SATRA STD 133 Compression set apparatus
During walking, a shoe sole (or top piece) is subjected to compression forces and deformation due to flexing with each step, which temporarily increases its area. By the completion of the step practically all of this increase in area is lost. However, if the soling or top piece is slightly plastic they do not completely recover and some of the increase in area is retained. Such a soling, therefore, gradually increases in size during wear.
There is no reliable single laboratory test which predicts the spreading behaviour of soles or top pieces. Dynamic tests have been designed which apply repeated deformations as experienced during walking, but useful information can also be obtained from much simpler tests which use a single prolonged deformation. Such a test can either compress the sole with subsequent measurement of the permanent decrease in thickness, or stretch by measuring any permanent increase in length.
To accomplish these measurements easily and under standard conditions, SATRA has developed the compression set apparatus which can be used to assess solid, cellular and micro-cellular rubbers under constant stress conditions. Using this equipment sample discs are cut from the sole or top piece material and are held between sprung loaded-platens for a given time under a constant pressure. By using several spacers, tiers of specimens can be tested at the same time. The thickness of the discs is measured before and after the test and the decrease in thickness provides an indication of the compression set of the material.
A suitable gauge for measuring the thickness under standard conditions is available, Reference STD 495.