1
$\begingroup$

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.

$\endgroup$

1 Answer 1

2
$\begingroup$

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.

$\endgroup$
3
  • $\begingroup$ May I ask you please how did you determine the extension lengths? $\endgroup$ Jan 4, 2016 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, 2016 at 17:51
  • 1
    $\begingroup$ (+1) Good answer. Consider adding your last comment as an edit to your answer. $\endgroup$
    – OnStrike
    Jan 4, 2016 at 23:31

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.