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High speed rotating vacuum seal
Nuclear Instruments and Methods 211 (1983) 213-214 North-Holland Pubhshmg Company
213
Letter to the Editor HIGH
SPEED
ROTATING
J.L. G A R I B A L D I
VACUUM
SEAL *
and C.M. LOGAN
Lawrence Ltvermore Laboratory, Ltvermore, CA 94550, USA
Received 16 September 1982
In a prewous paper we described a high speed rotating vacuum seal Further experience and development effort on this dewce has led to considerably improved performance
Target N e u t r o n Source ( R T N S - I I ) [3] application, bearing stability and bearing load capacity. To satisfy the R T N S - I I requirements, we again separated the load-carrying function from the sealing function. A schematic of the target assembly is shown In fig 1 Details of the seal a n d bearing are presented in fig 2 Load is carried by a matched pair of precision angular-contact ball bearings. These bearings have an msxde & a m e t e r of 152 4 m m At our operational speed of 5000 rpm, h n e a r bearing velocity is - 40 m / s . The bearings are lubricated a n d cooled by oil mist a n d compressed axr. Oil mist is controlled by forced-air exhaust from the bearing region. Sealing is achieved by a m e t h o d conceptually similar to the previous air-bearing design Two very flat surfaces, one fixed and one rotating, are operated w~th a small ( - 4 / ~ m ) gap between them N e a r the outer &ameter, pressurized air at 4 arm is injected through small holes to form a n a~r cushion between the two flat parts.
The f u n c n o n of the seal described here is to permit rotation of a target system on an accelerator while m a i n t a i n i n g vacuum. R o t a t i o n of the target is necessary to distribute heat load. Our first design [1] used mechamcal bearings for support a n d utilized slidxng contact for seahng. It was adequate for operation at 1100 rpm for several m o n t h s In order to h a n d l e higher b e a m power density, it was necessary to increase rotation speed and rotating mass In a 1977 p u b h c a t i o n [2] a new design was presented in which an air bearing was used for mechanical support and differential vacuum p u m p ing of the bearing flmd prevented unacceptable leakage to the b e a m t r a n s p o r t vacuum Experience with this design revealed two problem areas for the R o t a t i n g
* Work performed under the auspices of the U S Department of Energy by Lawrence Llvermore National Laboratory under contract no W-7405-ENG-48 Turbine a=r supply~ Bearing/seal assembly .
d+ beam
\
\
/-Turbine wheel
S
C
L/
,tnE.E.:r;OFe.
Titamum-tntlde/,
coatmg~
/ T a r g e t cooling ~/ water fitting
/ ~ ~z~/~
-
'\"
/~ ~'~Supp,y
J ~
F~g 1 Schematic secnon of RTNS-II target system Shaded components rotate 0 1 6 7 - 5 0 8 7 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 00 '• 1983 N o r t h - H o l l a n d
Neutron s o u r c e Iocatmn
rn
214
J L Gartbaldl, C M Logan / High speed rotating vacuum seal
] 3 }
J .
'
- Vent to atmosphere Levitation air •
Coohng mr and o=1 re=st
~
Exhaust Coohng mr and off mtst
=] ~ ~
d+ beamj i e a r m g
Vacuum
shaft ',~
........
e
c~
F~g 2 Section of vacuum seal and bearing Components with single cross hatching do not move Those shown in double cross hatching rotate Power for rotation ~s provided by a radial-flow air turbme
Inward at smaller radii are three grooves in the static part The first groove is vented to atmospheric pressure. The two inner grooves are connected to separate mechanically p u m p e d vacuum systems. Leakage from the inner groove to the b e a m - t r a n s p o r t vacuum is less than the gas load from 70 m A of d ÷ beam stopping on the tritium target A n essential feature of this design is that the connection between the rotating sealing plate and the bearing shaft is compliant. A n elastomer O-ring seal between thxs sealing plate and bearing shaft (both rotating parts) allows the sealing plate to a c c o m m o d a t e the inevitable small runout.
