Shaft Lead Methods and Detecting Problems Through New Technology

By Greg Vassmer, technical coordinator, ARPM

Greg Vassmer ARPM Technical Coordinator

Last issue, the column discussed the connection between dynamic oil seals and the (mostly) metal shaft surfaces they contact. In one corner is the seal – which, through the magic of rubber formulation, design and wear characteristics – acts like a small pump. In the other corner is the surface on which the seal slides, either helping or hindering this sealing magic. Rubber processors need this two-part system to act to keep oil inside and air outside.

There is a long list of enemies to this arrangement, but the focus is on one: the shaft lead. Can can indirectly manufacturers affect the quality of the shaft since it is provided by someone else, but rubber processors can measure and understand some vital characteristics; one of them, the “lead” or “twist,” is the most important. Unfortunately, this characteristic is measured in a non-analytical way, using a piece of quilting thread and a weight. Using the thread method is a lot like defining a “meter” as one ten-millionth of the shortest distance from the North Pole to the equator passing through Paris, assuming an Earth flattening of 1/334, which is, in fact, the original 1790 definition of a meter. 1 The method could use some improvement.

There are more modern ways to measure lead. Contact profilometers drag a diamond tip across the shaft surface while recording how it moves vertically. If the resulting graph is wavy like a screw thread, chances are good there is lead on the shaft. Single-trace contact methods are not enough. Seals leaked on shafts even when there was no wavy trace. If several traces are taken close together, it is possible to match the peaks from one trace to the next and see if a given peak moved slightly left or right. That catches a few more bad shafts, but not all. Only the thread and weight technique seemed to catch all types.

Enter optical systems – shining a light source on the shaft and recording the reflection (or perhaps the diffraction) of the light, the user will get a two-dimensional “patch” of shaft surface to analyze. Do that enough times around the full circumference of the shaft and it is possible to generate a very complete picture of the segment of shaft along which the seal slides. The two primary optical methods are white light interferometer and scatterometry. These methods provide an image of part of the shaft surface and can provide depth and statistical surface structure information. To convert this height data to the shaft twist requires some mathematics and software. The vendors of these systems have taken the images generated and applied Fourier transform techniques that extract embedded screw-thread-like features and calculate their direction and depth. From this can be derived the volume of anticipated fluid movement, also known as leakage, for the most part, but not completely.

So here is where a good standards organization can get involved. There are four to five vendors providing equipment promising to measure lead using these techniques. The mathematical techniques mostly are unique to each vendor, and each has strength in identifying a specific type of lead. But the techniques do not always correlate with the original, trusty thread and weight method. Rubber processors have new technology that is less subject to error and more repeatable attempting to replace a well-tested but error-prone technique, but so far, without finding correlation in the results between the two, ARPM cannot yet recommend it as a standard. But rubber processors can do something about it.

In December last year, the ARPM Seal Committee began a project aimed at testing the more modern, optics-based shaft lead devices and software to determine whether there is a correlation with the thread method and their repeatability. Vendors of the new techniques have been pulled in and will provide sample shafts made with techniques known to provide both good and bad shafts for them to test. Along with establishing the validity of the new techniques, the goal is to provide a broad background of how the analysis algorithms work and how to properly prepare samples. Once concluded, an update will be given to OS-1-1 with a valuable addition representing a more robust, less error-prone technology. n

References

1. https://en.wikipedia.org/wiki/History_of_the_metre#