Each factor involved with the o-ring seal selection should be weighed with the elastomers capabilities and tested thoroughly to ensure the seal meets the applications demands.
Preliminary O-ring Design Considerations |
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| An o-ring is a simple and versatile ring shaped packing or sealing device. Having a circular cross section that functions as a seal, in both static and dynamic applications, by being compressed between the mating surfaces comprising the walls of the gland, in which it is installed. Although o-rings can be made from a variety of materials, they are most commonly molded in one piece from an elastomeric material. | |
| 1. What will be the function of the part | 2. What type of Environment will the product be subjected to |
| - Seal a fluid | - Any water, chemicals, or solvents that could cause deformation to the part |
| - Transmit a fluid | - Oxygen or Ozone |
| - Transmit Energy | - Sunlight |
| - Absorb Energy | - Wet or Dry Environment |
| - Provide Structural Support | - Continuous or cycled pressure |
| - Dynamic or Static stress | |
| 3. What is the desired product life | 4. What are the desired properties |
| - Elongation Strength | - Compression Set Resistance |
| - Resistance to Deformation | - Resilience (Rebound) |
| - Compression Sets | - Tear Strength |
| - Resistance to Embrittlement | - Heat Aging Resistance |
| - Etc. (see materials for more properties) | |
Basic O-ring Design |
| How O-rings Seal |
| An o-ring acts as a seal by blocking any potential leaching path, of a liquid or gas, between two closely matted surfaces. An o-ring is installed in a machined grooved gland (see gland design for examples) in one of the two surfaces to be sealed. As the two surfaces are brought together the o-rings cross section becomes deformed producing a tight seal, see image. The greater the squeeze the greater the deformation . The reason an o-ring acts as such a good seal is because of the elastomer material in which it is made from. An elastomer typically a highly viscous material having the tendency to remember its original shape for a long time, allowing it to be compressed and recompressed without losing its original shape and therefore its sealing ability. It is important to choose the right elastomer for each intended application. Each elastomer has different thresholds for heat, cold water, gas etc. For examples of different elastomers and there unique characteristics please visit Columbia's elastomer section. |
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Cross Section and Inside Diameter "I.D." Calculation |
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| Dynamic Cross Section | Static Cross Section | ||
| - The following refers to a dynamic application, see gland | - The following refers to a static application, see gland design | ||
| design section for listings | section for listings | ||
| 1. List the bore diameter | 1. List the gland depth and multiply by the minimum and | ||
| 2. List the piston groove diameter | maximum squeeze requirements ( see the gland design | ||
| 3. Subtract the groove diameter from the bore diameter, | section for listings). | ||
| and divide the difference by two. | |||
| 4. Refer to corresponding table (see gland design section | Static I.D. Calculation | ||
| for listings) to establish minimum and maximum squeeze | List the diameter of the part that the o-ring that will be | ||
| requirements. | stretched over during installation and reduce this figure by 1% | ||
| 5. Multiply the figure from step three, by the minimum | to 5% therefore reducing the o-rings I.D. to allow for stretch, | ||
| and maximum squeeze requirements obtained in step | similar to the dynamic I.D. calculation. Then look up the o-ring | ||
| four. The two figures obtained represent the minimum | in the gland design section by I.D. and corresponding | ||
| and maximum o-ring cross section diameters, for the | cross section. | ||
| particular application. | |||
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| Dynamic I. D. Calculation | |||
| The inside surface of the o-ring will be resting on the | |||
| bottom of the piston groove. To have a complete seal the | |||
| o-rings I. d. must be smaller than the piston groove diameter | |||
| (see above). The o-rings I. d. therefore will be slightly | |||
| stretched in the application. The stretch should be a minimum | |||
| of 1-2% but not exceeded 5%. The following formula | |||
| calculates the o-ring I. d. | |||
| O-Ring I. d. = Groove Diameter / % of stretch desired (1% - 5%) | |||
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Reasons for O-ring Failure |
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| Failures are usually due to a number of causes and typically | |||
| include environmental issues. For example, excess friction | |||
| causes heat, which in return causes the o-ring to swell, therefore |  |
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| exposing the o-ring to a possible harsher chemical or environmental | |||
| attack. | Successful O-ring | ||
| Such effects may be compounded due to human error in overlooking | |||
| critical elements of gland design; including but not limited to | |||
| preliminary testing, proper o-ring compound use, poor installation, | |||
| lack of proper maintenance or lubrication. Some of the more common |  |
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| reasons for o-ring failure are as follows: | |||
| Failed O-ring | |||
| -incorrect o-ring size | "Non-Fill | ||
| -incorrect installation | |||
| -poor maintenance or lack of lubrication | |||
| -incompatibility of the elastomer and the environment subjected too |  |
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| These problem causing attributes can be difficult to evaluate, so it is | Failed O-ring | ||
| strongly recommended that adequate testing be performed in actual | "Extrusion and Nibbling" | ||
| environmental settings. | |||
| Failed O-ring | |||
| "Breaking or Cracking" |
Speciality Elastomers