Sealing surfaces of the required flatness can be prepared by conventional lapping Maintaining this flatness over time and under service conditions requires careful initial material choices and full process control Useful parts cannot be made from rolled plate or other material with oriented grain structure Care must be exercised in clamping during fabrication We use special fixtures to avotd point contact such as would occur in a lathe chuck Our experience indicates that some ver~ light contact probably occurs between the sealing surfaces when runnlng. We have found that 6061 aluminum alloy and leaded Muntz metal, C D A alloy no 36500, is an acceptable combination of materials to survive this contact under vacuum conditions. The sealing plate is made of aluminum alloy, and brass is used for the non-moving part which contains many machining details Performance of this assembly has been very ~atlsfactory Three h u n d r e d mllhon revolutions without maintenance has been achieved
References [1] R Booth, Nucl Instr andMeth 59(1968)131 [2] R Booth and C M Logan, Nucl Instr and Meth 142 (1977) 471 [3] C M Logan and D W Helkkmen, Nucl lnstr and Meth, in press
213
Letter to the Editor HIGH
SPEED
ROTATING
J.L. G A R I B A L D I
VACUUM
SEAL *
and C.M. LOGAN
Lawrence Ltvermore Laboratory, Ltvermore, CA 94550, USA
Received 16 September 1982
In a prewous paper we described a high speed rotating vacuum seal Further experience and development effort on this dewce has led to considerably improved performance
Target N e u t r o n Source ( R T N S - I I ) [3] application, bearing stability and bearing load capacity. To satisfy the R T N S - I I requirements, we again separated the load-carrying function from the sealing function. A schematic of the target assembly is shown In fig 1 Details of the seal a n d bearing are presented in fig 2 Load is carried by a matched pair of precision angular-contact ball bearings. These bearings have an msxde & a m e t e r of 152 4 m m At our operational speed of 5000 rpm, h n e a r bearing velocity is - 40 m / s . The bearings are lubricated a n d cooled by oil mist a n d compressed axr. Oil mist is controlled by forced-air exhaust from the bearing region. Sealing is achieved by a m e t h o d conceptually similar to the previous air-bearing design Two very flat surfaces, one fixed and one rotating, are operated w~th a small ( - 4 / ~ m ) gap between them N e a r the outer &ameter, pressurized air at 4 arm is injected through small holes to form a n a~r cushion between the two flat parts.
The f u n c n o n of the seal described here is to permit rotation of a target system on an accelerator while m a i n t a i n i n g vacuum. R o t a t i o n of the target is necessary to distribute heat load. Our first design [1] used mechamcal bearings for support a n d utilized slidxng contact for seahng. It was adequate for operation at 1100 rpm for several m o n t h s In order to h a n d l e higher b e a m power density, it was necessary to increase rotation speed and rotating mass In a 1977 p u b h c a t i o n [2] a new design was presented in which an air bearing was used for mechanical support and differential vacuum p u m p ing of the bearing flmd prevented unacceptable leakage to the b e a m t r a n s p o r t vacuum Experience with this design revealed two problem areas for the R o t a t i n g
* Work performed under the auspices of the U S Department of Energy by Lawrence Llvermore National Laboratory under contract no W-7405-ENG-48 Turbine a=r supply~ Bearing/seal assembly .
d+ beam
\
\
/-Turbine wheel
S
C
L/
,tnE.E.:r;OFe.
Titamum-tntlde/,
coatmg~
/ T a r g e t cooling ~/ water fitting
/ ~ ~z~/~
-
'\"
/~ ~'~Supp,y
J ~
F~g 1 Schematic secnon of RTNS-II target system Shaded components rotate 0 1 6 7 - 5 0 8 7 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 00 '• 1983 N o r t h - H o l l a n d
Neutron s o u r c e Iocatmn
rn
214
J L Gartbaldl, C M Logan / High speed rotating vacuum seal
] 3 }
J .
'
- Vent to atmosphere Levitation air •
Coohng mr and o=1 re=st
~
Exhaust Coohng mr and off mtst
=] ~ ~
d+ beamj i e a r m g
Vacuum
shaft ',~
........
e
c~
F~g 2 Section of vacuum seal and bearing Components with single cross hatching do not move Those shown in double cross hatching rotate Power for rotation ~s provided by a radial-flow air turbme
Inward at smaller radii are three grooves in the static part The first groove is vented to atmospheric pressure. The two inner grooves are connected to separate mechanically p u m p e d vacuum systems. Leakage from the inner groove to the b e a m - t r a n s p o r t vacuum is less than the gas load from 70 m A of d ÷ beam stopping on the tritium target A n essential feature of this design is that the connection between the rotating sealing plate and the bearing shaft is compliant. A n elastomer O-ring seal between thxs sealing plate and bearing shaft (both rotating parts) allows the sealing plate to a c c o m m o d a t e the inevitable small runout.
Sealing surfaces of the required flatness can be prepared by conventional lapping Maintaining this flatness over time and under service conditions requires careful initial material choices and full process control Useful parts cannot be made from rolled plate or other material with oriented grain structure Care must be exercised in clamping during fabrication We use special fixtures to avotd point contact such as would occur in a lathe chuck Our experience indicates that some ver~ light contact probably occurs between the sealing surfaces when runnlng. We have found that 6061 aluminum alloy and leaded Muntz metal, C D A alloy no 36500, is an acceptable combination of materials to survive this contact under vacuum conditions. The sealing plate is made of aluminum alloy, and brass is used for the non-moving part which contains many machining details Performance of this assembly has been very ~atlsfactory Three h u n d r e d mllhon revolutions without maintenance has been achieved
References [1] R Booth, Nucl Instr andMeth 59(1968)131 [2] R Booth and C M Logan, Nucl Instr and Meth 142 (1977) 471 [3] C M Logan and D W Helkkmen, Nucl lnstr and Meth, in press