in the compressor until the 

rattle was noticed. Several rod failures also 

occurred due to the fatigue exposure when 

the nut backed off. After discussing the 

situation it was explained that the customer 

was only applying roughly 500 ft•lb to the 

nut. This translated to only 8,500 lb of preload, 

which was significantly under the separating 

force of the rod load. To achieve the necessary 

rod stress of 25,000 psi (289,250 lb), a torque 

of 17,000 ft•lb would need to be applied to 

the standard nut. Using the MJT system, the 

user was able to achieve the required rod 

stress with only 175 ft•lb on each jackbolt. 

Due to past practices the end user wanted 

to install the locking set screw but was later 

convinced that the preload would hold the 

system tight. Fifty MJT style Piston end nuts 

of various sizes have been retrofitted with 

this user over a seven year period with no 

loosening of the pistons or nuts. 

Many bolted joints in reciprocating machinery 

are subject to fatigue failures. The constant 

reversal of stresses from tension to 

compression weakens the metal structures, 

which leads to cracks and eventually 

breakage. The time to failure depends 

on the metal used and its treatment, the 

design of the structure (stress risers), the 

size of stress reversals, and the number 

of cycles. The amount of stress reversals


is more important than the level of stress 

at which the reversals occur. For example, 

fatigue occurs more often if the reversals 

are between 0 and 30,000 psi than if they 

are between 40,000 and 45,000 psi. In most 

joints, the amount of stress reversal can 

be substantially reduced, if not eliminated, 

by preloading the joint to a level where the 

bolt does not “see” the load change. This is 

shown in Fig 16. 

Fig. 16: Chart showing joint fatigue life. 

TIGHTENING STUDS IN PURE 

TENSION 

MJT’s tighten studs/bolts in pure tension 

created by the axial thrusting of the 

jackbolts on the nut body. As mentioned, 

this prevents thread galling “picking up of 

material” on the surfaces of the threads 

that can eliminate the need to drill/ 

machine out frozen studs. Aside from this, 

tightening in tension eliminates galling 

of the spot faces or machined faces of 

the equipment where a nut usually slides 

under load. It also eliminates situations 

such as where a hex nut is not machined 

perfectly perpendicular to the main thread, 

which can cause the nut to dig in to the 

bearing surface. This can add a large 

friction that will affect tension output due 

to torque. Up to 5º stud misalignment can 

be compensated on applications such as 

anchorbolts, which can be slightly off line. 

Tightening in tension can also be helpful 

when it is critical not to have a turning 

moment due to tightening a standard nut. 

There have been instances where 

great time and expense have been 

spent laser aligning a machine only 

to throw it out when banging on 

the standard nuts that anchor the 

machine down. Also, there are 

applications such as on the ends of 

shafts where it is difficult to hold 

the shaft steady while torquing the 

nut on the end. Tightening in pure 

tension eliminates these kind of 

scenarios. 

Tightening in tension also eliminates 

unwanted torsion in the bolt or stud 

that can create crack starters, weakening 

the fatigue capacity of the bolt. Fatigue 

breakage causes unwanted and expensive 

downtime not to mention the cost of 

expensive studs. 

EVEN TENSION AROUND THE 

FLANGE 

MJT’s are very repeatable with regard 

to bolt load (+-10% error or 5% with 

calibration). Commonly available bolting 

manuals have stated that standard nuts/ 

bolts have a 30-40% error in preload 

even with a very accurate torque wrench 

(+- 3%). Bolting organizations have been 

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investigating how many variables affect 

preload. There are dozens that mainly 

influence the friction factor, such as surface 

condition, materials, and lubricants... etc. 

The scope of this subject is beyond this 

paper but more information is available 

if desired. (Some studies have shown 

flange bolts from 0-200% tight due to 

multiplication of the error from multiple 

passes with a wrench.) Other methods 

also are prone to error and loss of preload 

when applying shrinkage methods such 

as thermal tightening and hydraulic 

tensioning. 

