By: Eduardo Niño de Rivera
Edition
by: John Amendola Sr
Artec-Machine Systems
INTRODUCTION
We tend to think of
gearbox inspections as a thorough examination of all the components of a
gearbox performed by a specialist. Although the final diagnosis and the
recommended course of action should be performed by specialists, unusual noise,
a change in vibration patterns or a sudden increase in temperature may require
an immediate inspection of gearbox components. If time, cost and/or not having
a specialist available make it necessary for plant personnel with limited
experience to carry out the inspection, obtaining useful data to convey to the
specialists requires planning, preparation and following proper procedures.
This article offers advice to non-specialists on how to carry out an onsite
gearbox inspection to obtain the maximum benefit out of it.
BACKGROUND
The first level of
inspection should come from maintenance and production personal in daily
contact with the gearbox. After an initial run-in period, a gearbox will settle
into a “normal” operating behavior, displaying steady levels of noise,
vibration and temperature. Ideally these levels are measured and monitored
regularly, but this is often not the case and it is up to the plant personnel
to be familiar with what the normal operating condition “feels like” and to
report when the machine does not seem to feel well.
ROUTINE INSPECTIONS:
Whether the condition
of the gearbox is being monitored or not, whenever plant personnel are near a
gearbox, they should look for and report any sign of distress:
·
Keep
the casing clean. A gearbox covered in dust will lose heat dissipating capacity
and it will operate at a higher temperature, reducing useful life of gears,
bearings, seals and lubricating oil;
·
Listen
and touch to the gearbox. Noise, vibration and temperature should always be
within acceptable levels;
·
Check
the level of the lubrication oil, it must always remain within the level
indicated by the manufacturer. If it needs regular replacement, check for leaks
and high operating temperatures;
·
Look
for signs of overheating, like smoke coming out of shafts or seals,
discoloration of metal components, burnt paint, surfaces too hot to touch or
water evaporating too fast when sprinkled on bearing housings or shafts;
·
Look
for oil leaks in seals, gaskets, mating surfaces and throughout the circulating
oil system;
·
Check
tightness of bolts on the base, housing split and bearing side plates;
·
Check
for sings of displacement between adjacent components (gaps, cracks in paint,
markings on surfaces).
These simple habits
can have a great impact on the time between repairs and provide waring for
potential failures. An inspection can then be scheduled and prepared for,
avoiding costly unexpected shutdowns.
VISUAL INSPECTION OF
GEARS:
Ideally, visual
inspections should be conducted by a specialist, but under certain
circumstances, such as a sudden change in temperature, noise or vibrations, a
non-specialist must perform a visual inspection. The following are recommended
practices for a fast and thorough inspection that may lead to well-informed
decisions regarding the immediate course of action.
GATHER INFORMATION:
It is always a good
practice to gather as much information as possible, specifically, name plate
data, drawings and manuals that should have been provided by the manufacturer.
The other important source of information are the observations, data and
reports that made the inspection necessary. The combination of these two
sources will provide an idea of the size and nature of the task ahead.
OUTSIDE INSPECTION
DURING OPERATION:
If possible, the
gearbox should be observed during operation to check for:
·
Cleanliness;
·
Noise;
·
Vibrations
(if the proper equipment is available);
·
Signs
of overheating (take temperatures if possible);
·
Casing
integrity (look for cracks or oil leaks);
·
Bolt
tightness;
·
Signs
of displacement relative to the base;
·
Signs
of displacement between mating parts;
·
Oil
levels;
·
Oil
samples and temperatures (if accessible);
·
The
condition of the circulating oil system (pipes, hoses, ports, valves, gages,
pumps, etc.);
·
The
condition of sensors and electric connections;
·
Spray
a soap in water solution into the base, the solution will start bubbling if the
base is not set properly on the baseplate or the foundation, a condition known
as soft foot.
PLANNING:
Discuss with the
specialists that will provide the diagnosis and recommendations, what data and
other information will be required and how it must be presented. The working
plan should include:
·
The
objectives of the Inspection (what will be delivered, data, information,
reports);
·
Complying
with safety protocols;
·
Special
tools and equipment required;
·
Personnel
requirements (numbers and skills);
·
Machine
down time allocation;
·
Clean
space required;
·
Tasks
to be performed (personnel, tools, time and sequence).
OUTSIDE GEARBOX
INSPECTION WHEN STOPPED:
Once the gearbox has cooled
down, further inspection may take place. Start by making sure that all safety
protocols are in place, then thoroughly clean the casing to ensure that no dust
or other contamination can fall into the gearbox when the inspection plate or
the cover are removed.
Make sure the
necessary brakes are applied to the motors and the machine to prevent them from
running loose once they are disconnected from the gearbox. Mechanically
disconnect the gearbox from the motor, the brake and the machine to allow the
gearbox shafts to rotate freely.
