By examining and minimizing the sources for frequent gearbox

A back-to-back
arrangement tests
Romax Technology
designed 2-MW
gearboxes.
Turbines that last:
By examining and minimizing the sources for frequent gearbox failures, there is potential for an improved design.
Today, re-engineering the gearbox, rather than just replacing it, is the method more likely to improve turbine reliability.
Zhiwei Zhang • VP Engineering • InSight • North America • Romax Technology, Inc.
Colin McNichols • Senior Design Engineer • Romax Technology, Inc.
I
t’s well known in the wind industry that gearboxes don’t
often meet their original intended design life before
requiring replacement or refurbishment. In the U.S., over
65,000 megawatts of wind capacity is installed and most of
it comes from turbines with gearboxes. Such a large fleet means
there’s great potential for improved design and cost savings for
wind-farm owners and operators.
Noting and correcting serial issues to ensure a reliable gearbox
makes good, economical sense during turbine upgrades. This means
examining how and why gearboxes fail, and re-engineering them to last.
Significant savings and lifetimes are possible through proper simulation,
testing, and monitoring of gearboxes currently operating in the field.
Proper analysis of these gearboxes can increase longevity and lead to
insight and future cost savings for the industry. For example, a gearbox
life-extension plan that provides for one instead of two replacements
over a 20-year wind-farm life can save owners $500,000 per turbine.
That adds up quickly at a multi-turbine wind farm.
The comparative study in Gearbox scenarios shows three potential
choices when a gearbox is removed from a turbine for repair or upgrades.
A wind-farm owner can replace a failed gearbox:
Cost analysis
A large number of gearboxes in the U.S. is ready for refurbishment.
Parameters for this high-level cost model will vary by technology
and location, but the study shows that after the first failure, a rebuilt
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1.
2.
3.
Completely but without a design upgrade,
With a refurbished box but no other upgrades, or
With a refurbished and re-engineered gearbox for a longer life.
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Turbines that last:
gearbox with a higher lifespan can lead to significant cost
savings at site level and across an entire wind farm.
Why gearboxes fail early
Typically gearboxes are designed for each bearing and gear to
maintain 20 to 25 years of calculated life.
The main drivers for a drivetrain design fall into these
categories:
•
•
•
•
•
•
High reliability
Low mass
Low turbine cost
Low cost-of-energy (CoE)
Ease of maintenance
Ease of installation
Each category can separate further to fit other design features or
requirements. Some common challenges include:
• Planet bearing life. Planet bearing failures are costly to repair
and usually lead to full gearbox replacement. An ongoing industry
challenge is to design a gearbox to maximize planet-bearing life
(especially at the low-speed stage) within the available design
space (dimensional envelope, load capacity, cost goals, and
others). Attempts have led to spherical roller bearings (SRB),
cylindrical roller bearings (CRB), taper roller bearings (TRB),
and integral bearing raceways (where the outer race is part of the
gear). Some newer gearboxes are even using journal bearings.
The main reason for new bearing designs is to extend
life, but many changes haven’t met expectations. Designs
A cost study produced these cost
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• Gear failure modes. Although most
gears are designed to meet ISO standards
(ISO 6336 series), not all failure modes
are covered under these standard. Only
bending and contact fatigue, static capacity
to remove failure modes may bring new challenges. For
example, integrating the outer race of a bearing eliminates
the possibility of outer ring fretting or creeping, and
potentially increases the torque density and overall envelope
of the planetary sections. But these changes also add
complexity to manufacturing and assembly. Journal bearings
greatly reduce the number of parts, but provide difficulties
for maintaining sufficient hydrodynamic lubrication in all
operating conditions.
WINDPOWER ENGINEERING & DEVELOPMENT
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Planet bearing failure.
(strength), and scuffing are considered
in standard calculations. Issues such as
micropitting and TIFF (tooth interior
fatigue fracture) are still in research stages
without a common standard.
Additionally, there isn’t a “catch all”
model for these failure modes — each has
different underlying physical mechanisms.
Good design, therefore, must avoid many
potential issues, maintain material quality,
and simplify manufacturing practices.
• White etch cracking (WEC). It is a
prevalent issue that causes premature
bearing failure through spalls or axial
cracks. WEC is characterized by irregular
micro-structural alteration in the
subsurface material.
Root causes of WEC are highly debated
within the wind industry, and no specific
calculation or procedures are currently
available to remedy it. Reports show that casecarburized bearings tend to resist WEC, but
they are expensive. Case carburizing provides
residual compressive stresses in the rings,
which is believed to help against cracking.
Other failure modes
There are many ways a gearbox can fail
and some reasons are not solved with a
computer model. Here are two examples
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of failure modes from a root-cause
analysis project that proves good design
goes a long way:
1. Proper fit counts. In this case, a
high-speed bearing raceway fit proved
insufficient. Bearing rings started to
spin on the shaft, the end cap bolt
came unwound and a bolt dropped
in the oil feed. This led to a blocked
oil feed that overheated the gear.
Eventually, the gear lost all its teeth.
A good design approach would have
meant ensuring the right fit and
reverse-threading the end cap bolt.
The diagram shows a
level of analysis possible when using RomaxWIND software.
When it comes to predictive maintenance,
one method does not solve all problems. Good
re-engineering means each failure mode is
analyzed and understood for best results.
White etch cracking. The Inner-ring axial
crack has grown from a white etched area.
2. Watch for wear. Here, an
intermediate bearing wore excessively
during run-in. The feed took oil to
the torque tube bearing and carried
the hard wear particles to the contact
surfaces of the bearing. Eventually,
the torque tube bearing failed due to
abrasive wear. A good design approach
would add a separate oil feed to the
bearing from the manifold.
