Chain and belt guides - Murtfeldt - #39

The Coefficient of Linear Expansion and Its Rôle in Design for Plastics

How to calculate the coefficient of linear expansion

Thermoplastics have a higherthermal coefficient of linear expansion than materials made from metal. That is why guides made from plastics must be fixed in a way which allows them to expand lengthways.

When calculatingtheir expansion, please take into account the anticipa­ted difference between temperature during assembly and min./max. operating temperatures. Usingthe coefficient of linear expansion, calculate their max. elongation when subject to heat and their max. linear contraction when subject to cooling.

For calculation examples, please see Appendix 'Tolerances for Machining Plastics' (or see calculation below). Depending on the application, various fixing methods are available. These are described on the following pages. In every case, their guiding principle is to allow the plastics material to expand and contract when subject to temperature fluctuations.

Chain/belt/goods conveyed travelling in one direction only:

During expansion, the plastic guide backs up against the direction of conveyor travel. Therefore, sufficient space for its anticipated expansion should be allowed at its start.

How to calculate displacement AL = L-a-AT

where Al_ = displacement L = initial length

a = linearthermalcoefficientofexpansion

see pg25, line 25 AT = temperature difference in C°

please note:

AT = upper temperature - installation temperature lower temperature - installation temperature please state +/- for AT

plastic guide

space for expansion

direction oftravel ■

bolt/dowel pin

Chain/belt/goods conveyed travelling in one or alternating directions:

bolt/dowel pin plastic guide

-direction oftravel-

expansion gap

expansion gap

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