MEASURING END PLAY:
Place a dial indictor
on a fixed surface with the stylus touching the shaft’s end face, push the
shaft into the casing with enough force to make sure it is placed as far as it
can go and slowly rotate to make sure the bearing balls or rollers make proper
contact with their races, set the dial indicator to zero, pull the shaft out as
far as it can go and rotate to measure proper ball or roller contact, record and
compare the reading with the end play specified by the gearbox manufacturer.
MEASURING BACKLASH:
It is not usually
necessary for this type of inspection, but it is good practice to keep track of
backlash evolution over time as an indicator of bearing and tooth wear. If the
maintenance manual provided for the gearbox includes a backlash specification
and a way to measure it, follow the procedure outlined in the manual and
compare your measurement to the manual’s specification. If there is no backlash
specification, it can be measured by clamping a straight edge to the shaft
coupling and measuring how much the input shaft can freely rotate when the output
shaft is held in a fixed position (TIR). Ideally it is measured at the pinion’s
pitch circle diameter. In order to be able to compare consistent date obtained
at different times, the measuring procedure and measuring position must be the
same on every inspection.
VISUAL INSPECTION:
Before removing the
inspection cover or the casing top, make sure the casing and its surroundings
are clean and no dust or other contamination may fall into the gearbox once it
is opened; remove all contents from your shirt pocket to prevent them from
accidentally falling into the gearbox; and go through the check list of
required tools, equipment, available space and personnel.
Remove the inspection
cover following the manufacturer’s instructions. If the instructions are not
available, slowly loosen the bolts in a crossed pattern to evenly release the
pressure from the cover. Always use the supplied separating or lifting threaded
holes and remove the cover perpendicular to its support surface to avoid
damaging the cover or the casing. Take care not to allow gaskets or sealing
compound to fall into the gearbox and do not allow the cover to swing, putting
people and equipment at risk.
 |
| Photograph courtesy of Artec-Machine Systems. |
In many gearboxes, the
contact surfaces of all gear teeth are in plain sight when the inspection cover
or the top of the casing are removed, in others, a borescope may be required to
see some of these surfaces. Although a non-specialist may not be able to
recognize the type of damage or what caused it, it is usually not difficult to
see wear patterns, pitting, cracked teeth, scratches, rust or other damage to
the gear teeth. Their description and photographs can be sent to the
specialists for analysis and recommendation.
In some cases, the
contact patterns obtained during original assembly at the manufacturers plant are
still visible, if not, a new no-load inspection may be done (details will be
covered in a future article).
It is important to
learn if the gear teeth are case hardened or thru hardened. This information
should be noted on the gear data sheets or drawing. If information is not
available a measured check can be done with a hardness checker on the end of
the tooth face very near the surface or top land of the tooth. Otherwise a
simple hand file passed of a tooth tip is a fair way to compare. Hardened
surfaces do not scratch easily. This simple test provides the inspector an
indication of tooth wear. Thru hardened gears will polish over time and initial
macropitting may be tolerated for continued use with periodic inspection of the
trended nature of the surfaces. If the pits grow spalling will develop an
indication that the gear is failing.
Macropitting in case
hardened gears are more critical and can quickly lead to rapid deterioration of
the surface. Hardened surface gears can develop different surface distress such
as micropitting or scuffing which may be an indication of inadequate load distribution
due to bearing wear, structural support, misalignment, or changes is external
load or environmental conditions. Corrective action may arrest the distress and
if the damage were not severe, the gear may remain in operation.
KEEP A LOGBOOK:
It is important to
keep records of the condition of the gearbox and all its components, as well as
of all the activities performed on it. When possible take photos of the rotor
elements and bearings and archive their respective condition into the report
for future comparison.
Should the anomalous
operation continue after finding no evidence of damage, a specialist must be
called in to do a second inspection as soon as possible.
REPLACING THE COVERS:
Ideally, a maintenance
manual with instructions to close the gearbox would be available, otherwise,
the procedure to remove the inspection cover or the top of the casing should be
followed in reverse sequence. In the Process DO NOT:
·
Bang
any items;
·
Allow
foreign objects or matter to fall into the gearbox;
·
Damage
any dowel pins.
Make sure all bolts
and lubrication connections are properly tightened; filters, gaskets, sealing
compound and seals are in place; and all instruments are working in order.
START UP:
Again, follow the
manual’s procedure when available (this topic will be covered in a future
article)
SUMMARY:
A sudden increase in
noise, vibration, temperature or metal debris in the oil may warrant an
immediate inspection of the gearbox. Time, cost or non-availability of an
expert, may make it necessary for plant personnel with limited experience to
perform a visual inspection of the gear teeth condition. Getting the most of
such an inspection requires good communication with the expert that will do the
final analysis and recommendation, and proper planning, preparation, execution,
and record keeping. Finally, there is no substitute to following sound
maintenance practices and common sense to perform these tasks in a safe and
productive manner.
REFERENCES
2.- https://www.powertransmission.com/articles/0314/Best_Practices_for_Gearbox_Assembly_and_Disassembly/
