The purpose of a seal is to contain fluids and eliminate leaks. Dynamic seals in wind turbines retain lubricant for bearings and exclude contaminants that can destroy them. Such seals protect bearings as well as the rural environments, in which many wind farms are located, from unwanted lubricant leakage and spills.
Protecting bearings in a wind turbine means establishing a leak-tight sealing system. Traditionally, rubber lip seals running against rotating shafts have been used for this purpose. Their effectiveness is subject to three factors – the seal design, the lubricant, and the surface of the rotating shaft. The lip angle on the lubricant side develops hydrodynamic pressure to facilitate sealing, while the lip angle on the environmental side controls the amount of lubrication under the lip. A seal hinge controls lip deflection and aids in loading it on the shaft. The spring position controls the amount and location of lip load relative to the shaft-seal interface. Internal diametric interference between the lip and shaft interacts with the spring to create the seal force, while external interference between O.D. and housing prevents leakage around the outside the seal.
Lip seal performance can be compromised by the condition of the shaft surface, characteristics such as roughness, lay or texture, and machining methods to mitigate the inevitable imperfections. It should be noted that the smoothest surface may not be optimal for lip seals, because small irregularities can actually improve sealing.
Because lip seals begin to mirror the surface on which they are running, it is important the shaft surface lay be perpendicular to the axis of rotation to create the miniscule peaks and valleys that act as leakage barriers. If the surface lay is parallel to the rotation axis, it will create a direct leak path. In addition, seal manufacturers recommend plunge ground rather than machined surface finishes. Tiny screw-like tren-ches of the latter pump lubricant out of seals.
If all these factors are within the seal manu-facturer’s specifications, shaft rotation will create a hydrodynamic action when micro asperities on the seal lip acting as small viscous pumps begins to displace in a steady direction.
Unfortunately, lip seals cannot meet the wind power industry’s objective of 20 years service before they require major maintenance. Continuous contact with rotating shafts results in wear, leakage, and periodic replacement.
Unlike lip seals, isolator seals are a non-contact means of retaining lubricant and excluding contaminants. There are many types of isolator seals including complex hybrid designs. The latter combine standard labyrinth and new technologies, including hydrodynamic pumping, cellular foams, and unique unitizing elements. These hybrid seals are typically used in applications where standard technologies will not perform reliably over extended periods.
Standard labyrinth type seals are characterized by close tolerances and intricate, circuitous paths with abrupt directional changes to prevent leakage. Hybrid isolator seals, on the other hand, have unique impeller features on the rotor to create hydrodynamic pumping. The spinning rotor generates negative pressure outside the lubrication and environmental chambers. This negative pressure allows lubrication or contamination that enters the seal to flow from the shaft to expulsion ports. Cross-contamination is mitigated by the labyrinth path between the two systems.
In some hybrid isolators, cellular foam filters out airborne particulates and absorbs leaked lubricant. The foam has limited capacity, but even when it is full, it acts as a barrier. The foam used is application-specific based on viscosity of the fluid and particulate size of environmental contaminants.
Other hybrid labyrinth seals incorporate unitizing elements that hold the assembly together but do not create a seal. In these designs, the isolator actually uses the labyrinth path to retain lubrication and exclude contamination. A unitizing element simply prevents the rotor and stator from contacting one another, which prevents the generation of particles that could contaminate the lubrication system and eventually lead to premature bearing failure.
Used with a short labyrinth path, these new ideas significantly reduce the axial space required by conventional isolator seals. Most isolators today can fit into a space as small as 0.625-in. (16mm) wide. Some designs, especially those using unitizing rings, can fit into spaces just 0.375-in. (9.5mm) wide, the space typically reserved for a lip seal.
Improved machining capabilities also allow producing large bearing isolators. The stainless-steel construction of some of these isolators makes them suitable for use in hostile environments, and specialty O-ring materials can be interchanged with standard fluoroelastomers to enhance chemical resistance. Combining these technologies with low-wear comp-onents, and low motor torque and power requirements can provide isolators that perform maintenance-free for many years.
More recently, seal designers have devised split versions. Solid isolators previously required costly disassembly of power train components to replace a seal or several had to be installed during the original assembly. In contrast, split designs allow refitting turbines with the latest in long-lasting bearing-isolator seals without disassembly.
The application data required for either type of sealing system includes size (shaft, housing bore, available seal width), temperature (continuous and maximum), application parameters (equipment, sealing surface misalignment to housing bore, dynamic shaft run-out), media (type and level of lubricant), pressure (continuous and maximum), and shaft surface speed (continuous and maximum). The table summarizes the differences between lip seals and isolators.
Isolator seals facilitate the installation and maintenance of sealing systems. Lip seals are typically retained in a housing bore by a rubber-to-metal or metal-to-metal press fit, requiring considerable installation force. Press fits can let metal shavings enter a bore housing, leading to contaminated lube and premature bearing failure.
Also, nicks, burrs, or scratches on a shaft surface can damage a seal lip, producing a leak. A mounting tool prevents damage such as lip roll-over.
By comparison, isolators are easy to install. They usually have O-rings on their inner and outer diameters to seal on the shaft and against the bore respectively. To prevent damage to O-rings, the sealing surfaces of the shaft and bore must be cleaned prior to installation. Since the O-rings are not dynamic sealing elements, they are not subject to wear. Once the equipment is cleaned and inspected, the isolator usually can be installed by hand pressure alone.
Garlock Sealing Technologies, Palmyra, NY
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