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System simulation
Today’s software can help design and
analyze gearboxes and drivetrain systems.
An ideal program will also simulate
behavior and performance of different
components to ensure optimal design.
For example, RomaxWIND is an
all-in-one system level software analysis
program used in gearbox and drivetrain
design. This software analyzes a full
drivetrain from hub load to generator
inertia, as shown in the Many possible
problems illustration. Flexibility of
structural components, such as housings
and planet carrier, are considered in the
program and used to more accurately
calculate potential bearing misalignments.
Non-linearities, such as gear
backlashes and bearing internal
clearances, are also included in the static
analysis and solved simultaneously by
numerical iterations. Capturing the
whole system deflection is especially
important in a wind turbine where the
drivetrain is lightweight (compared to a
gearbox designed to sit in a factory).
A RomaxWIND system model of a gearbox
provides key information, including planet-load share,
a distribution of gear-face contact stresses, maximum
bearing contact maximum, and fatigue lives. Additionally,
running sensitivity studies on manufacturing tolerances
and operating conditions can validate the robustness
of a design against manufacturing and operation. Gear
geometry and bearing arrangement can also undergo
optimization against multiple targets such as durability,
static strength, vibration, and efficiency.
Gearbox re-engineering
Re-engineering, or a design upgrade, is becoming
more commonly applied to improve the reliability of
existing gearboxes. It is often a more cost-effective
alternative when compared to a complete gearbox
replacement that incorporates an original design.
Successful re-engineering involves learning as
much as possible about a gearbox’s performance from its
operation and failure history. All design enhancements
should undergo validation testing before production.
One challenge is in obtaining load data for proper reengineering of a gearbox. Even without the original design
loads, it is possible to develop loads based on experience
or use of load-measurement equipment. This lets users
obtain site-specific loads to process into a redesign load set
— rather than relying on the OEM load set.
Case study
This case study involves a megawatt-class turbine
gearbox re-engineering effort for wind-farm owner,
Eurus Energy America. Romax Technology was asked
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Turbines that last:
Calculating contact stresses
to carry out a thorough
design upgrade and
provide gearbox remanufacturing support.
With a proactive
program to minimize
long-term O&M costs
currently in place,
Eurus Energy expects to
overhaul more than 200
wind-turbine gearboxes
over a period of
approximately 15 years.
According to Joe Stevens, Vice President
of Operations and Asset Management at Eurus
Energy America, timing of the overhaul is
important and long-term gearbox is essential. “It’s
imperative that we ensure drivetrain efficiency
and reliability to optimize the lifecycle of our
wind turbines. The end-of-warranty with our
OEM was the ideal time to upgrade the gearbox
design going forward across two of our important
projects. The agreement with Romax enables
us to proactively achieve optimization with a
Planetary stage model. The planet gear teeth
high degree of confidence that our fleets will be
(lower illustration) show a distribution of face
able to continue operating in challenging wind
contact stresses.
environments for years to come.”
So far, Romax has undertaken a full bearing
analysis, selection, assembly, and gearbox redesign
concept assessment, complete with a detailed
design and CAD (computer-aided design) analysis
using RomaxWIND software and other packages.
Full manufacturing and test support is provided
during this process, with the goal of improving
gearbox life and reliability.
Two major design
Flow chart of Romax Technology’s typical
upgrades implemented
re-engineering process
in this effort include a
planet-bearing change
and gear microgeometry modification.
The planet bearing has
been changed from SRB
to CRB. Bearing fatigue
lives have also been
improved, as shown in
SRB vs CRB graph. The
original SRB design
does not meet the 20
years requirement
(about 175,000 hours).
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In this case, gear microgeometry was
optimized in the planet stage. The original
contact pattern was printed based on a
measured gear chart and the optimized
contact pattern is shown in Combined
tooth loading (A and B). In this graphic,
the tip or root contact and end contact are
all considerably reduced. Furthermore, the
hard contact line at HPSTC (highest point
of single-tooth contact) and LPSTC (lowest
point of tooth contact) were removed after
optimization. The maximum contact stress
across the gear face were also reduced to
prolong gear life.
Prototype gearbox testing
Modeling and simulation is important for
gearbox design but they cannot substitute
for reliability testing nor predict the
day a component will fail. Instead, the
analysis compliments and reduces testing
and the number of prototype designs.
One of the goals of simulation is to save
money on prototypes.
Drivetrain testing is a key responsibility
of a systems integrator, and a company should
also have an established engineering practice
for the re-engineering process. This includes
a cross-functional approach with several key
areas, including:
1.
2.
3.
4.
Identification of potential failure
points during redesign and analysis,
Realistic drivetrain testing to
represent operating conditions,
Reliability and performance data
gathered during field operation, and
Test data and field data used to drive
better design practice and better
drivetrain testing methods.
Predictive maintenance that combines
engineering with analytic simulation
capabilities can reduce costs of gearbox
failures and extend turbine life. By
reviewing hundreds of gearboxes in
service, serial issues can emerge that
provide insight for design and redesign
processes. The best practices that develop
will result in reduced O&M costs for
manufacturers and wind-farm owners. W
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Fatigue life versus bearing design
It’s Aerodynamics 101
Smoother surfaces create less drag and
result in better efficiency.
The graph compares spherical roller bearings (SRB) to cylindrical roller bearings
when used in planet gears.
That’s where we come in.
The original contact pattern is based on measured data. Note that the gear
contact goes all the way to the tip and root (and over the ends) when the gear
rolled through contact.
Conference & Exhibition
New Orleans, LA May 23 – 26
2016
Booth #2949
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Using RomaxWIND, the graphic shows an optimized contact pattern when incorporat-
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contact no longer extends to the tip and root or edge.
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