One of the decisions equipment designers need to make when installing O-ring seals in their applications is how much the O-ring will be squeezed by its mating hardware to create an effective seal.
The squeeze is a ratio of the amount of deformation applied to the seal expressed as a percentage of the free-state cross-sectional thickness. Deforming the seal cross-section “energizes” the elastomer matrix much like compressing a spring; the inherent elasticity of the rubber material causes it to push back against the mating components. This contact force blocks the passage of liquids, gases, and dry powders, preventing them from flowing between the rubber seal and the mating hardware.
The greater the squeeze, the more force is applied against the hardware and the tighter the seal. But that doesn’t necessarily mean that designers should always specify the most squeeze (assuming they knew what that level was or why it was “the most”). There are a number of factors to consider, which include:
As just noted, tighter seals generally result from higher levels of a squeeze. Beyond a certain level, however, other factors intervene that can work against an effective seal, such as the stress the force causes on mating hardware.
If you increase the squeeze, and its compressive force, too much you can potentially damage mating hardware, depending on the materials and the design of the hardware.
With higher squeeze also comes more friction and faster wear in dynamic applications. This may be enough to affect device function. For example, in a medical device that involves manual adjustment, an O-ring that generates too much friction may prevent the physician from properly utilizing the device.
With higher squeeze comes higher risk that pinching will occur when the O-ring is installed — creating pathways for fluid or gas to flow around the seal. Figures 1 and 2 show finite element models of what happens when O-rings are installed with 40 percent and 25 percent squeeze, respectively. The models depict O-ring pinch damage during assembly in a male (piston-type) O-ring gland. At the 40 percent level, pinching is difficult to avoid while pinching is eliminated at the 25 percent level.
The force that the squeezed elastomer exerts against the mating hardware, creating the seal, tends to decay with time. When that force decays entirely, the O-ring will retain its squeezed shape even when it is no longer squeezed. Compression set is a measure of this decay, expressed as a percentage. When compression set reaches 80 percent, most O-rings are in danger of losing their ability to seal. O-rings at higher squeeze levels generally take longer to reach that 80 percent compression set level. Based only on compression set, then, higher squeeze levels generally translate to longer useful O-ring work lives.
Figure 1: Squeeze does not impact the shape of the compression set curve
Designers need to consider carefully all these factors — in addition to other application-specific factors such as temperature and the pressure of the materials being sealed — before they decide how much O-ring squeeze to apply. The correct decision translates to an optimum seal over the longest O-ring life span with the least damage to either the O-ring itself or it's mating hardware.
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This blog was written by Dan Ewing, engineering supervisor, Parker Hannifin O-Ring & Engineered Seals Division.
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