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O-Ring Information

Selecting High Performance O-Rings for Static and Dynamic Applications

O-Rings are a critical component of nearly every type of application. These donut-shaped engineered high performance seals, seal connections in equipment by forcing a rubber seal into a channel and applying mechanical or hydraulic pressure to prevent clearance of a less viscous material. These inexpensive, easy to replace and maintain components don’t always get the credit they deserve. But, when they are not properly selected or cared for, O-Rings can create leaks, stop production, damage equipment or worse.. O-Rings are among the sealing classics par excellence.

Endless O-Rings with a circular cross-section, ‘O’ ring, or toroidal seal, is an exceptionally versatile sealing device. Primarily used for static sealing of inactive machinery components against liquids and gases. Applications, ranging from garden hose couplings to aerospace or oil and gas duties, make it the world’s most popular volume-produced seal. O-rings offer many benefits to designers, engineers, maintenance staff and plant operators.

  • Suit many static and dynamic applications.
  • Are very compact and occupy little space.
  • Seal efficiently in both directions.
  • Can work between -76° to +428°F (-60° to +228°C) depending on material type.
  • Can function at temperatures down to -200°C when made of PTFE.

Designed to deform, the O-ring "flows" to fill the diametrical clearance and blocks any further leakage. Pressure, as well as many other considerations, determine the effectiveness of a seal. These considerations are highlighted throughout this design guide. O-Rings are inserted into cavities defined as glands, and are typically used in one of two seal designs, axial or radial.

An O-Ring is specified by its inner diameter, its cross-section diameter, its material hardness/durometer (typically defined by the Shore A hardness), and its material composition.

In order for an O-Ring to seal against the movement of fluid, it must be compressed when seated inside the gland. A standard set of design guidelines exist to determine the proper O-Ring dimensions for radial and axial seals of a given dimension.

Technical sealing has been defined by DIN Standards as follows:

  • Static Seal - The sealing action created between two mating surfaces with no leakage of liquid or minimal diffusion of gas.
  • Dynamic Seal - The mating surfaces have relative movement with minimal leakage of liquid (useful to protect the sealing efficiency, acting as lubricant)

The simple geometry is the main characteristic of an O-Ring which, in conjunction with proper elastomer selection results in a low cost, easy to use and efficient sealing system. Elastomeric materials, when compressed, react like a high viscosity fluid which transmits applied stress in every direction; consequently, the O-Ring serves as a barrier, blocking the leak paths between the sealing surfaces.

O-Rings offer several advantages over other sealing systems: simplicity of construction, standardized seal dimensions, wide selection of materials, suitability for both static and dynamic applications, standard dimensioning of glands, low cost due to high volume manufacturing.

Sealing is always achieved through a positive compression or squeezing action, resulting in a deformation of the O-Ring cross-section. The most important sealing characteristic of an O-Ring is its resistance to compression set or residual deformation.

O-Ring Squeeze Compression: (O-Ring C/S) - Gland Depth / (O-Ring C/S)

  • Face seal:20-30%
  • Static Male/Female: 18-25%
  • Reciprocating: 10-20%
  • Rotary: 0-10%

O-Ring Installed Stretch: 

  • General rule is 0-5%
  • Excessive stretch can overstress material, thin cross section, and reduces % squeeze
  • % cross section reduction due to stretch about half of the % ID stretch

  O-Ring Application Pressure  Vs. O-Ring Gland Clearance Gap Size  

  • Excessive clearance and or pressure can result in seal-extrusion and failure
  • Durometer vs. Pressure and Clearance Gap Chart
  • Consider use of Back-Ups or product selection with large gaps or > pressure

O-Ring Tolerances:  

  • Tolerances should be considered for the O-Ring and the gland
  • Can impact sealing performance and life

O-Ring Gland Surface Finish:   

  • Seal material must fill in voids in surface
  • Static surfaces       
    • 32Ra to seal liquid
    • 16Ra to seal gas
  • Dynamic Surfaces: 8 to 16Ra
  • Too rough of a surface can result in abrasion or spiraling, even with a static seal
  • Lower durometer materials can be used to seal rough surface finishes

