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Bearing Knowledge Is Power

It has often been said that knowledge is power. This is especially true in the design and servicing of power transmission systems. Utilizing the knowledge of recent advances in bearing design and manufacturing technology provides a cost-effective ability to increase power handling capability or product life in both original equipment and service replacement applications.
     The term "power density" is commonly used to describe this concept of applying technology enhancements to maximize performance (life and power thruput) within a minimum space or within an existing system. For example, one manufacturer recently optimized his pinion bearing and shaft design to increase system life by 39 percent, reduce weight by 8 percent and reduce costs by 20 percent. Power density has also enabled service replacement bearings to increase life and improve system performance without modification to the existing bearing envelope or support design.
     It should be understood that to effectively decrease equipment downtime, reduce regular service/replacement intervals and increase overall system life requires one to first identify the weak link of the given system''s performance chain of core components, namely bearings, gears, shafts, seals, lubrication, etc. Here the design and application of "enhanced" or customized tapered roller bearings, as well as CBN ground gears, profiled gear teeth, light weight (aluminum) or high strength housings, and synthetic lubricants, can typically have a significant effect on maximizing the performance potential and power density of a power transmission system.
     One major objective of higher power density is to improve service life. Achieving this objective for bearings requires minimizing fatigue damage in its three primary modes:
Minimizing Bearing Damage 
  • Inclusion-A fatigue crack starts just below the raceway surface at a nonmetallic inclusion (micro impurity) and propagates to the surface.
  • Geometric Stress Concentration-Damage occurs in a localized region of high stress at the raceway edges due to high rolling loads and misalignment.
  • Point Surface Origin-Damage originates at a localized, high-stress point on the raceway surface, typically caused by insufficient lubricant film thickness separating the bearing surfaces.
Bearing Advancements
     By incorporating special enhancements to address these modes of fatigue damage, power density can increase the cost-effectiveness of the entire system. Specifically, tapered roller bearings have improved both service life and power capacity through advances in:
  • Materials Improved micro-cleanness, alloys and processing techniques (Figure 1).
  • Design Improved internal geometry, such as roller/race profiles, to carry higher loads and handle misalignment within a given envelope (Figure 2).
  • Tribology Optimized interaction of bearing surface topography with lubricants, lubricant additives and debris (Figure 3).
  • Manufacturing Advances in processing techniques for surface finish, plus precision tolerances, improved profiles, material cleanness and heat treatment.
  • Application Analysis Advanced performance-prediction tools that consider the effects of bearing design features as well as significant bearing environmental influences such as load zone, misalignment, lubrication, temperature, housing rigidity, load and speed.
Figure 1
Figure 2
Figure 3
     These advancements are the tools of power density. Successfully incorporating the correct mix of performance features can typically more than double "standard" bearing life (within the same envelope) while increasing purchase costs only a fraction. The more severe the operating environment, the more performance benefits that can be gained and the more cost-effective enhanced bearings become.
Degrees of Life Enhancement
     Various degrees of power density can be applied to enhance bearing life and durability, Figure 4. Typically an enhanced bearing will provide a minimum of 1.5 times the life of a standard bearing, although in more severe operating conditions, performance gains of 4-5 times are not uncommon. To clearly identify the design and life enhancement potential of power dense bearings, the designer should make a detailed application analysis of the many bearing design variables as well as operating and environmental factors. Bearing manufacturers can apply advanced software to accurately quantify these effects.
Figure 4
     Fully-enhanced bearings increase life by minimizing the three primary modes of fatigue damage through the incorporation of cleaner steel, super-finished rolling contact areas (rollers and raceways) and optimized geometry. Super-clean, air melt steel provides an additional 30 percent increase in life to fully enhanced bearings.
     Selectively-enhanced bearings offer the flexibility of selecting or applying only those enhancements needed to economically optimize performance in specific applications. Service life can increase within the range shown in Figure 4 and depends both on the specific enhancements chosen as well as the specific operating conditions. Such bearings fill the performance gap between standard and fully enhanced bearings. Selectively enhanced bearings are especially cost-effective in large bearings (8 to 84" OD), where fully-enhanced features may not be readily feasible.
Date£º2012-12-26 Visited Times:4836