| Screw lubrication | ||||||||||||||||||||
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The ultimate goal of screw lubrication is to control friction and to prevent galling. Difference must be made between lubrication of screw joints, screw drives, coated screws, stainless screws, screws applied in vacuum, at high temperature etc. |
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 Friction
control |
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The coefficient of friction in screw joints needs to be predictable
within a small range. Most time deterministic value's of the
coefficient of friction are listed in tables, without any
information about the range within the friction may vary or the
standard deviation that might be expected. The value of the
coefficient of friction however is much less important than the
standard deviation. The tightening torque in screw joints is
predominantly defined by the friction coefficient. It follows that
the percentage over which the coefficient of friction may vary will
be approximately the percentage over which the tightening torque can
be controlled.
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Galling |
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Stainless steel fasteners / high temperature application:
Special high temperature pastes are used for the lubrication of
screw joints applied at high temperatures. These pastes comprise a
basic oil which is mixed with different solid lubricants and may
contain additional additives. Additives as sulfur, zinc, lead and
aluminum which may lower the friction may increase the risk for
stress crack corrosion at high temperatures. Screws for high
temperature applications are made of high heat resistant generally
austenitic steels with alloy components as nickel, chromium,
molybdenum which enable only very thin oxide layers to develop on
the surface of the thread flanks. The thin "protective" oxide layer
can easily be penetrated by roughness summits resulting in a high
tendency to adhesion (galling). Therefore special thread lubricants
are formulated to prevent galling and tribo-corrosion in order to
lead to a defined tightening and to ensure non-destructive release
of the joint. Dry solid lubricants can be applied as an alternative
to greases and pastas. The dry film can easily be applied in
centrifuges or spray drums. The main advantage of dry-film coated
screws is clean handling. |
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Vacuum
environment |
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In high vacuum and related semiconductor manufacturing equipment
vented screws using a coaxial hole down the middle of the screw are
applied. A stainless steel (vented) screw may seize inside a
stainless steel tapped hole or stainless steel nut. This makes it
difficult or impossible to remove the screw without destroying it.
To reduce the threat of galling, use of a vacuum stable lubricant
such as Molybdenum Disulfide (MoS2), Tungsten disulfide (WS2),
graphite, or silver plating is recommended. Silver seemed to be best
overall because of it’s lubricating qualities. When screwed in
aluminum silver becomes very sticky. In aluminum titanium nitride (TiN)
coated screws performs well (ref. LIGO In-vacuum Fastener Galling
Experiment Tim Thompson, Edward Romero, and Franz Biehl August 27,
1998). |
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Lubricant
selection |
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Lubricant selection for power screws: Lubricant selection for screw joints: Zn-plating To be edited. For details you are referred to Chapter 8 "Lubricant selection and lubrication management".
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