Preventive maintenance: an examination of the root causes of gearbox failure
By Steve Luchetta, Director of Engineering, Philadelphia Gear Corporation
When equipment fails, often a plant manager's biggest
concern is how to get the equipment running again. However, equally
important to getting it back online is discovering 1) why the equipment
failed, and 2) how such a failure can be prevented in the future.
But often, plant managers and their employees are ill equipped to
identify the cause of such problems, ultimately leading to their
Preventive maintenance is defined as the regular performance
of equipment maintenance practices in order to avoid future equipment
problems. An important first step in any preventive maintenance
program is learning to identify the causes of equipment failure.
Once the cause of failure is determined, one can take steps to avoid
the problem in the future. That is the purpose of this paper - to
provide plant managers and service technicians with the knowledge
to identify causes of gearbox failure (a crucial link in the power
transmission chain) with the hope that it will lead them to establishing
an effective preventive maintenance program of their own.
Always important when there is potential metal-to-metal
contact, effective lubrication is extremely critical to all
gearboxes. Proper lubrication will help prevent both gear
and bearing failure. In contrast, many gear and bearing failures
result from insufficient or interrupted lubrication.
Maintaining proper lubrication necessitates
following proper lubrication practices. These include using
the proper lubricant, keeping oil clean and free of foreign
materials, and maintaining a sufficient supply of lubricant.
Because selecting a lubricant is based on so many independent
factors - gear type, load type, speed, operating temperatures,
input power, reduction ratio - choosing a lubricant should
be left up to a gear lubrication specialist. This is especially
true when you consider the technical sophistication found
in gearing today, along with increased speeds and loads, and
the specialized lubricants and additives now available.
Lubrication problems can cause several gear
problems. Failures, like scoring and galling, are generally
caused by oil film breakdown resulting in metal-to-metal contact,
and high temperatures resulting in tooth surface damage. If
a gear continues to operate without adequate lubrication,
damage will progress until the gear's tooth profiles are degraded
to the point where replacement is the only remedy. Further,
abrasive wear is often the result of foreign materials present
in the lubricant.
Maintenance professionals have several important
tools at their disposal for diagnosing gearbox lubrication
problems. One of these tools, oil analysis, plays a crucial
role in not only preventing such problems, but in assessing
the overall health of equipment. By analyzing particulate
content and concentration in the oil, engineers are able to
monitor the condition of an operating gearbox. Further analysis
of the oil yields vital information concerning the condition
of the lubricant used in the equipment.
Analysis of oil used for lubrication can alert
engineers to possible problems within a lubrication system.
Equipment that has exhibited frequent mechanical problems,
or which would cause an outage if it fails - such as compressor
drives - needs to be checked regularly for lubrication problems.
Further, lubrication problems can be detected
by examining wear patterns on gears. Gear tooth "pitting"
is characterized by a large number of very small pits, distributed
evenly over the working surface of a gear. The appearance
of such pitting is usually an indication of gear overload,
but may also be indicative of lubrication problems caused
either by some corrosive medium within a lubricant, or by
improper lubricant additives.
Vibration - the motion of a body about a reference
point caused by an undesirable mechanical force - is a key
indicator in the diagnosis of machine faults. Each machine
fault generates a specific vibration profile, and a single
vibration measurement provides information concerning multiple
components. The frequency of the vibration is determined by
the machine geometry and operating speed.
By analyzing shaft vibration, engineers are
able to determine whether the cause of the machine fault is
imbalance, misalignment, general looseness or wear, bearing
defects, gear defects, or some other unforeseen problem.
Imbalance is the force created by a rotating
body when its center of mass is offset from its center of
rotation. High radial peaks at 1x shaft RPM, low axial vibration
at 1x shaft RPM, low harmonics of shaft RPM, and 1x RPM sinusoidal
pattern in the time waveform characterize imbalance. Imbalance
can cause other faults to appear. Once a structure is vibrating,
any number of ancillary components can become loosened - hardware
and piping are good examples.
Misalignment is the deviation from a common
centerline during operation. Misalignment can occur as offset
(shafts are meeting square, but not on a centerline), angular
(shafts are meeting at an angle from one another), or both.
Gear damage caused by misalignment is visible as a fracture
originating at one end of a gear tooth, occurring on a diagonal
line. Misalignment is also a common cause of broken teeth
on helical and bevel gears.
