To optimize seal performance, the equipment designer needs to first determine what function the seal must perform and then look carefully at all of the environmental and operating conditions. By matching a seal to all of these conditions, you can prevent leaks and enable both seals and bearings to give their full service life.

Seal function

When selecting lip seals for rotating shafts, first determine what the seals must do — retain fluid, exclude contaminants, or both.

• Retaining fluids. Grease is easy to retain because it doesn’t flow readily. Thus, a non-spring-loaded seal, Figure 1, is usually the most cost-effective approach. The sealing lip points toward the grease as shown.

Because oil is more fluid than grease, it is more difficult to retain. Thus, it requires a spring-loaded seal, Figure 2. One type of seal has a lip shaped like a sine wave, Figure 3, that pumps oil back into the sump, a feature called hydrodynamic action.

• Excluding contaminants. In most cases, a non-spring-loaded seal is sufficient to exclude contaminants. In such applications, install the seal with the lip pointing toward the contaminants. One method uses an inexpensive V-ring seal, Figure 4, which stretches over the shaft for easy installation and seals against any suitable face (end of a bearing, washer, steel stamping, or back of an oil-seal shell). Because it rotates with the shaft, the V-ring slings off dirt, water, and other contaminants.

• Exclusion and retention. Some applications require both excellent oil retention and dirt exclusion. One way to do this is with two seals, one of which faces the bearing to retain the lubricant and the other faces outward to exclude contaminants. For extremely dirty environments, a mechanical seal with two spring-loaded sealing surfaces may be required. Seals for less critical applications feature a spring-loaded lip for fluid retention and a second, non-spring-loaded lip for dirt exclusion.

Environmental conditions

Factors such as temperature, pressure, and contamination in the working environment influence seal performance.

• Temperature. Both low and high temperatures degrade seal performance. At low temperature, rubber seals harden and become brittle. And, when the sealing lip becomes stiff, it can leak. To accommodate temperatures below 265 F, consider seals made from silicone or special PTFE compounds.

If the housing is made of a nonferrous material (aluminum or plastic), cold temperatures cause the steel seal case and the housing bore to contract at different rates, which may cause leakage at the bore. To minimize thermal contraction problems, use a seal with a rubber covering on the case OD.

High temperatures can shorten seal life and cause a chemical breakdown of the lubricant film on which the seal rides. A mere 25-degree increase, from 200 to 225 F, cuts the life of an ordinary seal in half! Moreover, the seal underlip temperature can be 50 F higher than the sump. That means that a seal operating in a 200 F environment may be running at 250 F, the upper limit for nitrile seals.

If it is impossible to reduce high sump temperatures, consider substituting a fluoroelastomer seal, which is suitable for temperatures up to 400 F and will last longer than polyacrylate and nitrile.

• Pressure. Excessive pressure distorts general-purpose seals, which causes heel wear and heat buildup. And, it can even force the seal out of the bore. If possible, vent equipment to the atmosphere to prevent pressure buildup. Seals in hydraulic pumps and motors are exposed to constant pressure. In such cases, be sure to select a pressure seal .

• Contaminants. To prevent bearing corrosion and abrasion caused by contaminants, install heavy-duty seals. If the amount of contamination is minimal, and a primary seal is used to retain a fluid, a V-ring or a seal with a secondary lip can be used to exclude the contaminants.

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