First published in Bolted #2 2016.
Q: Can I re-use bolts?
A: Before you re-use bolts, you should always consider the security level of the assembly and economic factors. Operators often lack important information, such as if the assembly working load or working temperature have been exceeded. As fatigue cracks are difficult to detect, the use of a new bolt (screw, nut and washer) is often recommended for security and legal reasons.
You can re-use bolts:
Before re-using threaded fasteners, always make a visual inspection of the head and/or nut for signs of damage or corrosion. Check that the coating on coated fasteners is not damaged or worn away. Ensure that there is no permanent deformation of the threads by running a new nut over the thread engagement length of the screw. Inspect the fastener shank for signs of reduction in diameter “waisting”, indicating that the yield strength of the fastener has been exceeded.
Clean the internal and external threads and all contact areas. Using the same or similar tightening condition as on the initial installation – such as torque level, tool class or lubricant type – ensures that the same clamp load is achieved.
If the fastener is initially assembled un-lubricated, the surfaces of the bearing area and threads will degrade under pressure when untightening, increasing the coefficient of friction between these surfaces. Tightening on re-use to the initial torque value will result in a reduced clamp load, because of the higher coefficient of friction of these surfaces. Subsequent re-use will progressively reduce the clamp load and then be consistent at a low preload and a lot of problems could take place, such as fatigue, vibration or joint separation.
First published in Bolted #2 2016.
One way is adding excitement to the mix. Several universities are, quite literally, trying to accelerate student interest through extracurricular motorsports projects. Two American projects – Gator Motorsports at the University of Florida, Gainesville, and the Virginia Tech Baja SAE Team in Blacksburg – along with Dutch Forze Hydrogen Racing at Delft University of Technology are sponsored by the Nord-Lock Group.
These projects cover all steps, from design and development to assembly and racing the vehicles. Gator Motorsports highlights how students acquire technical skills while practicing teamwork and project management, which is beneficial for their transition into careers in the industry later on.
Gator Motorsports takes part in the yearly Formula Society of Automotive Engineers (FSAE) design competition in Michigan, USA. Here, they compete against more than 100 teams from all over the world with their high-performance Formula-style race car.
Combining motorsport with sustainable technology, Forze Delft focuses on race cars powered by hydrogen fuel cells. Their latest model, the Forze VI, is one of the first fuel cell system race cars globally, and the first and fastest hydrogen-powered race car ever measured on the Nürburgring Nordschleife in Germany. Recently, it broke the electric-lap record on Circuit Park Zandvoort in The Netherlands.
True to its “Fun, fast, dirty” slogan, the Virginia Tech Baja SAE Team has chosen to go off-road. Its single-seat vehicle competes each year in an international competition arranged by the Society of Automotive Engineers (SAE).
These student projects rely heavily on sponsors, but it is not all about getting free parts. “Networking with industry sponsors exposes us to engineering after college,” says Jess Barton of the VT Baja SAE Team. “We have the opportunity to learn from the companies, including how they operate and what they design and manufacture. Real-world skills and advice from sponsor companies make us great engineers and professionals.”
Johanna Persson, Sales and Marketing Director, Nord-Lock Group
1. What kind of student projects does the Nord-Lock Group sponsor?
“The projects that we select should resonate and align with our business and brand strategy. We wish to connect with future engineers, designers and innovators that will play an important part in developing solutions for both current and future challenges within our world.”
2. What are the long-term gains for the students?
“We want to help them to develop their creativity. We support thoughtful, innovative projects and programmes that build the capacity of students to succeed in a constantly evolving world.”
3. How do I get my student project sponsored?
“All student organisations are welcome to submit an inquiry through our website if they see a use for our products. We can’t support activities asking for a cash contribution, but we will support projects with knowledge and market-leading products.”
First published in Bolted #2 2016.
Couplings are as old as industrialisation itself with even early simple machines, such as cotton mills and windmills, needing some way of connecting shafts. However, ever since the invention of the steam turbine back in 1884, shaft couplings have become essential in the power generation and shipping industries. As both turbines and shipping vessels have increased in size, so too has the amount of power and torque that needs to be transmitted. This in turn has greatly increased the demands on shafts and couplings, and in the case of bolted couplings, on the bolts themselves.
“In the power generation and marine industries, couplings have always been there and they really haven’t evolved much,” says Martin Walsh, an engineer with over 30 years experience working with large-scale bolted couplings. “If you look at a coupling from 60-70 years ago, it is pretty much the same design and concept as today. However, the engineering behind them has evolved a lot. Bolts in particular is an area where couplings have become a lot more sophisticated and this has allowed smaller couplings to transmit more torque.”
