Optical metrology ensures automotive dynamic seal effectiveness

Automobiles have numerous rotary dynamic seals that can fail, resulting in leaks. Such leaks cost the transportation and power equipment industries tens of millions dollars a year in warranty repairs, recalls, and fines. New three-dimensional optical metrology methods can reduce such failures and their associated costs by measuring the surface texture and lead angle characteristics of rotating shafts to levels of precision unattainable using traditional approaches.

Advances in the design and materials used in rotating shafts and their corresponding seals have extended the life and reliability of vehicles and power equipment. To minimize fluid leakage and friction in rotary dynamic seals, however, these designs have a critical dependence on shaft surface texture and machine lead angle. Without proper control of these factors seals can prematurely fail.

Surface roughness, for instance, affects both the lifetime and effectiveness of a seal. The lip of a new seal makes contact with the shaft and abrades as the shaft rotates. If the shaft is too rough, the seal abrades quickly and will begin to leak. If the shaft is too smooth the seal will not "bed" correctly and will also leak. The shaft must be rough enough that the initial lip wear will allow a small film of fluid (typically between 1µm and 3µm thick) to enter the shaft/seal interface. When this occurs, the seal begins riding on a thin layer of liquid and further wear ceases. The fluid's meniscus at the seal's outside edge prevents subsequent fluid leakage.

Machine lead also affects the seal's effectiveness. All machining and polishing processes for shafts leave some degree of residual grooving on the shaft's surface. If these grooves are at an angle (called the machine lead, lead angle, or simply lead) to the shaft's axis they can move the fluid within the shaft/seal interface. Angled one way they pull the fluid out past the seal, creating a slow leak. Angled the other way they push the fluid in the interface zone back into the housing, depleting the film layer and thus leading to excessive wear and early failure. Current standards for shafts and seals call for shaft leads less than 0.05°

The traditional measurement method for surface roughness uses a contact or stylus profilometer. This device rests a precision tip on the surface of the shaft and measures the tip's vertical movement as it translates along the shaft parallel to the axis.

Conventionally, measuring lead angle requires looping a cotton quilting thread around the shaft, hanging a weight from one end, and measuring with calipers or a micrometer any translation of the thread along the shaft as the shaft rotates.

An updated approach
Three-dimensional optical metrology devices are able to measure both surface roughness and machine lead simultaneously. The measurement unit is essentially a modified microscope with an illumination beam that splits along two pathways (see below).


Along one path the beam reflects off of a reference mirror. Along the other path, the beam reflects off the surface of the shaft. When the two beams recombine they interfere with one another, producing a pattern of light and dark bands that mimic the surface texture and can be captured by a CCD camera. These bands have their maximum contrast when the surface feature that created them is in focus. Thus, by vertically sweeping the microscope's focal depth and recording where maximum contrast occurs for each point on the surface within the camera's field of view, the instrument can build a full 3D map of the target surface.

 
To read the complete article, which includes surface mapping methods and a discussion of repeatability, click here, courtesy of EE Times Europe Automotive and Bruker Nano Surfaces.

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