Determining the exact size of an O-ring is one of the more challenging parts of ordering O-rings for your application. As you know, O-rings are used to create an impenetrable seal. If the O-ring is too large, fluids, gasses, dirt, water vapor outgassing under vacuum, and more will flow around it. If the O-ring is too small, overstretching it to fit it into place will compromise the O-ring’s integrity and can cause it to fail. Knowing how to properly size an O-ring is important to avoid O-ring failure and avoid wasting time and money. Sizing an O-ring isn’t tricky, but it is crucial that you are precise.
The best way to size an O-ring is to measure the cross-section (CS) as well as the inside diameter (ID) of the O-ring with a caliper or micrometer. The outside diameter (OD) is good to note but is a non-critical measurement. If you do not have these tools, you can use any measuring tool that you have available; a caliper or micrometer will be more accurate. For larger IDs over 6”, it is best to use an O-sizer or Pi-Tape for precise measurements.
Metric VS Standard Sizes
All O-rings either come in metric (mm) or standard (inch) sizes. If your CS matches one of the standard AS586 sizes (such as 0.70”, 0.103”, 0.210”, etc.) then your O-ring is most likely a standard size. If it does not match, then the odds are good that you have a metric O-ring.
Here is how to properly measure an O-ring:
Tolerances
One important consideration you need to keep in mind when sizing an O-ring is tolerance. Due to manufacturing limitations, there will always be some variance in the O-ring’s dimensions. For example, an O-ring size of 0.103 with a tolerance of ± 0.003’, the cross-section can measure between 0.100” and 0.106”. It is best to consult the manufacturer for their specific tolerance variance based on your selected O-ring material.
UC Components, Inc. RediVac® O-rings are designed for clean-critical applications
Our standard RediVac® O-rings are available in a wide range of standard AS568A inch-measure sizes and are manufactured from top-quality fluoroelastomer materials. Buna, silicone, and other materials are available, and specific chemistries may also be available upon request.
Our O-rings are cleaned and packaged in a certified Class 100/ISO Class 5 Cleanroom, making them suitable for immediate use in most HV, UHV, or EUV applications. Vacuum-baked O-rings are available for reduced outgassing under vacuum.
UC Components, Inc. has been the world leader in high vacuum hardware since 1974. We specialize in O-rings and fasteners for High Vacuum, Ultra High Vacuum, and other critical applications. View our parts catalog online to find the components you need, request a quote, or contact us for more assistance or additional information.
You can have the most innovative and high-tech equipment, but it may be rendered useless if you don’t have the correct o-ring in place. The importance of choosing the suitable 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.
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.
Aflashttps://www.callapg.com/o-rings: 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.
The temperature the o-ring will be exposed to is an important consideration when choosing the proper o-ring for your application. Temperatures can adversely affect the performance of even the best-made o-ring. 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. The ability to see the temperature ranges will help you determine if the material is best suited to your specific application.
Operating pressure is also an important consideration when you are purchasing o-rings. Typically, 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 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.
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 groove will require 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.
As you can see, a lot goes into choosing the correct 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.
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