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bolt securing

All posts for the tag “Tightening”

The Experts: Improving fatigue resistance

First published in Bolted #2 2015.

A: The fatigue capacity of a bolted joint is very small, as compared to its static capacity. To improve fatigue resistance, designers can increase the thread capacity and decrease the alternating stresses at the threads.

To increase the thread capacity, it is recommended to use a rolled thread instead of a cutting process. To increase the bolted joint capacity, utilize multiple smaller fasteners instead of a single larger fastener.

The capacity is also increased by using an improved connector, such as a Superbolt MJT (Multi-Jackbolt Fastener) or Flexnut, which improves the load distribution in the threads and adds elasticity to the bolted joint.

The best way to improve fatigue resistance is to reduce the alternating stresses at the threads. There are three main ways of doing this: Assembly design, assembly tightening, and assembly security.

The assembly design process provides an opportunity for improvement of the load distribution on bolted joints and to reduce the level of external stresses supported by each joint. To facilitate that, keep these principals in mind:

1. Use the highest possible preload
2. Minimize the bolt to load eccentricity
3. Use the largest possible contact surfaces
4. Use the largest possible clamping lengths
5. In most cases, use a preload higher than the working load

Other assembly design options include the use of necked-down studs or bolts, and the use of elastic washers, which counter the effects of relaxation, creeping, and thermal differential elongation.

With regard to assembly tightening, achieving the necessary preload is the main factor in reducing alternating stresses. It is recommended to use calibrated tools with high accuracy. It is also recommended to use a proper lubricant to achieve preload accuracy, and to reduce the risk of seizing. A suitable tightening sequence should be used to mitigate the risk of un-evenly loaded bolts and to ensure overall bolted joint integrity.

Regarding assembly security, it is recommended to secure the bolted joint against loss of preload. Further, secure the assembly against environmental effects, such as corrosion that could initiate a fatigue crack. This may be done through the selection of suitable materials and/or coatings for parts and fasteners.


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Lubrication: The when and why for tensioning

8 June 2017

Text: Amaris Neidich & Joseph Vernam

First published in Bolted #1 2017.

Q: How does lubrication affect hydraulic tensioning and multi-jackbolt tensioning?
A: With the traditional method of tightening a nut, using a spanner, lubrication is very important, as there are a lot of surfaces moving against each other – the threads of the nut against the threads of the stud and the underside of the nut against the surface of the joint. Overcoming these frictional forces accounts for approximately 90 percent of the work (energy input) applied to generating the load in the joint.

When a hydraulic tensioning tool applies a clamping load to a joint, lubrication has no effect as it is applied directly to the stud and joint. A tension force is a linear force applied in an axial direction, so there is no rotation required to generate the load. This allows the nut to be turned down against the joint face under minimal friction.

As there is no friction to consider, there is no need to reduce the coefficient of friction using lubrication. Also, the lack of friction in the application permits much more accurate and repeatable results.

With multi-jackbolt tensioners (MJTs), the use of a lubricant on the main bolt thread does not affect the preload. It is advisable to use a very light film of lubricant with anti-seize characteristics to facilitate tensioner removal.

A more tangible effect of lubrication for MJTs is from the required lubricant use on the individual jackbolt threads, jackbolt bottoms of the tensioner, and washer face.

Proper use of lubrication is crucial to safeguarding repeatable and precise preload control in Superbolt installations. Superbolt mainly uses a graphite-based lubricant with a low friction coefficient and steady performance to achieve a positive impact on the preload. The MJTs are delivered with lubricant pre-applied to the installed jackbolts. Additional lubricant is included for application to the jackbolt bottoms. For subsequent installations, reapplication of lubricant is required to provide the intended performance.


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The Experts: Hydraulic bolt tensioning tools

First published in Bolted #2 2016.

Q:   What are the key advantages of hydraulic tensioners?
A:  A hydraulic bolt tensioning tool provides a quick, easy and safe method for tightening large diameter bolts to high and accurate preloads. It does not use torque or require forceful turning of the nut or bolt, like impact wrenches, flogging spanners or hydraulic torque wrenches, where friction is a common enemy.

A hydraulic bolt tensioner is an annular jack, which fits over the bolt and nut to be tightened. The jack pushes against the bolted joint and pulls on the end of the bolt. Because the force produced by the jack is applied directly to the end of the bolt, a tension equal to the load generated by the jack is developed in the shank of the bolt. With the jack applying tension, it is possible to rotate the nut with zero torque until it is tight. The load applied by the jack is then relaxed and a high percentage, depending on the length of the bolt and its diameter, is retained in the shank of the bolt.

