FKM O-rings

To properly size an o-ring, you need to measure two things; the inside diameter (I.D.) and the width or cross-section of the o-ring. If your o-ring is used and show some wear, the measurements you take may not be accurate to what the proper replacement o-ring should be. In cases where the o-ring is too heavily damaged to properly size a replacement, the groove or gland the o-ring came out of should be measured.

The durometer, or hardness, of an o-ring can affect how well the o-ring is able to handle higher pressures and larger gaps between mating surfaces. The trade off is that the higher the durometer, the more difficult it is for the o-ring to deform and be installed. Because of a number of factors influencing each application, it is impossible to set a guideline for which durometer to use in each situation. If you are unsure of what durometer to be using, ask your R.E. Purvis & Associates, Inc. representative to assist you with your application.

Flash is a thin film of extra material that follows the line between the two mating halves of a part mold. This extra material is the result of a small gap occurring between the mold halves and the high pressure the rubber experiences as it is formed into its finished shape. Though most rubber manufacturers have processes in place to remove flash, it is near impossible to get all of the flash off every part. Typically, the allowance for part flash is .003" wide and .005" thick max.

Most nitrile and neoprene rubber materials undergo a process called “blooming” when they are stored. “Bloom” is a milky dusting of dry powder on the surface of the rubber. Typically, this is caused by unused vulcanizing agent(s) migrating to the surface of the rubber part. This blooming is entirely superficial. If the gray color is not acceptable, wash the rings in water or light mineral oil to remove it. Since blooming is entirely normal and does not affect the function of a rubber seal, it is not considered a rejectable defect. Likewise, it is not considered a contaminant in the rubber material.

The phrase "standard material" is an interesting one. To some it may mean a specific compound, while to others it may be something much broader than that. As a basic rule, calling out a durometer and material is sufficient to allow us to find you a part from any number of sources that is able to meet your needs.

Nitrile - 70 Duro

Fluorocarbon (Viton) - 75 Duro

Ethylene Propylene - 70 Duro

Silicone - 70 Duro

Hydrogenated Nitrile (HNBR/HSN) - 70 Duro

Polyurethane - 90 Duro

Neoprene - 70 Duro

Fluorosilicone - 70 Duro

Yes, R.E. Purvis & Associates, Inc. stocks metric o-rings and seals in a variety of configurations and compounds.

Yes, in addition to preconfigured kits that are available, R.E. Purvis & Associates, Inc. would be happy to discuss any special o-ring kitting you may require for a specific project or application.

Based on our manufactures recommendation, R.E. Purvis & Associates, Inc. uses the ARP 5316 recommended practice for setting shelf lives of elastomeric seals. Based on that recommendation, we suggest the following:

5 years: Polyurethane (milleable)

15 years: Nitrile, Neoprene, SBR, HNBR, Polyacrylate

Unlimited: Ethylene Propylene, Fluorocarbon, Perfluorinated Elastomer, Butyl, Silicone, Tetrafluoroethylene Propylene (Aflas®), Fluorosilicone.

Compression set is defined as the percent of deflection by which the elastomer fails to recover after a fixed time under specified squeeze and temperature. What this means is that it is a measurement of how much a seal changes when it is compressed to a certain level for a set amount of and then allowed to return to its free state. To calculate the compression set of a material, you first take the original thickness of the sample and subtract the thickness of the sample at the conclusion of the test. You then divide that number by the original thickness subtracted by the thickness you compressed the sample to. Once you have done that, multiply your final number by 100 to get your compression set.

A cross section is what you'd get if you took a finished part, cut it in half, and looked at the ends. The cross section can tell you a lot about how the seal functions. For example, the cross section of a u-cup can give you a better understanding of how the lips function while the system is pressurized. Cross sections are often used to gather information on why a seal may have failed and to assess the locations where wear may be occurring.

A static application is used to describe situations where the seal is located between two fixed components where no movement will be occurring. An example of a static application would be a face seal or gasket.

A dynamic application is used to describe situations where the seal is located between two components that are moving relative to each other. An example of a dynamic application would be a piston seal or rotary shaft seal.

Extrusion is the effect of having too large a gap and too high a pressure. The result is that the seal material is pushed through a gap between the mating hardware. The severity of extrusion can be anywhere from a slight ridge on the face of an o-ring resembling flash to the entire part being pushed out of the gland.

The material that is selected for a specific application depends on several factors. The fluid it will come in contact with, the temperature it will deal with, the pressure the part is exposed to, the length of time the part must last, and how much the customer is wiling to pay are all contributing factors. Your best solution to finding the right compound for your application is to contact us. The trained staff of R.E. Purvis & Associates, Inc. will be able to isolate the compound that will best suit your application's needs.

Yes, R.E. Purvis & Associates, Inc. has been ISO certified for 20+ years. Please visit the About Us page for a copy of our ISO certificate.

Our certification as a 100% Women-Owned Business is just another way we stand out in the marketplace. Being able to help you meet goals in this area, while also receiving top-notch service and quality products, is a multi-faceted win we provide! Please visit the About Us page for a copy of our WBE certificate.