O-rings find use in countless applications for being simple sealing solutions. Not only do they seal moisture and environmental contaminants out, but they also separate surfaces to prevent friction, and keep in essential fluids/lubricants. Despite their general simplicity, O-rings used in the aerospace sector display an unexpected level of complexity that should be factored in when making a purchasing decision.
On an average commercial jetliner, there are thousands of O-rings, each of which must withstand varying environmental conditions such as pressure fluctuations, aggressive lubricants and fluids, industrial performance requirements, and more. It is worth noting that there is no one-size-fits-all when it comes to O-rings; thus, manufacturers rely on their supplier’s material, testing, and industry expertise to identify the best O-rings for every use. To better understand these mechanical seals, we will cover 5 major things to know about O-rings according to Dr. Robert W. Keller, director of O-ring Technical Sales at Freudenberg-NOK Sealing Technologies.
1. How Important is O-ring Specification?
While there are a variety of customer-part number standards and customer-approved sources of supply, the industry standard for O-rings is AS3209-214. If you opt out of the standard, this requires extensive research, dimensional tolerances, and material specifications to ensure that the correct dimensions are used to manufacture your O-ring.
2. How Can O-rings be Diagnosed Via a Picture?
It is also important to note down the mating parts’ dimensions and tolerances, and you must be informed on the details of your applications such as temperature and pressure extremes, fluids and gasses the O-ring will be exposed to, and assembly procedures. Then, carry out an in-depth microscopic examination to get any additional information. This can be done with a 3D microscope to measure the dimensions and depth of any damage or unusual surface conditions of the O-rings. For leaking O-rings, the problem can usually be attributed to installation damage. Furthermore, O-rings are typically installed over metal splines, which can cut or carve into their surface. More commonly, the misalignment of mating parts can result in pinching or cutting of the O-ring surface.
3. How Can You Tell if an O-ring and Gland Design is Appropriate?
Gland and O-ring designs that have been installed for long periods of time should be inspected. While there are a multitude of O-ring design software packages, they do not often take into account the variables and peculiarities of the actual customer gland designs. This can introduce numerical rounding and truncating which can cause calculation errors. Dr. Keller explains that there are set in-house calculation routines that can factor in a number of coefficients and variables to get a detailed look at the designs. These calculations tackle “what-if” scenarios, allowing manufacturers to take a second look and change gland design, O-ring dimensions, tolerances, and material recommendation.
4. What Is the Best Material for O-rings that Interact with Typical Fluids or Gasses?
Generally, parent industry standard material specification or customer material specifications will provide a comprehensive understanding of the material at hand. However, there are still some things that can go overlooked. As such, chemical compatibility tests are conducted with numerous fluids and gasses. Keep in mind that it is not uncommon for fluids, temperatures, and pressures to produce situations where some materials may be incompatible. In the aviation world, there is a lot of unpredictability. With this in mind, Dr. Keller details that EPDM materials with improved low-temperature performance for aircraft hydraulic systems and FKM materials with improved low-temperature flexibility for high-pressure aircraft fuel systems are being developed.
5. What Is the Risk of Using Differing Fluids with Your O-ring?
For the application of any fluid, you require as much detail as possible about the fluid including pressure and temperature extremes. To understand how certain fluids interact with each other and how cross-contamination will affect your O-ring, you need to look at volume changes, property degradation, and compression set resistance.
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