First published in Bolted #1 2013.
Imagine if a bolt came loose on a crane, resulting in a 10-ton load dropping on passersby on the street below. Or what would happen if slackening on the bolted joint of a conductor on a power transmission line led to a power outage for several days in a large urban area?
Bolts are the crucial components that hold many critical products in our everyday life together. They need to be robust enough to withstand all types of weather conditions, extreme wear and tear, and, sometimes, being installed incorrectly. All too often, they must battle all of the above, meaning that sometimes the bolt can come loose or slacken. Over time, these miniscule shifts in the bolted joint turn into defects and end up as a costly, time consuming, and, in a worst case scenario, dangerous situation.
For years physicists and experts have battled to come up with solutions to combat loosening, relaxation and creep in bolted joints. More recently, they have been investigating the use of bolted joints in heavy industry where there is a risk of bolt loosening as a result of vibration and dynamic loads from spontaneous movement such as wind or inconsistent usage.
Two terms, ‘settlement’ and ‘relaxation’, often crop up in issues related to bolt failures. Settlement is the amount of microns lost between the contact surfaces in the joint, for example, the adaptation of the surface roughness. Relaxation is mainly caused by the relaxation of the stressed materials over time. The bolts or the clamped parts might lose their elastic strain, creating a loss of preload into the bolted joint.
“The challenge is related to the balance between the elongation of your bolt and the loss of compression in your parts,” explains Maxime Thonnerieux, Global R&D Director at Nord-Lock. “If you lose microns because of settlement then you will lose elongation of your bolt because everything is connected. The challenge for our customers is to first of all figure out if they have significant settlement in their joint or not.”
If the customer faces settlement or relaxation because of failing to install an adequate secure bolting solution, the next challenge, according to Thonnerieux, is ‘how to fix the problem?’ “If they can access the joint they can retighten it if they have figured out the problem, but this will be a time-consuming process. Sadly, in a lot of cases, they don’t figure it out until the issue has escalated.”
Having spent over 22 years at Austrian company Mosdorfer working with fittings for overhead transmission lines in the energy industry, Wolfgang Troppauer, Innovation Director, has had first-hand experience about how the combination of creep and poor installation techniques risked jeopardising power lines in Asia. This is one of the reasons why the 300-year old company is taking the problem of creep very seriously.
“This phenomenon of loosening bolts did actually occur a while ago,” reveals Troppauer. “What happened was a combination of a simple bolt and washer connection and relatively poor installation work by the linesman on site, resulting in undue pressure and, in the long term, failure of the bolted joint.”
Creep, especially in conductors, is one of the biggest challenges facing Mosdorfer, which supplies utilities and transmission system operators (TSOs) worldwide with tension and suspension towers, tension strings, fittings from low temperature steel, vibration dampers and roller suspension clamps for low voltage lines. In addition, the quality, and means, of installation on transmission lines varies dramatically from country to country.
“Generally in Europe, workmen are well trained and use cable carts to install the damping system on to the conductors, which makes it much easier for them to work,” he explains. “However, in some other countries, the linesmen have to physically climb onto the conductor bundles and hang 30 to 40 metres in the air. If the bolts are not tightened properly, there’s a real danger that the clamp will loosen and the conductors get damaged or fail completely. There are a lot of cost-saving issues which, at the end of the day, mean a higher risk of the product failing.”
One of Mosdorfer’s core products is spacer dampers which are fixed in bundled configurations to keep conductors at a certain distance from each other on the transmission line and are used to dissipate energy within the conductors. “These are really very important products because if you do not dampen the conductors and dissipate the wind induced energy, and, if, in a worst case scenario, they fall down, the line could blackout for hours or even days,” says Troppauer.
The spacer dampers have bolted clamp connections where the clamp is bolted on to the conductor. The hinged joint is tightened with bolts and nuts. The conductors are usually very dynamic as they tend to vibrate because of wind. If the connection is not robust, there’s a risk that the bolt may come loose.
The conductors are fixed between two towers which are 30-80 meters high; they are weights with static loads which must withstand dramatic temperature differences. At peak times such as lunchtime, the conductors become very warm as a result of the high demand for electricity, while during the night they cool down due to a reduction in the demand for power, as well as cooler evening temperatures. This can mean a temperature difference of 50 to 70-degrees Celsius, resulting in a high speed creeping process.
