The easiest way to obtain exact machine thermal growth characteristics is with the Easy-Laser XT770 EasyTrend with DM brackets. For more information please reach out to us.
Calculating Thermal Growth
Another method to calculate alignment offsets for machinery that operates at elevated temperatures can be carried out as follows:
Understanding Thermal Growth
Metal expands when heated. Rotating machinery typically operates at temperatures above ambient, causing the machine components to grow in all directions. The shaft centreline rises as the machine heats up because the expansion occurs from the mounting surface upward.
A machine aligned perfectly when cold may run significantly out of alignment once it reaches operating temperature. The amount of growth depends on the temperature rise, the distance from the mounting surface to the shaft centreline, and the thermal expansion properties of the machine materials.
To achieve correct alignment at operating temperature, the cold alignment must incorporate an intentional offset that compensates for the predicted thermal growth.
The Thermal Growth Formula
Calculation
TG = T × L × C
| TG | Thermal growth (the offset to apply during cold alignment), in millimetres |
| T | Temperature change from ambient to operating temperature, in degrees Celsius |
| L | Distance from the shim plane (mounting surface) to the shaft centreline, in millimetres |
| C | Coefficient of linear expansion for the material |
Coefficients of Linear Expansion
| Material | Coefficient (per °C) |
|---|---|
| Steel | 12 × 10⁻&sup6; |
| Cast Iron | 10.5 × 10⁻&sup6; |
| Stainless Steel | 16 × 10⁻&sup6; |
| Aluminium | 23 × 10⁻&sup6; |
Worked Example
Example: Cast Iron Motor Housing
Given:
- Motor operates at 80°C, ambient temperature is 20°C
- Distance from base to shaft centreline: 200mm
- Housing material: Cast iron
Calculation:
T = 80 - 20 = 60°C
L = 200mm
C = 10.5 × 10⁻&sup6;
TG = 60 × 200 × 0.0000105
TG = 0.126mm
The motor will grow 0.126mm upward at the shaft centreline when it reaches operating temperature. To compensate, set the motor 0.126mm lower than the driven equipment during cold alignment.
Compensation Strategy
The component that runs hotter will grow more. During cold alignment, this component should be set lower than the cooler component by the calculated thermal growth difference.
For example, if the motor grows 0.15mm and the pump grows 0.08mm, the net difference is 0.07mm. Set the motor 0.07mm low during cold alignment so that both shaft centrelines meet when both machines are at operating temperature.
- Calculate thermal growth for each machine separately
- The offset applied is the difference between the two growth values
- Set the hotter machine lower during cold alignment
- OEM data may provide recommended offsets for specific equipment
Verification Process
Calculated thermal growth values are estimates based on assumed operating temperatures and published material coefficients. Actual growth may differ due to variations in operating conditions, cooling arrangements, and material properties.
- Complete cold alignment with the calculated offset applied
- Run the machine until it reaches stable operating temperature
- Stop the machine and immediately check alignment before it cools
- Compare the hot alignment reading with the target (should be within tolerance)
- Adjust the cold offset if the hot reading differs from prediction
Additional Factors
Pipe Strain
Connected pipework that heats up can exert forces on the machine, causing additional movement beyond thermal growth. This is particularly relevant for pumps handling hot fluids.
Foundation Effects
Concrete baseplates can absorb heat from machinery, causing gradual settling or movement over time. This is separate from thermal growth but can affect long-term alignment stability.
Differential Temperature
If the inboard and outboard feet reach different temperatures, the machine may develop an angular misalignment in addition to the vertical offset.
Seasonal Variation
Outdoor installations or unheated buildings may require different thermal offsets in summer and winter due to changes in ambient temperature.
Shim Selection for Thermal Growth
When compensating for thermal growth, the shim material must maintain its properties at the operating temperature. AISI304 stainless steel shims are suitable for most industrial applications, retaining their mechanical properties up to approximately 870°C.
The thermal growth adjustments themselves are typically small, often less than 0.5mm. Fine thickness increments (0.05mm, 0.10mm) allow accurate shimming to achieve the calculated offset.