Coefficient of Thermal Expansion
Coefficient of linear expansion is the ratio of the change in length per °C to the length at 0°C. Coefficient of volumetric expansion for solids is approximately three times the linear coefficient. As a rough approximation, elastomers have a coefficient of expansion ten times that of steel (an exception to this is perfluoroelastomers). This can be a critical factor at high temperature if the gland is nearly filled with the seal, or at low temperature if squeeze is marginal. See the table below.
|Family||Test Range||Reported CLTE (m/m/°C)||Calculated CLTE (in/in/°F)|
|NBR||25 °C to 125 °C||1.97E-04||1.1E-04|
|HNBR||25 °C to 160 °C||1.95E-04||1.1E-04|
|AEM||25 °C to 175 °C||2.29E-04||9.5E-05|
|EPDM||25 °C to 125 °C||2.01E-04||1.1E-04|
|FKM||25 °C to 200 °C||1.98E-04||1.1E-04|
|Low Temperature FKM||-20 °C to 200 °C||2.67E-04||1.5E-04|
|Aflas||25 °C to 200 °C||2.85E-04||1.6E-04|
|FFKM||25 °C to 320 °C||3.91E-04||2.2E-04|
|VMQ||25 °C to 230 °C||2.30E-04||1.3E-04|
|FVMQ||25 °C to 175 °C||2.00E-04||1.1E-04|
|High-Temperature Type Aluminum, 2017||n/a||2.34E-05||1.3E-05|
|Stainless Steel, Type 302||n/a||1.73E-05||9.6E-06|
|Test method ASTM E831|
There are certain reactions that in some circumstances cause a seal to exert relatively high forces against the sides of a groove. These forces are generated by thermal expansion of the rubber and/or swelling effect of a fluid.
If the seal is completely confined and the gland is 100% filled, the dominating force is the force of thermal expansion of the rubber. There have been instances where a seal has ruptured a steel gland due to expansion when heated.
Effective force exerted by the seal due to fluid swell is another potentially large factor if the gland volume exceeds that of the seal by only 5 to 10% (see Figure 2-20). Depending on the interaction between the rubber and the fluid being sealed, the effect may be quite pronounced even at larger gland void conditions.