Conductor creep, due to constantly changing temperatures, can cause the diameter of the clamp to decrease and lose preload. Creep is also exacerbated by the fact that the conductors are made from aluminum, a relatively cheap, lightweight and high conductivity material with high corrosion resistance.
“In our business we have millions of these bolted connections so for each bolted connection that relaxes, there is a major risk of loosening. If there is too much relaxation and the bolt loosens, this could loosen the clamp from the conductor and the clamp could end up moving on the conductor. Even if it moves by just one millimetre, this will damage the aluminium conductor, which is a very serious issue,” says Troppauer.
“Digging a bit deeper into this incident, we discovered that in order to combat creep, the washer would require additional elasticity. We needed to come up with an alternative product that could withstand faults that arose during the installation process, plus extreme weather conditions and heavy loads, in such critical applications,” he continues.
Nord-Lock has recently launched the patented X-series washer that features a unique wedge-effect design combined with an exceptional spring effect. X-series washers have been specifically designed to protect bolted joints from spontaneous loosening and compensate for loss of preload caused by slackening. “The X-series is the result of our goal to design a system that would eliminate insecurity,” says Maxime Thonnerieux, who is behind the development of the X-series. “Beyond vibration and dynamic loads, the X-series allows us to serve customers with a multitude of other challenging application areas, such as painted or powder-coated surfaces, soft metals, composites and polymers.”
Wolfgang Troppauer and his team are currently testing the anti-loosening feature and the static and dynamic behaviour of the new X-series washers in the Mosdorfer vibration test lab in Austria. The final results will be available in 2013.
“As most of our products are designed for a 30 to 50-year lifespan, we need to simulate them in a 30-year environment. Once they’ve been installed on high voltage lines, it’s not easy to switch the lines off and leave people without power for hours,” Troppauer explains, adding that, so far, the results look promising.
Slackening: an explanation
Slackening is loss of preload due to plastic deformations without any rotary movement, and can cause loosening among other problems. There are three mechanisms that can cause slackening:
In bolted joints, creeping and relaxation occur simultaneously, thus both fall under the same category of relaxation, i.e. loss of preload due to plastic deformations from material restructuring over time.
FACTS: ABOUT MOSDORFER
Mosdorfer was founded in 1712 and initially made knives and blades before moving into manufacturing machine parts after the World War II. Mosdorfer specialises in parts for overhead transmissions, supplies utility and grid companies, contracting companies and wholesalers worldwide. The company produces over 30,000 different kinds of overhead transmission line fittings for voltages from 1kV up to 1200 kV. They also make low temperature steel fittings, vibration dampers, and roller suspension clamps for low voltage lines. Mosdorfer’s has clients worldwide, although they are mainly in Europe, the Far East, India, USA, South America and Canada. The company is located two hours south of Vienna, Austria.
COMBINING WEDGES AND SPRING
Nord-Lock’s new patented X-series washer combines the company’s wedge-locking technology which prevents spontaneous bolt loosening with an exclusive spring effect that protects against slackening caused by settlement and relaxation. This unique combination means the X-series can offer the highest security for critical bolted joints.
As with Nord-Lock’s original washers, each washer pair has cams on one side and radial teeth on the other to secure the bolted joint through tension instead of friction. The conical shape of the X-series washers also creates an elastic reserve in the bolted joint to compensate for the loss of preload and prevent slackening.
How it works
Upon tightening the fastener, the washers flatten and the serrations engage the contact surfaces. Since the cam angle (α) is greater than the thread pitch (β), the wedge-locking effect will prevent any rotation of the fastener. Directly after tightening, the joint settles and the fastener sinks into the surface material. The washers immediately deflect and the spring effect (Fs) counteracts the slackening movement (ΔL) of the bolt, thereby preventing loss of preload in the joint.
These multiple functions continuously act on the bolted joint to maintain preload and prevent spontaneous bolt loosening, serving as an effective solution for vibration, dynamic loads, settlement and relaxation. (More information: www.x-series.com)
First published in Bolted #1 2012.
Rolf Steinbock’s eureka moment, which came in 1974 while on a service call to one of his scrap choppers in a US steel mill, solved at a stroke two basic problems with big bolts. One is that they take a massive amount of torque to tighten. The second is that they have a nasty habit of working loose. When told that the bolts on the gearbox of the otherwise perfectly-functioning scrap chopper required tightening a couple of times a day, Steinbock had a flash of inspiration which he hastily scribbled down on the proverbial napkin. His solution worked perfectly, and the bolts never came loose again.
