Sealing Shielding

Selecting the Right O-Ring Seal Squeeze Ratio

O-rings, O-Ring Squeeze, Seals, O-Ring Materials, O-Ring Compression SetOne 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.

What is O-ring squeeze

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:

Seal tightness

As just noted, tighter seals generally result from higher levels of 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.

Mating hardware stress

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.

Application friction and wear

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.

O-ring damage

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.

Compression set

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

Figure 1: Squeeze does not impact the shape of the compression set curve

Last word of caution

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 its mating hardware.

You can find more support info and FAQ's on our website at this Support Link. If you have an immediate questions, we have live chat with our application engineers available for a number of our divisions - look for the "Chat with an Engineer Button."

To read more from the Sealing and Shielding Team, click here O-Ring Squeeze - More is Not Always Better.

This blog was written by Dan Ewing, Engineering Supervisor, Parker Hannifin O-Ring Division.

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Comments

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Comments for Selecting the Right O-Ring Seal Squeeze Ratio

Cindy Harbaugh
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.
Dan Ewing
Increasing the squeeze increases the compressive load force generated by the seal. This increased force can damage or deform the mating hardware in certain applications, particularly when that hardware consists of plastic components or thin walled metal items. In cases where increasing the squeeze is necessary to obtain a reliable seal or improve the service life, it may also be necessary to structurally reinforce the gland area or to use a softer seal material to keep the compressive load force within the structural limits for the existing hardware.
Yvan Simmons
I was looking into the Parker O-ring Handbook ORD5700, in page 114 to calculate o-ring friction force. How do you calculate the Percent Seal Compression required in Figure 5-9?
Also, do you happen to have "Figure 5-10: Friction Due to Fluid Pressure" for a 8000 to 15000 psi range of fluid pressures (differential)?
Tyler Karnes
The equation for calculating compression is (O-ring CS - Gland height) / O-ring CS. For a radial seal application, gland height is calculated by (Bore diameter - groove diameter) / 2.

Parker OES does not have data for fluid pressure from 8000 - 15000psi. If your application is a dynamic application at those pressures, it would likely require a PTFE or thermoplastic solution from Parker EPS division.

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