Machine Thermal Growth Offset for Cold Alignment

Because the hot machine's centerline rises as its supports grow. Support growth is ordinary thermal expansion of the pedestal between the mounting plane and the shaft centerline, so it depends on that HEIGHT, the material, and the temperature rise -- not on the machine's power or its size, which is the intuition it defeats. What matters is the DIFFERENCE between the two machines. A motor and pump that grow equally stay aligned however hot they get, and a hot pump on a short pedestal beside a cool motor on a tall one can grow the wrong way entirely, so the correction has a sign and the sign is not guessable. The correction is applied as a cold TARGET: the machine is deliberately out of alignment cold and comes INTO alignment at operating temperature. A pump on an 18 in cast steel pedestal at 6.5e-06 per degF running 90 degF above ambient rises 10.5 mils, so it is set 10.5 mils low cold; skip that and the running offset is 10.5 mils against an acceptable figure near 2 mils for a 3,600 rpm machine, which is roughly five times the tolerance from a correction that takes one line of arithmetic. The target is reported as a signed offset to be entered directly into the rim-and-face or reverse-dial move, which is the form a millwright can use without a second conversion. This is uniform expansion of a support of one material and one height. It does not model transient states -- a machine passing through startup is momentarily aligned for neither condition, and on a large machine the transient can be the worst case; it does not handle piping strain, which moves a pump centerline in ways temperature does not explain and which is a far more common cause of a machine that will not stay aligned; and it does not address the horizontal growth that a long, hot casing produces, or bearing-to-bearing differential growth within one machine. Measured growth from laser targets or optical readings taken hot beats any calculated value, and where a manufacturer publishes a growth target that target governs. The machine manufacturer's data, the plant's precision maintenance procedure, and hot alignment verification govern.

growth = coefficient of thermal expansion x support height from the mounting plane to the shaft centerline x (operating temperature - ambient); relative growth = movable growth - stationary growth; the cold target offset is the NEGATIVE of the relative growth.

The support thermal-growth relation as standard millwright practice, by name. Measured hot growth from laser or optical targets beats any calculated value, and a manufacturer's published growth target governs where one exists. The machine manufacturer's data, the plant's precision maintenance procedure, and hot alignment verification govern.

One line of thermal expansion arithmetic on dimensions and temperatures the user supplies.

Estimate. AHJ and licensed professional govern.

Field names used by the API: stationary_support_height_in, stationary_alpha_per_f, stationary_operating_temp_f, movable_support_height_in, movable_alpha_per_f, movable_operating_temp_f, ambient_temp_f, tolerance_offset_mils, stationary_growth_in, movable_growth_in, relative_growth_in, cold_target_offset_in, tolerance_multiple

• Uniform expansion of one material and one height transient states during startup are not modelledmillwright practice
• Pipe strain is not addressed it moves a pump centerline in ways temperature does not explainmillwright practice
• Measured growth beats calculated laser or optical targets taken hot governprecision maintenance practice