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1、1Chapter 3 Vacuum seal2342. virtual leakA virtual leak is a source of gas thats physically trapped within the chamber with only a small, very low conductance, path from the trapped pocket of gas into the chamber.567Common Solutions to Trapped Volumes8Seal Methods1. Welding2. Elastomer seal3. Metal S
2、eal4. Standard vacuum feed through9Welding 1. MethodsTIG weldingMIG welding EBWBrazing2. Maximum leak rate:10-7Pa m3/s.m101. low vapor pressure at the service temperature2. reliable, reusable and long life seals-most common type of seal used3. no components or materials which will vaporize at the pr
3、essures and temperatures to which the joint will be subject.4. seal vacuum as low as 10 -8 TorrElastomer seal1112Typical seal method Bolt and clamp1314KF Flange15Typical O-ring seal1. Dimensional. 2. Pressure. 3. Vapor Pressure4. Temperature5. Surface6. Permeability7. Vacuum Weight Loss8. Squeeze16E
4、very groove has a slight gap between the two mating surfaces forming the grooves internal cavity. O-ring volume to be larger than the cavity, allowing seal compression to block the diametrical gap, preventing leakage and providing 25% compression in the O-ring. The O-ring is held firmly in place and
5、 has no tendency to drop out of the groove when the chamber is open.For vertical O-ring applications (as a chamber), it is desirable to use an undercut groove. 1 Dimensional. 17Total clamping pressure should be approximately 100kg/cm of O-ring. 18Vapor Pressure. The O-ring must have a very low vapor
6、 pressure at the service temperature. no components which will vaporize at the pressures to which the joint will be subject. hold vacuum as low as 10-9Torr .19Temperature. The vapor pressure of most or all organic materials is proportional to temperature. It is therefore very important that O-rings
7、temperature be limited.The best of the materials must be held below 120C, and for minimum outgassing an even lower temperature is recommended. For temperatures held below 15 C, it is possible to use carefully constructed O-ring joints at pressures as low as 109Torr. Excessive heat, over time, degrad
8、es O-ring materials physically, and/or chemically, which may render them nonfunctional as seals. The O-ring temperature must be reduced to values between zero and -30 C during ultra vacuum operation.20Surface. It is important that the groove surfaces be so ground and polished that no residual tool m
9、arks or scratches occur at right angles to the length of the groove. Even minute scratches can produce a leak, which is difficult to locate and repair. The mating flanges, which are flat and serve to compress the O-ring must have, at the point where the O-ring contacts them, an equally good finish.
10、The surface finish in the O-ring groove and on the flat mating flange must be at least 32 micro inch and preferably 16 micro inch.21Permeability.In vacuum applications, high material resistance to gas permeation is directly equated with low vacuum leakage of the O-ring. Butyl and Fluorocarbon excel
11、as the most impermeable performers. Increased O-ring compression reduces permeability by increasing the length of the path the gas has to travel (width of ring) and decreasing the area available to the entry of the gas (groove depth).22Vacuum Weight Loss. It is particularly important in many space a
12、nd other vacuum applications that optical surfaces and electrical contact surfaces remain clean to serve their intended purpose.Some rubber compounds contain small quantities of oil or other ingredients that become volatile under high vacuum conditions and deposit as a thin film on all the surroundi
13、ng surfaces.Where sensitive surfaces are involved, the higher weight loss compounds should be avoidedTable 1 indicates the weight loss of several compounds due to vacuum exposure.2324252627Vacuum Leak Approximation. The leak rate of a gas through an O-ring seal may be roughly approximated using the
14、following formula :L = 0.7 F D P Q (1-S) 2 L = Approximate leak rate of the seal. F = Permeability rate of the gas through the elastomer at the Anticipated temperature D = Inside diameter of the O-ring. P = Pressure differential across the seal. Q = Factor depending on the percent compression and wh
15、ether the O-ring is lubricated or dry. S = Percent compression on the O-ring cross section expressed as a decimal (i.e. for 20% compression, S = 0.20)28293031Rotating SealsO-rings are used as seals for rotating shafts, with the turning shaft protruding through the internal diameter of the O-ring. Th
16、e inside surface of the O-ring is continuously exposed to friction generated heat from the rotating shaft. The most important factors to consider when designing rotating seal glands are frictional heat buildup, O-ring stretch, compression, application temperature limits, and shaft glandular machining.O-rings should be comprised of compounds featuring maximum heat resistance and minimum friction generating properties. To help mini
