Sealing solutions for industrial bearings
By By Patrick Rhodes
To perform properly and last, bearings must be protected from loss of lubrication and external contamination. Yet traditional contact oil seals can severely damage equipment. This conundrum can be solved by using non-contact isolators that...
June 1, 2014
By By Patrick Rhodes
To perform properly and last, bearings must be protected from loss of lubrication and external contamination. Yet traditional contact oil seals can severely damage equipment. This conundrum can be solved by using non-contact isolators that protect both the bearings and the equipment in which they are installed.
Oil seals continue to be used, even though it typically takes at least two and sometimes three such seals to deliver the same level of performance as a single bearing isolator.
Bearing isolators originally featured metal components with simple pathways for dynamic sealing and O-rings for static sealing of the shaft and bore housing. Since then, they have evolved into highly engineered devices with close tolerances and complex pathways with sharp changes in direction.
Among the conditions leading to bearing failure are equipment washdown, inadequate labyrinth excluder ring (LER) seals and chemical attack. These can be prevented with standard bearing isolators.
Although size and temperature are commonly assumed to be enough information for an effective sealing solution, the application itself, media, pressure, speed and any special features required, also must be known. Application data sheets should be completed and submitted to application and product engineers for review to ensure all relevant information has been provided.
Bearing isolator construction
Industry standards further narrow the choice of viable bearing isolators. For example, the American Petroleum Institute specifies non-sparking materials for bearing isolators to be used in the petroleum, heavy-duty chemical and gas industries.
This requirement has been adopted by other industries as well, making bronze the material of choice for bearing isolators. If metal construction is not required, PTFE mixtures can be used to make chemical-resistant bearing isolators suitable for use in pharmaceutical and other applications.
Standard bearing isolators are typically fitted with O-rings, which if improperly specified will be the first component to fail under chemical attack and extreme temperatures. Depending on the application, PTFE-encapsulated FKM, AFLAS or silicone can be specified, instead of the standard brown FKM O-rings.
Bearing isolators last up to seven times as long as traditional oil seals, providing both lubricant retention and exclusion of contaminants. In addition, they can now provide electrical sealing as well. The torque and horsepower of electric induction motors controlled by pulse-width-modulation (PMW) variable-frequency drives (VFDs) can be adjusted to run at 40% lower rpm for significant power savings.
Uncontrolled induction motors operate on a three-phase, sine wave power scheme, in which the frequency, phase and amplitude of the input power add up to zero for a balanced circuit. When VFDs are used, power is supplied in controlled pulses called square-wave or six-step voltage.
These pulses create capacitive-coupled, common-mode voltage (CMV) on the shaft, which follows the path of least resistance to ground, usually through the bearings. The oil film does not provide sufficient insulation to prevent this excess, unbalanced voltage from arcing from the inner to the outer races of the bearing. This arcing acts as tack welding, and electric discharge machining (EDM) pitting results in fluting damage and eventual failure of the bearing. This damage produces a high-pitched noise as the bearing elements roll over the fluted races.
To eliminate this damage, conductive brushes have been added to traditional bearing isolators to prevent voltage from passing through the bearings. Providing a path of even less resistance than the bearings, this allows the excess voltage to travel safely to ground.
Any CMV from the shaft is dissipated by the brushes, preventing EDM from occurring. Additional dynamic sealing can be provided on the non-drive end of a motor by installing a traditional bearing isolator. For large, high-power motors, insulated bearings are recommended to force stray voltage through the conductive brushes.
Bearings also must be protected from the ingress of external contamination. Industries such as mining, power generation and primary metal production subject equipment to harsh conditions. In these environments, bearing isolators last up to three years, compared with just three to six months for a typical contact lip seal.
Filters for external contamination
For particularly dirty applications such as coal pulverizers and mine cars, air filters are being integrated into bearing isolators. Consisting of closed-cell foam, these filters prevent contamination from entering the seal pathways. The durable foam is inserted into a groove in the isolator, directing contamination to a drain port.
Split oil seals have long been used to eliminate the need to dismantle equipment for installation. Bearing isolators also are available in split designs with some restrictions, which include non-metallic, flooded and hybrid isolators that cannot be split due to their material of construction or application requirements.
Applications where the lubrication level is above the drain port preclude the use of traditional bearing isolators, where lubricant will travel through the labyrinth pathways and leak.
They also will leak if used with unvented forced lubrication systems, which create pressure differentials in the pathways. The pathways do not retain pressure, either positive or negative, so these pressure differentials will cause lubricant to leak through the labyrinth as it would under flooded conditions.
This problem has been solved with the development of a hybrid seal with capabilities beyond those of traditional isolators. These seals for flooded applications can be fully submerged, retaining as much pressure as most general-purpose oil seals.
The diversity of industrial sealing applications call for matching them with bearing isolators on an individual basis, collecting and taking into account all relevant data. This requires due diligence, but the reward is an optimal solution for protecting the bearings and extending the service life of plant machinery and equipment.
Patrick Rhodes is an applications engineer with Garlock Sealing Technologies, Palmyra, NY.