vacuum-suitable fasteners

„This is just as much in vacuum technology as it is in cycle sport.“

vacuum-suitable fastenersThose, who wants to be fast, has to avoid the use of standard parts. Cycle sport parts therefor are fitted with special low-weight fasteners. Mechanical In-vacuum-installations are mounted with special vacuum suitable fasteners for low outgassing.

Screw joints inside a vacuum environment have to prevent virtual leaks and the presence of lubricants and screw waxes. Vacuum-suitable screws achieve that through a high cleanliness grade and through the venting of cavities inside the screw connection. 

Virtual leaks in screw joints

Through virtual leaks the vacuum vessel pump down time increases dramatically.

Avoiding virtual leaks through vacuum suitable design

Mechanical installations inside a vacuum environment essentially need screw joints for non-permanent connections. With screw joints internal cavities are formed under the screw head and at the ground of tapped blind holes. While evacuating the vacuum vessel, the gas inside the cavities only can escape through tiny spaces of low conductance, formed through roughness and geometrical deviations between the threads or under the screw head. Because of the ratio between the tiny spaces and the gas-filled cavities is in the scale of 1:1,000 … 1:100,000 a permanent gas flow occurs from the cavities to the vacuum vessel. A virtual leak, so called because its effect is the same than a real leakage. In worst case the target working pressure will not be met.


Avoiding virtual leaks through vacuum suitable design

Venting orifices between gas filled cavities and the vacuum environment avoid virtual leaks in screw joints.

Avoiding virtual leaks through vacuum suitable designVacuum suitable screws therefor are modified with a center venting hole ore with a lengthwise venting slot along the screw shaft and under the screw head. Vacuum suitable washers are modified with venting grooves at the annulus area. The venting orifices provide a much more higher conductance between the cavities and the vacuum environment. The ratio between the orifice and cavity lies in the scale of 1:10. Trapped gas inside the cavities can flow into the vacuum environment while the vessel gets evacuated. The vacuum vessel performance keeps well-preserved.

Cleanliness Requirements  in vacuum suitable screws

There is one important rule for Bicycle Racing Sports as well as for vacuum Technology, Stay Clean!

vacuum suitable screwsPresence of several substances in concentration above allowed limits can cause a lot of trouble. In vacuum technology, areic outgassing rates of vacuum-contiguous parts must be less than 1*10-9 mbar*l/(s*cm²). In UHV-systems areic outgassing rates less than 1*10-12 mbar*l/(s*cm²) are required. In UCV-applications an accumulated molecular hydrocarbon contamination maximum of 5*10-9 g/cm² is allowed. The required cleanliness level of vacuum suitable fasteners is achieved by a precision cleaning process. In industrial scaled production, ultrasonic wet cleaning using surfactant cleaning agents and cascade rinsing baths with DI-water are established. Required Purity Classes are reached reliable and guaranteed.

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