Experimental Investigations concerning virtual Leaks

Setup:

To determine the impact of virtual leaks in screw joints to the performance of a vacuum system, experimental investigations under real live conditions have been performed. Screw joints have been investigated inside a vacuum vessel with well-known vacuum characteristic. Under identical conditions the test was performed once with non-vented screws and once with screws featured with a center venting hole with a diameter of 1.8 mm.

Procedure:

16 screws with a size of M8 x 30mm have been tightened in a tapped blind hole on a test sample. The sample was applied in a vacuum vessel with well-known volume. The vessel was evacuated and the pressure drop was recorded.
 
Experiment:

The chart shows recorded pressure drop graphs of a vacuum vessel with 16 internal screw joints once with non-vented screws once with center vented screws.

Experimental Investigations concerning virtual Leaks

Results:

From the difference of the pressure drop graphs a “virtual leak rate” of about 3E-7 mbar*l/s was calculated for a single, non-vacuum suitable screw joint. With the vacuum suitable configured screw joints the evacuation time decrease by up to 60%, the achieved base pressure is reduced by 1E-7 mbar.

Conclusion: Impact of virtual Leaks

Non-vacuum suitable screw Joints have an enormous impact in the performance of vacuum systems.

In High vacuum Applications the Use of vacuum suitable Fasteners strongly is recommended. For Ultra High vacuum Applications this is essential. The calculated “virtual Leak Rate” of a vacuum unsuitable screw Joint, compared to specified Leak Rates of standardised vacuum Flange Joints, is enormously high. One vacuum unsuitable screw Joint occurs a 300 times higher Gas Load than an Elastomer-O-Ring sealed Flange Joint and a 30,000 times higher Gas Load than a metal sealed UHV Flange Connection.
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Das Diagramm zeigt aufgezeichnete Abpumpkurven einer Vakuumkammer mit 16 innenliegenden Schraubenverbindungen einmal ohne Entlüftung und einmal mit zentrischen Entlüftungsbohrungen an den Schrauben.

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