The I.D. and O.D. of an O-ring gland is primarily influenced by diameter of the mating surface of the rod or piston and bore. Although the cross-section of the O-ring is seen as fairly arbitrary, there are some distinct advantages to either a larger or smaller cross-section O-ring. Listed below are the advantages of small cross-sections and the advantages of large cross-sections: |
Advantages of Smaller Cross-Section |  | Advantages of Larger Cross-Section |
• More compact
• Lighter weight
• Less expensive; especially for higher cost elastomers like FKM or fluorosilicone
• Less machining required for machined grooves since grooves are smaller
• More resistant to explosive decompression |
• Less prone to compression set
• Less volume swell in liquid on a percentage basis
• Allows for larger tolerance while still maintaining acceptable compression squeeze and compression ratio over full stack-up range
• Less prone to leakage due to contamination; dirt,
lint, scratches, etc. |
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O-rings are primarily used to prevent the loss of a fluid or gas. However, O-rings can be used as dust seals, drive belts or on rotating shafts. Most O-ring seals can be classified into one of the three arrangements shown below.
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Piston Configuration |
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Rod Configuration |
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Face Type Configuration |
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I.D Stretch/O.D. Interference |
For hydraulic and pneumatic piston sealing applications
The O-ring inside diameter (I.D.) should be stretched between 2% and 5% for dynamic applications and 2% and 8% for static applications. For O-rings with an inside diameter smaller than 20 mm, this is not always possible which can result in a wider range of stretch. To minimize this range and 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 outside diameter (O.D.). The O-ring's outside diameter (O.D.) should not exceed 3% of the rod gland diameter for O-rings with an inside diameter (I.D.) greater than 250 mm, or 5% for O-rings with an inside diameter (I.D.) smaller than 250 mm. For O-rings with an inside diameter (I.D.) smaller than 20 mm, this is not always possible due to tolerance issues, which can result in a greater O-ring outside diameter (O.D.) interference.
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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.
O-RingSeries | Original O-Ring C/S | Reduced O-Ring C/S at % ID Stretch (Inch/mm) |
Inch |
mm | 1% | 2% |
3% |
4% |
5% |
000 | 0.070 | 1.78 | 0.069/1.76 | 0.069/1.74 | 0.068/1.73 | 0.068/1.71 | 0.068/1.69 |
100 | 0.103 | 2.62 | 0.102/2.59 | 0.101/2.57 | 0.100/2.54 | 0.100/2.52 | 0.100/2.49 |
200 | 0.139 | 3.53 | 0.138/3.49 | 0.137/3.46 | 0.136/3.42 | 0.135/3.39 | 0.134/3.35 |
300 | 0.210 | 5.33 | 0.208/5.28 | 0.206/5.22 | 0.205/5.17 | 0.204/5.12 | 0.203/5.06 |
400 | 0.275 | 6.99 | 0.272/6.92 | 0.270/6.85 | 0.268/6.78 | 0.267/6.71 | 0.266/6.64 |
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The difference between the original O-ring cross-section and the final O-ring cross-section once installed is known as the compression squeeze.
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| This can usually be expressed as a percentage: |
O-ring C/S Squeeze (%) = |
Compression Squeeze  C/S |
x 100 |
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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.
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Area of a circle = πr2 and r = | d2 |
, where d = diameter (C/S) |
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Gland CSA = D x w*
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| Gland Fill (%) = |
O-ring CSAGland CSA |
x 100 |
* Effect of gland angle and extrusion gap not addressed. |
It is important to consider the groove fill or occupancy 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, measured in percent (%). For static O-ring applications volume swell up to 30 % can usually be tolerated. For dynamic applications, 10 or 15 % swell is a reasonable maximum unless special provisions are made in the gland design itself. This is a general rule and there may occasionally be exceptions.
It is also important to note 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 of metal, such as steel. |
Extrusion is a concern for radial seals where there is gap between the piston and the bore for a piston type seal or between the rod and the bore for a rod type seal. It is not typically a concern for face type seals where the metal parts to be sealed are in contact line-to-line. The issue is that at higher pressures and especially for softer O-ring elastomers, the O-ring can be forced by the pressure into the small gap between the piston (or rod) and the bore. Unless the bore and the piston (or rod) are ensured to remain concentric by the hardware, we have to assume that entire possible gap can shift to one side (see diagram below).
| | Piston Type Seal
Radial Extrusion Gap = | Bore Ø- Piston Ø
2 | |
Rod Type Seal
Radial Extrusion Gap = |
Bore Ø- Piston Ø
2 |
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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 the use of anti-extrusion devices. These are thin rings made of hard plastic materials such as PTFE, Nylon, and PEEK. Once in place these rings will provide essentially zero clearance.
Reduce the clearance shown by 60% when using silicone or fluorosilicone elastomers.
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