When
it comes to mounted bearing installation and maintenance, there are
many industry myths and misconceptions that may affect plant uptime and
overall performance. Maintenance managers need to be aware of these
elements, so that they can maximize performance and keep the plant
running.
Bearings can fail for many reasons and studies have helped to
understand the reasons for these failures. As shown in the pie chart
(click it to expand it), a large majority of bearing failures are
related to lubrication problems and contamination. The following myths
and misconceptions address some of these issues, and provide insight
into how they can be resolved and avoided.
Installation Myth #1:
Using a hammer is okay to position a bearing right?
FALSE: Never deliver a direct blow to a bearing. The rolling elements
and raceway are hardened, but they can still be damaged. Impact from
the hammer can transfer to the raceway leaving permanent indentations,
and running the bearing with these indentations can cause noise and
dramatically reduce bearing life. You should check the shaft diameter,
look for burrs, dirt or corrosion on the shaft and if needed use a
press to slide the bearing on. If a press is used, pressure should be
placed equally on the face of the inner ring to help avoid damage to
the raceways and rolling elements.
Installation Myth #2:
Off-the-shelf TGP shafting is always the best option.
FALSE: TGP is turned, ground and polished. It's a manufacturing method,
not a tolerance range or a guarantee that the shaft meets the bearing
manufacturer's specified range for diameter and roundness. It's
recommended to measure and specify the proper shaft diameter and review
the bearing manufacture recommendations.
Installation Myth #3:
It's okay to hand-tighten setscrews, one at a time.
FALSE: Setscrews are an integral part of the locking system between a
shaft and bearing, and should be tightened to the manufacturer's
recommend torque. Under tightening may result in loss of lock and
slipping of the bearing on the shaft, and over tightening may result in
raceway distortion or inner ring cracking. The recommended approach is
a half-full/full method. Half-full/full refers to tightening the first
setscrew to half the recommended torque, the second setscrew to the
full torque then back to the first setscrew for the full torque.
Lubrication Myth #1:
Re-lubrication once a year is sufficient.
FALSE: Re-lubrication is necessary to replenish grease in the bearing
when the current grease breaks down or deteriorates. Re-lubrication is
a necessity because the base oil breaks down as a result of
temperature. The lubrication film between the bearing rolling element
and the raceway can diminish or is eliminated, resulting in
metal-to-metal contact. Re-lubricating the bearing replenishes the oil,
helping maintain the proper lubrication film.
Pumping new grease into the bearing also helps flush away
contamination. Many mounted bearings are designed to allow the grease
to enter the bearing cavity as close to the rolling elements as
possible. As more and more grease is added to the bearing, the old
grease is pushed out of the seals (if the seals are purgeable). The
action of purging grease pushes and helps keeps dirt away from the
seals.
Bearing manufacturers offer general re-lubrication recommendations,
including amounts and intervals, as suggested starting points. The
amount of grease used at re-lubrication can vary with bearing size and
type. Re-lubrication intervals can vary based on load, speed
temperature, or environmental conditions. For example, a mounted ball
bearing in lightly loaded, low-speed, clean environments may only need
re-lubrication every 12 to 24 months.
However, each application is different and applications with higher
speeds, temperatures, or heavy contamination would require more
frequent bearing re-lubrication, possibly daily to once a week. Review
of the bearing manufacturer's recommendations is encouraged. Specific
applications should be monitored regularly and lubrication intervals
and amounts adjusted accordingly.
Lubrication Myth #2:
Always add grease until grease purges from the seal.
FALSE: If you pump grease into the bearing until it purges out the
seal, you likely have completely filled the bearing cavity. If you
completely fill the bearing with grease, the excess grease can increase
bearing operating temperature and potentially create enough pressure to
blow the seal out. However, in a dirty and/or low-speed application
where contamination may easily enter the seals, filling a bearing full
of grease may help improve bearing performance. Application experience
will dictate when the entire bearing cavity should be filled with
grease.
Lubrication Myth #3:
If there is noise, it must be the bearing and grease should be added.
FALSE: If the bearing is making noise, internal damage has likely
occurred. If the bearing continues to run without being replaced, more
internal damage may occur to the bearing, with the potential for
catastrophic failure. Adding grease may provide temporary relief, but a
noisy bearing should be closely monitored and replaced as quickly as
possible. The root of the failure should also be investigated either
with independent or manufacturer failure analysis (manufacturer
analysis requires removal of the bearing as soon as possible to aid in
a more accurate diagnosis of the problem).
Lubrication Myth #4:
Any grease will do.
FALSE: Not all types of grease are the same. Some grease can be
incompatible because of the different thickeners (soaps) used. When two
incompatible greases are mixed, they may thicken and harden or become
thin and leak out of the bearing. For example, many electric motors use
a polyurea thickener and some mounted ball bearings use lithium-complex
thickeners. This grease is borderline compatible, and depending upon
the final make up, may or may not work together. Grease types can also
be incompatible based on the viscosity or type of the oil in the
grease, so consulting with a lubrication supplier is always recommended.
Lubrication Myth #5:
Simply shoot grease through the grease fitting.
FALSE: Before putting grease into a system, it's recommended to fully
wipe the grease fitting and ensure that the grease gun is clean. One
good practice is to put the grease gun tip in an oil bath or wrap it
with a plastic cover to protect it.
Misapplication Myth #1:
Bearings will not be hot to the touch.
FALSE: Normal bearing operating temperatures can range from 80 degrees
to 150 degrees Fahrenheit, but certain applications may run higher or
lower than others. Most bearings are rated for -20 degrees to 220
degrees Fahrenheit, but can be supplied with special grease, seals or
heat stabilizing processes that allow them to operate at higher
temperatures.
Bearings typically run hotter at start up or right after re-lubrication
because excess grease increases drag and friction in the bearing. The
bearing will typically reach steady state operating conditions, as
excess grease is pushed out by the rolling elements and purged from the
seals. Spikes of up to 50 degrees Fahrenheit at start up and a spike of
30 degrees Fahrenheit can occur after re-lubrication.
Misapplication Myth #2:
Bigger bearings are always better.
FALSE: Bigger bearings have a higher load capacity, which may show a
higher bearing fatigue life. If the load isn't high enough to achieve
the minimum load requirement, however, the rolling elements can skid
along the raceway instead of roll. Skidding along the raceway may
result in high operating temperatures, excessive wear, lubrication
breakdown and subsequent bearing failures.
Misapplication Myth #3:
Sealed/lubed-for-life bearings will last forever.
FALSE: The bearing life will depend on the grease life, which is
affected by operating conditions (speed and load) and environment
(temperature and contamination). Various things can be done to improve
grease life, such as enhanced seals, proper installation practices and
proper grease selection. Ultimately, the best bearing is the properly
lubricated bearing.
If you have a bearing that doesn't achieve the desired bearing
operational life, consult a bearing manufacture that can assist in
properly selecting the bearing for the application.
This feature was previously published in REM
magazine. Ian A. Rubin is marketing manager, mounted bearings for
Sealmaster and Browning-branded products at Emerson Power Transmission
Solutions. For information, visit www.emerson-ept.com.
It’s a whole new world for maintenance and engineering technicians. The use of mobile computing devices is on the rise, replacing the traditional pen and clipboard. Is it right for your facility?
When
it comes to mounted bearing installation and maintenance, there are
many industry myths and misconceptions that may affect plant uptime and
overall performance. Maintenance managers need to be aware of these
elements, so that they can maximize performance and keep the plant
running.
