INTRODUCTION

Shaft misalignment is said to occur when the centerlines of rotation of two machine shafts are supposed to be collinear but are not in line with each other. Thus, misalignment is the deviation of relative shaft position from a collinear axis of rotation (measured at the points of power transmission) when machinery is running at normal operating conditions. For example, consider a motor shaft which is connected to a pump shaft, with centerlines that are not collinear. Such shaft misalignment may result in excessive vibration, although there is not a direct relationship between the magnitude of vibration and shaft misalignment. (In some cases, a slight amount of misalignment may actually reduce the magnitude of vibration.) In addition, shaft misalignment may be the cause of any or a combination of the following conditions:

• Shaft failure resulting from cyclic fatigue

• Cracking of the shafts at, or close to, the coupling hubs or bearings

• Increased wear of the bearings, seals, or coupling, leading to premature failure

• Loose foundation bolts

• Loose or broken coupling bolts

• A coupling that runs hot

• High temperature of the casing or of the oil discharge near the bearings

• Excessive grease or oil on the inside of the coupling guard

• Excessive power consumption by the rotating equipment

The objective of shaft alignment is to reduce these detrimental effects and thereby extend the operating life span of the rotating machinery.

This part of this chapter describes the types of misalignment, describes the use of spectrum analysis...