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O-rings

An O-ring is a circle of rubber used as a mechanical seal. They are designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface.

The joint may be static, or (in a few circumstances) have relative motion between parts and O-ring (rotating pump shafts and hydraulic cylinders, for example). Joints with motion usually require lubrication of the O-ring to reduce wear. This is often accomplished with the fluid being sealed.

O-rings are one of the most popular seals used in machine design because they are inexpensive and easy to make, reliable, and have simple mounting requirements. They can seal tens of megapascals (thousands of psi) pressure. In some cases, O-rings are used with back-up rings.  

 

HISTORY

The O-ring was invented in 1936 by a then 72-year-old Danish-born man, Niels Christensen. During the second world war, the US government "bought" critical war-related patents after finding out the big businesses were in violation of Christensen's patent right. Christensen got a lump sum payment of US$75,000 for it. 

 

THEORY AND DESIGN

Successful O-ring joint design requires a rigid mechanical mounting that applies a predictable deformation to the O-ring. This introduces a calculated mechanical stress at the O-ring contacting surfaces. As long as the pressure of the fluid being contained does not exceed the contact stress of the O-ring, leaking cannot occur.

The seal is designed to have a point contact between the O-ring and sealing faces. This allows a high local stress, able to contain high pressure, without exceeding the yield stress of the O-ring body. The flexible nature of O-ring materials accommodates imperfections in the mounting parts.

O-rings are available in a large number of standard sizes and materials. Manufacturers or reference books supply application and machining data for the mounting. O-rings are one of the most common and important elements of machine design.  

 

MATERIAL

O-ring selection is based on chemical compatibility, application temperature, sealing pressure, lubrication requirements, quality, quantity and cost.

Synthetic Rubbers: Acrylonitrile butadiene copolymers (NBR); Butadiene rubber (BR); Butyl rubber (IIR); Chlorosulfonated polyethylene (CSM); Epichiorohydrin (ECH, ECO); Ethylene propylene diene monomer (EPDM); Ethylene propylene monomer (EPM);

Thermoplastics: Thermoplastic Elastomer (TPE) styrenics; Thermoplastic polyolefin (TPO) LDPE, HDPE, LDPE, ULDPE; Thermoplastic Polyurethane (TPU) polyether, polyester; Thermoplastic etheresterelastomers (TEEEs) copolyesters; Thermoplastic polyamide (PEBA) Polyamides; Melt Processable Rubber (MPR); Thermoplastic Vulcanizate (TPV).

 

Buna-N 70

Description: Standard Nitrile is also known as Buna-N. Excellent resistance to petroleum-based oils and fuels, water and alcohols. Nitrile also has good resistance to acids and bases, except those with a strong oxidizing effect.

Limitations: Avoid highly polar solvents (Acetone, MEK, etc.) and direct exposure to ozone and sunlight.

Chemistry: Copolymer of butadiene and acrylonitrile. By varying the acrylonitrile content, elastomers with improved oil/fuel swell or with improved low-temperature performance can be achieved. Specialty versions of carboxylated high-acrylonitrile butadiene copolymers (XNBR) provide improved abrasion resistance. And hydrogenated versions of these copolymers (HNBR) provide improve chemical and ozone resistance elastomers.

Buna–N, Also Known as Nitrile Elastomers or MBR Rubbers, is a copolymer of acrylonitrile and butadiene. Buna–N is considered the Standard for most general applications and has outstanding resistance to petroleum based lubricants, hydrolic oils, gasoline, fuels, alchohol and L.P gases. 

Temperature Range: 20F to 400F (-29C to 204C ) with intermittent service to 600F (315C). 

Hardness: 70 Shore A (Durameter ). 

Color: Black

 

VITON Compounds

Description:  Excellent resistance to petroleum products and solvents. Very good high-temperature  performance. Fluorocarbon elastomers make up the most widely
used seals in the semiconductor industry.

Limitations:  Avoid polar solvents, amines, anhydrous ammonia, hydrazine and hot acids.

Chemistry:  Copolymer of vinylidene fluoride and hexafluoropropylene, although many more exotic versions exist for improved chemical resistance or low temperature performance.  

 

Color

Many times the different colors of the O-ring have meaningful effect on their properties.  Michael Olsen has written an article on.. “Is there any difference between black and brown Viton O-rings?” at http://www.chem.colostate.edu/~glass/viton.html

 

TIPS & TRICKS (for getting optimal O-ring performance)

  • Make sure that your O-ring material is compatible with the solvents and chemicals that it will come into contact with.

  • Before installing coat the seal with a lubricant

  • When installing, try to minimize the stretching of the seal

  • The groove that you are installing the seal into must be shallower than the cross section height of your O-ring.

  • The higher the temperature and exposure to chemical reduces the life of a seal

  • O-rings that are not subjected to movement will last longer than in dynamic applications  

 

WHEN TO REPLACE O-RINGS

Over the years Services for Plastics, Inc. has seen many applications where O-rings have been used and performed without problems for years.  However as a key to any preventative maintenance program and safety program replacement before failure is always the best.

While it is almost impossible to predict when an O-ring will fail, there are good rules of thumb to use.  First is that when servicing an item that has an O-ring installed, look at the seal to determine if there is any cracking or hazing due to degradation or mechanical damage.   In dynamic applications it is very important to look for wear patterns and material being removed from the seal during the movement.

Secondly, when servicing a item that contains a seal, if you can see it, you should replace it.  This due diligence will do a long way to keep the machinery running longer.  If the O-ring has been in service and 50% of its effective lifespan has been used, overlooking this can cause additional downtime later.  And wouldn’t that be a waste of your time to tear the unit down again for something that you could have replaced in five minutes.  

 

WHAT MAKES SFP O-RINGS BETTER THAN THE REST?

Because our O-ring suppliers are ISO-9001:2000 certified, you are purchasing O-rings that have proven chemical makeup along with consistent manufacturing and lot traceability.  Don’t buy cheap foreign made O-rings that become brittle and fail when you can least afford it.  Buy the best - SFP O-rings.

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