Mechanical shaft seals for pumps - Grundfos

70
Tribology
The lapped surface has a plateau with some valleys. Consequently, the bearing area rapidly
increases with the depth until a large area has been reached. As opposed to the lapped
surface, the area of the grinded surface is slowly increasing with depths indicating a more
even distribution of valleys and peaks.
Fig. 4.10 shows how the leakage rate differs according to the direction of the scratches on the
surface. The arrows indicate the direction of rotation of the seal ring.
According to fig. 4.10, the lubricating film can be pumped to the pumped medium side or to
the atmospheric side, depending on the direction of the scratches on the surface.
Leakage rate [ml/h]
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0 0.05 0.1 0.15
0.2
Roughness, Ra [µm]
Fig. 4.10: Measured leakage rate as a function of the roughness value, R a
and the direction of the scratches on the surface
Direction of rotation
TheLeakage typical rate surface [ml/h] topography of seal rings is a statistic distribution of scratches in all
directions 1.8 obtained by means of a lapping process. A shiny surface with a small roughness can
1.6
be produced by lapping.
1.4
However, where both seal rings are made of hard materials, one of the seal rings should have
1.2
a dull 1.0 finish to prevent the seal rings from sticking together during standstill.
For0.8 a dull surface finish lapped to an Ra value of 0.2 the running-in period may last several days.
0.6
0.4
0.2
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Waviness [µm]