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Shaft misalignment occurs when the centre lines of rotation of two (or more) machinery shafts are not in line with each other.

As simple as that may sound there still exists a considerable amount of confusion to people who are just beginning to study this subject when trying to precisely define the amount of misalignment that may exist between two shafts flexibly or rigidly coupled together.

• How accurate does the alignment have to be?
• How do you measure misalignment when there are so many different
• coupling designs?
• Where should the misalignment be measured?
• Is it measured in terms of ... mils, degrees, millimetres of offset, arc
• seconds, radians?
• When should the alignment be measured ... when the machines are off-line or when they are running?
• In more precise terms, shaft misalignment is the deviation of relative shaft
• position from a collinear axis of rotation measured at the points of power
• transmission when equipment is running at normal operating conditions

Simply stated, the objective of shaft alignment is to increase the operating life span of rotating machinery.To achieve this goal, machinery components that are most likely to fail must operate within their design limits. Since the components that are most likely to fail are the bearings, seals, coupling, and shafts, accurately aligned machinery will achieve the following results ...

• Reduce excessive axial and radial forces on the bearings to insure longer
• bearing life and rotor stability under dynamic operating conditions.
• Minimize the amount of shaft bending from the point of power
• transmission in the coupling to the coupling end bearing.
• Minimize the amount of wear in the coupling components.
• Reduce mechanical seal failure.
• Maintain proper internal rotor clearances.
• Eliminate the possibility of shaft failure from cyclic fatigue.
• Lower vibration levels in machine casings, bearing housings, and rotors
• (*note ... frequently, slight amounts of misalignment may actually decrease
• vibration levels in machinery so be cautious about relating vibration with
• misalignment).