Once formed, microasperities are advantageous because they serve as reservoirs to hold lubrication that prevents further lip wear. They also contribute to an inherent pumping capability. Figure 144 shows the microasperities as they might look if seen through a glass shaft at rest.

As microasperities form on the lip’s wear path, the plunge ground surface on the shaft that is under the seal lip is worn smooth. A smooth path or wear band is created around the shaft in the circumferential direction. As the shaft rotates, the contact point of the lip is sheared in the circumferential direction. The microasperities are pulled so that they are directionally oriented at an angle to the shaft, and they are elongated, thus creating tiny helices. Because the oil side angle of the seal lip is larger than the air side angle, the helices on the oil side of the contact band are shorter with a larger helix angle and a larger pressure gradient than the helices on the air side of the contact band. Because of these geometrical features, the pumping activity of the helices on the air side is greater than the pumping activity of the helices on the oil side of the contact band. The net result is an in-pumping effect that prevents oil leakage from the oil reservoir (sump).

Figure 145 shows the directional orientation of the helices as they might look if seen through a rotating glass shaft. It’s important to note that the pumping action of most shaft seals is very directional (e.g. back toward the seal’s fluid side). It is therefore imperative that shaft seals be installed in the proper direction. Installing a seal backward, such that the air side angle is facing the fluid, will result in immediate leakage due to the pumping of fluid out of the sump.

HOW A SHAFT SEAL WORKS MAIN PAGE