Operation, design and maintenance

Let s analyze the evidence that pump mechanics have seen so many times. Consider the difference between a deflected shaft and a bent shaft. 

A bent shaft is physically bent and distorted. Placing the shaft into a lathe or dynamic balancer and rotating it will reveal the distortion. If a bent shaft is installed into a pump and run, it will fail prematurely, leaving evidence and specific signs on the eircumferenee of close tolerance stationary parts around the pump s volute circle. The shaft will exhibit a wear spot on its surface where the elo.se tolerance parts were rubbing. 

The basic difference between a bent shaft and a deflected shaft is the following. A bent shaft spinning inside close tolerances leaves a scratch mark around the eireumferenee of stationary elements corresponding to a damaged spot on the shaft. A deflected shaft. spinning within close 

Shaft deflection is the result of an external radial load. The external radial loading originates with the pump operator or proee.ss when the pump runs away from its best effieieney point on the curve. The resistance to deflection is a function of the shaft s overhang length and its diameter. The deflection resi.stanee, also called the flexibility laetor, is known as the L/D laetor. 

Rarely do design engineers request the L/D faetor in their quotes. Some engineers don t know they have the option. Most pumps are bought based on priee, and beeause a high defleetion resistanee (low L/D Faetor) indieates a larger diameter shaft with oversized bearings these U pe pumps don t normally win a competitive hid process. 

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