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 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.
Nord-Lock washers secure bolted joints with tension instead of friction. Watch this video and let us explain how it works!
First published in Bolted #2 2017.
Q: What do the markings on bolts and nuts mean?
A: Bolt heads and nuts are often marked with numbers, letters, dashes, slashes, dots, or an assortment of other marks. Fasteners commonly have two different markings: a unique manufacturer identification symbol – such as letters or an insignia – and information about the fastener strength. Such markings differ based on how the fasteners were made. See the table for the alloyed steel metric and stainless-steel metric fasteners that comply with ISO standards. UNC thread fasteners mainly comply with ASTM standards.
Due to lack of space, markings can be missing on smaller sizes, such as those with diameters below M5 according to ISO 898-1. However, the bolt class must be marked on the head above this size.
First published in Bolted #2 2017.
Q: Can I use Nord-Lock stainless steel washers with steel bolts?
A: You can, as there is no difference in thread pitch between steel and stainless steel bolts. However, it is always best to use the same material for all parts of the joint. If you use a stainless steel washer together with high-strength fasteners of grade 10.9 or 12.9, you might deform the washers. These are only surface-hardened, and with a very high pre-load, the softer inside might incur “plastic deformation”. Steel bolts of grade 8.8 or lower might work in many applications, since the mechanical strength of grade 8.8 is similar to the one for stainless steel washers.
Another important aspect to consider when designing a bolted joint, including stainless steel washers and steel bolts, is corrosion, especially so-called galvanic corrosion, which may reduce the product life dramatically. Galvanic corrosion damage is induced when two dissimilar materials are coupled in an electrolyte. When a galvanic couple forms, one of the metals becomes the anode and corrodes faster than it would by itself. The other material becomes the cathode and corrodes slower than it would alone. Nord-Lock steel washers with Delta Protekt coating use the principle of controlled galvanic corrosion. Zinc material in this coating protects the cathode (the washer steel material).
First published in Bolted #2 2017.
Any machine with moving pivots will eventually experience lug wear. The most common are applications subjected to heavy loads and vibrations, such as mining and construction equipment. Other common applications include industrial presses, wind turbines and moveable bridges. Any moving pivot in just about any application will experience lug wear at some point – the higher the demands, the faster the onset. When it happens, it will lead to a loss of precision and control.
There are three main reasons why lug wear is inevitable when using conventional straight pins:
The most common solution to lug wear is to repair the lugs with welding and line boring. The first step of this is to unload the pivot and dismount the pin. Then the line boring equipment needs to be lined up and “mounted” to the equipment. The worn lugs are rebored, filled up with weld, and finally rebored with a fine cut to the original diameter and tolerance. After removing the line boring equipment and repainting the lugs, a new replacement pin is installed. This whole process can take anywhere from a few hours to a few days, depending on the size and complexity of the installation. During this time, the machine is inoperable.
Despite the time and costs, this method is generally accepted as unavoidable. “It’s just something everybody does because everybody else does it, and they’re not even aware that there is another way,” says Jonny Wiberg, development & research engineer, Expander System. “Repairs are just accepted, and people don’t even look for another solution.”
Over the years, engineers have searched for better solutions to the lug wear problem. None of the previous attempts has proven universally effective. One option is to use a pin that fits as tightly as possible into the lug’s holes, practically eliminating the play between the two, and ensuring the best possible pressure distribution for a straight pin. Not only does this make the pivot expensive and the pin difficult to mount; over time, the lug hole will expand anyway.
Using the temperature method, the pin is frozen and then allowed to get warmer and expand once installed, creating a perfect press fit in the lugs. Tolerances of both pin and lugs need to be exceptionally tight – down to some hundredth of a millimetre, or tolerance grade 6. This significantly increases the cost of the pivot. Pivots with frozen axles are often considered maintenance-free, but they are impossible to maintain, as the axle can’t be removed.
Another solution is to improve the strength of the lugs with bushings. However, this will only prolong the onset of lug wear, and will not eliminate the problem completely, as the bushings need to be replaced several times during the equipment’s lifetime.
None of these solutions will completely remove the need for costly and time-consuming lug repairs. In contrast, the Expander System can potentially eliminate lug wear once and for all. It works by using a pin with tapered ends and expanding sleeves on each side. When it is installed, the sleeves expand radially, so that they fill the lug to create an exact press fit.
