Extrusion is the single most common cause of catastrophic seal failure. Seals begin to extrude when the exposed heel of the seal is forced through clearance gaps in the seal grooves from a variety of reasons. There are several ways to minimize or even prevent extrusions through better machine design or seal design. Below are four:
Reduce clearance gaps
At the design stage, the clearance gaps must conform to the manufacturer’s recommendations. The table below shows typical values for a polyurethane seal. Double check application parameters that might be tougher than expected. Look for unusual temperatures or pressures and unfamiliar fluids, and adjust the gaps accordingly.
Metric Series Inch Series
Select alternative materials
A seal’s ability to resist extrusion is a physical property of the material and is closely related to the yield strength. The chart below shows the stress strain curve for a PTFE seal compound. Stress on the seal’s heel is easily computed. This allows engineers to choose a more suitable material to ensure that yield stresses are not exceeded. Be aware that the yield strength in polymers can change dramatically with temperature gains.
Use an active anti-extrusion device
There are several seal designs that incorporate an anti-extrusion device, such as a backup ring, that enhances a seal’s performance and extends its life by eliminating extrusion altogether. Examples include the System Seals 135 and 188 series.
Add a full-face, anti-extrusion ring
Outside forces on the rod can cause pressure peaks well beyond a system’s operating parameters. In many cases a full-face, anti-extrusion ring can be easily added to a seal, as shown below to protect a seal’s integrity from pressure spikes. These rings fit neatly inside the seal for easy installation.
It’s often beneficial to conduct tests and Finite Element Analysis to predict a potential failure. This is particularly necessary when seal geometries are complex or a definitive safety factor is required. The example below shows FEA of PTFE seal under high pressure. For more information, contact the System Seals Engineering Center at 216.220.1800.