Types and Consequences of Misalignment

Misalignment exists when the centreline of the driver shaft and the centreline of the driven shaft are not collinear at the operating condition. It takes two forms, usually together.

• Offset (parallel) misalignment — the shafts are parallel but displaced.
• Angular misalignment — the shafts meet at an angle.

Each is measured in two planes — horizontal and vertical — so a full alignment corrects four values.

What it costs

• Bearing life falls sharply, because bearings carry a continuous cyclic side load they were not designed for.
• Seal life falls, because the shaft deflects and the faces open once per revolution.
• Coupling wear and, eventually, coupling failure.
• Energy consumption rises measurably.
• Vibration rises, classically at twice running speed with significant axial vibration.
• Shaft fatigue in severe cases.

The flexible coupling misconception

A flexible coupling accommodates misalignment in the sense that it does not break immediately — it does not eliminate the forces. The load is transmitted into the bearings and seals regardless. "It has a flexible coupling" is not a reason to align loosely, and couplings advertised as misalignment-tolerant are tolerant of the coupling failing, not of the machine surviving.

Alignment is cheap, done once, and pays back across the whole life of the machine. It is also the maintenance task most often rushed at the end of a shutdown.