Fasteners and Sealing
residual pressure and preventing the equipment from functioning properly. A ‘leak’ is quantified by measuring a ‘certain volume of gas per second’ traversing the connector, expressed in mbar.l/s. In the vacuum industry, 1E-4mbar.l/s is considered a ‘big leak’, tolerated only in very rare low vacuum applications, while 5E-7mbar.l/s is considered a very small leak, tolerable in most vacuum applications.
Pressure gradients
A low vacuum of 10mbar induces a ‘pressure differential’ with atmospheric pressure (1000mbar) of 990mbar. An ultra-high vacuum of 1E-7mbar (0.0000000001 bar) induces a differential of 0.9999999999 bar. More than one bar differential pressure is unattainable because a vacuum chamber cannot contain ‘fewer than zero air molecules’, which explains the ‘standard gradient’ of one bar. A given leak for a given connector differs if the connector is exposed to overpressure. The leak is larger if the vacuum chamber is replaced by a ‘10bar pressurised chamber’. Since leaks are most commonly measured using ‘helium
tracers’, they are often called ‘helium leaks’. However, since vacuum applications do not work with helium, leaks are also indicated for air molecules.
Air leaks ‘2.6 times less’ than helium (standard calculation based on the square root quotient of the molecular mass of N2
and He). A connector leak of 4E-7mbar.l/s measured
under a helium atmosphere is actually 1.5E-7mbar.l/s in the real vacuum application (air).
Leakage or diffusion?
Leakage and diffusion are two major mechanisms whereby gas can permeate a hermetic barrier. ‘Leak’ phenomena are due to ‘gaps’ through which gas passes, while diffusion occurs when gas passes through the material. Defects or cracks in the barrier generally result in gross leakage and device failure. Yet even intact barriers allow small quantities of gas to diffuse through them, a natural phenomenon observed in most plastics and rubber. Connectors or other devices mounted on hermetic systems
therefore exhibit residual leakage. Connectors for high- vacuum applications usually have O-rings made of Viton, which produces a tiny residual leak. Optimising the design of connectors or other devices mounted on hermetic systems, and selecting material carefully, ensures that the residual leakage rate remains extremely low and suitable for most ultra-vacuum applications.
Easier mating-thread alignment P
ennEngineering has announced two new fastening solutions: PEM fasteners with ‘float’ capability
enable easier mating-thread alignment during attachment and promote design flexibility for relaxed tolerance applications, and PEM self-clinching concealed-head studs and standoffs which provide strong, permanent, and reusable threads for mating hardware in thin-metal assembly applications. The family of floating fasteners includes
captive panel screws (Types PF11MW and PF12MW) and self-clinching nuts (Types AS, AC, LAS, LAC, A4, and LA4). All install easily and permanently in thin sheets. Floating captive panel screws (Type PF11MW with knurled cap and Type PF12MW with smooth cap) reliably install into metal or non-metal panels as thick as 0.63-in/1.6mm, enable subsequent access
to an assembly, and compensate for up to .060-in/1.52mm mating hole misalignment. Patented MAThread
anti-cross threading technology corrects off- angle installations, aligns components, and slides through clogged internal threads, and a shoulder on the retainer portion of the fastener helps simplify installation. Thread sizes range
from #4-40 through ¼-20 and M3 through M6 and available options include a scratch- proof DuraBlack finish. Assorted coloured plastic caps are also available. PEM self-clinching concealed-head
studs and standoffs provide strong, permanent, and reusable threads for mating hardware in thin-metal assembly applications. Their concealed-head feature offers design and aesthetic benefits by allowing the side of the sheet opposite installation to remain smooth and untouched. This feature additionally serves as an ideal solution for applications where the host metal sheet must remain completely sealed from
air, liquid, dust, gases, or other potential contaminants. Concealed-head studs and standoffs
install permanently in steel or aluminium sheets as thin as .062-in/1.6mm. Studs (aluminium or stainless steel) are available in thread sizes #4-40 through #10-32 and M3 through M5 and in lengths from .250-in through 1-in and 6mm through 25mm. Standoffs (stainless steel) are offered in thread sizes #4-40 through 1/4-20 and M3 through M6 and in lengths from .187-in through 1-in and 4mm through 25mm. l
Enter 39 or ✔ at
www.engineerlive.com/ede
PennEngineering is based in Galway, Ireland.
www.pemnet.com
www.engineerlive.com 39
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