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Understanding Thermal Growth and Its Impact on Shaft Alignment

Thermal imaging close-up of overheated mechanical connection

When machines heat up, they grow – and that growth can destroy shaft alignment if not accounted for.
– The Science of Thermal Growth

When rotating equipment operates under load, temperature changes cause the machine casing and shafts to expand. This thermal growth directly affects alignment between coupled machines.

The magnitude of movement can be calculated using the standard formula:

ΔL=L×C×ΔT {Delta L = L times C times Delta T}

  • L: Vertical distance from shaft centerline to machine foot
  • C: Coefficient of linear expansion (e.g., 0.0000059 in/°F for cast iron)
  • ΔT: Temperature change (operating temp − ambient temp)
Diagram showing how thermal growth affects shaft alignment in motors and driven machines via formula and example calculations

Example:

For a motor with a 26″ shaft-center height experiencing a 250°F rise:

26″×0.0000059×250°F≈0.038″26″ \times 0.0000059 \times 250°F \approx 0.038″

That’s nearly 1 mm of vertical movement—enough to cause severe misalignment if left uncorrected.

How Thermal Growth Affects Alignment

  • Uniform Growth at Both Feet: Creates a vertical offset.
  • Unequal Growth (Front vs Rear Feet): Produces angular misalignment, leading to coupling stress, bearing wear, and vibration issues.

 

Practical Example:
A motor aligned “cold” with its driven pump may look perfect during installation. Once the pump reaches operating temperature and one end rises more than the other, the shafts shift, resulting in coupling misalignment and premature wear.

Corrective Approaches

1️⃣ “Hot” Alignment (Cold Targets)

Pre-adjust machines by adding shims or offsetting alignment values to compensate for predicted thermal growth. Once the machine heats, shafts settle into proper alignment.

2️⃣ Live Thermal Monitoring

Advanced laser systems (e.g., Easy-Laser, RotAlign Touch, Pruftechnik) measure live shaft movement during warm-up, allowing precise dynamic corrections.

3️⃣ Use Built-In Calculations

Many modern alignment tools integrate the ΔL = L × C × ΔT formula, applying thermal growth compensation automatically.

Key Takeaways

  • Thermal growth is a predictable, measurable factor in rotating machinery alignment.
  • Even small movements can create significant coupling stress.
  • Always account for ΔL = L × C × ΔT when aligning motors and pumps.
  • Use cold targets or live monitoring to achieve operational alignment, not just installation alignment.

Need thermal growth analysis or precision shaft alignment? Contact THERMOInspect for expert services with advanced laser and infrared diagnostic tools.