Thermal Movement Comparison for Shaft Alignment
Compare two measured or OEM-specified hot-minus-cold shaft-centerline movements at one common reference plane. This worksheet does not predict shaft movement from casing temperature, generic material coefficients or machine type.
Translation-only comparison
This target is algebra for the stated translation-only convention, not a shim instruction. Enter the signed target into the governing alignment procedure only after checking its sign convention, foot geometry, angular targets, tolerances and OEM requirements.
Coordinate convention and equations
gN = yN,hot − yN,cold
r = yDriven − yDriver
Δrhot−cold = gN − gD
For rHot = 0 in the translation-only model: rCold = gD − gN
Positive position is upward. A positive rCold means the driven shaft centerline is above the driver shaft centerline at the common cold reference plane; a negative value means it is below. The equations only compare two already established movements.
Acceptable inputs
- OEM-specified movements.
- Validated live-trend measurements.
- One common plane/state/sign convention.
Not inferred
- Movement from surface temperature.
- Generic motor/pump/turbine growth.
- CTE from an unsourced material label.
Not represented
- Angular or horizontal movement.
- Foot/shim corrections.
- Soft foot,piping strain or foundation motion.
- Alignment acceptance tolerances.
Primary sources and standards boundary
- API Digital Catalog — lists API RP 686, Machinery Installation and Installation Design, Edition 3, as an active recommended practice currently in pre-ballot revision. Exact alignment requirements are not reproduced here and remain a licensed-source matter.
- Fluke Reliability, Shaft Alignment on-board help, Version 2.0, Edition 10.2025 — page 45 defines thermal growth as shaft-centerline movement between idle and operating conditions and enters specified values at required foot positions; pages 46–47 describe the coefficient-based fallback; page 113 defines an explicit sign convention and distinguishes actual, specification and actual-minus-specification results.
- Fluke, Understanding Thermal Growth and Its Impact on Shaft Alignment — explains that heating can be uneven and movement can be multidirectional, recommends OEM targets/actual operating data, and describes live trend monitoring of shaft movement.
- NIST Engineering Metrology Toolbox FAQ — cautions that generic material names and coefficients averaged over unspecified ranges can create large uncertainty.
- ASTM E228-22 — active test method for measuring linear thermal expansion of rigid solids; ASTM notes that accurate results require calibrated equipment and accurate length and temperature measurement. This worksheet does not implement E228.
Clasificación: signed comparison of controlled movement inputs. It is not an ISO 20816 vibration-severity calculation, an ISO 21940 balancing calculation, an API RP 686 conformity decision or a substitute for the OEM/alignment-system procedure.
