Why can correct cold alignment differ from the required running alignment?

The short answer

Because the machine axes can move differently as the machines and supports heat up. The desired cold relationship may intentionally account for predicted operating movement. A wind-generator coupling should therefore use the equipment designer’s alignment target, rather than assume that zero cold offset always produces the required running condition.

Locate the thermal reference points

Identify each machine’s support geometry, material, locating features and relevant temperatures. Expansion from a fixed foot or bearing reference can move the shaft centre differently from uniform growth about its own centre. The base may also move. A temperature rise alone is insufficient; the direction and length over which expansion occurs determine the resulting displacement at the coupling.

Separate prediction from permission

An alignment instrument measures the relationship in its actual measurement state. It does not predict future thermal movement unless an appropriate model or target is supplied. An invented cold offset can create excessive misalignment during startup or fail to compensate the real running movement. KTR’s displacement information describes coupling accommodation, not a general instruction to deliberately misalign shafts by a particular amount.

Request state-specific targets

Ask for the cold alignment target, measurement temperature/state, predicted operating relationship and allowable range across the actual duty. Clarify whether the coupling’s combined displacement and reaction forces have been checked. No MARSTON thermal growth or alignment offset is inferred here. If the target is unavailable, report that missing design input rather than choose an offset from a similar machine. Alignment acceptance should connect the measured cold geometry with the intended operating geometry through an explicit equipment-specific basis. A running target should cover the expected range of temperatures, rather than one ideal warmed condition only. Startup and cooldown remain part of the coupling displacement envelope unless the documented equipment duty excludes them.