Conventional wisdom says that the more an O-ring seal is squeezed (i.e. deformed relative to it’s “unsqueezed” state), the tighter the seal. More squeeze equals greater force between the O-ring and its mating hardware — which means that liquids, gases, and dry powders are otherwise prevented from flowing between the rubber seal and mating hardware.
An O-ring that’s being squeezed more also tends to maintain that force (and therefore the better seal) longer than an O-ring that’s being squeezed less. The failure of an elastomer to maintain its “push back” force over time is called the compression set. An elastomer with high compression set (generally more than 80 percent) no longer returns to its original uncompressed shape when it is not being squeezed. However, the assumption that increasing the squeeze on the O-ring will result in a better seal may not always be correct, depending on other factors. These include:
The more forcefully you squeeze O-rings during installation, the more likely you are to pinch them, creating pathways for leaks. That was the finding when Parker compared two finite element models used to predict the behavior of O-rings installed at 40 percent and 25 percent squeeze (Figures 1 and 2, respectively). At 40 percent, pinching of the O-ring between mating components was difficult to avoid, while at 25 percent installation pinching was virtually eliminated.
Mating hardware damage
As Figure 3 shows, the relationship is not linear between the amount of squeeze and the compressive load force required to maintain the squeeze. In fact, the compressive load force rises much faster beyond 30 percent squeeze. An O-ring requires almost two and a half times more load force to compress to 40 percent than it does to 25 percent. This level of compressive load could crush or deform lightweight or fragile mating components.
The data presented in these figures are for 70 durometer nitrile rubber. Other materials, such as perfluorinated elastomers and compounds with low elongation (i.e., they only stretch “so far”) may rupture when squeezed more than 30 percent. Other materials could experience accelerated compression set and reduced service life at 40 percent squeeze.
While many variables affect an O-ring's form, fit and function, the most important parameter behind leak-free sealing is the amount of squeeze applied. Yet, as these data indicate, the ideal amount to squeeze is itself influenced by many factors. You can't just assume that more is better.
Keep in mind that other factors that can cause an O-ring seal to perform poorly, such as thermal degradation, chemical interactions, gas permeation, mechanical damage such as extrusion or wear, or low-temperature loss of resiliency. In these situations, adjusting the amount of squeeze is not likely to fix the problem.
Figure 1: At 40 percent squeeze, an O-ring is likely to be pinched during installation
Figure 2: Even with poor lubrication, installation with 25 percent squeeze shows little risk of damage
Figure 3: Compressive load force increases substantially beyond 30 percent squeeze
This article was written by Dan Ewing, engineering supervisor at Parker's O-Ring Division in Lexington, Kentucky.
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