Even tension from bolt to bolt is especially 

important when sealing up a gasketed 

flange or pressure vessel joint. This helps 

eliminate leakage problems associated 

with one end of a flange having low load 

comparatively by creating even pressure 

around the flange interface. 

ELASTICITY 

It is desirable for most bolting systems 

to have as much elasticity as possible. 

This is also true for high temperature 

bolting. Due to creep, joints become 

loose after prolonged service and start 

to leak. Repeated cooling and reheating 

of the system often aggravates this. Tests 

show that turbine-type MJT’s will add 

the approximate equivalent of four stud 

diameters to the elasticity of bolting 

systems; this is an increase of 50-100% 

in elasticity for an average bolting system. 

Since creep continues at a uniform rate at 

a given temperature, this extra elasticity 

can significantly prolong the service life of 

high temperature joints. 


ELASTICITY OF MJT’S IN 

GASKETED JOINTS 

In conventional gasketed joints, a gasket 

is placed between the raised faces of two 

flanges and then clamped by a number 

of bolts. The clamping forces have to 

be sufficient to resist the blowout of the 

gasket by the pressure acting on the flange. 

The gasket is more or less compressed by 

the clamping forces and will stabilize at a 

certain thickness. If conditions in the joint 

remained stable, the joint would probably 

last forever. 

In an actual working joint, however, 

conditions are not stable. The gasket can 

be attacked by its environment and shrink 

or expand. Internal pressures can fluctuate, 

changing load conditions on the gasket 

and thus fatiguing it. Heat can act on 

the gasket in several ways. It can change 

the chemical state of the gasket, it can 

change the gasket’s elasticity and strength, 

and it can cause the load on the gasket 

to fluctuate due to thermal expansions 

and contractions of the various joint 

components. If during the operation of a 

gasketed joint the gasket gets thinner for 

any of these reasons, the joint may lose its 

integrity, and the gasket may blow out from 

the pressure acting on the joint. 

Whether a gasketed joint loses its integrity 

or not depends partly on the gasket 

material, but the bolting system employed 

is just as important to the integrity of the 

joint. For example, a gasket may be 0.0625 

inches thick when new, but after tightening, 

it may be compressed to 0.050 inches. 

While the gasket is being compressed by


0.0125 in., the bolts will be stretched. The 

elongation of the bolt depends on its 

original length and the stress in the bolt 

after tightening. 

Example 1: If the grip length of a bolt is 

4 in and the bolt is tightened to a stress 

of 45,000 psi as shown in Fig. 17, it will 

stretch 0.006 in. Now assume that the 

gasket only shrinks 0.003 in. The remaining 

elongation in the bolt will only be 0.003”, 

which means that the stress in the bolt 

will be only 22,500 psi. The joint has lost 

50% of its tension. What we have in this 

example is a stubby rigid bolting system. 

Example 2: If a bolt is three times as long 

as the one in Example 1, it will be 12” long 

initially. The elongation = 3 x 0.006 (which 

is 0.018) at 45,000 psi stress as shown in 

Fig 18. If the gasket now shrinks the same 

0.003” as in Example 1, the remaining 

elongation in the bolt is still 0.015”. The 

stress in the bolt is still 37,500 psi, and the 

joint has lost only 16.7% of its original 

tension. This is an example of a relatively 

elastic bolting system. 

Tests have shown that nut-type MJT’s 

can add elasticity to an average bolting 

system. Standard nut-type MJT’s will add 

as much as 4 to 12 diameter equivalents 

to the elasticity of bolting systems. If one 

considers that the average clamping length 

of a bolt is around four diameters, it can 

be concluded that nut-type MJT’s will triple 

or quadruple the elasticity of the average 

bolting system. It is rather easy to establish 

the relative elasticity in an MJT bolting 

system. The procedure is illustrated in Fig 

19. 

