Optimize shaft surface finish for maximum seal performance

Shaft roughness and machine lead are not the only variables that must be monitored to ensure leak-free sealing.

By Glenn E. Gabryel

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Contacting radial lip seals are used in a range of mechanical power transmission equipment. Seals extend the service life and efficiency of motors, gear drives and other devices by retaining lubrication and excluding contamination. Whether fabricated from rubber or plastic, they depend on precisely machined shaft surfaces to establish and maintain a controlled oil film that is approximately 0.00001 inches (0.25 microns) thick.

Shaft lead angle plays a significant role in leakage.Shaft lead angle plays a significant role in leakage.
Shaft lead angle plays a significant role in leakage.

If the shaft surface is too smooth, there can be an absence of  "pockets," or asperities, to maintain a lubricant film, resulting in possible lip damage and premature leakage. If it is too rough, metal peaks protrude through the oil film, causing severe wear and short seal life. For many years, the Rubber Manufacturers Association (RMA) published an industry standard recommending shaft surface finishes with a roughness of Ra 10 to 20 microinches (.25 to .50 lm) and less than 0.05 degree of lead angle.

Lead is a well-known leakage factor. It is a spiral groove, created during the initial turning process and, when oriented outside the housing, can create outflow that overcomes the pumping ability of seal to retain oil. The presence of lead can be confirmed by a simple but effective string and weight test  using the formula1:  

Lead angle =  arctan                           string advance__________
                                (shaft circumference) * (number of revolutions)


However, engineers and technicians have discovered that leakage sometimes occurs in a variety of applications even when the limits are observed. This indicates that Ra and machine lead alone are insufficient to ensure consistent, leak-free sealing. Consequently, RMA commissioned a task force to determine what other factors contribute to leakage.

Surface roughness

The taskforce found that focusing on the correct roughness parameter alone will not reduce the incidence of seal leakage. For example, Ra does not describe the surface profile, because it is an averaging technique for the micro-surface deviations above and below a mean line in a given assessment length. Therefore, various shafts with the same Ra value may have different profiles and a given seal will react with dissimilar results on each (see Figure 1).

Figure 1. Various shafts with the same Ra value may have different profiles, and a given seal will react with dissimilar results on each.

Figure 1. Various shafts with the same Ra value may have different profiles, and a given seal will react with dissimilar results on each.

In the early stages of the study, it was also determined other shaft conditions were found to have a negative effect on sealability. For example, out-of-roundness and lobing could result in a dynamic run-out exceeding the ability of a lip to follow the shaft, particularly at high speeds and cold temperatures. Therefore, roundness and lobing (see Figure 2) were added to lead along with grinding chatter in an initial set of recommendations (see Figure 3).

Figure 2. Rounding and lobing also have an impact on sealability.

Figure 2. Rounding and lobing also have an impact on sealability.

Figure 3. In addition to shaft lead, these characteristics influenced sealability.

Figure 3. In addition to shaft lead, these characteristics influenced sealability.

They incorporated a variety of methods used to finish the shaft at the seal contact area, including plunge grinding, paper polishing, “as drawn,” roller / diamond burnishing and ceramic turned with high-speed tool. Twelve variations of the five methods selected for evaluation are in Table 1.

They incorporated a variety of methods used to finish the shaft at the seal contact area, including plunge grinding, paper polishing, “as drawn,” roller / diamond burnishing and ceramic turned with high-speed tool. Twelve variations of the five methods selected for evaluation are in Table 1.

Thirty-six test shafts were fabricated using SAE 1045 steel at 30 Rc hardness. The diameter was 3.000 inches (76 millimeters). Test conditions were:

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