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The primary rotary seal face rotates at the same speed as the
pump shaft while maintaining
equally distributed seal face loading. There are two methods in
which to drive the rotary
face of mechanical seals; friction drive, and positive drive.
Mechanical rotary seal face
compression is generally accomplished by incorporating a single
coil spring, bellows, or
multiple spring arrangement within the respective seal
design.
Mechanical seals using a single coil spring have a wide range of
applications. Such seals
feature a simple, easy to use design with large spring cross
sections which resist corrosion.
Its chief limitations are its tendency to distort at high
surface speeds and the large axial and
radial space it requires within the pump stuffing box.
High speed centrifugal pump applications prohibit the use of
single coil springs as high
rotational speeds promote distortion of the rotary seal face
which may induce excessive fluid
leakage. Applications of this nature can incorporate a number of
small coil springs spaced
around the rotary seal face unit. These small springs are not as
susceptible to distortion at
high speeds as is the single coil spring, and they consequently
exert an even compression on
the seal face at all times.
Friction drive seal use the secondary sealing member to
drive the rotary seal face. the friction between the
secondary seal in a pusher design, or elastomeric bellows ina
non-pusher seal, and the pump shaft can provide
adequate resistance to drive the rotary seal face. Because
it
depends on this friction to drive the rotary face this type
of
seal driving can only be employed with fluids having
relatively low viscosities as starting and operating viscous
torque can easily exceed the frictional force available.
Multiple spring positive drive mechanical seals
generally will incorporate a rotary compression
assembly where the collar of a multiple spring seal is
normally secured to the shaft by set screws, or placed in
alignment with rotation drive pins fixed to the pump
shaft. Pins link a compression ring to the collar. This
compression ring presses against and energizes the
shaft secondary sealing o-ring and drive the rotating seal face.
The collar, compression ring,
pins, springs, and set screws comprise a single assembly.
Other methods and designs for positively driving a mechanical
seal exist and it is
recommended the mechanical seal manufacturer be involved during
the pump selection
process to assist in determining which is most suitable for a
specific application.
Mechanical Seals .... Seal driving and springcompression
relating to mechanical shaft seals
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