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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. |
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When two surfaces are brought into
contact as with fasteners, the high points as seen in fig 3
take the initial load. |
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When pressure or static load is
applied, these high points squash (deform) until the real
contact area is increased to take the load. |
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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. |
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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. |
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Galling, also known as seizing, cold
welding or pick up, occurs under higher stresses where
stronger bonds or welds are formed between base metals,
mainly because the contact surfaces being deformed are
larger. |
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Generally the causes behind galling
are due to high torque/tightening levels and fast
application methods like speed/air guns. |
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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. |
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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.
- Sulphur to Sulphur atoms are weak and break easily.
- Sulphur to Molybdenum and base metals are strong. |
