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Basic O-Ring Information

Introduction

The ‘O’ ring, or toroidal seal, is an exceptionally versatile sealing device. 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

Information within is believed to be accurate and reliable. However, The O-Ring Store, LLC makes no warranty, expressed or implied, that parts supplied in this material will perform satisfactorily in specific applications. It’s the customer’s responsibility to evaluate the material prior to use.