The 2017 September issue of Bolted magazine is available now! As with every issue we have filled the magazine with interesting cases and insights from the world of bolting.
In this edition, our theme article focus on lug wear which is a common issue for pivot joints. But why does it occur, what solutions exist and do they solve the root cause of the problem?
You will find out more about Expander System in our customer case, where we visited Danish company Viggo Benz who delivers solutions and equipment for crushing, demolition and sorting.
Also, see how Nord-Lock washers secure containers that transport radioactive substances, where there is no room for equipment failure.
Last but not least, don’t miss out on how and why we now offer lifetime warranty throughout our product range.
Want to receive your complimentary copy of the Bolted magazine? Subscribe here now!
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.
First published in Bolted #1 2017.
Dutch company Akkadia manufactures specialist CCTV systems for demanding situations in various industries. Its cameras can be found everywhere, from offshore, to polar regions and deserts.
Since 2010, Akkadia has had a contract with the Dutch Ministry of Infrastructure and the Environment (IenM), providing CCTV technology in tunnels throughout the Netherlands. Tunnel cameras must be particularly durable, as the vehicles that constantly pass close by create huge wind pressure. Standard security cameras normally don’t last more than a year and a half.
Akkadia developed a specific camera for tunnels – the PTZ – as a stainless steel, vibration-resistant application, built to last more than ten years. To create and maintain such robust technology requires a high-quality locking solution.
The IenM were keen to use its usual self-locking nuts, but Akkadia were already employing a product that would ensure the ability of its cameras to provide long-term, trouble-free traffic surveillance.
Akkadia had entered into partnership with Nord-Lock and fit its washers in the CCTV camera technology. The IenM was subsequently won over by convincing evidence presented by Akkadia about the quality of the Nord-Lock washers, including test reports from TÜV in Germany. The PTZ tunnel camera utilises NL6 and NL8 steel and stainless steel washers on the inside, and NL10 stainless steel washers on the mounting and base.
All moving parts and internal connections on Akkadia CCTV systems are secured by Nord-Lock washers; from the housings to the base plates.
No problems have been reported in the six years that Akkadia has supplied tunnel cameras to the IenM. Akkadia has now manufactured around 800 systems featuring the Nord-Lock solution. Currently, over 400 PTZ tunnel cameras cover two large motorway projects in the Netherlands – the Amsterdam Orbital, and the Maastricht motorway near the Belgian border.
The success of Akkadia’s tunnel cameras highlights the importance of total cost of ownership. In thinking long term, the IenM has saved money on the cost of maintenance and replacing broken cameras.
First published in Bolted #1 2017.
Starting with ROLLING STOCK some 20 years ago, Nord-Lock wedge-locking washers are now found in all safety-related areas of the railway industry, such as bogies, coupling devices, brake systems, rail dampers, housing, and many more.
Nord-Lock washers ensure the functionality of bolted joints, even at the highest levels of vibration caused by rail traffic. One example is in railway switches, highly complex structures that must be used as long as possible with minimal maintenance, while exposed to the elements and to considerable stress.
Nord-Lock steel construction washers (NLSC) help railway companies avoid operational downtime, thus saving money. When renovating or replacing old bridges, small temporary bridges are assembled on site with a bolt preload of 100 percent to achieve secure bolted joints. However, these bolt connections can only be used once. Using Nord-Lock washers, the preload can be reduced.
Overhead line masts and signal masts must cope with severe dynamic loads caused by passing trains. Given the large number of masts, reliable bolt connections are crucial to reducing maintenance costs. With their special geometry, Nord-Lock X-series washers increase security, including on bolted joints with short clamp length, as well as in softer materials.
A recent application for the Nord-Lock X-series is noise-absorbing walls, which cope with extreme vibrations from passing trains. In Germany alone, 3,000 km of noise-absorbing walls are to be installed by 2030. A combination of concrete bolts and X-series washers is used to attach noise-absorbing barriers on existing bridge decks, many of which were not designed for noise barriers.
