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Elastomer bellows
This type of seal uses an elastomer
bellows or diaphragm as the secondary sealing member, forming
the seal between the rotating mechanical seal and the shaft upon
which it sits.
Usually there is a ring or other component holding the elastomer
in compression against the shaft and, once installed, this
interface between seal and shaft remains static, any wear or
misalignment being taken up by the flexibility of the bellows.
This is very useful where there is excessive shaft end play and
run out, and this type of seal is very widely used in pumps,
mixers, compressors, etc. although the range of elastomers
available may be more restricted than with pusher type seals
which use O rings instead of an elastomer bellows. Also,
pressure differentials handled by elastomer bellows seals are
lower than for pusher seals - typical values are up to 40 bar.
This type of seal normally requires a positive abutment at the
back of the bellows to ensure that it is positioned to give the
correct working length to the seal.
Very short seal arrangements can be made with bellows type seals
such as with the Cyclam PRR type where the profile is of a
larger outside diameter, shorter working length than other
manufacturers. A typical example is a 40mm diameter PRR
requiring only 20 mm working length, compared to 30 mm for a
typical DIN seal such as the John Crane type 502.
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Metal Bellows This type of seal
uses a flexible metal bellows which replaces the elastomer bellows and
the spring and offers a metallic surface in contact with the
fluid being sealed. Widely used in the petroleum, chemical,
pharmaceutical and industries pumping abrasive fluids such as
paper stock, they have a smooth surface which resists clogging.
Bellows can be found which have been formed from a single piece
of material by hydraulic means, but usually they are edge
welded. The secondary seal to the shaft or sleeve is normally by
an O ring compatible with the fluid being pumped.
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