INTRODUCTION
(English
version of the article published in Spanish in this blog on March 11, 2019)
Maintenance and Production personnel are usually well aware of the financial consequences of unscheduled shutdowns due to gearbox failures in critical equipment. Given today´s technology, the best way to prevent unscheduled shutdowns is through condition monitoring systems with sensors fixed to the gearbox. These systems continuously measure, record and analyze vibrations, temperatures, oil pressures and other variables to detect impending damage on gears, bearings and other gearbox components. In a perfect world, condition monitoring would give us the necessary information to take timely corrective action, thereby avoiding unscheduled shutdowns. However, not all gearboxes are equipped with these systems, and even when they are, an inspection by a qualified technician may be necessary to determine the underlying cause of failure. Depending on a plant’s systems, inspections may be scheduled based on the number of hours the equipment has been in operation; when unusual vibration, noise, temperature, lubricant contamination or other anomalous conditions indicate there is a risk of failure in the near future; or to determine the cause of an incidental or recurring failure.
Maintenance and Production personnel are usually well aware of the financial consequences of unscheduled shutdowns due to gearbox failures in critical equipment. Given today´s technology, the best way to prevent unscheduled shutdowns is through condition monitoring systems with sensors fixed to the gearbox. These systems continuously measure, record and analyze vibrations, temperatures, oil pressures and other variables to detect impending damage on gears, bearings and other gearbox components. In a perfect world, condition monitoring would give us the necessary information to take timely corrective action, thereby avoiding unscheduled shutdowns. However, not all gearboxes are equipped with these systems, and even when they are, an inspection by a qualified technician may be necessary to determine the underlying cause of failure. Depending on a plant’s systems, inspections may be scheduled based on the number of hours the equipment has been in operation; when unusual vibration, noise, temperature, lubricant contamination or other anomalous conditions indicate there is a risk of failure in the near future; or to determine the cause of an incidental or recurring failure.
Proper planning and
preparation are essential to perform a quick and thorough inspection of all
gearbox components for an accurate assessment of their condition, cause of
initial damage, expected remaining useful life, immediate actions that may
reduce the probability of failure before the next scheduled shutdown and
remedial measures that may offer longer time between gearbox related shutdowns
in the future. This article discusses the roll that non-gear-specialist maintenance and production personnel play in
planning, preparing and collaborating with gear specialists to make inspections
of critical gearboxes as quick, thorough and accurate as possible.
CAUSES OF FAILURE
The identification and
analysis of gear failures is beyond the scope of this article, there is plenty
of literature on the subject [references]. Here, we will only offer a list with
some of the most common causes of gear failure:
Cause
|
Damage
|
Contact fatigue
Sub-Case Fatigue
|
Initial macro pitting
Advanced pitting
Spalling
Macropitting
Tooth breakage
|
Bending stress fatigue
|
Root fillet cracks
Partial Tooth factures
Tooth Fractures
|
Lubrication film
brake down
|
Fine grain spalling
Micropitting
Latent micropitting
Scuffing
Plastic flow
Tooth Burning
Fretting corrosion
and false brinell
|
Lubricant
contamination
|
Tooth corrosion
Abrasive scoring and
ware
|
Electric arcing
|
Electric pitting
|
Manufacturing
defects
|
Premature pitting
and spalling
Premature tooth
braking
Hub destruction
Grinding fractures
|
Overload
(insufficient design service factor, unexpected peak or shock loads, increase in output demand)
|
Cracks
Fractures
Macro pitting,
pitting and spalling
Plastic flow
|
PLANNING AND
PREPARATION
BEFORE THE SPECIALIST
HAS ACCESS TO THE GEARBOX,
DO NOT:
1. Remove
the gearbox from the machine if at all possible. A better diagnose can be
rendered if, prior to inspecting the internal components, the specialist is
able to observe and measure vibrations, temperatures and oil pressures under
regular operating conditions and a tooth contact check to assure proper load distribution
over the tooth flanks.
2. Open the gearbox. It is important
for the specialist to inspect the gear casing for leaks and cracks while the
gearbox is still closed with its seals and bolts as they have been during
operation.
3.
Clean
or drain the lubricant without taking a complete set of samples. A proper oil
analysis requires samples from different spots, including lubricant input and
output ports for the oil cooling system, top, middle and bottom of the oil
sump, gear lubrication nozzles, filters, magnetic particle traps and lubricant storage
drums. The condition of the oil in the different locations may provide
information regarding the probable sources of lubricant contamination and on
where and how the failure originated.
PLAN:
1. Gather as much information as
available on the gearbox and its application: manufacturer, model, size, serial
number, power rating, speed ratio, date purchased, date of commissioning,
drawings, parts lists, service history and normal operating temperatures, oil
pressures, speeds and power actually delivered by the motor.
2.
Describe
in detail the reason for the inspection (suggested by the service manual,
unusual noise, temperature or vibration, etc.).
3.
Request
from the gear specialist a description of the tasks to be performed, a time
table and a list of spare parts, tools, equipment and personnel required for
the inspection.
4.
Reach
a clear understanding with the gear specialist regarding the inspection
procedures, the laboratory tests to be performed and the reports that will be
submitted upon completion.
5.
Schedule
the necessary test and shut down periods.
6. Make sure the spare parts, tools,
equipment and personnel will available during the inspection
DURING THE INPECTION:
1. Be available to take care of any
contingencies.
2.
Coordinate
the timely availability of spare parts, tools, equipment and personnel.
3. Overview and record the sample
gathering, lab testing, inspection and report presentation procedures.
AFTER THE INSPECTION:
1. Review all reports
2.
Overview
the startup procedure
3.
Keep
all recorded data for future reference
4. Follow the operation and maintenance
procedures recommended as a result of the inspection
CONCLUSION
Rather than something
spectacularly new, this article follows sound practices that have proven their
effectiveness through many years and across many industries. Planning and
preparation are an essential part of any productive activity and the inspection
of gears is no exception.
References
2.- The Gear Reducer
Book. Peerless-Winsmith, Inc 1980