Steinbock’s solution, to split one big torque into a number of smaller torques, was commercialised as Superbolt. “He was actually surprised that nobody had thought of it before,” says his son, Allan Steinbock, who is the company’s Vice-President. Word spread to customers beyond the steel industry and, today, Superbolt’s multi-jackbolt tensioners can be found in a wide range of applications, from satellites to submarines to the Large Hadron Collider at CERN.
The problem Steinbock tackled is the fact that for bolt diameters bigger than about M24, it is very difficult to create enough torque to tighten or loosen a bolt. Simple physics shows that the torque needed to properly pre-stress a bolt increases by the third power of the bolt diameter. Therefore as the bolt diameter increases, the torque requirement for tightening it soars.
Traditionally this has required some heavy-handed methods, but all have their drawbacks. The sledgehammer gives little control, is inconsistent and often causes injuries. Thermal tightening, crane wrenching, hydraulic wrenching and hydraulic tensioning can be expensive, inaccurate, time consuming and unsafe.
However, Superbolt tensioners, which are designed as direct replacements for standard nuts and bolts, allow for the tightening of large bolts with simple hand tools, making bolting more accurate, faster and safer. “The primary benefit of Superbolt is the reduction of torque required,” says Steinbock. “You only need a handheld torque wrench or air tool.”
Superbolt tensioners utilise a ring of hardened jackbolts threaded into a nut body. The Superbolt tensioner is first threaded by hand onto a new or existing bolt or stud. Once positioned, bolt tensioning is accomplished by tightening the circle of jackbolts. A number of different product lines are based on the same basic concept.
“Because we are able to generate the proper preload, we achieve the proper holding power and we don’t have bolts or nuts coming loose, even in high-vibration situations,” says Steve Busalacchi, Engineering Manager. “This reduces expensive downtime: both downtime required for maintenance to retighten loose bolts, but also downtime caused by studs breaking due to insufficient holding power.”
Because Superbolt can be installed with hand tools, there are also time savings. “We install quicker and we remove quicker,” says Busalacchi. “People’s first impression is that it will take longer because they have several bolts to tighten instead of one. But once they see that you spin it on by hand and perform a quick tightening pattern similar to mounting a wheel on your car with hand-held tools, then they are impressed by the time savings we provide.”
The safety benefits of the Superbolt solution are highly appreciated at a time when many industries have implemented safety programmes. “Safety is a huge factor for us,” says Steinbock. “Alternative methods require equipment which can create extremely dangerous conditions. Our products are safe to use, and this is a huge benefit to our customers.”
While Superbolt products are available off-the-shelf, about half its sales are for special non-standard items in sizes all the way from M16 to over M1450. “What makes Superbolt unique is our ability to adapt to different situations, whether this is reviewing temperatures and changing materials, or customising designs to fit customer requirements. We are very adaptable and want to make sure that the customer receives the right solution and is satisified,” says Busalacchi.
Users of Superbolt products including General Electric, Siemens and Rolls-Royce appreciate the fact that that they can receive finite element analysis (FEA) carried out by independent organisations. “These organisations can provide an independent assessment of what we are telling our customers about our calculations,” says Norbert Schneider, Head of Engineering at Nord-Lock AG. “It boils down to safety and customers’ peace of mind.”
A current installation of Superbolts shows just how extreme an environment they can handle. About 2,000 tensioners made from exotic nickel-based alloys have been installed at the Max Planck Institute for Plasma Physics in Greifswald, Germany, where research will be carried out into the principles of a fusion power plant, which, in the future, could provide safe, green energy from the same process that takes place inside the sun.
“The Max Planck Institute has chosen Superbolt because the ambient conditions under which the research is to be carried out are quite horrendous – we are talking about -270°C, radiation and an absolute vacuum, as well as extremely high loads,” says Schneider. “Basically, we are dealing with outer space conditions found close to the sun.” And highly compact machines mean that there is no access for heavy tools. “The forces are so immense that they require large bolts and the only way to tighten large bolts without heavy tools is the Superbolt principle,” says Schneider.
Such characteristics and performance mean that once customers have tried Superbolt, they rarely return to the troublesome methods of the past.
