Surface plating of vacuum suitable screws

Surface plating provides well defined friction conditions in screw joints.

Cleanliness Requirements in vacuum Technology prohibit the Presence of Lubricants and screw Waxes. Undefined Friction Conditions and a dramatic Increase of Galling Risk within the screw Joint are Consequences.

Stainless steel is the most popular material in vacuum technology. Typically stainless steel grades A2-70 and A4-70 are used for vacuum suitable screws. High chromium content leads in the formation of a surface passivation layer from chromium oxide. Especially while creating a screw connection, the passivation layer can crack through locally high contact pressure. Atomic bonds are formed between the threads, the parts are cold welded, galling has occurred.
    
Surface plating of vacuum suitable screwsVacuum suitable screws are plated with silver or with gold. Plating provides a barrier layer between the threads. The risk of galling is minimised considerably. Screw joints can be applied with reproducible and reliable preload forces.



Vacuum suitable screws lubrication

Adjusting screws and elevating screws inside a vacuum environment often have to be lubricated. When lubrication is necessary, coatings from molybdenum sulphide (MoS2) or tungsten sulphide (WS2) can be applied on vacuum suitable screws. This wear reducing coatings are established solutions in mechanical engineering and plant construction. Coatings are applied to the screw surface as microscopic particles without using organic paste. Due to the comparatively poor adhesive force the generation of particle contamination while using such screws has to be considered.

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