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.
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.
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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Besides environmental factors, operating conditions such as the type of lubricant and lubrication method, shaft finish and eccentricity, and speed can also affect seal performance.
• Lubricants. Special additives are increasingly being used in lubricants to extend bearing life. But, these additives may adversely affect common seal materials. For example, disulfide additives give lubricants antiwear properties, but they also harden conventional seals, causing them to leak. Other lubricants may soften the seal and cause it to swell so it becomes imbedded with abrasive dirt, which causes wear. Before selecting a lubricant-seal combination, consult the manufacturer for fluid compatibility charts.
As sump temperature or speed increases, the lubricant film between seal and shaft becomes thinner and breaks down. Also, in many applications, the amount of lubricant at the seal is sparse or nonexistent for a short time after startup. Either condition causes seal wear or “stick-slip” that permits leakage. Fluoroelastomer or PTFE seals can compensate for inadequate lubrication.
• Shaft factors. Both shaft finish and eccentricity affect seal performance. The shaft should have a burr-free chamfer or radius at the end and a smooth finish, from 10 to 20 min. Ra. A plunge-ground shaft that is free of machine lead is a key element in leak-free performance. Use shafts that are hardened to Rc 30 and have diameters within acceptable tolerances, typically 60.003 in. for shafts up to 4-in. diameter. The bore of the machine housing should be chamfered, free from burrs, and finished to at least 125 min. Ra.
Too much shaft eccentricity exceeds the ability of the flexible lip to follow the shaft and may cause early leakage. Shaft eccentricity consists of two types: shaftto- bore misalignment (STBM), which is the offset distance between shaft and bore centerlines, and dynamic runout (DRO), the amount by which the shaft does not rotate around the true center.
Spring-loaded seals usually perform satisfactorily if the total eccentricity (STBM + DRO) falls within ranges specified by the manufacturer for given shaft speeds. For example, a typical maximum total eccentricity value for a shaft speed of 1,500 rpm is 0.013 in.
• Speed. The maximum speed at which a seal is effective depends on factors previously mentioned: shaft finish and eccentricity, pressure, temperature, and type of lubricant. Consult the manufacturer for recommendations.
Why seals fail
When a seal fails prematurely, the usual reaction is to replace and forget. But, that won’t work for long — the cause of failure probably still exists and the new seal will fail as well. On the other hand, a better seal may offer two or three times the life of the old one. Therefore, it pays to do a little detective work, find the cause, then eliminate it or substitute a seal that can handle the problem.
Improper installation, usually resulting in a cocked seal (not square to the bore) that wears unevenly, Figure 5, causes most premature seal failures. To avoid installing a seal cocked, first lubricate the seal and make sure the shaft and bore are free from wear or damage. Then, install the seal, using the proper tools, in a clean area. Prior to installation, store seals in a cool area at 40 to 70% humidity and avoid distortion due to handling or hanging on a peg or nail.
Seals may leak due to a nick or cut in the seal lip, Figure 6. Other conditions that lead to early seal failure include excessive heat, a change in lubricant, insufficient lubrication, and shaft misalignment or roughness. The simultaneous occurrence of several conditions (pressure and speed) at their limiting values can also adversely affect seal performance.
Where seals fail consistently, inspect the seal lip for brittleness or cracking. Excessive heat produces a hardened lip that loses its ability to follow the shaft and fails to seal.
The additives in new lubricants may produce adverse reactions in sealing materials. If sealing failures suddenly increase in an area that was previously without such problems, determine if either the seal or lubricant was changed so that they are now incompatible.
Safety, environment, and quality implications
In an era when companies are faced with a flood of regulations from sources such as OSHA and EPA, the need to avoid oil leakage is becoming more pressing. Oil leaks not only cause downtime; they also lead to problems with quality certification (ISO 9000) and government regulations. An oil leak, for example, causes a slippery floor, a fire hazard, and a possible environmental problem. This condition comes under the “general housekeeping” standards of OSHA, which is actively citing such violations.
Materials play an important role
When seals fail at short intervals, consider substituting a more durable material. Though seals made from premium materials, such as fluoroelastomers, are more expensive than conventional nitrile rubber seals, reduced downtime and labor can make fluoroelastomer seals a cost-effective approach.
Nitrile seals are excellent for use with most mineral oils and greases. They operate at temperatures from 265 to 225 F. Abrasion-resistant compounds are available for applications involving scale, sand, grit, or dirt. Polyacrylate elastomers are well suited for use with extreme pressure (EP) lubricants and they resist oxidation and ozone. They operate at temperatures to 300 F. But, don’t use them with water or at temperatures below 240 F.
Silicone seals, which operate at temperatures from 2100 to 325 F, minimize friction and wear. But, they are not compatible with oxidized oils, some EP additives, and abrasives.
Fluoroelastomer seals operate over a wide range of temperatures, from 240 to 400 F. They resist most of the lubricants and chemicals that destroy nitrile, polyacrylates, and silicones.
PTFE seals are custom manufactured to meet the demands of extreme temperatures — from 2100 to 500 F. They accommodate higher speeds and pressures than other materials and offer resistance to various chemicals.
Glen E. Gabryel is an industrial product/services manager for CR Services, Elgin, Ill.