The effect of open space conditions before docking (vacuum and other types of radiation); Large intervals between the points of coupling on flanges when the coupling forces slightly exceed the forces which open the joint, which leads to a partial and uneven opening, which changes due to the effect of external loads; The possibility of significant (up to 1-2 mm) transverse shifts in the faces of the frames from the coaxial position during docking; Limited compression forces of the seals during coupling of the joints; coupling may occur in a wide range of temperatures (usually from ); The requirement of the absence of adhesion after the effect of the conditions of open space, and a long period in a coupled state; The need (for APDAs) for seals with contact of resin on resin. The usual requirements for reliability and small leakage are made on the seal of the joint (for domestic docking devices leaks should not exceed several hundredths of a gram of air per hour). To meet these requirements the seal is made of two duplicate concentric end seals. Studies have shown that the best characteristics (minimal size, forces of compression and adhesion, stability, multiple use, fastening) belong to seals with profiles which are tapered on the side of the contact surface (Figure 3.22). The fastening of the seals to the frame is very important. A T-shaped mounting groove is recommended. The actual degree of compression is defined by the protrusion of the seal above the plane of the joint and the height of its deformed part. The degree of compression is calculated considering gaps in dimensions, possible nonlinearity and opening of the seal, and temperature changes in dimension. The latter factor is due to the fact that the coefficient of linear expansion of resin is 100-150 times greater than that of aluminum. For example, for a temperature drop of 100'C the protrusion of the seal changes by 0.2-0.3 mm. A seal with a specific compression of 10-25% is used, which is sufficient to insure hermeticity with the given leakage value.
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