“The hardest part, as with any product, is to get people to try it,” says Steinbock. “But once people have tried it, our repeat order rate is phenomenal. Customers now count on us to provide a quality product and we receive tremendous loyalty from them in return.”
SUPERBOLTS IN USE
Gold award from CERN
Superbolt supplied more than 1,500 high-strength mechanical tensioners, expansion bolts and multi-jackbolt tensioners to the CERN Large Hadron Collider in Switzerland. Multi-jackbolt tensioners were used for an application requiring a very high clamping force, but with limited space for tightening the bolts. Since only hand tools were required, the need to create anchor points for heavier tightening equipment was eliminated. Superbolt won the CMS gold award for its contribution to the project.
Ideal mine solution
The severe environment of an underground coal mine takes a brutal toll on crusher drum bits. It is a violent application with high RPMs and constant pounding as the bits cut into coal and rock and often break off, requiring repair underground, where accessibility is limited. With Superbolt, space restrictions are of little concern because only small hand tools are required, making the process easier and faster.
Cheaper and faster
Large ammonia reactors often require the use of large and expensive hydraulic tensioners, and tightening or untightening can take several days, working around the clock, and using cranes. At one such reactor, not only was the initial cost of the Superbolt tensioners only a fraction of the cost of the hydraulic tensioners they replaced, but the installation took two labourers only 5 hours.
First published in Bolted #2 2011.
Böllhoff is a family-run business based in Germany that goes back four generations. Today, it has more than 2,000 employees and its technology can be found all over the world. Over time, it has built a solid reputation for high quality, continuous improvement and a zero-tolerance policy when it comes to errors.
“AS FAR AS QUALITY is concerned, we have no room for compromise,” says Frank Nientiedt, executive board member at Böllhoff. “The same is true of Nord-Lock.” For the past ten years, the two companies have been working together. This has proven to be a highly successful partnership.
To ensure that its high standards such as ISO, IRIS (International Railway Industry Standard) and DIN (German Institute for Standardisation) are maintained, Böllhoff is regularly audited by several accredited certification bodies. Nord-Lock washers have also been subjected to similar testing and have had their effectiveness verified by independent institutions, including the German Federal Railway Authority (Eisenbahn-Bundesamt, EBA) and Deutsche Bahn.
Such accreditations have given Böllhoff full confidence in Nord-Lock’s locking capabilities, which is vital for the safety and reliability of its applications. It also welcomed Nord-Lock’s decision in 2011 to provide laser marking on all its washers. “By labelling its products, Nord-Lock ensures that the original can be distinguished from a copy at a glance,” says Nientiedt. “This is particularly important in applications where copies could cause problems.”
First published in Bolted #2 2011.
If the wheel nuts on your car come loose while you’re driving, the whole car will probably sustain damage. If a fastener in the engine breaks the entire car is also at risk of sustaining damage. The consequences of a fastener failure tend to be significant.
“Joining parts together is one of the most critical steps when delivering a product or a system. And the fastener is the key component in this,” says Maxime Thonnerieux, Global R&D Director at Nord-Lock.
It is absolutely crucial that fasteners fulfil their function. They must be safe and dependable throughout the product’s life cycle, and this is where the quality concept enters the picture.
When we use poor quality fasteners there is always a risk that parts cannot be assembled at all, or that they break during assembly. However, the most serious consequences often occur when everything seems to be in order during assembly, and the poor quality gives rise to faults at a later stage. This may be corrosion, loosening or fastener failure due to metal fatigue.
“We have seen many such cases where people have installed fasteners in good faith only for them to fail later,” says Bengt Sehlå at the Swedish Institute of Steel Construction.
For example, the less experienced manufacturer may accidentally expose fasteners to so much heat when applying a coating that hydrogen migrates into the steel causing embrittlement and steel strength to fall below the stamped value. The first step in ensuring good quality is to choose fasteners that meet the relevant standards. For example, in Europe all construction materials must be CE-marked, which entails their conformance with basic function and safety standards and compliance with prescribed inspection procedures.
“But there may be manufacturers that CE mark their products without having the right to do so, which is why it’s advisable for purchasers to frequently carry out random sample tests to see if the fasteners meet their standards,” says Bengt Sehlå.
Unfortunately it’s not possible to inspect every single fastener as it is often a matter of very large quantities, and the tests destroy the fasteners. There are also parameters that are difficult or impossible to check on the finished product, and which are therefore not covered by the standards. The best thing the purchaser can do to ensure he gets good and consistent quality is to set up a quality acceptance level depending on the potential consequences affecting the final product and accept only lower quantities of defected fasteners (PPM) from the manufacturer.
