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
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.
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.
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
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.
selection is based on chemical compatibility, application temperature,
sealing pressure, lubrication requirements, quality, quantity and cost.
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
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).
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.
highly polar solvents (Acetone, MEK, etc.) and direct exposure to ozone and
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.
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.
Range: 20°F to 400°F (-29°C to 204°C ) with intermittent service to
Hardness: 70 Shore A (Durameter ).
resistance to petroleum products and solvents. Very good high-temperature
performance. Fluorocarbon elastomers make up the most widely
used seals in the semiconductor industry.
polar solvents, amines, anhydrous ammonia, hydrazine and hot acids.
of vinylidene fluoride and hexafluoropropylene, although many more exotic
versions exist for improved chemical resistance or low temperature
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?”
& 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
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.
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
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.
MAKES SFP O-RINGS BETTER THAN THE REST?
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.
here for more information on O-rings