Why Trending is Important to Predictive
Activities
This case study takes place in a packaging facility. The
packer on module 8J0 was checked for vibration integrity during a pre-machine
care evaluation. A problem was detected in the folding arm gearbox. The
frequency characteristics indicated that a bearing was deteriorating.
This seems straightforward, but is it?
The gearbox is located in a section of the packer that is not easily
accessible. The necessary
repairs require separating the two sections of the machine. When the
packer is split, it will take an additional two days of work before the
unit can be restored to service. The maintenance leader does not believe
in the technician's ability to make a vibration call correctly. He
has vowed to bring the technician and the technician’s boss under
the scrutiny of leadership for poor performance if the vibration assessment
is wrong.
The added pressure from the maintenance leader has caused
the vibration technician to look for other means to confirm the call to
replace
the gearbox
bearing. The original call was based on a vibration signature that showed
a frequency peak at 36.9 with harmonic side bands at amplitude of .10 inch
per second (IPS). This means that there is frequency where there
should not be. The amplitude of this frequency by most
standards is not considered to be a serious problem and the machine was
running fine.
This is not a good set of circumstances to request repair work to be performed.
To be confident in the call, several steps must be followed:
1. Go back and look at the trend data on this particular
gearbox. This was done and the data was taken on a regular basis (once
per month).
2. Retrieve the packer’s gearbox drawings and find out exactly what
kind of bearing was installed in the gearbox, what it was driving, the
particular gear that was installed, etc.
3. Explore other alternative tests that could increase
the confidence of a correct call.
After looking at the drawing, it was determined that the
bearing was a 1208 roller bearing. With this information, the technician
could look more
closely at the match of frequency placement to the characteristics of the
bearing. The results were close but not exact. The frequency placement
and the bearing characteristics were off by a small amount, adding
some uncertainty about the original call.
The trend data had to be reviewed again. The frequency
was so close and had all the characteristics of a bearing based on
experience.
Reliability management was convinced the problem was a bearing that
had doubled in amplitude for more than four months. If this gearbox were
to fail catastrophically during operation,
several million
dollars would be lost in parts, labor, and lost production opportunity.
The technician
explored other tests to add confidence to the
call and decided to use fiber optic technology to take a visual
look at the bearing. A monitor attachment allowed several people (including
the maintenance leader) to view the test live as the camera was viewing
the bearing. The packer was shut down, the oil drained, and the inspection
covers were removed for access. There was just enough room for a smaller
person to get far enough in to do the inspection. The results were conclusive
as the visual images showed the damage on the bearing rollers (see below).
The repair could now be completed with full confidence.
When the maintenance crew removad the gearbox assembly,
a thorough visual
inspection was conducted to determine the failure mechanism. The
results of the inspection revealed the cam driven by the gearbox
had significant damage because the cam follower bearing had failed.
The closest
gear to the gearbox had tooth damage, and the 1208 bearing that showed
up on the vibration signature was a double roller bearing instead of a
single roller bearing as the drawing had indicated. This explained why
the frequency placement didn't line up. The primary failure was the
cam follower bearing. The secondary failures were the 1208 bearing
damage, cam damage, and gear damage.
After the work was completed and the equipment restored
to service the lessons learned were explored. These included the following:
1. The trend data was critical in making an accurate
call.
2. There are two different roller configurations with
the same stock number in the stock room.
3. The cam follower bearing was too delicate for the service it provided.
4. The maintenance leader is now confident in the technician's
vibration calls.
5. Other reliability tools should be purchased for the facility,
such as a fiber optics camera.
The job of the reliability department, as far as this
writer is concerned, is to "trend, trend, and trend". The understanding,
documentation, and sharing of the reliability lessons learned are the
future of a company's
success.
Submitted by Mark A. Latino, SrVP Operations, Reliability Center, Inc.
|
