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The Practical Pumping Handbook

5.3.3.3 Bearings
In the conventional end suction process pumps, there are two factors in
the bearing that can be readily evaluated.

The Size (shaft diameter) and Series (width) of the bearing are directly
related to the load carrying capability (LCC) of that bearing. An
increase of one Series of the bearing will represent an increase of
approximately 50% in the LCC of the bearing. An increase of one Size
will allow an increase of approximately 15% in the load carrying
capability of the bearing. Specific values can be identified in the bearing
manufacturer's engineering data. The larger bearings will last longer.

When one Bidder has one size smaller bearing than the others on both
the radial and the thrust side, an assessment of one set of bearings per
year will be made at the cost of one set of bearings.

5.3.3.4 Lubricant protection
The L-10 life formula for bearings includes a Life Adjustment Factor
which considers the effectiveness of the lubricant. Consequently,
whichever lubricant is used, it must be protected against contamination.
While many pumps still standardize on the use of lip seals in their
bearing housings, most quality pump manufacturers offer (at least as an
optional extra) bearing isolators in the housing. These isolators offer
protection to the lubricant which is not available from Lip Seals. Any
Bidder that does not offer isolators can be assessed the cost of one set
of bearings.

5.3.3.5 Rotational speed
From the same formula, it can be seen that the life of the bearing is
inversely related to the rotational speed (N) of the pump. Any Bidder
operating the pump at the higher speed, can also be assessed the cost of
one set of bearings.

5.3.3.6 Shaft slenderness ratio
The Shaft Slenderness Ratio is a simple formula used to identify the
strength of the overhung shaft on a centrifugal pump. In this formula,
the distance (D) from the centcrlines of the impeller and the closest
bearing to that impeller is compared to the effective diameter (L) of the
shaft at the face of the stuffing box.
A low value of the Shaft Slenderness Ratio will minimize the possibility
of deflection and bending, and the amplitude of any vibration that may
take place. As a high vibration level and excessive shaft deflection both
contribute to premature to premature seal and bearing failure, a more
severe evaluation assessment would be appropriate if the pump is in

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