One of the most important functions of a bolted coupling is to maintain shaft alignment. In marine applications, where shafts typically turn at low speed, any misalignment will cause vibration, which in turn puts unnecessary loading on the bearings. In power generation applications, where rotation can be as high as 3,600 rpm, even the slightest vibration or uneven loading is unacceptable and would severely limit the turbine’s ability to run at full power. For this reason, a lot of time and resources are invested in optimising shaft alignment.
“Once you’ve achieved the alignment, then the bolts need to hold it as it was set and maintain that alignment in service,” says Walsh. “At some point in the future, you will take those bolts out and disconnect the shaft. When you put it back together, you want it back to exactly where it was before because you’ve already invested a lot in getting the alignment right.”
For many years the most common bolting solution was standard through bolts, which are relatively cheap and readily available. A bolt is simply inserted through the bore and tightened with nuts on either end to create a friction connection. But the amount of torque that can be transmitted through friction is severely limited and excessive torque can lead to slippage and misalignment. The resulting micro movements and uneven loading can then lead to damaged bolts and bores. The coupling therefore needs to be rebuilt and shaft alignment re-established.
In theory, fitted bolts, which fill the bore, can offer greater torque capacity, since torque is then driven through direct shear across the cross-section of the bolt. In practice it is difficult to achieve a truly fitted bolt, since the bolt’s diameter will reduce as it is tightened. This creates a gap between the bolt and the bore, leading to the same problems of slippage and bolt failure.
This need to establish and maintain shaft alignment, even after a coupling has been dissembled and reassembled again, has led to the increased use of expanding sleeve bolts. Since expanding sleeve bolts expand into the bore, they can ensure a truly fitted bolt and a far more even load distribution. This eliminates movement and slippage, so that shaft alignment should automatically be re-established once the expanding sleeve bolts are reinstalled.
“The expanding sleeve bolt has probably been the biggest single advance in accuracy over the past 30 years,” says Steve Brown, Global Product Manager – Expansion Bolts, Nord-Lock. “They offer many pros and little in the way of cons – ease of installation, accuracy of fit, ease of removal, regaining of alignment and with correctly prepared holes, regaining of concentricity and re-usability.”
A key factor driving the development of bolted couplings has been the evolution in engineering analysis. “70-80 years ago, couplings and bolts were over-engineered and bigger than they needed to be, as engineers erred on the side of caution,” says Walsh. “It was a situation that existed in many industries because the ability to do sophisticated calculations and simulations was not available.”
Now many OEMs have the ability to test the affects of temperature, different materials and operation conditions using computer modelling and simulations. Due to the complexity of rotating couplings, the finite element method (FEM) is becoming increasingly common for identifying weak points and torque tolerance of specific installations. Shear tests have also been used successfully to demonstrate the physical limitations of different bolting solutions.
“There is still scope for further analysis and it would be helpful to see exactly how the newer design of bolts with expanding sleeves compare to the older bolts when it comes to transmitting higher torque,” adds Walsh. “Having a full FE analysis could be a significant advantage since it shows the potential for reducing the number of bolts and size of the coupling, particularly in industries such as wind turbines, where they tend to avoid bolted couplings due to space restrictions.”
The need to design smarter and smaller couplings will continue to be important as turbines and shipping vessels keep on growing in size and output, and need to transmit even more torque.
C.A. PARSONS ”TURBINIA”
In 1884, British engineer Sir Charles Algernon Parsons invented the first steam turbine.
His first model only generated 7.5 kW of electricity, but it demonstrated huge potential for generating electricity and for powering ships. In 1893, the Parsons Marine Steam Turbine Company was set up and in order to demonstrate the capabilities of the new technology, began developing the experiential vessel Turbinia.
The new ship was equipped with three axial-flow turbines fitted to three shafts, with each shaft driving three propellers. On completion in 1894, the Turbinia was the fastest ship in the world reaching speeds of up to 34 knots (63 km/h) – in comparison the Royal Navy’s fastest vessels could only reach 27 knots.
In 1897, the Turbinia turned up unannounced to the Navy Review for Queen Victoria’s Diamond Jubilee, and in front of royalty and senior Navy figures, it was able to clearly demonstrate its superiority in speed and power. Within two years, Parson’s turbines were adopted by the Royal Navy and shortly afterwards used to power transatlantic passenger ships.
