The force required to achieve the proper seal squeeze is a result of three main factors: the cross section of the o-ring, durometer of the material, and the percent squeeze required. These relationships are demonstrated in Illustration 5.3. Please note that if the o-ring is stretched a considerable amount in its operating environment it may be important to incorporate the reduction in the cross section due to the stretch when determining the percent squeeze. Below is the formula for the calculated reduced cross section.
The calculation assumes the o-ring volume does not change and the cross section remains round when stretched. In actual applications it has been observed that when stretched the o-ring cross section does not remian circular. Therefore, the actual cross section reduction is slightly larger than what the formula estimates.
Illustration 5.3 is comprised of the combinations of durometer, percent squeeze, and cross sections. The chart provides data for incremental percentages of squeeze, typical durometers, and standard o-ring cross sections. The force required for a different percent squeeze, non-standard cross section, and/or durometer can be inferred from the generally linear relationship between the three factors and force required.