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Tribology and Lubrication Technology Lubrication Fundamentals December 2013 : Page 24

LUBRICATION FUNDAMENTALS The mysterious world of seals These important structures play a major role in making sure a lubricant stays in place. (Photo courtesy of SKF USA, Inc.) IN THE WORLD OF TRIBOLOGY, seals are a bit mysterious. Most things that move relative to something else need to be lubri-cated in some fashion—we all pretty much have that figured out. And, intuitively, it seems to make sense that to lubricate two surfaces, we need a way to keep the lubricant where we want it. However, many systems do not have that requirement, including chainsaw bars, trackside lubricators and the rap-idly becoming obsolete two-cycle engines. These all use the lubricant once and then pass it on irretrievably to the envi-ronment. In other more common systems, something must help hold the lubricant in place, and that something is a seal. But here’s where it gets interesting, the lubricant may not necessarily be a lubricant as we most commonly think of it— but I’m getting ahead of myself. The main function of seals can be summarized as follows: seals are present to prevent or minimize passage of fluid through gaps between mechanical elements of a machine. There are many different types of seals that are used in vari-ous applications. First, seals can be classified into two major categories: stat-ic and dynamic. Static seals, which are the simplest form, seal two objects together that have relatively little or no motion such as thermal expansion. These seals are usually in the form of gaskets or sealant compounds. The more dynamic applica-tions (pun intended) require dynamic seals. Of these seals, there are two major types: those sealing rotating motion and those sealing reciprocating motion ( see chart on page 24 ). As you can see, there are a lot of different kinds of mo-tions that must be sealed and a lot of different methods have been devised to do so. Often these varied designs were the result of the nature of the material being sealed and the nega-tive consequences of that material being released into the environment, especially with regard to employee safety. It is beyond the scope of this article to go into all the different designs and materials of construction, except to say that in addition to sealing some materials in, we may also be sealing an environment out—or vice versa. To be less abstract, first consider that many seal designs need a small amount of lubricant to function. The lubricant may not be a lubricant as we normally think of it but, rather, as in a chemical plant, the very chemical we are working with. Such is the case in sealing a pump shaft for a pump that is designed for pumping water, solvents, gases, caustic acids, WWW .S TLE. OR G 24 TRIBOL OG Y & L UBRIC A TION TE CHNOL OG Y

The mysterious world of seals

These important structures play a major role in making sure a lubricant stays in place.<br /> <br /> IN THE WORLD OF TRIBOLOGY, seals are a bit mysterious. Most things that move relative to something else need to be lubricated in some fashion—we all pretty much have that figured out. And, intuitively, it seems to make sense that to lubricate two surfaces, we need a way to keep the lubricant where we want it.<br /> <br /> However, many systems do not have that requirement, including chainsaw bars, trackside lubricators and the rapidly becoming obsolete two-cycle engines. These all use the lubricant once and then pass it on irretrievably to the environment. In other more common systems, something must help hold the lubricant in place, and that something is a seal. But here’s where it gets interesting, the lubricant may not necessarily be a lubricant as we most commonly think of it— but I’m getting ahead of myself.<br /> <br /> The main function of seals can be summarized as follows: seals are present to prevent or minimize passage of fluid through gaps between mechanical elements of a machine. There are many different types of seals that are used in various applications.<br /> <br /> First, seals can be classified into two major categories: static and dynamic. Static seals, which are the simplest form, seal two objects together that have relatively little or no motion such as thermal expansion. These seals are usually in the form of gaskets or sealant compounds. The more dynamic applications (pun intended) require dynamic seals. Of these seals, there are two major types: those sealing rotating motion and those sealing reciprocating motion (see chart on page 24).<br /> <br /> As you can see, there are a lot of different kinds of motions that must be sealed and a lot of different methods have been devised to do so. Often these varied designs were the result of the nature of the material being sealed and the negative consequences of that material being released into the environment, especially with regard to employee safety. It is beyond the scope of this article to go into all the different designs and materials of construction, except to say that in addition to sealing some materials in, we may also be sealing an environment out—or vice versa.<br /> <br /> To be less abstract, first consider that many seal designs need a small amount of lubricant to function. The lubricant may not be a lubricant as we normally think of it but, rather, as in a chemical plant, the very chemical we are working with. Such is the case in sealing a pump shaft for a pump that is designed for pumping water, solvents, gases, caustic acids, crude oil or whatever. This obviously makes the job more difficult, as some of these substances are not only potentially hazardous but also not very good lubricants and may be deleterious to many types of sealing materials.<br /> <br /> Thus, there are a number of considerations when it comes to the proper use of seals in machinery. In this article, we’ll review the major issues. Not all seal designs require physical contact with a mating surface to seal. A good example is labyrinth seals, which provide a tortuous path progressively building sufficient back pressure to prevent leakage.<br /> <br /> However, with the more common seals that do have contact, the sliding surfaces of the seals are always lubricated by the fluid sealed or by supplementary lubrication fed directly into the space ahead of an exclusionary seal or from impregnated solid lubricants at the seal interface. Therefore, one has to consider wear of both the seal and its mating surface.<br /> <br /> Ideally, the mating surface should be of a hard, wear-resistant material relative to the seal and the substance being sealed (which also can include slurries or other entrained particulate material). The hardness of the seal material and its compatibility with the fluid sealed are clearly critical factors. Thus, ideally, when it is necessary to replace the seal, only the seal should be replaced and the mating surface only cleaned or lightly polished.<br /> <br /> High surface-sliding speed and/or excessive pressure between sliding surfaces increases temperature. At low speed with high pressure, lubrication may be difficult. These factors can change seal/face geometry, which results in seal failure. As alluded to above, abrasives in the fluid sealed or rough seal seats or mating surfaces can cause damage and leakage. Abrasion resistance of the seal material is an important factor. Interestingly, too much or too little surface roughness on sealed surfaces can equally be a problem.<br /> <br /> SEAL FAILURE<br /> <br /> Given the proper design considerations of speed, pressure, abrasion resistance, chemical resistance, surface roughness and materials of construction, there are other important factors that can result in seal failure. Probably the most common is mishandling, which results in scratches, dings or chips in the seal face. This damage to the seal or its mating surface usually occurs during installation, storage or handling. Misalignment during installation is another major source of failure.<br /> <br /> Even with proper design and installation, we are not out of the woods yet. We can have failure at startup where the seal faces are not lubricated. An example would be running the seal dry because a valve on the suction side of a pump was left closed.<br /> <br /> Finally, like most mechanical systems, proper maintenance is important. Two key factors are “unscheduled movement,” which can be excessive axial or radial movement of a shaft against the seal; that is, excessive vibration or any other kind of related movement that is not part of the design. This could arise from a pump and motor that are firmly attached and aligned to a solid base. The other key factor is excessive contamination, which is often due to improper maintenance of filtration systems.<br /> <br /> Like so many things we encounter in this tribologically influenced world of ours, there is often more to it than meets the eye, and seals are no exception. Without effective seals, our cars burn oil (bad rings), our environment is polluted due to leakage, employee safety can be jeopardized and the workplace becomes a pig sty with material leaking across the floor and a haze in the air.<br /> <br /> In 1993 the Clean Air Act was introduced and since then the seal industry has implemented many very effective practices when it comes to the proper installation and maintenance of seals. Even the most cursory look at our environment versus 20 years ago clearly shows these advances.

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