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10.2.6 Material Fatigue Pump shafts, bearings, gears, impellers and all other moving metal components of the pump are candidates for material fatigue failure. Material fatigue occurs with all types of pumps, but may have more of an effect on displacement pumps, which have higher fluctuating stresses. One component that often creates a material fatigue problem is an unbalanced impeller resulting in the vibration of components throughout the pump. 10.2.7 Bearing Failure Although bearings are relatively inexpensive, they can cause costly shutdowns of complete systems. Short bearing life for centrifugal pumps, for example, can be caused by a number of problems including the following: misalignment, a bent shaft, a rotating part rubbing on a stationary part, an impeller out of balance causing vibration, excessive thrust caused by mechanical failure inside the pump, excessive bearing temperature caused by lack of lubrication, dirt or other contaminant in the fluid, excessive grease or oil in an anti-friction bearing housing, and rusting of bearings from water in housing. Most bearing problems can be classified by the following failure modes: fatigue, wiping, overheating, corrosion, and wear. Fatigue occurs due to cyclic loads normal to the bearing surface. Wiping occurs as a result of insufficient lubrication film thickness and the resulting surface-to-surface contact. Loss of sufficient lubricant film thickness can occur from under-rotation or from system fluid losses. Overheating is shown by babbitt cracking or surface discoloration. Corrosion is frequently caused by the chemical reaction between the acids in the lubricants and the base metals in the babbitt. 10.3 MODEL DEVELOPMENT Reliability models have been developed to address the difference between pump types. Because of the physical design differences between centrifugal and displacement pumps they have specific performance and reliability advantages and disadvantages. Centrifugal pumps are limited by pressure but can supply almost any amount of capacity desired. Displacement pumps lose capacity as the pressure increases due to the increase in slippage which occurs with an increase in pump pressure. The amount of slippage can vary from pump to pump depending on the actual manufactured clearances in the pump chamber. The slippage can also increase with time as wear increases. Slippage will be inversely proportional to the fluid viscosity. Equation (10-1) shows that since slippage "S" increases as the pressure requirements increase, the value of capacity "Q" is decreased. Pumps Q=7.48(DN-S) 10-9 (10-1) 10-10 Revision C

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