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Thermal Growth – why should you care?

buckled rails img

Engineering and technical people often use technical phrases that sound complicated. Thermal growth is one of those terms. It sounds like it could be an engineering marvel – one that would help us perform complicated alignments of high temperature, high speed machines. So, here’s the “secret” of thermal growth.

When metal gets hotter, it gets bigger.

Just how big is based on three things:

  • (What it is) The coefficient of linear thermal expansion, C
  • (How hot it gets) The differential temperature of the material being heated, vs the ambient temperature ∆T
  • (How tall it is) The linear dimension of the material being calculated, L (How much material do you have)

It is calculated using the following formula:

C x ∆T x L = amount of thermal growth

Coefficients for Linear Thermal Expansion for commonly used machine metals

Cast Iron0.0000059
Mild Steel0.0000063

Still sounds complicated? Try this; 12 inches of these commonly used materials, heated to 100°F above room temperature, yields the following growth:

Cast Iron   0.007”

Mild Steel  0.008”

Stainless   0.009”

The pump in this photo will be used to pump 180°F water, and is installed in an 80°F room. Its shaft centerline height is 24 inches, and it is a cast iron frame. So:

12 in. of cast iron, heated 100° over room temperature = 0.007”. But we have 24”, so:

0.007 x 2 (12 inches twice) = 0.014”.

It sounds right, but not so fast!

Thermal growth is not calculated by the fluid temperature, but by the pump casing temperature. That’s what’s holding the pump shaft up. And it almost never heats evenly from the bottom to the split line. So you must either:

  • Make an estimated calculation of the thermal growth by sampling the temperature in several places, and getting an average temperature,
  • Use the HotCheck function on your laser alignment tool (if available), to get a comparison between the hot and cold alignment values,
  • Use a dynamic measuring process, such as Fixturlaser’s OL2R (off line to running) system, to get an extremely accurate measurement of thermal growth, which would also include dynamic factors, such as piping changes.

Thermal growth is not always a factor in aligning machinery, but if your machine is:

  • Big,
  • Hot,
  • Fast,
  • or must be aligned to a very close tolerance,

thermal growth must be factored as part of the alignment process.

Remember, precision alignment, must include thermal growth calculations, or else, it is not precision alignment.

Nathalie Drouin

Services Techniques Claude Drouin Inc.


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