What to Consider When Choosing an O-Ring?

 You can have the most innovative and high-tech equipment, but if you don't install the right O-ring, it can become useless. The importance of selecting the right O-ring for any application should never be underestimated, as the right size and material selection can be the difference between start-up and operation and failure.

 

Material Selection

Selecting the right material for your O-Ring is important and is the starting point for your O-Ring purchase. Common material choices include

 

Nitrile (Buna-N): This is the most widely used O-ring material. It has excellent resistance to petroleum products. It also has excellent compression permanent deformation, tear resistance and abrasion resistance. It does not resist ozone, sunlight or weather very well unless it is specifically compounded. Temperature range: -40° to +250°F.

 

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

What to Consider When Choosing an O-Ring?

Oil Resistance Black NBR Rubber O-ring

Chloroprene: This elastomer has moderate resistance to petroleum and offers good ozone and weathering resistance and good compression set. Ideal for sealing refrigerated fluids, such as Freon®. Temperature range -65°- +250°F.

 

Silicone: This elastomer is well suited for weatherability and ozone resistance. It has 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 NBR: This material offers better heat resistance, superior physical properties and better chemical resistance than traditional NBR compounds. It has improved resistance to ozone and sunlight, and excellent resistance to permanent deformation by compression. 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 hydraulic fluids. Resistant to certain acids, alkalis and ozone. Not recommended for petroleum fluids or diester lubricants. Temperature range: 65° - +300°F.

O Ring

O Ring

PTFE: This is a white thermoplastic fluoroethylene resin with heat resistance, low friction, dielectric strength, weatherability, toughness, non-absorbency and flexibility. Temperature range: -65°- +500°F.

 

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

 

As you can see, there are a variety of materials to choose from, each with its own unique advantages and disadvantages. It is important to choose the right material.

 

Temperature

The temperature to which the o-ring will be exposed is an important consideration when selecting the right o-ring for your application. Even with the best oring, temperature can have a detrimental effect on performance. If the operating temperature is too low, the O-ring will lose its elasticity and become more brittle, making it susceptible to breakage. On the other hand, if an O-ring gets too hot, it becomes too flexible and degrades.

 

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

O Ring

O Ring

Operating Pressure

Operating pressure is also an important consideration when purchasing an O-Ring. Generally speaking, a standard 70 Shore hardness will usually work well in a static seal setup. However, once pressures approach the 1500+ psi range, you may want to consider an 80-90 Shore hardness.

 

Surface finish

Surface finish is also an important consideration. In general, the ideal surface finish for dynamic surfaces is 5 to 16 µ in. RMS. Smoother and rougher surfaces can cause increased friction and shorten seal life. Rougher surfaces up to 64 µ in. RMS can be used for static applications.

 

Stretch

When purchasing an O-Ring, you also need to consider how far the ring will stretch. To seal effectively, the inside diameter (ID) of the O-ring must be smaller than the groove in which it sits. This will require a slight stretch in the O-ring when seated in order to fit snugly into the groove. Stretch should be between 1-5%, with 2% being ideal in most applications.

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