SEALING CHARACTERISTICS OF ALL
ALUMINUM VACUUM SYSTEH USING ELASTIC HETAL GASKETI.

S;:tkai, H. Ishimaru and G. ll.orikoshi

National Laboratory for I:Iigh Energy Physics
Sealing charact~rtstics of combination of aluminum alloy £ranges andelastic metal gasket\.1_ "Helicoflex" for ultra high vacuum is described.
ll.elicoflex is a flexible metal Q-ring composed of an elastic helical core
and a metal sheet which wraps the elastic core. Fig. 1 shows the structure ofHelicoflex. The elastic helical core supplies sealing force to the metalsheet. The material of the metal sheet can be selected among Al, copper, SUS
etc. All of which have an enough ductile characteristics. As the metal sheet
material, we selected pure Al. because Al is strong against corrosion and softenough. It is not necessary to be baked to a high temperature. Practically,
the sealing performance depends largely on the condition of work surfaces ofthe flange and the gasket. ~or the reliable sealing with a leak rate less than
10- 11 atm. cm 3 s- 1 , one of the following conditions must be satisfied. (1) Thesurface of the flange is finished in a lathe using sharp single point tool asillustrated in Fig. 2. The roughness of this surface is about 5 llm "' 50 llm.
(2) The surface of the flange is superfinished with the roughness less than
0.5 llm using oil impregnated emery paper or.diamond tool. Especially using
diamond tool, the smooth mirror finish is easily obtained in spite of thehardness of the aluminum alloy 2219-T87.
The flange surface finished by a lathe, as illustrated in Fig. 3 thepeaks of the flange surface cut into the aluminum sheet of the gasket,as theThe picture of the surface of the gasket before use isfla~ges are tightened.
shown in Fig. 4. The picture of the pressed surface of the gasket is shown in
Fig. 5. The surface of the dales do not contact with gasket and it seemsto
continue from the inside to the outside. But the conductance is negligible
small for leakage. This sealing mechanism is based on the same principl.e. as
ordinary cutinto-t:ype metal gasket in detail, but the existence of elastic- core
improves reliability and easy using. The features of this sealing a.re relatively
insensitive to the scratch of a flange and gasket, but the same gasket cannot beused repeatedly.
The superfinished surface of the flange, as illustrated in Fig. 6 thealuminum sheet contacted with flang~surface is flowed in the ~p between
elastic core and flange surface,as the flanges are tightened. The surface ofthe gasket in contanct with flange surface is superfinished too with this flowing. The picture of the pressed surface of the gasket is shown in Fig. 7. Thissealing is performed by superfinished metal surfaces which a.re placed in closelycontact· each other. This is a new type metal seal. The features of this sealing is relatively sensitive to the scratch of flange and gasket, but especiallyin the case of smooth mirror finish by diamond tool, repeated use of the samegasket is possible.
!he elastic sealing characteristic of the ll.elicoflex o-ring in room
temperature is shown in ~ig. 8. Yo is the initial sealable tightening force
along the Q-ring. Y1 is the minimum sealable tightening at decreasing th~tightning force. Yz is the maximum sealable tightening force. For the flange
surface finished in a lathe, initial tightening force increases infinitely as
repeated use of the sam~ gasket. And it is nearly stable for smooth mirror
finished surface of the flange.

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About two cond~t~ons of the flange surface, the seal~ng mechan~sms are
So the caus~on for using ~s d~ffe~ent, but each casehave a special feature :respectively. So the prove~r use ;i.s needed ;for eachpurpose.
essent~ally d~fferent.

ReferenceH. Ishimaru and G. Horikoshi, IEEE Transactions on Nuclear Science Vol.,
NS-26, No.3, p.4000.

thightening force

elasticcore
purealuminium

cutting in a"lathe~~...

/11111//l!}jfo!Jl;I/Jjfo

Fig. 1

Fig. 2~cutintn.....

Fig. 3Fig.S Surface of the gasketpressed by the flanges finishedin a lathe
X1 00

Fig. 4 Surface of the gaske~
before use
X100

Fig, 7 Surface of the gasketpressed by the smooth mirror
finished flanges
X1000Fig. 8 Elastic sealing
characteristic of"Helicoflex"-200-0.5mm