Page 159 - Know and Understand Centrifugal Pumps

Pump and Motor

Alignment

I n trod uction
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Pump shaft and driver shaft alignment is very important for long useful
equipment life, and to extend the running time between repairs.
Besides, good alignment reduces the progressive degradation of the
pump.

Why do we use the word driver? We tend to think that pumps are powered by electric
motors. However, some pumps are powered by internal combustion engines, or with I
turbines or hydraulic motors. Not always are pumps and drivers connected with a
direct coupling. Some pumps are coupled through pulleys, chain drives, gearboxes or
even transmissions.

If the pump shaft and impeller assembly were perfectly balanced and
aligned, it would rotate in a perfect orbit around the shaft centerline.
This condition is practically impossible. There is always some imbalance
in the shaft and impeller assembly due to its casting and machining
process, and perfect alignment doesn’t exist. Because of this, the shaft
spins eccentrically around the centerline. We could call this movement
‘eccentric rotation’. The implications of a pump exhibiting rotary
assembly imbalance (eccentric rotation) include:

Excessive running noise.
Vibration and excessive loads on the bearings causing premature
failure.
Rapid wear of the coupling and eventual premature failure.

Premature packing or mechanical seal failure.

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