STRESS RELIEF THROUGH 

MJT’s 

The interaction of the jackbolt force and 

the opposing force of the main bolt creates 

a bending moment. Since the bending 

moment is created in a circular manner, the 

resulting stress in nut-type MJT’s is a hoop 

stress. The hoop stress causes an increase 

of the tensioner diameter at the bottom 

and a decrease of the diameter at the top 

as shown in Figure 20. 

Fig. 17: Application example 1. Fig. 18: Application example 2 - a more elastic system. 

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It is well known that most bolts fail at 

the bottom of the nut in the first two 

or three threads because of the stress 

concentrations that occur in that region of 

a rigid nut. Tests have shown that the flex 

action of a nut-type MJT can sufficiently 

Fig. 19: The elasticity of an MJT can be 

established by measuring the bolt elongation 

after loading. This indicates the actual tension 

in the bolt. Measuring the gap between the nut 

body and the hardened washer measures the 

elasticity of the bolt plus the nut body. There is 

more elasticity in the pintle of the jackbolts. The 

elasticity in the pintle cannot be measured, but it 

can be calculated using Hooke’s Law. 

(a) 


relieve the concentration of stresses to 

increase the static bearing capacity of high 

strength bolts by as much as 15%. (Low 

strength, ductile type bolts are not much 

affected by the stress-relieving action of 

MJT’s). MJT’s also increase the fatigue 

resistance of dynamically stressed bolts, 

but it is difficult to put a numerical value to 

that. The amount of stress relief depends 

primarily on the detail design of the MJT. 

Basically it can be stated that the higher 

the hoop stress in the MJT, the more stress 

relief one can expect. Most MJT’s are 

designed to have hoop stress equal to twothirds 

to three-fourths of the main bolt 

stress. This ensures that the bolt will always 

fail before the MJT does; it also establishes 

hoop stresses large enough to substantially 

relieve stress concentrations in the critical 

bolt thread areas. 

The stress relieving action of nut-type 

MJT’s is simulated in a flexible flange 

nut as shown in Fig 21. The flanged nut 

(or Flexnut) bears on the outside of the 

flange, thus creating the desired bending 

Fig. 20: Schematic of stress-relieving action by torquing a nut-type MJT. (a) Before torquing; (b) after torquing. 

Flexing as shown is highly exaggerated. 

(b)


moment. Flexnuts are used 

extensively in combination 

with bolt-type MJT’s. 

However, they are not 

very suitable for replacing 

standard hex nuts because 

they require excessive 

torque if tightened directly. 

Flexnuts are designed to 

flex out at the bottom and 

flex in toward the top. 

This distributes the bolt 

along more threads, adds 

elasticity, and prevents stress 

concentrations in the first few threads. 

CONCLUSION 

Multi-jackbolt tensioners offer the benefits 

of properly loading studs and bolts with 

the use of small hand and air tools. Cost 

justification is accomplished on many 

levels; the most significant of which is 

related to reducing expensive downtime. 

The speed of applying the system along 

with the reliability and fatigue resistance 

gets equipment on line faster and keeps it 

together once it is installed. 

In most cases, the system is more 

economical than other bolting methods 

to purchase and it provides a means for 

personnel to do their job safely. Reliability 

is enhanced with the added elasticity and 

the uniform preload capabilities. Safety, 

economic and environmental concerns 

created by leakage problems are eliminated. 

Fig. 21: Schematic of stress-relieving action of flanged nuts in 

combination with bolt-type MJT’s. 

About the author: Allan Steinbock is Vice 

President of Superbolt ® , Inc. and has been with 

the company since 1984. He has a Bachelor of 

Science Degree in Mechanical Engineering from 

the University of Pittsburgh and is a member of 

SME and other industry specific organizations. 

Allan has consulted on thousands of bolting 

projects and has designed products for a broad 

range of industries. 

16

Cost Justification and Reliability Benefits of Multi ... - Nord-Lock

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