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I have a heater rod inside a brass rod, which is wrapped by PTFE insulation. The dimensions of the assembly are:

  • Heater diameter = 9.5 mm
  • Brass rod diameter = 25 mm
  • Insulation diameter = 100 mm

When the heater heats the assembly, will the assembly get tighter or looser? The final temperature that the brass rod would reach is 100 °C and the initial temperature is 20 °C.

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Plastics have a thermal expansion rate of approximately 5 to 10 times that of metals. The PTFE will expand the most and the brass expansion is about 50% more than a steel heater, so the assembly will get looser.

Here is a link to a chart of linear expansion coefficients. For a 9.5 mm part with 80 °C temperature change, the steel extends .0082 mm, brass .0142 mm and PTFE .0851 mm.

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  • $\begingroup$ May I ask you please how did you determine the extension lengths? $\endgroup$ – Mohamed AbdelAlim Jan 4 '16 at 17:36
  • $\begingroup$ Change in Length (mm), ∆L = C*∆T*L, Mech. Eng. Design p33, C is constant from engineeringtoolbox.com/linear-expansion-coefficients-d_95.html (read metric column values, not english), ∆T is 80 degrees, L is 9.5mm $\endgroup$ – Greg Marsh Jan 4 '16 at 17:51
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    $\begingroup$ (+1) Good answer. Consider adding your last comment as an edit to your answer. $\endgroup$ – OnStrike Jan 4 '16 at 23:31

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