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Article By Anders So¨derman & Patrik Lundström Törnquist, BUMAX
Galling is a common form of mechanical wear caused by excessive friction between two moving surfaces, usually metal surfaces. The process involves material being torn up and transferred between the two surfaces when they are under sufficient load to compress the surfaces together.
Standard fasteners prone to galling
Standard stainless steel bolts and fasteners have a tendency to gall under certain conditions due to their specific properties. Thread galling can occur with standard fasteners when pressure and friction cause the bolt threads to seize to the threads of a nut or a tapped hole. Severe galling, known as “cold welding”, can cause the two surfaces to fuse together, which makes the joint impossible to remove without cutting the bolt or splitting the nut.
The consequences and implications of galling should not be underestimated. A galled fastener may not be able to achieve the necessary pre-load – especially in case of dynamic loading. This jeopardizes the entire reason for using fasteners, i.e. to securely fasten two surfaces together, which is complicated by the fact that an application may involve hundreds or even thousands of stainless steel fasteners.
Imagine critical fasteners in the rotating parts of a chemical pump or propeller galling. The joint is likely to be subject to a fatigue breakage, which will at least require costly maintenance and downtime for the customer or end-user. However, in the worst-case scenario, the fatigue breakage of critical fasteners can have serious safety implications that might result in accident or injury. Galled fasteners are also much more susceptible to corrosion, which can ultimately result in breakages.
Risk of catastrophic failures
Galling is an issue in many segments and applications, wherever stainless steel fasteners are used, such as the marine, automotive, oil and gas, chemical processing, energy, defense, pulp and paper, construction and general engineering sectors. There is a risk of catastrophic failures if the thread galls before sufficient torque has been obtained. That covers most if not all machines and devices with moving parts. In extreme cases, galling can lead to the fastener breaking and failure of the application. The risk is highest for applications that are exposed to dynamic loads. However, galling can be overcome even in the most challenging of applications – particularly by selecting premium quality stainless steel fasteners.
Let us look at a real-life example of the galling issue and how it can be solved: NARA Machinery, a Japanese specialist manufacturer of powder processing equipment for e.g. the pharmaceutical sector, experienced severe galling and gorging problems with its machines. Many of the machines require a high degree of air tightness with firmly tightened stainless steel screws. However, as screws that have powder contact cannot be lubricated, they are highly susceptible to thread galling issues. The standard stainless steel screws used were simply not good enough. Besides galling, they also had air leakage issues and required frequent removal for cleaning and maintenance purposes.
Such issues and maintenance needs resulted in costly procedures and downtime for NARA Machinery’s industrial customers. In the worst instances, galled screws had to be cut and carefully removed and replaced to protect against dust getting into the machine. The remedy to this quality problem was to use premium fasteners (the BUMAX 88 with higher molybdenum content that has been specifically developed for demanding applications) instead of standard screws. Since installing these premium fasteners, none of NARA’s customers has reported any issues with galling or air tightness. The use of premium fasteners has even allowed NARA to develop brand-new high-quality powder-handling machines.
How can galling be prevented?
It is important to consider how galling can be avoided from the outset when designing a particular fastener application in order to avoid a great deal of rework, maintenance and costs further down the line. Basically, to minimize galling and its negative consequences, it is essential to be aware of the concept of galling and ensure that only premium stainless steel fasteners are used in critical applications.
Premium fasteners may look similar to standard stainless steel fasteners, but their superior material properties can make all the difference with galling.
Explain to your fastener supplier exactly how the fasteners will be used to ensure you optimize galling resistance. Premium stainless steel fasteners also typically offer other properties that are essential in many critical fastener applications – including corrosion resistance and high tensile strength.
The benefits of greater galling resistance
Using premium fasteners that are at a lower risk of galling brings the following general benefits:
- Less maintenance, stoppages and other production disturbances. Most machines used in production are designed to clock up thousands of work hours with very little downtime. Any downtime can be very costly in terms of repairs and lost operational revenues.
- Increased safety and security. If a fastener malfunctions or breaks, the result can be dangerous and even disastrous, depending on the application. Just imagine a weakened vehicle, vessel or building…
- Increased cost-effectiveness. Using the best fasteners from the very start can save a lot of money down the line. The extra cost for premium fasteners pales in comparison to repair and replacement costs.
“Galled fasteners are much more susceptible to corrosion”
Besides choosing the correct fasteners, there are other preventative measures to take:
Correct material selection
Selecting the correct material for your specific application is extremely important as certain types of stainless steel are more prone to galling due to their atomic structure. In particular, strain-hardened stainless steels that have been cold formed provide excellent galling resistance due to their improved strength and reduced ductility.
Choose fasteners with a perfect fit
As galling is common between metal surfaces that are in sliding contact with each other, premium fasteners that are designed to perfectly fit together can significantly reduce the risk of galling as they minimize movement and friction. Here, high-quality threads with less surface deviations that can rub together and lead to galling issues are particularly essential.
Lubrication is an important way to reduce galling by allowing the two materials to slide past each other without causing friction. Some leading premium fastener manufacturers use custom-made wax to ensure an optimal friction coefficient. Anti-seizing and anti-galling lubricants can also help reduce galling.
Don’t use damaged fasteners
A bolt with dented or damaged threads has a significantly increased chance of galling. Check all fasteners for damage that may have occurred during shipping. Dirty bolts with debris in the threads can also greatly increase the risk of galling – so make sure you only use clean bolts.
As most stainless steel is more sensitive to high temperatures, tightening bolts more slowly can reduce friction and heat that can cause galling. This may involve avoiding the use of power tools that can cause excessive friction and heat. Calculations should be made for particular applications to determine the kind of tools that can be used.
You must ask yourself: How valuable are your production and operations assets, all the machinery and property that keeps the company going and brings in the revenues? Is it worth gambling on cheaper, galling-prone fasteners and bolts rather than premium stainless steel fasteners that are specifically developed to minimize galling? Executives in many industries must be aware of the potentially disastrous implications of fasteners failing due to galling, and do their utmost to prevent this from happening. This cannot be left to chance.
About the authors
Anders So¨derman is Technical Director of BUMAX. He has worked in the stainless steel industry since 1995 and has an M.Sc. in Materials Science from the Dalarna University in central Sweden.
BUMAX is a Swedish specialist manufacturer of premium stainless steel fasteners, see www.Bumax-fasteners.com/