Labyrinth bearing seals can offer cost advantage

It is argued that only contact seals, including lip seals and face seals, are able to hermetically seal a bearing enclosure.

07/30/2004


By David Orlowski, Inpro/Seal Co., Rock Island, IL.

Key concepts

  • Labyrinth bearing isolators positively seal without contact.

  • Lip seals consume, on average, 147 W of power.

  • Life-cycle costs for labyrinth isolators are less than for lip seals.

    • It is argued that only contact seals, including lip seals and face seals, are able to hermetically seal a bearing enclosure. Hermetic sealing is defined as an absolute prevention of voluntary transfer of air or vapor in or out of a bearing enclosure that can only be accomplished with a contact-sealing device.

      Whether or not this degree of sealing is necessary is open to debate. Labyrinth bearing isolators have been refined to the point of positively sealing while the equipment is at rest and dynamically sealing when the equipment rotates. In any event, the arguments are theoretical in nature and in practice the empirical data is at best inconclusive.

       

      Both lip and face seals hermetically seal bearing housings for a time, but both are subject to wear and deterioration at a fraction of the design life of the bearings they are supposed to protect.

       

      Compact, inexpensive face seals, sometimes powered by permanent magnets, were applied to pump bearing housings primarily because of their ability to hermetically seal the bearing environment (Fig. 1).




      Fig. 1. Face seals can be energized by magnets.

      With the exception of difficult-path, simple labyrinth isolators in API pumps and steam turbines, there was no known viable alternative to long-established methods of sealing bearing enclosures.

       

      Compound labyrinth isolators function in place of a common lip seal and are non-contacting, non-wearing, and do not generate heat.

       

      Labyrinth isolators cost at least ten times more than a rubber lip seal, making whole plant conversions difficult to justify. Conventional wisdom points out that for the price of a labyrinth isolator, one could buy a dozen or so lip seals. Rarely would anyone rationalize the periodic change-out of a lip seal, since it was almost never done.

       

      Grooving of the shaft by a lip seal can be repaired by installing a thin, specially made sleeve. Power consumption, while the lip or face seal was sealing, was never thought to be important.

       

      Contact seals consume power

      According to industry sources, the electric power consumed by contacting radial lip seals, and other contact seals, can be substantial.


      Although a lip seal’s duration of contact with the shaft is short lived (the mean life is 1800 hr); it has frictional drag and an average power consumption rate of about 147 W. When the lip seal ceases to contact the shaft, its usefulness as a bearing and lubricant protection device also comes to an end.

       

      For maximum performance, lip seals depend on a controlled oil film at the contact point between the sealing lip and the shaft. The shaft can be neither too rough nor too smooth. Machining marks on the shaft can cause almost instant leakage into or out of the bearing enclosure (Fig. 2).




      Fig. 2. Lip seals cannot tolerate shaft imperfections.

      Face seals also depend on a constantly replenished lubrication film between their optically flat sealing surfaces.

       

      Do the math

      An average of about 38 million lip seals are installed each year. The mean operating life of a lip seal is approximately 20% of the 8760 hours in a year, so that 20% of the yearly installation of lip seals is engaged with shafts and actually sealing at one point in time.

      This would put 7.6 million lip seals per year sealing and gripping shafts and each consuming an average of 147 W of power. Even these conservative estimates show 1117 mW of power to be wasted in an ineffective bearing sealing strategy.

      There is a very small amount of data produced showing the magnitude of power consumption relegated to contacting face seals, but estimates put that number equal to or greater than that of lip seals.

      If all 38 million lip seals were engaged and sealing, the total power consumption would be 5586 mW, or about half of what a large utility, such a Florida Power and Light, produces in 24 states. It may be fortunate that only 20% of the lip seals produced in one year are engaging shafts and sealing at any one time.

      Noncontact labyrinth bearing isolators consume zero power in operation and have no finite life limitation. Their performance does not deteriorate with time in service. They are capable of outlasting the bearings that they are designed to protect (Fig. 3).


      Fig. 3. Labyrinth isolators have no wearing parts.

      Heat generation is not a problem and bearing life is enhanced by running in cool, clean lubricant. Even with these advantages, labyrinth bearing isolators command only a tiny segment of the available market.

      Rubber lip seals have their place, but not necessarily in the process industries. Their effectiveness is short-lived and even while they are working, their energy drag is costly. Contact bearing sealing in industrial applications cannot be avoided entirely because of situations including flooded lube conditions such as worm gear housings and vertical down housings where there is a pool of oil above the output shaft protrusion.

