Millwright Certification Precision Shaft Alignment Techniques 1

Question 1

What type of misalignment exists when two shaft centerlines are parallel to each other but do not share the same centerline?

Accepted Answer

Parallel (offset) misalignment

Answer

Parallel (offset) misalignment means the two shaft centerlines run in the same direction but are displaced from each other radially. Angular misalignment, by contrast, occurs when the centerlines meet at a non-zero angle. Both types often exist simultaneously and must be corrected separately.

Question 2

What is bracket sag (indicator sag), and how does it affect alignment readings?

Accepted Answer

It causes the indicator to droop under gravity, making the bottom reading appear more negative than the true shaft position — requiring a sag correction to be subtracted

Answer

When a dial indicator and its bracket are cantilevered off a shaft, gravity causes the assembly to sag downward. This makes the indicator read a more negative (lower) value at the 6 o'clock position than the actual shaft relationship warrants. Sag must be measured and mathematically corrected before calculating moves.

Question 3

What is a 'cold alignment target' (also called a cold offset or cold correction), and why is it used?

Accepted Answer

An intentional misalignment introduced during cold alignment so the machine will be in true alignment at normal operating temperature

Answer

Many machines grow vertically and horizontally as they reach operating temperature. A cold alignment target intentionally offsets the shaft positions in the cold state by a calculated amount equal to predicted thermal growth, so the machine achieves true alignment only after warming up to operating conditions.

Question 4

Why do higher-speed rotating machines require tighter shaft alignment tolerances than lower-speed machines?

Accepted Answer

Because misalignment-induced vibration forces and bearing loads increase with rotational speed, accelerating wear and reducing service life

Answer

Dynamic forces generated by misalignment grow with rotational speed. Even a small offset or angular error produces significantly higher cyclic stresses on bearings and couplings at 3600 RPM than at 900 RPM. This is why alignment tolerance charts reference tighter allowable limits as speed increases.

Question 5

When taking dial indicator readings during shaft alignment, why should both the driver and driven shafts be rotated together as a single unit?

Accepted Answer

To eliminate errors caused by shaft runout and coupling eccentricity, ensuring readings reflect only the positional relationship between the two shaft centerlines

Answer

If only one shaft rotates while the indicator rides on the other, eccentricity in the coupling hub or runout in the shaft will contaminate the readings. Rotating both shafts as a unit means the indicator sees only the change in relative position between the two centerlines, which is the true alignment condition.

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Millwright Certification Precision Shaft Alignment Techniques 1