Designing the proper gland for a seal is usually a straightforward, easy process. Not only does the Parker O-Ring Handbook outline recommendations for stretch, squeeze, and volume fill necessary to achieve a good design but the Parker inPHorm design tool can be used to directly output the recommended dimensions making the design process quick and painless for most situations.
While most may be straightforward, there are certain applications, like those which experience high pressure, that require special consideration to ensure a robust design is achieved. Parker O-Ring Division defines a high pressure seal as one that may see 500 psi or greater. When an application pressure exceeds these limits, critical parameters like modulus of elasticity and diametral clearance become vitally important. If these factors are not considered, the design may run the risk of extruding seal material which can result in leakage, equipment downtime, equipment recall, or even injury to personnel. In the following paragraphs, I will help identify what modulus of elasticity and diametral clearance are, how they are impacted by pressure and how you can determine the pressure rating of a system.
Modulus of elasticity, often abbreviated as modulus, is defined as a material’s resistance to elastic deformation. A rubber material’s modulus is the ratio of stress over strain and can be measured experimentally by pulling samples on a tensometer. Modulus is difficult to express numerically because of rubber’s non-linear behavior. Oftentimes modulus at 100% elongation is given on a material test report. This value is the ratio of stress over strain after a material has been stretched 100% of its original length. Modulus can also be reported at other percentages of elongation with the expectation that the value may not be exactly the same as modulus at 100%. Because of the non-linear behavior of rubber and the destructive nature of tensile testing, we choose to focus on material hardness as an easy measure of how an O-ring will perform in relation to pressure. When comparing two materials of the same hardness, we then utilize modulus at 100% elongation as a way to indicate which material will resist extrusion better.
In the world of O-rings, we measure material hardness using the Shore A durometer scale, which runs from 0 to 100. On the scale, 0 represents the softest materials and 100 represents the hardest materials. For example, a typical rubber band has a Shore A durometer of 20, while a plastic hard hat would measure 100. As the Shore A durometer hardness of a material increases, the modulus of elasticity also increases.
The standard O-ring material hardness is about 70 to 75 Shore A. These materials can handle most application pressures, but when pressure increases the force exerted on the material can become great enough to deform and push the O-ring. Parker O-Ring Division has specialty compounds designed with a hardness of 90 to 95 Shore A, making the seals more suitable for higher pressure.
While hardness governs how the material reacts to pressure, diametral clearance governs the size of the gap that an O-ring can extrude out of. Diametral clearance is the total gap between the bore diameter and the piston diameter. Figure 1 identifies the radial clearance in both a male and female radial seal application. To convert radial clearance to diametral clearance, just multiply the radial clearance by two.
To understand how diametral clearance affects pressure rating, it is easier to think about an example outside of O-rings. If you have a piece of metal with a pinhole and you push Silly Putty™ through the hole, you’ll find that material does not push through to the other side. If you slowly increase the size of the hole, you’ll find that eventually, at a certain size, the material will begin to “extrude”. This helps show that the size of a gap plays an integral role in how likely it is for material to be forced out.
In light of the effect that clearance and material hardness plays in extrusion, Parker has provided baseline recommendations to help determine the likelihood of extrusion in a given application. The “Limits for Extrusion” chart, shown below and published on page 3-3 of the O-Ring Handbook, gives guidelines for pressure, clearance, and material hardness. The “Limits for Extrusion” chart was generated from actual testing performed by Parker Engineered Materials Group.
Interpreting the “Limits for Extrusion” chart can be done in two ways. When given gland design information (including clearances) and material choice, a pressure rating can be obtained. For example a gland design with a 0.010” clearance gap utilizing a 90 durometer material can potentially see 2,000 psi before extrusion would occur. If given a required pressure, then the required clearance can be found for a given material hardness. For example, an application at 1,000 psi would require a 0.018” clearance gap for a 90 durometer material, a 0.013” clearance gap for an 80 durometer material, and a 0.008” for a 70 durometer material. As you can see, the softer the material gets, the smaller the clearance needs to be to prevent extrusion.
By following the design recommendations outlined per the O-Ring Handbook and “Limits for Extrusion” chart, a robust design can be achieved which helps eliminate the risk of seal failure. This helps give you the safety and peace of mind needed when designing critical components utilizing sealing materials.
For additional assistance you can contact Parker’s Application Engineering department for detailed recommendations on applications which fall outside the scope of this post.
This blog was contributed by Eric Uehlein, O-Ring Division Applications Engineer