What is thermal growth in shaft alignment?

Thermal growth is the vertical expansion of a machine's centerline height as it heats up from ambient to operating temperature. The formula is ΔH = H × α × ΔT, where H is foot-to-centerline height, α is the coefficient of thermal expansion, and ΔT is the temperature rise.

Why pre-offset cold alignment?

Machines are aligned cold but run hot. If both machines grow by different amounts, perfect cold alignment results in misalignment at operating temperature. Pre-offsetting the cold alignment by the expected differential growth ensures alignment when hot.

How do I determine operating temperature?

Operating temperatures can be measured on running equipment using infrared thermometers or contact probes at the bearing housing or machine feet. Typical values: electric motors 60–90°C, pumps 40–70°C (process dependent), steam turbines 80–200°C.

What are typical temperatures for motors and pumps?

Electric motors typically operate at 60–90°C at the casing. Centrifugal pumps range from 40–70°C for cold water service, but can be much higher for hot process fluids. Steam turbines may reach 150–300°C. Gearboxes typically operate at 50–80°C.

What is a hot alignment check?

A hot alignment check is performed while the machine is at operating temperature (immediately after shutdown). It verifies that the thermal growth predictions were correct and the machines are aligned under actual operating conditions. This is the gold standard for validating alignment.

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軸對中熱膨脹補償

計算驅動機與被驅動機因熱膨脹產生的垂直中心線增長量。確定冷態對中預偏移目標值，以確保在運轉溫度下達到對中。

驅動機 + 被驅動機 ΔH = H × α × ΔT 冷態偏移目標

環境溫度 (°C)

在下方輸入兩台機器的資料。差值將決定冷態預偏移量。

驅動機（例如：馬達）

機器類型

材料

機腳至中心線高度 H (mm)

操作溫度 (°C)

被驅動機（例如：泵）

機器類型

材料

機腳至中心線高度 H (mm)

操作溫度 (°C)

快速預設

結果

冷態預偏移目標（差額膨脹）

—

驅動機熱膨脹量

—

被驅動機熱膨脹量

—

驅動機 ΔT

—

被驅動機 ΔT

—

熱膨脹公式

當機器加熱時，其機架會垂直膨脹，導致軸中心線升高：

H — 機腳至中心線高度 (mm)
α — 熱膨脹係數 (×10⁻⁶ /°C)
T_op — 運轉溫度 (°C)
T_amb — 環境溫度 (°C)

材料熱膨脹係數數值

材料	α (×10⁻⁶ /°C)
碳鋼	12.0
鑄鐵	10.5
不鏽鋼 (304/316)	16.0
鋁	23.0

典型運轉溫度

機器類型	典型機殼溫度 (°C)
電動機	60 – 90
離心泵（冷水）	40 – 70
汽輪機	80 – 200
齒輪箱	50 – 80
壓縮機	60 – 120

實務範例

範例 — 馬達 + 泵組

已知： 環境溫度 = 20°C

馬達：H = 200 mm，鋼材 (α = 12×10⁻⁶)，T_op = 80°C

泵：H = 250 mm，鋼材 (α = 12×10⁻⁶)，T_op = 60°C

ΔH_馬達 = 200 × 12×10⁻⁶ × (80 − 20) = 0.144 mm

ΔH_pump = 250 × 12×10⁻⁶ × (60 − 20) = 0.120 mm

Difference = 0.144 − 0.120 = 0.024 mm

The motor grows more → set motor 0.024 mm LOW during cold alignment.

💡 提示： The machine that grows more should be set LOW at cold alignment. When both heat up, the differential growth brings them into alignment.

⚠️ 注意： This calculation assumes uniform thermal expansion of the machine frame. In practice, thermal gradients, piping strain, and foundation effects may cause additional movements. Always verify with a hot alignment check when possible.