The 2018 March issue of Bolted magazine is available now! As with every edition we have filled the magazine with interesting cases and insights from the world of bolting.
In this issue of Bolted, we take a closer look at what goes into the manufacturing process of traditional bolts – from raw steel to tailor-made applications.
We ask expert Filemon Schöffer about the potentials with 3D-printing, and we meet German company “MMG” who is a world leader in production of propellers for large container ships.
And of course a lot more.
Want to receive your complimentary copy of the Bolted magazine? Subscribe now!
First published in Bolted #1 2017.
How do you define ideal fastening, which you also covered in your book?
“Ideally, fastening should be based on the use of widely available, standardised fasteners, rather than specially designed parts. More importantly, ideal fastening should ensure a bolt fastening design that won’t lead to any kind of failure. The entire product design becomes invalid if a single failure occurs. You must pay attention to every aspect. I consider ‘evaluation without any omission’ most important.”
Is using lubricants an advantage in bolt fastening?
“Yes, if the fastened objects don’t slip against each other, lowering the friction coefficient is favourable in all aspects. If fastened objects are in a ‘loosening environment’, they are more likely to loosen if the friction coefficient is low, but it does not necessarily lead to loosening.
They are in a ‘loosening environment’ if they are repeatedly subject to slip against each other with a force exceeding a certain threshold.
How do external forces cause slip, based on shear direction, axial direction and torsion?
“If an external force is applied in the shear direction, it would cause slip. If it is applied in the axial direction, the fastened objects would separate from each other – separation. Under these conditions, the lower the friction coefficient, the more likely loosening is to occur.
When writing Bolted Joint Engineering – Fundamentals and Applications, I used the conventional view of the slip phenomenon, explaining the slip of fastened objects on the contact surface – so-called ‘macro-slip’. You can observe this with your eye, as this type of slip needs to be only 0.1 mm for visual confirmation. Around 1988, it was found that invisible ‘micro-slip’ actually occurs before the macro-slip and that it causes rotation, which is so micro that, whether turned in the direction of loosening or not, it can’t be confirmed with the naked eye. This phenomenon, ‘micro-slip’, gradually diminishes the axial force. It was introduced in an article in the Journal of the Japan Society for Precision Engineering.
“If fastened objects are in contact with each other, conventional experiments can’t measure the slip amount of a certain section of the contact surface or of other sections. But all of these values can be calculated using the finite element method, FEM. It has been used in the fastener industry since around 2000 and today most research on threaded fasteners utilises it. An article by Doctor Satoshi Izumi et al. in 2006 announced that gradual rotational loosening was found to occur with micro-slip (invisible minute slip)rather than macro-slip (clear, visible slip). I was shocked when I first read the article, which states that when micro-slip occurs repeatedly, it causes minute rotational loosening as small as 1 degree per 1,000 times or 1/1000 degree each time. A 1/1000-degree rotation is not at all observable to the eye. With the finite element method, it can be studied perfectly and it was demonstrated that micro-slip causes rotational loosening. I felt I was in trouble! [Laughs] The results drastically shook the concept of critical amount of slip.
I had thought that micro-slip would naturally lead to fretting wear, but didn’t consider that it could cause rotational loosening. I had no way of testing that at the time. It was an eye-opening experience.”
A slip not visible to the naked eye. Gradually diminishing the clamp force, it can ultimately lead to visible rotational loosening (macro-slip). Settlements and relaxation of the material can also decrease the clamp force. Nord-Lock Group has developed X-series washers that deal with both forms of slip. They counteract all kinds of clamp force losses with the spring effect, while the wedge effect prevents spontaneous bolt loosening.
Facts: Doctor Tomotsugu Sakai
► Have a bolting question? Contact us
First published in Bolted #2 2017.
Daher Nuclear Technologies GmbH, located in Hanau close to Frankfurt am Main, Germany, develops containers for transportation of radioactive substances. For obvious reasons, these containers must be extremely safe.
Designing a new container for uranium hexafluoride transports, the company had to consider the very stringent international and national requirements, including the recommendations of the International Atomic Energy Agency (IAEA) for transport by road, rail and sea. A container that fulfils these requirements must, for example, be resistant to the mechanical and thermal loads that can occur in case of an accident.
These mechanical accident loads are defined by a sequence of tests that include a 120-centimetre fall, followed by a 9-metre fall, followed by a fall from 1 metre onto a spike. The container must remain sealed, so that the subsequent thermal test, a fire, doesn’t jeopardise the container’s safety.
Daher set out to design the container locks so that the locking bolts would, under no condition, come loose or be lost during the loading of the container onto a lorry or during transport. The company’s intensive search for the optimal solution led to Nord-Lock wedge-locking washers of type NL16-254SMO. These safety washers are an important component in Daher’s triple-protected locking system: the lock is secured with a bolt, which in turn is locked in position by another bolt. The wedge-locking washers from Nord-Lock are located under the second of these bolts. Each container has six locks and each lock is equipped with a Nord-Lock washer pair.
Thanks to the use of Nord-Lock wedge-locking technology, the locking systems on the Daher transport container for the nuclear industry can no longer be worn by vibrations or stress, but remain tightly and securely locked. Daher was also pleased to find how cost-effective the use of the Nord-Lock product is, and how easy the maintenance is. If needed, the wedge-locking washers can be replaced at any time to ensure that the transport containers remain in top condition. The containers have a service life of more than 30 years – something that the Nord-Lock washers contribute to.
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.
In this video we explain how you choose the right size of washer for your bolted joints.