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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. |
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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. |
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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. |
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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 |
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Inclusion-A fatigue crack starts
just below the raceway surface at a nonmetallic inclusion (micro impurity)
and propagates to the surface.
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Geometric Stress Concentration-Damage
occurs in a localized region of high stress at the raceway edges due to
high rolling loads and misalignment.
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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.
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| Bearing Advancements |
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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: |
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Materials -- Improved micro-cleanness,
alloys and processing techniques (Figure 1).
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Design -- Improved internal
geometry, such as roller/race profiles, to carry higher loads and handle
misalignment within a given envelope (Figure 2).
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Tribology -- Optimized interaction
of bearing surface topography with lubricants, lubricant additives and
debris (Figure 3).
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Manufacturing -- Advances in
processing techniques for surface finish, plus precision tolerances, improved
profiles, material cleanness and heat treatment.
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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.
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Figure 1
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Figure 2
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Figure 3
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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 |
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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. |
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Figure 4
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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. |
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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. |
| Putting Power Density
to Work |
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Despite the detailed analysis associated with optimizing power density,
the concept is not limited to original equipment design. Its effectiveness
has been well-documented with replacement bearings in existing envelopes-doubling
life, decreasing downtime and reducing regular maintenance intervals. |
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In fact, most industrial vehicle drivelines experience demanding operating
criteria, and as such, can significantly benefit from enhanced bearings.
These criteria include: heavy loads, high deflections, high temperatures,
slower speeds (generates a thin lubricant film separating the bearing contact
surfaces) and low viscosity lubrication. Power density attributes can be
applied to directly address the operating environment's most prevalent
mode of fatigue. Specific applications include: off-highway truck wheel
and rolling mill chock bearings,countershaft transmissions, pinion shafts,
planetary drives, sprockets, hydraulic pumps and motors, sheaves, large
gear drives and most industrial gearbox and axle units. |
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Bearings with high power density are currently being applied by manufacturers
to reduce equipment cost, size and weight through techniques such as 1)
upgrading -- increasing power capabilities while maintaining existing bearing
envelopes, and 2) downsizing -- using smaller, lighter weight bearings
to handle current loads. |
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Similarly for service replacement these cost-effective, power dense bearings
enable users to: 1) increase bearing life, or 2) correct a field performance
problem, without modifying the system to incorporate a larger bearing. |
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In this context, the knowledge to improve performance and reduce downtime
through increased bearing power density has proven to be a "powerful" means
to maximize competitive advantage. Give it a try. |
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This article is furnished
courtesy of PTDA
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