O-Ring Surface Inspection with 3D Metrology

May 26, 2011
Understanding surface roughness and finish is possible, and may be vital to quality control and effectiveness.

An O-ring is a very simple, yet precise, elastomeric seal that’s placed between two or more parts of an assembly. This critical ring of elastomer is one of the most widely used mechanical components, and its applications continue to develop in line with advances in elastomer research. Like virtually all materials industries, nanotechnology research is opening new doors to greater understanding of elastomer at the nano level.

An O-ring is designed to seal between two or more parts, so understanding how the O-ring surface interacts with these parts can be critical to quality control.

Because an O-ring is designed to seal between two or more parts it is vital to understand how the O-ring surface interacts with these parts. For example, if the O-ring surface is too rough, it may cause abrasive wear; on the other hand if the surface is too smooth it may create problems with the seal. Understanding surface roughness/finish is just one of many surface parameters that are vital to quality control and ultimately to the effectiveness of an O-ring.

Other parameters to be observed include surface shape, form, and topography defect, among others. Ensuring the quality control of such parameters will rely heavily upon quantifiable, reproducible, and reliable inspection of the O-ring surface. Precise measurement and evaluation of an O-ring surface can lead to the best selection surface roughness/finish and control measures.

Nanovea 3D non-contact profilometers use chromatic confocal technology with the ability to measure O-ring surfaces. The Irvine, Calif., company designs and manufactures profilometers, mechanical testers and tribometers that combine the most advanced testing capabilities available: scratch adhesion, indentation hardness, wear friction and 3D non-contact metrology at nano, micro and macro range. Nanovea also provides laboratory services.

Nanovea's instruments are in use internationally in educational and industrial organizations ranging from automotive, cosmetics, biotechnology and medical devices to microelectronics and space applications.

The Nanovea ST400 is used to measure the surface profile of an O-ring. Several surface parameters are automatically calculated from the O-ring profile including the most common, Sa (average surface roughness), shape and form. Additionally, the cross section of the O-ring will also be further analyzed.

Measurements were randomly selected on the sample, and drastic changes in surface topography were not an issue for Nanovea profilometers, which can easily measure height variations from nanometers up to 27 mm.

The axial chromatism technique uses a white light source, where light passes through an objective lens with a high degree of chromatic aberration. The refractive index of the objective lens will vary in relation to the wavelength of the light. In effect, each separate wavelength of the incident white light will re-focus at a different distance from the lens (different height).

A spectral analysis of the O-ring surface using a diffraction grating.

When the measured sample is within the range of possible heights, a single monochromatic point will be focalized to form the image. Due to the confocal configuration of the system, only the focused wavelength will pass through the spatial filter with high efficiency, thus causing all other wavelengths to be out of focus. The spectral analysis is done using a diffraction grating. This technique deviates each wavelength at a different position, intercepting a line of CCD, which in turn indicates the position of the maximum intensity and allows direct correspondence to the Z height position.

Nanovea optical pens have zero influence from sample reflectivity. Variations require no sample preparation and have advanced ability to measure high surface angles. They are capable of large Z measurement ranges, and can measure any material — transparent/opaque, specular/diffusive, or polished/rough.

A three-dimensional profile of an O-ring.
A two-dimensional profile extraction of an O-ring.
A three-dimensional profile of the O-ring surface.
A three-dimensional profile of the O-ring cross section.
A two-dimensional profile extraction of the O-ring cross section.

In this application, the Nanovea ST400 3D profilometer demonstrated its can characterize both the topography and the nanometer details of the O-ring surface with precision. (Many other measurements also are possible.) With this information an O-ring surface can be investigated for R&D or quality control procedures.

For further detail, a 2D cross section can quickly be chosen to analyze special areas of interest at nanometer range. Special areas of interest could be further analyzed with integrated AFM or Microscope module. Nanovea 3D profilometers speeds range from 20 mm/s to 1 m/s, for high-speed inspection in laboratory or research applications, and the systems can be customized for size, speed, scanning capabilities, Class 1 clean room compliance, with an indexing conveyor, and for inline or online integration.

Jorge Ramirez is an applications specialist with Nanovea , which develops and supplies profilometers, mechanical testers and tribometers. Visit or contact [email protected] for more information.