Manufacturing fasteners is a very complex process. A fastener is not just an object but a product that must fulfil its function safely and reliably. Every stage of the manufacturing process is important to final quality, some stages more so than others.
Supervision of the manufacturing equipment is an important element to verify consistent quality. If a machine performs the same operation on bolt after bolt, then its load curve should appear identical every time the machine carries out the operation. If the load begins to deviate from the usual curve it’s time to stop the machine and look for the problem.
The stage most critical to quality depends on the type of fastener. For nuts and bolts, thread rolling and the coating are good examples of critical stages; the coating, because it often involves temperature variations that may have an adverse affect on steel strength. It is also important that the coating is good, particularly in respect of its capacity to grip the steel, as well as its ability to ensure sufficient friction. It is friction that determines the amount of tension in a joint when it is tightened to a given torque.
“A poor coating may induce a higher or lower tension than anticipated,” explains Maxime Thonnerieux.
Excessive tension may cause a fastener to shear during assembly, while insufficient tension may lead to bolt loosening or fatigue failure at a later stage.
However, for washers it is the final coining that is of greatest importance. A quality-conscious manufacturer will naturally make certain that every stage of the manufacturing process is checked, and be extra thorough with those stages that are especially critical.
The quality initiatives of a manufacturer should also extend beyond the factory. Batch numbers on every fastener permit traceability, which forms an important part of the total quality management system. If a customer suffers problems with a fastener the batch number allows the manufacturer to re-check the manufacturing batch and perhaps identify the root cause of the problem. Sometimes it is also advisable to notify other customers that have purchased fasteners from the same batch.
So, as customers what do we do to make sure we’ve bought good quality fasteners? Identifying certification carried out by 3rd party bodies can be helpful in the search for quality-conscious manufacturers. The certification is a guarantee that the company fulfils precise standards.
Maxime Thonnerieux recommends the purchaser chooses fastener suppliers that can prove they meet the required standards and have approvals and certificates from different industries.
“I would ask the manufacturer how he could assure me that all his products fulfil performance requirements.
Are there any occasions when the choice of high-quality fasteners is of the utmost importance?
“Perhaps in structural applications for bolts exposed to high loads, temperature variations, shocks or vibrations, where safety really matters. But in my opinion, safety matters every time,” says Maxime Thonnerieux.
First published in Bolted #1 2011.
The Life Cycle Profitability (LCP) Calculator spells out in black and white the long-term economic advantages of using Nord-Lock washers. Data on component costs, labour costs, assembly time and maintenance demands are entered into the web-based tool, and within minutes the user receives a report on total life cycle costs, both in figures and in text form.
“The unit price of our products may sometimes be a bit higher than for alternative design solutions,” says Nord-Lock Applications Engineer Frida Cullin. “However, this tool shows that if you consider all the costs related to a bolted joint, not only the component cost, you will most likely save a substantial amount of money over time by using Nord-Lock.”
The LCP Calculator shows a step-by-step comparison of the total costs for Nord-Lock’s washers against competing bolt securing solutions, although it does not take into consideration the costs related to a bolt failure. “This is not a tool to show that Nord-Lock is always the cheapest solution over time,” says Global Account Manager Martin Schneider. “It can also show that Nord-Lock is more expensive, depending on the parameters and the application. But unlike other solutions, the chances of our solution failing are negligible, and the cost of bolt failure is the highest cost of all.”
All Nord-Lock sales staff have access to the calculator for demonstration to customers. Customers can also get a free temporary software licence to further explore the long-term cost savings of using Nord-Lock. The LCP Calculator is targeted at purchase managers and others at management level, but also at design engineers. “Twenty years ago design engineers were mainly focused on function, but now they have more responsibility for ensuring that their designs are also cost-effective,” says Schneider.
See also Performance Services – Sourcing.
First published in Bolted #1 2011.
It is a question that many an engineer or project manager has been faced with: whether to take the cheaper short-term option now and worry later about the consequences of machine failure; or to invest more from the outset in quality components, but wait years to see a return on investment in terms of lower cost of ownership. Increasingly, companies are seeing the benefits of the latter approach, a concept known as life cycle cost (LCC), which is defined as the total cost, from acquisition to disposal, of operating a machine or plant. But still many fail to recognise the economic advantages of taking the long-term view – not least when it comes to bolting.
