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Shaft alignment: shims and tolerances

Enter the foot distances and the measured offset and angular misalignment to get how much to shim and move each pair of feet and whether the set is within tolerance for its speed.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Pre-sizing
Geometry
Measurement on the movable machine (relative to stationary)

Signs: positive offset = movable machine higher (or toward the reference side); positive angularity = movable centerline rising away from the coupling.

If the movable machine grows more when hot, set it lower when cold: a negative target equal to the growth difference.

—Correction at the movable machine feet
—

Side view (vertical plane)

Movable machine centerline relative to the stationary one, from coupling to rear feet. Vertical scale exaggerated.

Tap the chart to see the values.

Top view (horizontal plane)

Same reading in the horizontal plane.

Tap the chart to see the values.

The report gathers this page's inputs, results, equations, assumptions and references, with fields for project, engineer and revision. In the print dialog, choose Save as PDF.

Method, assumptions and references

Equations used

Movable centerline position at distance x from the coupling: y(x) = offset + angularity × x
Front feet shim = −(y(A) − target); rear = −(y(A + B) − target); positive = add
Horizontal plane: same expression, no thermal target
Tolerance: offset and angularity compared with the usual table by speed for short flexible couplings

Assumptions and limits

  • Readings already converted to offset and angularity at the coupling center (laser systems do this; with dial indicators, use the reverse indicator method).
  • Tolerances from the usual industry table for short flexible couplings; spacer couplings and the manufacturer may set their own values.
  • Soft foot corrected beforehand; baseplate and bolts in good condition.

Standards and technical documents

  • ANSI/ASA S2.75-2017 Part 1: Shaft alignment methodology — General principles, methods, practices and tolerances
  • API RP 686, 3rd ed. (2021): Recommended practice for machinery installation and installation design
  • Piotrowski, J. Shaft Alignment Handbook, 3rd ed. CRC, 2006
  • ISO 20816-3:2022: vibração de máquinas industriais (efeito do desalinhamento)

U.S. regulations

  • OSHA 29 CFR 1910.147: control of hazardous energy (lockout/tagout) during alignment
  • OSHA 29 CFR 1910.219: guarding of shafts, couplings and power-transmission parts

Frequently asked questions

What is the alignment tolerance for an 1,800 rpm motor?

Per the usual table for short flexible couplings, offset up to 2 mils is excellent and 3 mils acceptable; angularity up to 0.3 mil/in is excellent and 0.5 mil/in acceptable.

How do you calculate motor shims?

Add the offset measured at the coupling to the angularity times the distance to each pair of feet. The result with its sign reversed is the shim: positive adds, negative removes.

What is thermal growth compensation in alignment?

It means leaving the movable machine deliberately misaligned when cold by the amount it will grow more than the stationary one when hot, so that it runs aligned.