First published in Bolted #1 2017.
Look up into the sky in the coastal city of Miri, in north-eastern Sarawak, Malaysia, and you may see Nord-Lock washers in action: inside a radio-controlled aircraft. These hobby airplanes are sold by Byond Horizon, a business that otherwise uses drones to take aerial photos and videos for companies.
“Unlike our drones, these recreational aircraft use gasoline-powered engines,” says Mr. M. Fadzly of Byond Horizon. “These engines cause a vibration problem. Since the frame is made of wood, which is soft, and the engine and its mounting are made of steel, the bolts holding them together can come loose after only four or five flights.”
Fadzly says that it can be difficult to access the plane’s engine, so the loosening is a real problem. A friend of Fadzly’s, however, works at Mayura Engineering and is a supplier of Nord-Lock washers.
“I ordered a box, tried them out, and now I recommend them to all of my customers when we are assembling their airplanes,” says Fadzly. “The Nord-Lock washers never need to be retightened, and they also make sure the engine stays in place. It would be quite dangerous if the engine were to fall out during flight.”
Fadzly not only sells the radio-controlled airplanes, he flies them himself. “I’ve been doing it since 1997,” he says. “It’s a passion, and a fun thing to do on the weekend. It gives you the feeling of flying a real airplane.”
First published in Bolted #1 2017.
ENERGY. With oceans covering more than 70 per cent of the earth’s surface, wave power is potentially a huge untapped source of renewable energy. The problem is that most wave energy converters are too large and costly to be commercially viable. Swedish company CorPower Ocean could have the answer.
The company’s compact Wave Energy Converter works by oscillating in resonance with waves, amplifying their motion and then converting that energy into power. CorPower Ocean founder, cardiologist Stig Lundbäck, invented the initial concept based on the pumping principles of the human heart. In the same way that a heart uses hydraulically stored energy to form back in place, the Wave Energy Converter uses a pneumatic pre-tension system to pull down the buoy after it has been lifted by a wave.
This allows for a relatively small device to harvest a large amount of energy. It is estimated that one buoy, eight metres in diameter, can generate around 250 kilowatts of power. That is enough electricity for around 200 homes.
“If you look at wave energy potential, somewhere between 10 to 20 percent of global electricity consumption could be provided by wave power,” says Patrik Möller, CEO, CorPower Ocean. “It has the potential to become the most competitive source of renewable energy. It offers five times more energy density than wind and ten times more than solar power. Waves have fewer variations and are more predictable than sun and wind, so you know a few days in advance what the energy flow will be.”
Currently, the Wave Energy Converter is undergoing tests with simulated wave loading, while a full-scale demonstration is being set up to begin in 2017. One of the key challenges has been keeping the buoy small and lightweight, while at the same time strong and durable enough to survive the toughest storms at sea.
This has presented a number of fastening challenges. On the mainframe inside the buoy, CorPower Ocean has elected to use Superbolt tensioners due to their lower torque requirements compared to a single bolt, which makes assembly far more manageable. Superbolt can also guarantee reliability over the buoy’s intended 20-year lifespan. At the base of the buoy, Nord-Lock washers are used, since they can maintain the correct tension over many load cycles over a long period of time.
First published in Bolted #1 2017.
The people of Nuremberg are proud of their underground system, which is among the most modern in Europe. The city, located in Bavaria, Southern Germany, has the only underground network in Germany where two of the three lines operate automatically, without train drivers. Nuremberg trains travel the equivalent of the circumference of the earth twice each day, carrying more than one hundred million passengers per year.
After 40 years of continuous use, it comes as no surprise that a renovation of the track beds is required to ensure passenger safety. The main beam, also known as a concrete stringer, which attaches the tracks to the tunnel floor, has simply sustained damage in too many places.
