Conventional wisdom holds that the more an O-ring seal is squeezed (ie, deformed relative to its "squeezed" state), the tighter the seal. More squeezing equals greater force between the O-ring and its mating hardware - i.e. liquids, gases and dry powders that would otherwise flow from between the rubber seal and the mating hardware.
An O-ring that is squeezed more also tends to hold its force (and thus a better seal) longer than an O-ring that is squeezed less. The failure of an elastomer to maintain its "push-back" force over time is known as compression set. An elastomer with a high compression set (usually over 80%) no longer returns to its original uncompressed shape when it is not squeezed. However, it may not always be true to assume that increasing the squeeze on the O-ring will result in a better seal, depending on other factors. These include:
The harder you squeeze the o-rings during installation, the more likely you are to pinch them because of the way leaks are created. This was found when Parker compared 2 finite element models used to predict the behavior of the O-ring installed at 40% and 25% extrusion (Figures 1 and 2, respectively). At 40%, pinch-fit O-rings between components are unavoidable, while at 25% mounting clamping is virtually eliminated.
The relationship is linear between the amount of uncompressed and the compressive load force required to maintain the squeeze. In fact, the compressive load force rises much faster than 30% extrusion. The O-ring takes almost two and a half times the load force to compress to 40% than it does to 25%. This level of compressive load can crush or deform lightweight or fragile mating components.
The data shown in these figures is for 70 Shore nitrile rubber. Other materials, such as perfluoroelastomers and compounds with low elongation (ie, they only stretch "so far") are likely to break when squeezed more than 30%. Other materials can experience accelerated compression set, reducing service life by 40% in extrusion.
While many variables affect the form, fit and function of the seal, the most important parameter behind a leak-free seal is the amount of extrusion applied. However, as these data suggest, the ideal amount of extrusion itself is affected by many factors. You can't just assume that more is better.
Keep in mind that other factors can cause O-ring seals to perform poorly, such as thermal degradation, chemical interactions, gas permeation, mechanical damage such as crushing or abrasion, or low temperature loss of elasticity. In these cases, adjusting the amount of squeezing may not solve the problem.