جبران رشد حرارتی برای هم‌راستاسازی شفت

Thermal growth is the vertical expansion of a machine's shaft centerline height as the machine heats up from ambient to operating temperature. All materials expand when heated, and for rotating machinery this means the shaft centerline rises.

The growth is calculated as: ΔH = H × α × (T_op − T_amb), where H is the foot-to-centerline height, α is the coefficient of thermal expansion, and the temperatures are in °C.

Machines are aligned when cold (not running) but operate when hot. If the driver and driven machines grow by different amounts vertically, perfect cold alignment will result in misalignment at operating temperature.

By intentionally offsetting the cold alignment by the expected differential growth, the machines come into proper alignment when they reach operating temperature. The machine that grows more is set low during cold alignment.

Operating temperatures can be measured on running equipment using:

• Infrared thermometer — quick non-contact measurement at machine feet or bearing housings
• Contact thermocouple probes — more accurate surface temperature
• RTDs/thermocouples — permanently installed sensors

Measure at the machine feet or base, not at the top of the casing, as this best represents the temperature affecting vertical growth. Allow the machine to reach thermal steady state (typically 2–4 hours).

Typical casing/foot temperatures for common machines:

• Electric motors: 60–90°C (depending on frame size and load)
• Centrifugal pumps: 40–70°C (cold water), up to 200°C+ (hot process)
• Steam turbines: 80–200°C
• Gearboxes: 50–80°C
• Compressors: 60–120°C

These are approximate values. Always measure on the actual equipment when possible.

A hot alignment check is performed immediately after shutting down equipment that has been running at normal operating temperature. The alignment is measured before significant cooling occurs (typically within 15–30 minutes of shutdown).

This check verifies whether:

• The calculated thermal growth predictions were accurate
• The cold pre-offset was correct
• Other factors (piping strain, foundation movement) are affecting alignment

A hot check is the gold standard for validating alignment and should be performed whenever practical, especially on critical machinery.