Often, misalignment is the result of loose bearings,
resulting in shaft deflection and, later, a possible gear
tooth fracture. Part of any preventive maintenance program
would include the inspection of such bearings to ensure they
are running with the proper clearance and are in satisfactory
condition. Checking proper adjustment is often part of such
a program. Otherwise, significant gear damage is possible.
Wear is another fault determinable by vibration
analysis, and can cover a broader range of gear damage - from
scoring and galling, to abrasive wear, to plastic yielding.
Plastic yielding - a severe flow of surface
material resulting in lip ledges at the end of gear teeth
- may occur on gears subjected to heavy, continuous load,
as well as gears subject to intermittent heavy loads or overload.
Preventive maintenance measures are taken not
only to ensure equipment is kept running; these efforts are
also made to keep equipment running at peak output levels.
This often means uprating a gearbox for optimum output based
on its application. By performing a detailed review of a gearbox
and its application, engineers can determine the equipment's
uprate potential, and in many instances, a gearbox can be
uprated by upgrading rotation elements, without adversely
affecting existing gearbox interface requirements.
In contrast, lack of preventive maintenance
often results in the opposite. Equipment in use from different
manufacturers, and maintained irregularly, often is not operating
at peak levels, or worse, is used at levels exceeding maximum
output recommendations. In these cases, preventive maintenance
practices can be taken to calibrate machinery so that it is
operating at optimum levels - before it becomes a problem.
As potentially damaging as equipment failures,
environmental factors often place equipment under strains
that it was not designed for - whether it is corrosion from
a humid environment, or lack of maintenance due to inaccessibility.
Lack of accessibility to the cooling tower drives
(due to the fact that elevation of cooling towers often exceeds
80 feet) sometimes leads to poor-to-nonexistent maintenance
procedures. In addition, equipment found in moist, humid environments
is more susceptible to failure due to corrosion, as moisture
inside the gearbox can eventually accumulate and destroy the
Compounding the problem, safety concerns often
do not allow for inspection of the units while they are running.
This certainly was the case with the oil refinery mentioned
earlier. Consequently, this affected the inspect/repair procedures
negatively and tended to render cooling tower drive failures
ESTABLISHING A PREVENTIVE MAINTENANCE PROGRAM
While identifying the cause of equipment failure
can sometimes be as simple as looking closely at the damage,
discovering the root cause of such a problem is often considerably
more difficult. The bottom line is, most plants do not have
the sophisticated equipment needed to identify shaft vibration
anomalies or analyze oil samples for foreign materials. Without
these resources, how then can they establish a preventive
Outsourcing preventive maintenance functions
to an outside service provider is certainly an option. Outsourcing
these duties allows plants to focus on their own core competencies,
letting experts, with access to both a strong knowledge base
and a wide range of necessary equipment, handle maintenance
and repair duties.
When selecting a preventive maintenance provider,
there are several key services that should be included in
any agreement. Repair and overhaul services are a necessity,
as ultimately all equipment will need maintenance at one point
or another. The key is to plan that maintenance downtime so
that it does not negatively affect production. Such services
should cover breakdowns, scheduled maintenance, parts reconditioning,
service upgrades, reverse engineering, alignment and balancing,
and on- or off-site diagnostic services. Providers should
be thoroughly skilled in performing gearbox failure root cause
analysis. For repairs, full disassembly and cleaning, inspection
and measurement, engineering evaluation, uprate recommendations,
performance of repairs and spin test should all be accomplished.
In the case of catastrophic equipment failure, failure analysis
is an important service.
Troubleshooting is another necessary service,
as part of a comprehensive preventive maintenance program.
Such troubleshooting duties include engineering assistance
in the identification, and swift resolution of operational
problems. Other important services (mentioned earlier) include
vibration and oil analysis. These two monitoring techniques
are paramount in identifying equipment anomalies before they
become a problem.
Any good preventive maintenance contract should
include a strong warranty on all equipment serviced. Such
warranties often offer many of the services already mentioned,
and are ultimately a benefit to the refurbished equipment.
Also, as important as any warranty is the proper installation
of a gearbox*. Proper installation can assure years of trouble-free
operation, provided that adequate preventive maintenance procedures
While identifying the cause of equipment failure
is only the first step in establishing an overall preventive
maintenance program, it is an important step. The information
gathered will ultimately serve as the foundation for planning
future preventive maintenance - a particular necessity when
working with mission critical equipment. Such information
will also help service technicians avoid making the same mistakes
after initial equipment repairs. Once this information is
determined, working with a service provider to establish a
complete preventive maintenance program is important in maintaining
equipment for future use, as well as lowering equipment lifecycle