Screw lubrication

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.

Friction control
Galling
Vacuum
Lubricant selection

Friction control
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.
Typical coefficients of friction in power screws.
combination dry lubricated
steel-steel
steel-bronze
steel-brass
steel-cast iron
steel-plastic		 0.8
0.4
0.35
0.25
0.15		 0.15
0.15
0.20
0.15
0.12
Typical coefficients of friction in screw joints.
oiled MoS2-lubricated Zn-plating
0.12...0.18 0.08...0.12 0.12...0.18

Galling
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.

Vacuum environment
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).

Lubricant selection
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".