“The concept of LCC is not well practiced in many organisations because they are driven by short-sightedness and a focus on short-term cost reduction instead of a focus on what drives cost,” says Christer Idhammar, founder and CEO of Raleigh, North Carolina-based maintenance management consultants Idcon Inc. “The right equipment might cost more, but the cost of ownership is lower. Long term you will have much lower costs and better maintainability, and therefore higher reliability.”
Statistics show that more than 50% of accidents and failures in industries are related to bolt failures, and Idhammar has come across numerous such examples in his work advising companies around the world. In one extreme case, where a bolt came loose and fell into the machinery at a paper mill, a granite roll worth over $1million was destroyed and the plant put temporarily out of action. Investing in a better bolt securing solution at an early stage could have saved such a massive expense years down the line.
Siemens Industrial Turbomachinery, based in Finspång, Sweden, employs the LCC approach in its manufacture of gas turbines for power generation. Several years ago the company conducted a study of its assembly process and concluded that securing the roughly 2,000 bolts on each of its multi-million euro turbines was consuming too much time and causing too many injuries among its workforce. The method that was used to lock the bolts was a washer that was manually deformed with a hammer and tongs, a so called tab washer.
By taking a cradle-to-grave view on the bolt securing solutions it uses, Siemens has reduced costs throughout the lifespan of its turbines, and at the same time reduced workplace injuries and the costs associated with them. “Changing from the old method to Nord-Lock washers meant huge savings in time, injuries and money,” says Martin Lindbäck, Head of Project Office at Siemens’ R&D department. “We save about 50,000 to 100,000 Swedish kronor (5,500 to 11,000 euros) per gas turbine during assembly. And as we have to disassemble the turbine for servicing about four or five times during its life and unlock a lot of bolts and washers, there is a huge saving on maintenance costs during that life cycle. Downtime is very important for our customers, so every hour we can save when the machine is stopped is very important.”
Deciding on and investing in the right equipment from the outset can prove to be thousands of times cheaper over the life cycle of a product, but is often steered by internal politics or accounting procedures. Idhammar explains that as a rule of thumb, when a project has reached the halfway stage time-wise, only about 5 to 8% of the total cost has been spent. “But by that stage you have made decisions that will lock in about 85% of the future life-cycle cost,” he says. “You have decided at that point on having just one pump, instead of one pump plus a backup; on having stainless steel piping instead of galvanised; on having a bolt-securing solution that doesn’t guarantee that bolts won’t come loose. These are crucial decisions.”
If, at that halfway stage, you want to make modifications, it will cost 100 times more than if you had thought of it from the beginning. “And if you start operating the equipment and discover five years later that you have a problem, it typically costs you 1,000 times more,” says Idhammar.
See also Performance Services – Sourcing.
First published in Bolted #2 2010.
In more than 20 years of working with joining technologies, Bernard Tollet has never been bored. “There are so many levels to be involved in: design, purchasing, production and quality assurance,” says the 57-year-old, who had a long career at Volvo Trucks and who is now employed at the Technical Centre for Mechanical Industries (CETIM) in Saint-Etienne, France, as an assembly technology engineer. “I have worked in all four departments; never saw the same thing twice because everything is always evolving.”
Tollet’s long experience has taught him that most fatigue failures of bolts are caused by incorrect mounting or tightening. Manufacturers therefore have to focus on controlling their bolt tightening processes as the main way of reducing fatigue failure in their bolted joints. While other causes of fatigue failure include machining of parts, heat treatments, surface treatments, design, choice of materials and conditions of use, tightening accounts for more than half of all cases.
So how do you achieve this controlled tightening? Tollet speaks about three important steps in achieving a good assembly. The first step to a controlled tightening is of course controlling the design and dimensions of the joint to ensure that the aimed value of preload is correct, the second is controlling the fasteners, and the third controlling the tools and the tightening method.
“The issue for a design engineer is: ‘What amount of preload do I need to make the joint withstand static and dynamic loads?’” says Tollet. “Whereas all that matters for a producer is: ‘How do I control the amount of preload installed in the joint using available tools?’”
