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21/08/2009
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Stainless steel fasteners are used in many applications where zero foreign object debris (FOD) is essential.
 Such applications include the food industry, clean rooms, electronics, aerospace, satellites, safety critical applications, space and other vacuum environments.
Kato first introduced the Tangless insert to eliminate the potential problem of tangs being broken off after installation and being left or getting into critical parts of equipment.
A further problem with stainless steel inserts is that when they are used with mating stainless steel fasteners, they can be susceptible to galling. At the least this may cause some damage to thread forms or in severe cases prevent disassembly of the threaded joint. To lessen the effect of galling, some users apply lubrication before joint assembly. However, this can lead to unreliable torque/ tension results in the joint and also have FOD issues that may not be acceptable for a critical application.
Kato's latest development combines the benefits of a Tangless insert with special anti-galling material so that customers with critical applications can have what it describes as 'a truly FOD free' solution. The material used in the company's new insert is an anti-galling wear resistant austenitic stainless steel, comparable with materials like Nitronic 60 said to provide galling prevention without the need for lubrication or plating.
The problems with using lubrication
Galling is a cold welding process which can occur when the surfaces of male and female threads come into contact. Stainless steel fasteners are particularly susceptible to thread galling, although it also occurs in other alloys which self generate an oxide film for corrosion protection, such as aluminium and titanium.
During assembly, pressure builds between the contacting thread surfaces and breaks down the protective oxides. With the absence of the oxide coating, the metal high points can shear and lock together. Minor galling may cause only slight damage to the thread surface and the bolt may still be removed. However, in severe cases galling can completely weld the thread and prevent disassembly.
Thread lubrication is one of the most effective measures to lessen the potential for galling. The lubricant reduces friction which is the key element in thread galling. However, attention must be given to the torque tension relationship which can be altered with the use of lubrication. You must also be aware of the end use of the fasteners before settling on a lubricant.
Stainless steel is frequently used in applications that require zero FOD. This would mean that lubricants or plating cannot be used.
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Author
Chris Shaw
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