Introduction

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.

AUTHOR'S NOTE

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 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.

■ Wear and rubbing between close tolerance rotary and stationary elements in the pump leading to their failure.

■ Premature driver bearing failure.

■ Increased energy consumption.

■ Excessive operating temperatures and lubricant failure.

One of the most important and least considered points of correct alignment is the relationship with the power transmitted from the motor to the pump. An almost perfect alignment (0.003 inch) with an adequate and new coupling transmits almost 100% of the motor's power (there will always be some small losses). The Pump performance curve identifies the BHP or brake horsepower required for the pump to perform at its duty point.

The next graph (Figure 10-1) indicates the expected continuous running time of rotating equipment with increasing misalignment. As you soon see, the alignment improves and so does the service time.

ROTATING MACHINERY FAILURE TIME

ROTATING MACHINERY FAILURE TIME

TOTAL MISALIGNMENT IN INCHES

Know and Understand Centrifugal Pumps Types of misalignment

There are two basic types of misalignment, angular and parallel. Within each of these basic types of misalignment there are combinations of both. These are the most common combinations:

■ Vertical/angular misalignment (Figure 10-2

■ Vertical/parallel misalignment (Figure 10-3)

■ Horizontal/angular misalignment (Figure 10—4)

■ Horizontal /parallel misalignment (Figure 10-5)

■ Combined angular and parallel misalignment

Figure 10-4 and Figure 10-5 Top view of horizontal/angular and horizontal/parallel misalignment

Bore Not Perpendicular to Face

Figure 10-6 and Figure 10-7 Misalignment can be transmitted through the couplings and coupling faces.

Figure 10-4 and Figure 10-5 Top view of horizontal/angular and horizontal/parallel misalignment

Figure 10-6 and Figure 10-7 Misalignment can be transmitted through the couplings and coupling faces.

Coupling with Excentric Bore

Bore Not Perpendicular to Face

Distorted Coupling Face

Distorted Coupling Face

Figure 10-8 Misalignment can be transmitted through the couplings and coupling faces.

Figure 10-8 Misalignment can be transmitted through the couplings and coupling faces.

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