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O-Rings
O-rings are designed for both static dynamic applications. A properly designed O-ring groove allows the O-ring to be squeezed diametrically out-of-round even before the pressure is applied. The distortion of the O-Ring's resilient elastic compound fills the leakage path, thus, creating a seal.
O-Ring Profiles
O-Ring Cross Section
Advantages of Smaller Cross Section Advantages of Larger Cross Section
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More compact
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Lighter weight
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Less expensive; especially for higher cost elastomers like FKM or fluorosilicone
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Less machining required for machined grooves since grooves are smaller
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More resistant to explosive decompression
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Less prone to compression set
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Less volume swell in liquid on a percentage basis
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Allows for larger tolerance while still maintaining acceptable compression squeeze and compression ratio over full stack-up range
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Less prone to leakage due to contamination; dirt, lint, scratches, etc.
O-Ring Gland Types
O-rings are primarily used to prevent the loss of a fluid or gas. However, they can be used as dust seals, drive belts or on rotating shafts. The majority of O-rings can be classified into one of the three arrangements shown below.
Piston Configuration
Rod Configuration
Face Type Configuration
I.D. Stretch/O.D. Interference
For hydraulic and pneumatic piston sealing applications
The O-ring's inside diameter (I.D.) should stretch between 2% and 5% for dynamic applications and between 2% and 8%
for static applications. For O-rings with an inside diameter smaller than 20 mm, this is not always possible as it can result in a wider range of stretch. To minimize this range of stretch (including the maximum stretch) it is necessary to minimize the
tolerance of the piston gland diameter, and have a less stringent requirement for the minimum O-ring stretch.
In dynamic applications, it is important to keep the maximum stretch to 5% or less to avoid detrimental effects on sealing performance.
For hydraulic and pneumatic rod sealing applications
The O-ring's outside diameter (O.D.) should be at least equal to or larger than the rod gland diameter to give interference on the
O-ring's O.D. The O-ring's O.D. should not exceed 3% of the rod gland diameter for O-rings with an I.D. greater than 250 mm, or 5%
for O-rings with an I.D. smaller than 250 mm. For O-rings with an I.D. smaller than 20 mm, this is not always possible due to tolerance
issues, which can result in a greater O-ring O.D. interference.
O-Ring
Piston
Rod
Reduction in Cross Section
If the I.D. of the O-ring is stretched, the cross-section of the O-ring will decrease. The following table gives the O-ring cross-sections that result from various percentages of I.D. stretch.
Reduced O-Ring C/S at % ID Stretch (Inch/mm)
Compression
The difference between the original O-ring cross-section and the final O-ring cross-section once installed is known as the compression squeeze.
This can usually be expressed as a percentage: O-ring C/S Squeeze (%) = Compression Squeeze
C/S
Gland Fill
The gland fill is the percentage of the gland that is occupied by the O-ring. It is calculated by dividing the cross-sectional area (CSA) of the O-ring by the cross-sectional area of the gland.
Area of a circle = πr2 and r = d ,where d = diameter (C/S) and π =i(3.14159) 2
Therefore, O-ring CSA = π
Gland CSA = D x w*
Gland Fill (%) = O-Ring CSA x 100
Gland CSA
* Effect of gland angle and extrusion gap not addressed.
It is important to consider the groove fill of the installed O-ring to avoid detrimental effects on radial sealing
performance. Groove fill of the installed O-ring should not be more than 85 % to allow for possible O-ring
thermal expansion, volume swell due to fluid exposure and effects of tolerances. Volume change is the
increase or decrease of the volume of an elastomer after it has been in contact with a fluid, and is measured
in percent (%). For static O-ring applications volume swell up to 30 % can usually be tolerated. For dynamic
applications, 10 or 15 % volume swell is a reasonable maximum, unless special provisions are made to the
gland design. This is a general rule and there may occasionally be exceptions. It is important to note that
there are significant differences in the coefficients of thermal expansion between the O-ring material and
the groove materials. Elastomers can have coefficients of thermal expansion 7 to 20 times higher than that ofmetals, such as steel.
Extrusion Gap
Extrusion is a concern for radial seals where there is gap between the piston and the bore or between the rod and the bore. However, it is not typically a concern for face type seals as the metal parts to be sealed are in contact, line-to-line. The concern with extrusion is that at a higher pressure (especially with soft elastomer O-rings), the O-ring can be forced into the small gap between the piston or rod, and the bore. Unless the bore and the piston or rod remain concentric by the hardware, this can cause the entire gap to shift to one side, creating a diametrical gap (see diagram below).
Piston Type Seal Rod Type Seal
Radial Extrusion Gap = Bore Ø- Piston Ø Radial Extrusion Gap = Bore Ø- Piston Ø
2 2
Limits for Extrusion
There are different methods to counter O-ring extrusion.
One of these methods is to simply increase the durometer
rating of the O-ring. However, as you increase the durometer,
the O-ring can become less malleable. Another option would
be to use an anti-extrusion device. Anti-Extrusion devices are
thin rings made of hard plastic materials such as PTFE, Nylon, and PEEK. Once in place these rings will provide essentially zero clearance.
Theory & Design
O-rings are available in various metric and inch standard sizes. Sizes are specified by the inside diameter and the cross section diameter (thickness). In the US the most common standard inch sizes are per SAE AS568C specification (e.g. AS568-214). ISO 3601-1:2012 contains the most commonly used standard sizes, both inch and metric, worldwide. The UK also has standards sizes known as BS sizes, typically ranging from BS001 to BS932. Several other size specifications also exist.
Typical Applications
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. Fortunately, the pressure of the contained fluid transfers through the essentially incompressible O-ring material, and the contact stress rises with increasing pressure. For this reason, an O-ring can easily seal high pressure as long as it does not fail mechanically. The most common failure is extrusion through the mating parts.
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. But it is still important to maintain good surface finish of those mating parts, especially at low temperatures where the seal rubber reaches its glass transition temperature and becomes increasingly crystalline. Surface finish is also especially important in dynamic applications. A surface finish that is too rough will abrade the surface of the O-ring, and a surface that is too smooth will not allow the seal to be adequately lubricated by a fluid film.