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    • February 2021
/ What Factors to Consider When Selecting an O-Ring

What Factors to Consider When Selecting an O-Ring

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You can have the most innovative and high-tech equipment, but if you don’t have the right o-ring in place it may be rendered useless. The importance of choosing the right o-ring for any application should never be underestimated as the appropriate size and material selection can be the difference between being up and running and failure.

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Material Selection

Selecting the appropriate material for your o-ring is very important and is your starting point when shopping for an o-ring. Common material selections include:

Nitrile (Buna-N): This is the most widely used o-ring material. It provides excellent resistance to petroleum products. It also offers an excellent compression set, tear, and abrasion resistance. It does not offer good resistance to ozone, sunlight, or weather unless it is specifically compounded to do so. Temperature range: -40° to +250°F.

Fluoroelastomer: This elastomer offers excellent mechanical and physical properties with good resistance to petroleum products, low compression set, and high-temperature resistance. Ideal for vacuum service and low gas permeability. Temperature range: -15° to +400°F.

Chloroprene: This elastomer is moderately resistant to petroleum and offers good ozone and weather resistance and good compression set. Excellent for sealing refrigeration fluids such as Freon®. Temperature range -65°- +250°F.

Silicone: This elastomer is ideal for weather and ozone resistance. It offers limited oil resistance and is not recommended for dynamic sealing applications due to poor tensile and tear strength and low abrasion resistance. Temperature range -80° - +450°F.

Highly Saturated Nitrile: This material offers better resistance to high temperature, superior physical properties, and improved chemical resistance over traditional nitrile compounds. It has improved resistance to ozone and sunlight and excellent resistance to compression set. Temperature range -40°- +325°F.

Ethylene Propylene (EPM, EPDM) (EPR): Excellent resistance to Skydrol® fluids used in commercial aircraft hydraulic systems. Recommended for hot water, steam, and phosphate ester-type hydraulic fluids. Resistance to some acids, alkalies, and ozone. Not recommended for petroleum fluids or diester lubricants. Temperature range: 65° - +300°F.

PTFE: This is a white thermoplastic fluoroethylene resin that offers heat resistance, low friction, dielectric strength, weatherability, toughness, no water absorption, and flexibility. Temperature range: -65°- +500°F.

Aflas: This material offers high temperature, chemical, and electrical resistance properties. Recommended for use with oils, lubricants, hydraulic fluids, transmission, and power steering fluids, sour oil and gas, ozone, steam, acids, bases, alcohols, and other chemicals. Temperature range: +25° - +450°F.

As you can see, there is a wide variety of material options, each with its unique strengths and weaknesses. Choosing the right material is very important.

Temperatures

The temperature that the o-ring will be exposed to is an important consideration when choosing the right o-ring for your application. Temperatures can adversely affect the performance of even the best-made oring. If the operating temperature is too cold the o-ring will lose its elasticity and become more brittle, making breakage likely. On the other end of the spectrum, if an o-ring gets too hot it can become too flexible and degradation will occur.

The importance of temperature is why you’ll always see temperature ranges on our products. This will help you determine if the material is best suited to your specific application.

Operating Pressure

Operating pressure is also an important consideration when you are purchasing o-rings. Typically speaking, in a static sealing setting a standard 70 shore hardness will usually work well. Yet, once pressure approaches the 1500+ psi range you may need to consider a shore hardness of 80-90.

Surface Finish

Surface finish is also an important consideration. Generally speaking, the ideal surface finish for dynamic surfaces is 5 to 16 µ inch RMS. Smoother and rougher surfaces can result in increased friction and reduced life of the seal. Rougher finishes up to 64 µ inch RMS can be used in static applications.

Stretch

When purchasing o-rings, you’ll also want to consider the stretch of the ring. For effective sealing, the inside diameter (ID) of the o-ring must be smaller than the groove where it will sit. This will require that there is a slight stretch in the o-ring when it is in place, fitting snuggling into the groove. The stretch should be between 1-5% with 2% being ideal in most applications.

APG is Your O-Ring Resource

As you can see, there is a lot that goes into choosing the right o-ring for your specific application! APG offers a great selection of standard o-rings and we can offer vulcanized options if you need something other than standard sizes. Ready to shop for o-rings now? If you are uncertain about which of our products will suit your particular needs, please do not hesitate to give us a call at 713-561-3309 and one of our experts would be happy to assist you!

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