O-ring Design Guide
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For the majority of standard applications,
the design of the O-ring seal has generally already been accomplished.
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The following are design guidelines
that enable designers to depart from the standard designs in order to meet special requirements,
or obtain improved performance.
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O-ring Dimensions
An O-ring is a circular torus ring (see figure below). Only two of the three dimensions-the inside diameter (ID), the cross section (CS) and outside diameter (OD) are required to completely specify the size of an O-ring.
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Calculation
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OD=ID+2×CS
ID=OD?2×CS
CS= （OD?ID）?2
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O-rings are available in various metric and inch standard sizes. Sizes are specified by the inside diameter and cross section.
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Classification of O-ring Application
All O-ring sealing applications can be classified as either static or dynamic. The static seals are further divided into static radial and static axial seals depending on the direction of compression. There are two types of static radial seals: piston seal and rod seal, and one type of axial seal called face seal. As for dynamic seals, the great majority of them are compressed radially and are subject to reciprocating motion, or less frequently, to an intermittent rotary or oscillating motion.
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Static
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There is no relative motion between sealing surfaces. Static seals are easier to design because they can handle wider tolerances, rougher surface finishes, and higher pressure limits.
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Dynamic
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There is relative motion between sealing surfaces. This motion introduces friction, which creates design problems different from those of static seals.
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Motion Types of Dynamic Application
| This type of application involves relative back and forth motion along the shaft axis between the inner and outer parts of the groove. |
In dynamic rotary sealing, a rotating shaft goes through an O - ring's inner diameter. The O-ring keeps rubbing against the same part of the shaft, generating friction heat there continuously. When an elastomer is stretched and heated, it contracts (the Gough - Joule effect). This can make the O - ring seize the shaft, shifting the dynamic contact to the O - ring's outer diameter and the groove. This leads to more friction heat and early seal leakage or failure.
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Measures should be taken to reduce heat buildup:
-Provide minimal initial compression
-Select O-ring with smallest cross section
-Maintain low system pressure
-Do not exceed temperature 100℃ (212°F)
-Use a shaft of diameter no greater than that of the relaxed O-ring ID.
-Relative motion must occur exclusively between the O-ring ID and rotating shaft.
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This type of application is commonly used in faucet valves. The shaft or groove rotates back and forth through a limited number of turns around the axis of the shaft.
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Since the surface speed in oscillating seals is so slow, groove design for reciprocating services is applicable.
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Direction of Compression
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Axial
The compression is on the top and bottom of the ring
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Radial
The compression is between the ID and OD of the ring
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Seal Types and Groove Dimensions
The majority of O-ring seals can be categorized into piston, rod or face seal. Typically the O-ring is captured in a rectangular groove.
Calculation of groove dimension for each seal type is shown below.
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ID Stretch/ OD Interference
The ID or OD of the O-ring should be chosen to minimize the potential for installation damage and to minimize wear during use.
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Axial (Face) Type Seal - Static
| Pressure from Inside |
Pressure from Outside |
| The OD of the O-ring should be slightly larger than the outside groove diameter (OGD) so when the pressure is applied, the O-ring is already where it would be as a result of the pressure. |
The ID of the O-ring should be slightly smaller than the inside groove diameter (IGD) so when the pressure is applied, the O-ring is already where it would be as a result of the pressure. |
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Calculation
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Interference= (OD?OGD)?OD
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Recommended Values
Maximum=3%? ? ?Minimum=0%
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Calculation
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Stretch=(IGD?ID)?ID
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Recommended Values
Maximum=3%? ? ?Minimum=0%
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Radial Type Seal - Dynamic and Static
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| Piston Seal (Outer Sealing) |
Rod Seal (Inner Sealing) |
| The ID of the O-ring should be smaller than the groove diameter so that the installed O-ring is always slightly stretched. |
The OD of the O-ring should be at least equal or slightly larger than the groove diameter so there is always some interference. |
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Calculation
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Stretch=(Groove?ID)?ID×100%
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Recommended Values
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Dynamic:? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?Static:
Maximum=5%? ? ? ? ? ? ? ? ?Maximum=8%
Minimum=2%? ? ? ? ? ? ? ? ? Minimum=2%
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Calculation
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Interference=(OD?Groove)?OD×100%
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Recommended Values
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Maximum=5%? ? ?Minimum=0%
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Exception
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For O-rings with ID less than 20mm, this is not always possible so the stretch range is wider. To minimize this range and the maximum stretch, it is necessary to minimize the tolerances of groove diameter, and have a less stringent requirement for the minimum O-ring stretch.
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Exception
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Not applicable for O-rings with ID less than 20mm due to tolerance issues and can result in a greater OD interference, which causes too great an increase in the O-ring cross section and can affect the service life of an O-ring.
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For more information on O-ring design techniques, please click the [DOWNLOAD] button below.
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