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This creates a more uniform closing force between the faces, and also
eliminates the need for a secondary seal at the seal face, which
automatically stops any possible fretting damage.

Although the main closing force is normally provided by the pressure in
the stuffing box, the springs and bellows compensate for any shaft
movement and keep the seal faces closed during startup and shutdown
of the pump.

6.3.4 Fretting seals
A pump shaft will undergo both radial and axial movement for a variety
of reasons, including bearing tolerances, end play, vibration and shaft
deflection.
In addition, movement within the mechanical seal is also quite normal
due to the difficulties in maintaining the two faces absolutely parallel,
which is caused by
9 Equipment and installation tolerances,
9 Thermal growth,
9 Pipe strain, or shaft misalignment.

To keep the seal faces together, the springs arc constantly adjusting the
seal in relation to the moving shaft.
When an clastomcr is used between the rotating face and the shaft
under these conditions, the elastomer moves back and forth on the
shaft. This creates a 'polishing' action which repeatedly removes
the protective oxide coating from the corrosion resistant material of the
shaft, and eventually creates a groove at that point on the shaft. The
groove causes leakage and necessitates repetitive repair or replacement
of the shaft.
To combat this problem, the use of the shaft sleeve has been continued
as a sacrificial part in the way of the stuffing box.
However, the only lasting solution to the problem of fretting corrosion
lies in the elimination of the dynamic seal. Most major seal manu-
facturers now produce 'non-fretting' seals which protect the pump
parts from fretting corrosion. The Bellows Seal shown in Figure 6.6 is a
typical example of such a seal.

6.3.5 Balanced or unbalanced seals
The balance of a mechanical seal determines the magnitude of the
resultant closing force on the faces. It is created by varying the effective
cross-sectional areas of the seal in conjunction with the pressure in the
stuffing box.

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