TECH TALK: Thermal Expansion of Engineering Plastics

Like most other solid materials, engineering thermoplastic materials expand upon heating and contract when cooled. Thermal expansion and contraction must be taken into consideration when designing constructions/assemblies composed of materials having different coefficients of thermal expansion which can or will be exposed to (high) temperature changes or when the plastic components have to meet stringent tolerances over a wide temperature range.
It also has to be noted that thermal expansion can cause significant stresses in a component or can lead to undesirable deformation in case the design does not allow for proper expansion and contraction of the components (example see Fig. 1).

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Fig. 1: a UHMW-PE wear strip bolted to a steel substrate near the outlet of an oven having a temperature of up to 300°C/570°F during operation.

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Figure 2

The COEFFICIENT OF LINEAR THERMAL EXPANSION (CLTE) is a material property indicating the extent to which a material expands upon heating. The CLTE of thermoplastic materials is generally not a constant but varies with temperature (example see Figure 2: CLTE of Ketron 1000 PEEK as a function of temperature). Average values of the coefficient of linear thermal expansion within specified temperature ranges of the Quadrant Engineering Plastic Products are given in the product data sheets and Quadrant’s technical literature.

Calculation of linear thermal expansion:
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EXAMPLE:
A Ketron 1000 PEEK sleeve bearing OD 127 mm / 5 in. x ID 101.6 mm / 4 in. x Length 76.2 mm / 3 in. assembled in a steel housing at room temperature (20°C / 68°F) is going to be retained by means of a steel flange in order to prevent axial displacement.
What’s the minimum width S of the gap between sleeve bearing and steel flange in order to prevent the bearing from buckling in case the environmental temperature could rise up to maximum 150°C/302°F?
[CLTE of Ketron 1000 PEEK between room temperature and 150°C/302°F: 55 x 10-6 m/(m.K) or 30.6 x 10-6 in./in./°F]

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ANSWER: 0.55 mm / 0.0215 in.
Please note that enough clearance between shaft and inside diameter of the bearing should be provided in order to prevent the bearing from seizing on the shaft during operation at high temperature.

Please consult Quadrant’s Design & Fabrication Reference Guide for proper bearing clearance to be applied at assembly or contact Quadrant’s Technical Service Department for support.


 

Koen_002Koen Verhoeyen
Application Development Engineer

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