Shaft surface finish is defined by two main features: surface texture and machine lead. The first of these, surface texture, has a significant effect on the seal’s elastomeric lip. As the Rubber Manufacturers Association (RMA) defines it in RMA OS 1-1, surface texture consists of three components: Ra, Rz, and Rpm.

The first step in determining Ra (roughness average) is to filter the profile obtained when a stylus moves across the surface of a shaft so that the areas of the profile above and below a certain line (called the mean line) are equal. The cutoff length (le) is the distance that the stylus travels across the shaft surface to obtain a set of readings. It is recommended that data from a total of five cutoff lengths (which equal one evaluation length, lm) be collected and analyzed after an appropriate pre-travel (lv) and post-travel (ln) of the stylus. The Ra is the average of the absolute value of the deviations from the mean line over the evaluation length (see Figure 111). The RMA recommendation for Ra is 8-17 µin (0.20-0.43 µm) with a cutoff length of 0.010 in.

But studies have shown that Ra numbers alone don’t tell the whole story. In practice, a shaft surface finish that falls within recommended specifications can still cause seal failure. This is because, due to the averaging inherent to the Ra calculations, surfaces with widely varying profiles can all generate the same Ra value. An example of this phenomenon is illustrated in Figure 112. Note the different capacities for seal lip abrasion represented by the three surface profiles. All have the same Ra value, yet each would affect a seal lip differently. Because Ra numbers alone may mislead, the RMA suggests using two additional parameters—Rz and Rpm—to more completely define surface finish.

As shown in Figure 113, Rz is the average peak to valley height, or the average value of the greatest peak-to-valley distances in five consecutive sample lengths (le) taken over the assessment length of the (filtered) profile.

As shown in Figure 114, Rpm is the average peak to mean height, or the average value of the five highest peaks above the median in five consecutive sample lengths (le) taken over the assessment length of the (filtered) profile.

The RMA shaft surface texture recommendations are shown in Table 34. These specifications are important because you want the shaft surface to be rough enough to hold pockets of oil to lubricate the seal lip without being so rough that lip damage will occur.

But proper numbers alone don’t suffice; even if the surface texture meets the specifications shown in Table 34, the presence of screw threads or spiral grooves on the shaft surface can create early leakage. Known as shaft lead (or machine lead), these threads or grooves result from relative axial movement of the finishing tool during the finishing operation. Improper shaft finishing can easily contribute to seal leakage and/or contaminant ingestion.

As detailed in RMA Handbook OS-1 (Shaft Finishing Techniques for Radial Lip Type Shaft Seals) and illustrated in Figure 115, testing for the presence of shaft lead is a relatively simple process. With the shaft mounted in a holding chuck, the surface to be tested should be lightly coated with silicone oil. After ensuring that the shaft is level, a length of quilting thread (or, alternatively, unwaxed dental floss) is draped over the shaft. A one-ounce weight is suspended from the thread so as to create a string-to-shaft contact arc of 220 to 240 degrees.

The shaft is rotated at 60 RPM and the string is carefully observed to determine if axial thread movement exists. If movement is observed, the shaft is reversed to see if the string direction of motion is reversed. Thread movement combined with reversal when the shaft direction of rotation is reversed will betray the presence of lead; no movement means no lead is present. A full interpretation of thread movement is listed in Table 35.

As important as surface texture is, the other shaft surface parameter—machine lead—is also critical. As previously shown in Table 34, the specification for shaft lead is 0 ± 0.05 degrees. There are a variety of shaft surface finishing methods, but keep in mind that many of these can generate varying amounts of lead and are therefore problematic. Next is a closer look at many of these surface finishing methods, and the pitfalls to watch out for with each.

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