       

      Because of user demand, most ANSI pump manufacturers equip their products with metallic, noncontact labyrinth bearing isolators. The API has prohibited the use of lip seals for decades because of their concern for pump reliability in the hydrocarbon processing industry. Pump warrantees have been extended to as much as five years because of field experience with labyrinth bearing protected products.

       

      Severe duty electric motors, that previously had little or no bearing protection, now come equipped with labyrinth bearing isolators because of the IEEE-841 standard. For the ultimate motor, some are buying IEEE-841 motors with NEMA premium efficiency. Warrantees have been extended to five years to take advantage of the new level of motor reliability.

       

      The initial cost of a metal labyrinth bearing isolator is ten times that of a rubber lip seal of comparable size. The time for installation of a labyrinth bearing isolator is about the same as that of a rubber lip seal. Even though the effective life of the rubber seal is a matter of a few months, hardly anyone changes them out when they cease to function. If lip seals were replaced when worn, considering the costs of labor and downtime, they would be much more expensive than labyrinth bearing isolators.

       

      Metal labyrinth bearing isolators, once installed, are considered to be permanent bearing protection. Their performance is measured in hundreds of thousands of hours (years) and their life expectancy meets or exceeds the projected design life of the bearings they isolate and protect.

       

      The initial performance of a labyrinth bearing isolator is the same 3 or 10 yr after installation. If the equipment they protect is ever dismantled for any reason, the labyrinth bearing isolator can be used over and over again.

       

      More info

      For more information about the basics of bearing isolation, go to bearingisolators.com . Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, jfoszcz@reedbusiness.com



Top Plant
The Top Plant program honors outstanding manufacturing facilities in North America.
Product of the Year
The Product of the Year program recognizes products newly released in the manufacturing industries.
System Integrator of the Year
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
October 2018
Tools vs. sensors, functional safety, compressor rental, an operational network of maintenance and safety
September 2018
2018 Engineering Leaders under 40, Women in Engineering, Six ways to reduce waste in manufacturing, and Four robot implementation challenges.
July/Aug
GAMS preview, 2018 Mid-Year Report, EAM and Safety
October 2018
2018 Product of the Year; Subsurface data methodologies; Digital twins; Well lifecycle data
August 2018
SCADA standardization, capital expenditures, data-driven drilling and execution
June 2018
Machine learning, produced water benefits, programming cavity pumps
Spring 2018
Burners for heat-treating furnaces, CHP, dryers, gas humidification, and more
October 2018
Complex upgrades for system integrators; Process control safety and compliance
September 2018
Effective process analytics; Four reasons why LTE networks are not IIoT ready

Annual Salary Survey

After two years of economic concerns, manufacturing leaders once again have homed in on the single biggest issue facing their operations:

It's the workers—or more specifically, the lack of workers.

The 2017 Plant Engineering Salary Survey looks at not just what plant managers make, but what they think. As they look across their plants today, plant managers say they don’t have the operational depth to take on the new technologies and new challenges of global manufacturing.

Read more: 2017 Salary Survey

The Maintenance and Reliability Coach's blog
Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
One Voice for Manufacturing
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Maintenance and Reliability Professionals Blog
The Society for Maintenance and Reliability Professionals an organization devoted...
Machine Safety
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
Research Analyst Blog
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Marshall on Maintenance
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
Lachance on CMMS
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
Material Handling
This digital report explains how everything from conveyors and robots to automatic picking systems and digital orders have evolved to keep pace with the speed of change in the supply chain.
Electrical Safety Update
This digital report explains how plant engineers need to take greater care when it comes to electrical safety incidents on the plant floor.
IIoT: Machines, Equipment, & Asset Management
Articles in this digital report highlight technologies that enable Industrial Internet of Things, IIoT-related products and strategies.
Randy Steele
Maintenance Manager; California Oils Corp.
Matthew J. Woo, PE, RCDD, LEED AP BD+C
Associate, Electrical Engineering; Wood Harbinger
Randy Oliver
Control Systems Engineer; Robert Bosch Corp.
Data Centers: Impacts of Climate and Cooling Technology
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
Safety First: Arc Flash 101
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
Critical Power: Hospital Electrical Systems
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
Design of Safe and Reliable Hydraulic Systems for Subsea Applications
This eGuide explains how the operation of hydraulic systems for subsea applications requires the user to consider additional aspects because of the unique conditions that apply to the setting
click me