As the sleeves of the Expander System expand into the lug, they can take up unevenness or deformation, eliminating the need for welding and line boring. This significantly reduces the time needed for installation as well as the machine downtime. The most time-consuming process for installing the Expander System is the dismantling and removal of the original pin – a process that is also necessary before welding and line boring. In a recent example, the Swedish Expander System company was asked to do a cost comparison for a 70-millimetres axle. Considering the cost of the expansion bolt, the cost of pin removal and installation, plus the income loss from downtime, the total cost of the Expander System solution was calculated at around 500 euros. A conventional pin was around a third of the purchase price, while the costs of removal and installation remained the same. The time needed for line boring, in addition to the time taken for the transportation of line boring equipment, and the loss of income from significantly higher downtime, all contributed to a total cost estimation of over 2,300 euros.
Using the Expander System will not totally eliminate the need for boring, but for the welding process. It will eliminate the lug wear problem for the lifetime of that pivot. Using conventional pins, lug wear would inevitably return and the repair procedure would need to be repeated. In a typical application, this happens three to four times during a machine’s lifetime or every 3,000 or 4,000 hours. This means that the cost savings can amount to thousands of euros – for each machine.
How a rusty nail led to an award-winning innovation
In the 1950s, twin brothers Everth and Gerhard Svensson were building roads throughout Sweden, and becoming increasingly frustrated with the downtime and repairs caused by lug wear. One day, when a pivot pin was coming loose, Everth improvised and took an old rusty nail to fix the pin in the lug hole.
As a temporary solution, the rusty nail worked quite well and inspired Everth to develop the Expander System. For many years, the twin brothers used expander products as they continued to build roads. However, it wasn’t until 1986, when Everth’s son Roger realized the ingenuity of his father’s solution, that the concept was patented and the company Expander System Sweden AB was founded. In 1987, the Swedish Minister of Industry awarded the Expander System with the Innovation Development Award, in memory of Alfred Nobel. Today, the Expander System is installed in millions of machine joints globally.
Getting over 6,000 extra operating hours
Lug wear is a widespread problem for machinery pivots. It has cost users of machinery lots of money through the years – for repairs as well as for downtime. This is something that the Expander System can put an end to.
The Expander System will in most cases cost more than a traditional straight pin. But when all costs are fully calculated, including the time and costs associated with welding and line boring, and the loss of production due to downtime, the Expander System will prove to be significantly more cost-effective. The full extent of savings depends on many different variables, but it is fair to say that the higher the frequency of lug wear and the higher the costs of downtime, the greater the potential savings.
For Swedish construction machine supplier Maskinia AB, every minute of downtime for machine repairs means lost income. This is why they have been using the Expander System since 1999.
Recently, an excavator was brought in for repairs after 3,700 hours of operation. Using the Expander System, the boom mounting axle was replaced in just 6 hours. By contrast, the repair would have taken 3–4 days if it was replaced by a traditional pin, using the common method of welding and line boring.
Lars Malmén, Aftermarket Manager at Maskinia, says that, “The Expander System admittedly costs more than a traditional axle, but if you include repair time and stoppages with loss of income, the difference is clearly to the advantage of the Expander System. If you add the fact that Expander offers a 10-year function warranty, you can count on at least 10,000 problem-free operating hours – compared with the 3,700 that is regarded as normal for a traditional pin.”
First published in Bolted #2 2013.
Q: When and why does galling and seizing appear?
A: Galling is caused by a combination of friction and adhesion between metallic surfaces during sliding. When galling occurs material is adhesively pulled from one surface leaving it stuck on the other in the form of a lump. This process spreads rapidly as the built up lumps induce more galling.
The tendency of material to gall is affected by the material’s ductility. Typically, softer materials are more prone to galling while harder materials are more resistant.
In bolting, thread galling appears during fastener tightening as pressure builds up between the contacting and sliding thread surfaces. Thread galling commonly occurs with fasteners made of stainless steel, aluminium, titanium, and other alloys.
In extreme cases galling leads to seizing – the actual freezing together of the threads and bolt lock-up. Continued tightening may lead to the breakage of the fastener or result in torn off threads.