Page 187 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

These seals work well within their designed life. Their designed life is
about 2,000 hours. An automotive drive shaft spinning at about 1,800
rpm would move the car at approximately 50 miles per hour. 2,000
hours would be equivalent to about 100,000 miles on a car. 2,000
hours (at 1,800 rpm) on a pump would be equivalent to about 83 days
at 24/7 operation. Many mechanics have questioned the logic of
installing a 3-month seal to protect a 5-year bearing.
At about 2,000 hours, one of two things can happen to cause failure to
a lip seal. Either the frictional heat from the spinning shaft burns and
cooks the rubber lip, or the rubber lip eats a groove into the shaft. How
can a soft elastomeric rubber lip cut a groove into a stainless steel shaft?

One of the components of stainless steel is chromium. A layer of
chromium oxide is visible on the surface of stainless steels. That’s why
stainless steel appears to be chromed. As the stainless steel shaft spins
under the rubber lip, the chromium oxide particles imbed into the
rubber lip.

Chromium Oxide is present in just about eve ry maintenance shop in the WOI ,Id. We
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call it the GRINDING WHEEL The abrasive materi: 11 in your electric grinding wheel is
Chromium Oxide. Cheap wheels may tend to use a1 luminum oxide.
Mer a few revolutions, the rubber lip of the oil seal becomes an
abrasive lip, which eats a groove into the stainless steel pump shaft. The
rubbing action abrades the pump shaft, removing metal, and depleting
the chromium content of the stainless steel, which hrther accelerates its
erosion.
When the rubber lip can no longer maintain contact with the spinning
shaft, the oil or grease can leak out of the bearing housing. Con-
taminants can enter into and destroy the bearings. When the lip seal is
changed with the bearing change, the new lip rides into the old groove
cut by the previous lip. That’s how a $6-dollar rubber lip seal can take
out a $300.00 bearing and an $800.00 stainless steel pump shaft ...
about 4 times per year.