O-Ring Gland Sharp Corners:

    Corners should be chanpfered to limit  damage during seal installation

O-Ring Gland Fill % (O-Ring Volume) / (Gland Volume) : 

  • Gland volume vs. O-Ring volume
  • About 25% void space or 75% nominal fill
  • Need space in groove to allow for volume swell, thermal expansion, and increasing width due to squeeze
  • Narrower groove for sealing vacuum or gas
  • O-Ring can extrude into clearance gap or get squeezed in two directions

O-Ring Eccentricity & Side Loading:

  • Too much squeeze on one side and not enough on the other or none at all
  • Can open too wide a clearance gap and result in extrusion of one portion of seal leading to leaks

There are a number of factors to consider when selecting the proper O-ring for your application. Sizing, design and installation are all important, but the basic element of choosing the proper polymer material while designing the equipment is key. Choosing the proper O-ring material when replacing in the after-market is equally important.

Common Causes of O-ring Failure:

O-ring failure can be traced back to design, installation, production quality and material choice. Choosing the proper materials, size, compression and surface finish will make a big different in machine performance and longevity. Design related failures are particularly common, and can be avoided with certain considerations:

  • Know the depth – O-rings sit in a channel known as a gland. Knowing the gland depth allows a machine designer to calculate compression and choose an O-ring that will avoid extrusion and tearing.
  • Leave space – O-rings should not fill the entire gland, but leave space for O-ring swelling and possible thermal expansion of the seal.
  • Avoid stretching – Stretching the O-ring past five percent of the centerline diameter should be avoided, as it can flatten the O-ring cross section. If the O-ring must be stretched past the five percent mark, the gland depth should be reduced to retain the necessary compression.
  • Proper installation – As with any component, proper installation is important to successful operation. In the case of O-rings, installers should use the correct lubricant, keep both the O-ring and gland clean, and install the O-ring without stretching or pulling on the device with a screwdriver or other tools. The surface finish should also be inspected before installation, and an O-ring with scratches, nicks or imperfections should not be used.
  • Choose a reputable supplier – Sourcing O-rings from a reputable supplier or manufacturer can help avoid potential production-related quality concerns. Poorly made O-rings will fail sooner than expected due to low quality materials.
  • Chemical Compatibility: In its role as connection seal, an O-ring will regularly come in contact with the material that is being sealed into the system. In the AC system example, that material is likely a refrigerant, such as R-1234yf, the latest AC refrigerant on the market. The differing properties of each polymer mean that an O-ring built for an AC system may not function as well in a system where it is sealing in oil instead of refrigerant – and vice versa.
  • Thermal Degeneration and Weathering: The chemical structure of the base polymer will dictate the thermal, ultra-violet (UV) and ozone compatibility of the O-ring. If you choose a polymer without the correct temperature compatibility, the O-ring can form radial cracks on the highest temperature surfaces.

When specifying an O-ring, designers must consider all of the factors. Frequently, only the required hardness is indicated, which can lead to a poor quality O-ring being chosen for the application. By using an ATSM D2000 “call out,” along with necessary morphology and dimensions, a designer can specify not only the hardness but all required properties and temperatures that your O-ring requires, including tensile strength, compression set, temperature resistance and more. O-ring manufacturers and suppliers can match the ASTM D2000 parameters to the right component for your application, helping extend machine life and prevent unexpected downtime.

Once you have selected and installed the right O-ring for your application, equipment owners should plan to replace O-rings each time the system needs to be opened. Recalling the significant costs of repairing or replacing a leaky system, correct O-ring material selection, design, installation and maintenance serves as a low cost insurance against leakage.

O-rings, the common components that help keep equipment running, might not seem critical at first glance. Using the correct, high quality O-ring for your application will require a small investment up front but pay dividends in reduced downtime and equipment maintenance costs.