Control of the fasteners is normally linked to control of the torque-tension relationship (see figure 1, overleaf) of the assembly, namely the friction involved in the assembly. Lubrication is the most common way to get low and even friction conditions in the joint. Low friction means less tightening torque is needed to achieve the right clamp load, and also gives much more predictable behaviour of the joint. Another way to decrease friction is by using coatings on the fasteners. “Only 20% of nuts and bolts sold in Europe feature a friction coefficient requirement (a surface treatment for friction control)” says Tollet. “These are mainly used by railway, automotive, aeronautics and steel construction industries. And when all is said and done, applying a surface treatment for friction control represents an increase of only 3–5% in the base cost of a fastener. Choice of fasteners and verification of their specifications are the responsibility of both the design and the purchasing department.”
Controlling tools and tightening method means investigating each characteristic and parameter of the tightening process. The most common tightening method is torque tightening, which involves applying a known rotational force to the assembly to achieve what’s known as preload, or clamp load. This is the tension in the assembly which has been specified by the design engineer for best performance of the machinery. According to Tollet, torque tightening using correctly calibrated tools is normally sufficient to achieve a good clamp load for most industrial bolted joints. But sometimes the function of the bolted joints only allows small differences between maximum and minimum allowable clamp load (see figure 2, above). “Engineers need assurance that the preload achieved in production is within the required range. For these cases you might need to consider switching from the torque tightening process to another method which might be more accurate or more suitable for the particular production conditions. The big responsibility of control of the tools rests with the production department. It has the responsibility for ensuring the required tightening operation has been completed, in sequence when necessary, that no bolt has been tightened twice and that all the other assembly items (especially washers) are in place.
The business case for tightening
First published in Bolted #1 2010.
If you hold a steel fastener in your hand, it is almost never the actual steel that you touch. There is a thin coating between your fingers and the steel – just a few micrometers thick – that improves the fastener’s performance in one or more ways. It can protect against corrosion; it can give the material reduced friction; it can also enhance the aesthetic value, which is the case with chromed wheel bolts.
“The cheapest and simplest type of corrosion-preventive coating is pure galvanisation. Galvanisation offers adequate corrosion protection in many, but not all, cases,” says Lars Askengren, MD of SYF (the Swedish coating association).
When pure galvanisation is not enough, zinc alloys – a blend of zinc and other metals – can be used instead. If even greater protection is needed, zinc flakes can be used. Zinc flakes offer a number of advantages such as equal or better corrosion protection with a thinner layer and eliminated risk of hydrogen embrittlement.
Another purpose of the coating can be to lubricate the material. In order for torque-controlled pneumatic screwdrivers to tighten a bolt correctly, the right amount of friction is required. Friction-reducing substances such as polytetrafluoroethylene (most commonly known by the brand name Teflon) can be mixed in with the coating to control the amount of friction. Waxing is also used as a coating method for the same purpose.
Environmentally driven development. The transition to more environmentally sound techniques is a clear trend with respect to coatings.
“Above all, there is a major transition underway to minimise the use of hexavalent chrome (Cr6+). Since 2000, the EU has introduced tough restrictions regarding the way in which Cr6+ may be used in e.g., cars and electronic products,” says Ingegerd Annergren, department manager at the corrosion and metal research institute Swerea Kimab.
“Hexavalent chrome has been used extensively as the outermost layer on top of the actual coating. This applies, not least, to screws, which are handled in large quantities where they come in contact with each other and the outer coating is damaged. Cr6+ can repair this damage by forming different surface compounds but this reactive characteristic also makes it hazardous to the environment. As an alternative, nano-additives are being studied to see if they can provide the same self-repairing effect,” says Ingegerd Annergren.
Acceptable replacements for Cr6+ are available today but they are currently more expensive. Other coatings also have an environmental impact to one extent or another, through e.g., the use of chemicals and energy consumption. However, in many cases they nevertheless result in a reduced overall environmental impact in the opinion of Csaba Madru, an Applications Engineer at Nord-Lock.
“Dealing with corrosion damage not only costs companies vast sums of money; it also has an impact on the environment since replacement parts have to be produced, shipped, kept in stock and so on. Consequently, a coating that extends the service life of a metal can result in a reduction of the overall impact on the environment,” says Csaba Madru.
More functions save money. Another trend in this area is the demand by more and more customers for coatings with a greater number of functions. Companies are increasingly conscious and it is becoming more common to pay a bit more for e.g., self-lubricating screws since this eliminates one working step and results in more efficient assembly.
