Vacuum motor "Sealed Bearings" the meaning of:
This subject comes up often and has been beat around a bit. There is a basic misconception about seals. These seals are designed to prevent air from being drawn through the bearing when used in, typically, electric motors where the motor rotation pumps air that would centrifugally be drawn through the bearing. If this were permitted, the lubricant would act as fly paper and capture all the dust that passes, rendering the lubricant uselessly contaminated.
Seal practice requires a seal to leak if it is to work. The seepage lubricates the interface between shaft and seal and without this small amount of weeping, the seal lip would burn and develop a gap. In the presence of water on the outside, the weeping oil emulsifies and circulates back under the lip to introduce moisture into the bearing. This is usually not fatal, in some situations because it is only a small amount, but the displaced grease on the lip dries out and leaves the lip unlubricated. Excessive water and/or foam, will prematurely cut the life of any sealed bearing. Further, once the water/moisture produces rust, this rust (abrasive)will grind away at the bearing and cause friction; thus over-heating and shut-down.
The next time water contacts the interface, it wicks into the gap by capillary action and begins to fill the bearing. This is an expected result for seal manufacturers who live by the rule that no two fluids can be effectively separated by a single seal lip. Two oils, for instance, must have separate seals with a ventilated air gap between them. If a seal is to work with only one lip the contained fluid must be at a higher pressure so that the flow is biased to prevent circulation.
The JAGUAR MX-2 ETM @ cross-american.com