Sir Charles Algernon Parsons’ designs also saw steam turbines quickly scaled up, making it possible to generate cheap and plentiful electricity. In 1899 the first megawatt turbine was built in a power generation plant in Germany, and within Parsons’ lifetime, his invention was adapted by all major power stations in the world.
LET’S STICK TOGETHER
The need to assemble components in bolted joints goes way back. For long the through bolt was the standard threaded fastener. It is inserted through the bore and tightened with nuts on either end to create a friction connection. Excessive torque may lead to slippage and misalignment, which in turn can lead to damaged bolts and bores.
The expanding sleeve bolt is a more recent invention, which overcomes these problems. It expands into the bore and ensures a truly fitted bolt along with more even load distribution. It also simplifies installation and removal as well as retrofitting.
Find out more:
The September issue of Bolted magazine is out now! Prepare yourself on a reading journey to explore interesting cases and insights from the world of bolt securing.
In this issue, our theme article focuses on coupling challenges. Couplings are essential in the power generation and shipping industries, and are arguably the most demanding of all bolting applications. Learn what the bolting experts have to say about couplings.
Engineers from the Nord-Lock Group answer questions on how to get the most out of your fasteners and cover the key advantages of hydraulic tensioning.
We witness the largest public works project in California’s history, where Boltight tensioners are installed on the San Francisco-Oakland Bay Bridge. We then move on to Oman to find out how broadcasting antennas owned by the BBC are being secured against windstorms. Last but not least, we ride on the time machine as we simulate 20 years of wear and tear on a wind turbine in just a fraction of that time.
We are glad to inform that Bolted is now available in Korean language! Bolted magazine are published in 8 languages, download them now:
Want to receive your complimentary copy of the Bolted magazine? Subscribe here now!
First published in Bolted #1 2016.
The Myth: A small thread pitch reduces fatigue life due to the notch effect.
The Truth: A general statement is, as so often, not possible in this case since various parameters interact. In the literature there are partially contradictory statements. Heinrich Wiegand et al have conducted comprehensive studies on the influence of thread pitches. Diagram (a) shows the influence of thread pitches on bolt fatigue life.
Different property classes result in different characteristics. The fatigue life of high-strength 12.9-class bolts decreases with increased thread fineness, caused by the increasing notch effect. With decreasing strength and increasing ductility this effect is not visible anymore.
For bolts of property class 8.8, a decrease in fatigue life can also be seen. A larger cross-section of the fine thread will compensate for the decreasing fatigue life. The tolerable load amplitude (diagram b) is practically continuous.
There are more factors that influence the thread pitch than those mentioned. These include, for example, the displacement of the force application point, the root radius, the notch depth, the distribution of load force at the thread pitch and the notch sensitivity of the bolt material.
First published in Bolted #1 2016.
Each year objects dropped from height damage equipment, disrupt operations and cause a number of injuries – in the worst case fatal ones. To counteract this, the work group DROPS (Dropped Objects Prevention Scheme) was started in the late 1990s.
Oil and gas operators, major engineering and drilling contractors, inspection specialists, industry bodies and suppliers come together in DROPS to share and discuss best practice. The organisation produces publications and offers resources such as training to its members.
“While the collaboration of all the key players is not so unusual in the oil and gas industry, our single-issue focus is unique and means we can be very effective at what we do,” says Greg Reid from Silverdot Ltd, who administers the activities of DROPS.
With over 200 member organisations, interest in DROPS continues to grow. “When DROPS started it was solely focused on the drilling sector within the oil and gas industry,” Reid says. “Today, we are a truly global organisation that covers all aspects of the industry. In fact, we are attracting attention from mining and construction companies, as they see the benefits of industry-wide collaboration.”
The Nord-Lock expertise in bolted connections is showcased in the third revision of Reliable Securing, which highlights the company’s washers as a recommended solution for mechanical and structural connections where maintaining the clamping force is critical.
“Nord-Lock has been a very active member of DROPS for around ten years,” explains Reid. “Our organisation can only be successful if all our members bring a collaborative attitude to the table. Nord-Lock are always free and open with their advice, offering unbiased expertise that doesn’t have their own interests at heart.”
For more information:
The white paper “New innovations to solve difficult shaft coupling bolting problems” is written by Stephen J. Bussalacchi, Product Manager of Superbolt Division, Nord-Lock Group.