Hydraulic bolt tensioning provides:

  • Accuracy – very high bolt load tolerance accuracies, often 5% or better. No frictional losses to consider nor need for lubrication. Easy to calculate load transfer and to prevent overload.
  • Speed – the tools are remarkably fast to install and operate.
  • Simultaneous tensioning – multiple bolt tensioners can be hydraulically actuated at the same time.
  • Uniformity – tensioning several bolts at the same time guarantees the same applied load to each bolt.
  • Safety – no pinch points or reaction issues. The safety engineered into Boltight systems surpasses the industry safeguards.
  • Versatility – using adaptor kits, one tensioning kit can often be used to cover a large variety of bolting sizes.
  • Cost efficiency – combining the factors above, hydraulic bolt tensioning provides operational value for the customer.


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The Experts: Why does torque vary when tightening and untightening?

First published in Bolted #1 2010.

Q: Why is the untightening torque lower than the tightening torque when tightening a bolt or nut?

A: When tightening a bolt or nut, the applied torque must overcome the thread friction, the friction under the bolt head or nut and the inclined plane of the thread, to obtain a bolt preload (see Fig 1A).

While untightening, provided that the assembly remains in the same condition, the applied torque only has to overcome frictional forces. As a result, untightening a fastener requires a lower torque (see Fig 1B).

There are some exceptions to this rule. Since friction conditions vary, higher torque can be required to untighten a fastener. Corrosion, seizing, or surface roughness can considerably increase friction and subsequently the untightening torque.

With Nord-Lock washers, the difference between the ­tightening and untightening torque is even more significant. During ­tightening, sliding occurs between the bolt head or nut and the serrated surface of the upper Nord-Lock washer (see Fig 2A).

However, during untightening, sliding occurs between the cam faces of the washers, where ­friction is ­significantly lower (see Fig 2B).

This is a valuable feature of Nord-Lock washers, as a low untightening torque facilitates maintenance and thereby reduces downtime.


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The Experts: How to determine a suitable tightening torque

torque tools chart

First published in Bolted #2 2012.

Q:  I don’t know the tightening accuracy of my tool. How should I proceed with my torque calculation?

A:  All tightening tools have a certain inaccuracy that must be accounted for in order to determine a suitable tightening torque.

The accuracy of a tightening tool can generally be obtained from the manufacturer or retailer. However, there are standards that categorize tightening tools and their accuracy level including the tool operator. If you do not know the tightening accuracy of your tool, the values in the table could be applicable.

It is also important to remember that friction variance influences the final preload. It often has a greater impact on the final preload than the tool inaccuracy. A cost effective way for an accurate tightening process is therefore to properly lubricate the fasteners.


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The Experts: Should I change my tightening torque with Nord-Lock?


First published in Bolted #2 2010.

Q: Does the use of Nord-Lock washers affect my tightening routines? How?

A: When a tightening torque is applied to create a clamp load in a joint, friction under the bolt head (or nut) and in the threads must be overcome before any torque goes to achieving the clamp load.

Since the Nord-Lock washers feature serrations, the coefficient of friction under the bolt head (or nut) will be slightly increased. The thread friction remains unchanged. A slightly higher tightening torque is therefore required to achieve the same clamp load as for a flat faced joint.

Nord-Lock’s general tightening guidelines are taking the increased friction into account and permit to achieve a very accura te tightening.

Some users choose to keep their torque values when implementing Nord-Lock washers. It will result in a somewhat lower (-10% to -20%) but still acceptable preload.

It is also important to remember that friction varies significantly depending on bolt grade, bolt size, lubricant and joint material so real torque values may actually differ from torque guidelines.


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The Experts: What effect will Nord-Lock washers have on my surfaces?

Effects of Nord-Lock washers on the surface of your application

First published in Bolted #1 2011.

Q: Will Nord-Lock wedge-locking system damage the surface in my application?

A: Nord-Lock washers are specially designed to create impression marks without scoring the mating surface. The impression marks are important since they prove that the serrations grip correctly into the mating surface. In bolted assemblies rotation always takes place where the friction is lowest. Provided that sufficient impression marks are created any tendency to rotate will occur between the cams of the washers. Consequently, any rotation is prevented by the wedge effect of the cams.

When a joint secured by Nord-Lock washers is tightened, rotation always takes place between the upper washer and the bolt head or the nut. The serrations are simply pressed down into the material, no scoring of the mating surface occurs. The bolt head/nut is hard enough to withstand the minor scoring that it is exposed to. During untightening rotation takes place between the cams of the Nord-Lock washers. Therefore there is no risk of damaging the surface in the application. The impression marks slightly increase the compressive stresses in the surface, but they will not score the material or damage it.


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Watch the new Junker test video here!

Nord-Lock washer junker vibration test video

We are pleased to present our brand new Junker test video. It is now available on the Nord-Lock Group YouTube channel, and also embedded below:

The Junker test, according to DIN 65151, is considered the most severe vibration test for bolted connections. In this video we compare the performance of Nord-Lock washers with other locking methods.

Interested in learning more?