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Hydrodynamic Effect


Hydrodynamic Effect

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Ever wonder how a radial lip seal can stop a reservoir of oil from leaking around a shaft rotating at 2000 rpm? The frictional heat created between two surfaces at this speed is enough to damage the shaft, let alone an elastomer material. So how does the lip material keep from burning up? The answer can be explained by taking an in-depth look at the hydrodynamic effect.

The contact width, shown above, of a radial lip seal is created by the interference between the primary lip and shaft diameter. When these two bodies are at rest the oil is retained due the pre-load stresses of the elastomeric material. As the shaft begins to rotate the seal lip remains stationary. The fluid is then drawn along the dynamic surface creating shear flow between the two bodies. There are many affects of this dynamic motion but the most important happen at the contact point of the seal lip and shaft. As the fluid converges toward the contact width, a pressure gradient is created. Because the two surfaces have roughness or micro-asperities, the hydrodynamic pressure profile of the contact width is shown in the following illustration.

y2Hydrodynamic Pressure Profile

This pressure profile, combined with the increased pressure gradient, lifts the seal lip away from the shaft, creating a film of fluid. This phenomenon is known as the hydrodynamic effect, and is the method by which the lip material avoids immediate hardening due to excessive frictional heat. In essence, the lip is riding on a layer of fluid and not the shaft.

Hydrodynamic Sealing Aids

Hydrodynamic Sealing Aids
The Hydrodynamic Effect and the Dynamic Sealing Mechanism provide an explanation of how a radial shaft seal works in application. The pumping action provides continuous lubrication for the contact width of the elastomer lip. When certain parameters are pushed to extremes such as shaft speed, lead and viscosity the pumping mechanism needs some support.

Hydrodynamic sealing aids assist in the dynamic sealing mechanism, and increase the pumping rate back to sump. To explain this phenomenon in more detail, the image shows the footprint of a unidirectional hydrodynamic aid. The “L” style aid is a series of molded ribs located on the air side at an angle of 15° to 20° to the circumferential contact width. Oil that escapes past the contact width is forced back by the rotating shaft into the converging space between the rib and the lip.

The benefit of using hydrodynamic aids is the increase of underlip pressure, decrease in friction, lower running temperature and ultimately can provide longer life. In cases where there is potential for small scratches and nicks on the shaft in the contact region, aids are used to overcome the potential for dynamic leakage.

Hydrodynamic aids are available in different patterns and as both unidirectional and bidirectional. Unidirectional aids are for either left or right shaft rotation. Mixing these two can result in catastrophic leakage.
Figure 9: Schematic of Hydrodynamic Aids

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