This is a daunting issue for the provider of the Nuremberg metro services, VAG (Verkehrs-Aktiengesellschaft Nürnberg). Normally, metro companies need to completely shut down a track for weeks during the renovation of such concrete stringers. Employing water pressure to remove the concrete, it is a time-consuming and extremely dangerous job, considering the many power lines inside the tunnel. Long delays caused by closed tunnels are costly for the track operators, adversely affecting train traffic and irritating passengers.
Coming to the rescue, a brand new innovation caught VAG’s attention just as they started planning the renovation. Local Nuremberg dowel and concrete bolt manufacturer TOGE Dübel won a railway innovation award for a new concept that enhances the sustainability of existing concrete bridges. Present in the audience, VAG representatives were intrigued and had the idea of trying the concept for the first time in an underground rail environment. Currently, work is under way at the first three stations: Bärenschanze, Gostenhof and Maximilianstrasse. Work on the second largest station in the network, the “Plärrer”, with 98,000 passengers daily, is planned for 2017.
Instead of complete reconstruction, concrete bolts measuring 36 centimetres and weighing 1 kilo are utilised as load-bearing components to improve the life span of the overall track bed construction. The bolts are fitted with a patented special thread that cuts into the borehole wall upon application. The force of the bolt is mechanically transferred to the anchor base and the concrete is fixed in place.
“Completely removing a concrete stringer and installing a new one could never be accomplished without service disruption,” says Waldemar Gunkel, Technical Director of TOGE and one of the two inventors of the new generation of concrete bolts.
“In Nuremberg, however, our system is only being installed between the hours of 23.00 and 04.00. By the morning, everything is running normally.”
During these working hours, only one track is shut down and trains are redirected via a single track, while the porous areas of concrete on each stringer are chipped away and replaced. Finally, the stringers are fixed into the ground utilising the concrete bolts. Since the bolts need to be drilled into the concrete, there is a risk of inclination as the drilling machine might not be positioned at an exact 90-degree angle. That’s why all concrete bolts that are being used in this first project are secured by Nord-Lock X-series washers. Their conical shape can compensate for the inclination, while the wedge effect prevents spontaneous bolt loosening due to vibration.
The Nord-Lock connection came via Deutsche Bahn – Germany’s national railway operator – where Nord-Lock original wedge-locking technology has long been prescribed as the standard.
Jochen Süssenbach, Nord-Lock Project Account Manager, sees great potential in this new approach to metro renovation. “We’re looking at a huge renovation of the tunnels that virtually doesn’t affect the timetable at all”, he says. “In terms of costs, it’s also a solution that beats any conventional method.”
So far, the renovation is running as planned. The first construction phase has even been completed a week ahead of schedule and the total time for construction carried out at all three metro stations will last six weeks instead of several months, which could have been the case with the previous method.
The concrete bolts themselves are designed to last for 50 years. No concrete will last that long, but further renovations will not be necessary for decades.
Describing TOGE’s Innovation Award-winning solution, Bavarian Interior and Transport Minister Joachim Herrmann said the following: “We have our fingers right on the pulse.” He hinted at the billion-dollar losses that Germany faces due to the poor condition of some 120,000 highway bridges and 30,000 railway bridges.
THE UNDERGROUND RAIL SYSTEMS are in a similar state. Just as in Nuremberg, most metro networks in Germany, as well as in the rest of Europe, were established in the 1970s. Gunkel thinks TOGE has found an important application for its concrete bolt: “This project gives us the boost to further drive our product development forward.”
Facts: The Nord-Lock Solution
Client: TOGE Dübel GmbH & Co.KG.
End customer: Metro services provider, Verkehrs-Aktiengesellschaft Nürnberg (VAG).
Location: Nuremberg, Germany.
Project: Renovating concrete stringers under metro tracks without affecting traffic.
Solution: Using concrete bolts with a patented special thread to reinforce the existing structure.
Nord-Lock Product: X-series multifunctional wedge-locking washers with enlarged outer diameter (NLX24sp)
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