Demanding bolting applications in the hydro industry, such as shaft couplings, have typically required the use of fitted or interference fit bolts for proper torque transfer. However, these bolts require large and expensive tools for tightening, precision machining and extreme tolerances/surface finishes. These extreme machining requirements also apply to the mating coupling bores. Assembly may require further mechanical adjustments and disassembly is often time consuming and cumbersome. Additional concerns with these methods include worker safety, stuck bolting, and failures as a result of fatigue. This paper will examine a possible means to overcome these bolting challenges and achieve a bolted connection that is pre-loaded safely, cost effectively, and with as little downtime as possible. One such system is a radial fit ‘expansion bolt’ that utilizes split expanding sleeves and low input torque multi-jackbolt tensioners to achieve a true radial pre-load into the coupling bores as well as high axial clamping of the split line. Shaft coupling flanges are a critical component of hydro turbines all over the world. Utilizing the latest procedures and tools to achieve a secure bolted joint is essential to ensure many years of safe operations.
To receive the full white paper, please submit the form below. This white paper is currently only available in English.
► Request for paper: “New innovations to solve difficult shaft coupling bolting problems”
From the Empire State Building in New York to the Eiffel Tower in Paris and Burj Khalifa in Dubai, steel has been used in some of the world’s most famous landmarks and is used everywhere around us, in buildings, bridges, highways or other civil engineering projects.
Steel is one of the most commonly used materials in commercial and industrial building construction as it provides high design flexibility, durability, and low long term maintenance costs. Since steel has higher strength than concrete, it allows for bigger and more slender constructions. It is also much lighter compared to frameworks made of concrete. Using steel frames is also time efficient since pillars, beams and other constructions elements can be delivered to the construction site ready to install. This, in turn, gives great savings potential.
In the past, riveting was a common way to connect structural steel. Today, bolting and welding are the primary connection methods. Both methods have their advantages and constraints. A general practice is that welding is done at an earlier stage, in a workshop, where it can be executed in a controlled environment and then larger frameworks can be assembled by bolts at the construction site.
Performing welding at a construction site, however, may lead to uneven joint quality and is often very costly. It requires specialist skills and equipment, and is sensitive to the surrounding environment since uneven heating and cooling members may distort, which can result in additional stresses.
With bolting it is possible to assemble structural steelwork parts quickly and cost-effectively. It is also easier to get the steel parts fitted and adjusted during assembly on site. Unlike welding, bolting is less susceptible to poor weather conditions and has less inspection requirements.
Steel construction elements are safety critical and need to ensure that the final result is strong and durable. Connections in such structures must comply with strict quality requirements and standards for the design (the Eurocodes) and fabrication (EN 1090-2) of structural steelwork. This means that both welding and bolting have regulations to follow according to Eurocode 3.
Bolted connections can be commonly divided into preloaded and non-preloaded structural bolting assemblies. As recommended in EN 14399, preloaded bolts are almost always used on bridges. These bolts are specially designed to withstand vibration and dynamic loads, and also recommended when slip between joining parts is to be avoided. In buildings, preloaded bolts may be utilized where oversized or slotted holes are used to increase tolerances during assembly.
For preloaded bolted steel connections there are two different bolting systems developed and standardized in Europe (EN 14399): HV-sets (in accordance to EN 14399-4 and EN 14399-8) and HR-sets (in accordance to EN 14399-3 and EN 14399-7). Both systems are similar and consist of bolts, pre-lubricated nuts and washers. The radius under the bolt head is a particular characteristic for those bolt sets. It is larger than for normal standard bolts in order to decrease the notch effect.
The manufacturer of these bolt sets provides the customer with tightening guidelines that must be followed according to the European regulations. In spite of this practice, it is known that the bolts may still loosen due to dynamic loads and require time-consuming and costly retightening.
Using standard locking washers would be the easiest solution to cope with loosening of bolted joints or to decrease retightening frequency. For structural steel parts using HV/HR-sets this is, however, not the case. The radius under the bolt head makes the use of common locking methods with HV/HR bolt sets impossible.
It is not only impossible design-wise but also from a legal perspective since regulations in Eurocode only allow dedicated washers according to EN14399 for these types of bolt sets.
The Nord-Lock Group has developed a locking washer specially designed to fit the HV/HR sets. Each Steel Construction (SC) washer pair has chamfers on the inner diameter to ensure an optimal contact surface between the bolt and the washer. Nord-Lock wedge-locking technology secures the bolt with tension instead of friction preventing the bolt from rotating loose. The washer is CE approved to be used together with HV bolt sets.