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Understanding and preventing Metal Thread Galling.

galling

Stainless steel is a popular fastener material due to its inherent resistance to corrosion and generally higher tensile strength than commercial low carbon steel. However, during installation users may encounter galling resulting in
excessive wear, or in the worst case, seizing of the mating components, Figure

By identifying the likely causes, designers and assemblers can safeguard against this mischievous, and sometimes misunderstood, phenomenon. While galling can occur between a multitude of mating surfaces, this bulletin focuses on the causes and prevention associated with threaded fasteners.

In technical terms Galling is a type of wear associated with the joining of two parts of material, and is actually precluded by another type of wear ‘adhesion’, which takes place before Galling can occur.

Surface construction plays a large role and when greatly magnified as shown in the diagrams, we find the thread surface is in reality rough and irregular.

galling

• When two surfaces are brought into contact as with fasteners, the high points as seen in fig 3 take the initial load.
• When pressure or static load is applied, these high points squash (deform) until the real contact area is increased to take the load.
• If relative motion is introduced (ie spinning a nut on a bolt or tightening) then wear may occur due to the protective oxides rubbing off at the high points, exposing the base metals and causing them to weld together.
• Adhesive wear occurs when the pressure/load is small and the weld is weak. A small amount of base material either transfers to the stronger side, or floats independently in the joint and is known as plastic deformation. This can be evidenced by spinning a nut on a bolt with your fingers, and noticing when the nut catches or sticks. When you push the nut over that point the transference of materials has occurred even though not seen by the eye.
• Generally the causes behind galling are due to high torque/tightening levels and fast application methods like speed/air guns.
• In particular, stainless steel presents the majority of galling problems, mainly due to its Low Heat dissipation at the
point of contact where the build up occurs (eg the high points) and what is recognised as a high co-efficient of friction, which basically means it heats up very quickly when rubbed together.
• Solid type lubricants (Molybdenum Di-Sulphide) work as seen in fig 4, by creating layers over the materials that form a weaker bond than the base metals and forming a barrier that the high points cannot push through and therefore not allowing contact of the base materials.

 

Best results for preventing galling are achieved when you;
• Lubricate where possible before use with a solid type lubricant.
• Keep torque within guidelines.
• Use low speed applicators.
• Clean, grit free product is best.
• Select right quality and grade combination.
• Adjust torque guidelines for lubrication.

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