“Companies focus more and more on the total cost. They see that a small increase in the purchase cost can lead to a considerable reduction in the ‘life-cycle-cost’,” says Csaba Madru.
It is also becoming more common to colour the fastener when the coating is applied. This is sometimes for aesthetic reasons but it is also customary to use colour-coding to distinguish different components from one another.
Corrosion protection is a complex process since it all depends on the ambient conditions. Material that is highly resistant to corrosion in a particular environment may be completely unsuitable and corrode quickly in other environments. Carbon steel and low-alloy steel, which eventually rust in damp environments, are otherwise very corrosion-resistant to concentrated sulphuric acid. And aluminium holds up well against nitric acid despite the fact that it corrodes readily when exposed to other strong acids and alkalis. Therefore, the choice of corrosion protection is determined by the exact conditions that the material will be exposed to: which acids, alkalis, salts, organic compounds, etc., will there be in the ambient environment? This makes it essential to always consult an expert when choosing a coating.
Salt spray tests
Standardised salt spray tests are used to measure the effectiveness of different types of corrosion protection. The results show the number of hours that the material can withstand a sprayed saline solution before visible white rust (zinc oxide) and red rust (ferric oxide) are formed.
Approximate values for different types of corrosion protection (hours to red rust)
By creating a thin outermost layer of corrosion products, e.g. a metal oxide on a coating, the rate at which corrosion takes place can be reduced. This is called passivation and it is often done as a follow-up coating of zinc or a zinc alloy.
Hot dip galvanisation, hot dipping process
The material is given a coating by being dipped in molten metal (above 420 degrees Celsius for pure zinc). In most cases, this provides better protection than electrogalvanisation.
Chromisation, i.e., follow-up coating with hexavalent chrome (Cr6+), is now used less and less due to the fact that Cr6+ is very hazardous to the environment. Passivation is used instead (see above).
In the process of electrogalvanisation, the material is dipped in a solution containing metal ions and is connected to a source of electrical current, resulting in the formation of a thin coating on the surface of the material. One negative side effect is that hydrogen embrittlement – a reduction in structural strength – can occur in, for example, high-strength steel. However, this can be eliminated by follow-up treatment.
During phosphatising, a thin coating of iron or zinc phosphate is deposited on the material. The phosphate coating offers a certain amount of corrosion protection while at the same time providing a good surface for painting or lubrication. Both untreated and zinc-coated steel objects can be phosphatised.
The material is dipped in a solution containing metal flakes/scales. It is then heated so that the flakes melt together, forming a durable shell that bonds strongly to the material. Coating with flakes is becoming more and more common for several reasons: it provides very effective corrosion protection, it can be combined with a mixture of e.g., lubricants and it is a relatively environmentally friendly process.
The plastic PTFE, known by the brand name Teflon, is often used in combination with metallic corrosion protection to give fasteners the desired amount of friction. In addition to extreme friction characteristics, PTFE is also a durable plastic that does not age, is not broken down by UV rays and is highly resistant to heat and chemicals.
Xylan is a brand name and a family of fluoropolymer coatings which often contains PTFE. Xylan® provides lubrication as well as wear, heat and corrosion resistance. The coating can be colored to suit different applications.
Zinc is less noble than iron, which in simple terms means that zinc will corrode first in water when zinc and iron are together. Even when the iron is directly exposed to corrosion, it gets a certain amount of protection from the presence of zinc. In relative terms, zinc is also an environmentally friendly metal that is available in large quantities and is reasonably cheap. And despite the fact that it “sacrifices itself” for the iron, it corrodes very slowly in most conditions. The combination of all of these characteristics gives zinc a unique position with respect to corrosion protection. Zinc is used both in pure form and in alloys. It is applied through the processes of electrogalvanisation, hot dip galvanisation or flakes (see the section Zinc flakes to the left).
Combining other metals enhances the zinc coating’s characteristics in various ways.
Zinc-iron contains a very small amount of iron – only about 1–1.5 percent – but this is enough to make the material better suited to passivation (see the section Passivation to the left).
Zinc-nickel normally has a nickel content of about 12 percent, which gives this alloy several advantages compared to pure zinc. Among other things, it can withstand higher temperatures and functions better in contact with certain other materials such as aluminium and pressure-treated wood.
Zinc-aluminium is applied in the form of flakes and is a common coating on high-strength steel used in very demanding conditions such as offshore and the auto industry.