Bearing Failures Cause
Failures ofbearingsinmachinerycan cause large losses. A recent bear- ing failure in a large self-propelled combine harvester caused the loss
of the machine valued at over $400,000.
There is a specific failure mode of bearings that has arisen in recent years which
is associated with bearing failure, often
referred to as cage failure.
Figure 1 is a view of a typical ball bearing. The balls roll in the gap between the
inner and outer race guided by the cage,
which keeps the balls separated. Roller
bearings are similar in shape with the exception that the rolling elements are cylindrical in shape and are separated by a
cage that matches this geometry.
Figure 2 is a view of a ball bearing cage,
usually made of metal. Recently, plastic
cages were introduced by manufacturers
All images: Charles C. Robers, Jr. Ph.D., P.E.
due to advantages in cost, smooth operation and ease of manufacture.
Figure 3 shows a typical plastic cage
made of a polyamide such as nylon. A
problem with the plastic cage is that the
operating temperature of the bearing is
restricted when compared to the metal
cage. Equipment manufacturers may substitute a plastic caged bearing for a metallic caged bearing, ignoring the operating
temperature bearing limitation.
Just because a particular metallic caged
bearing has operated successfully in a
machine since it was designed, does not
mean a plastic caged bearing will operate in the same application. The metallic
caged bearings tolerate temperature fluctuations caused by loading and environmental conditions.
The high temperature limitation on poly-
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amides is typically about 160 F. In an application, a plastic caged bearing may not
tolerate a temperature fluctuation easily
handled by a metallic caged bearing. The
plastic cage can melt or soften, causing loss
of bearing roller element spacing.
Figure 4 depicts what happens when
the cage fails. Bearing balls lose the
proper spacing in the bearing and rub together, creating excessive frictional heating and movement of the shaft off center,
which can add to frictional heating with
other components. The bearing can reach
approximately 3000 F, causing a fire in the
machinery and significant damage.
Figure 5 shows a failed bearing that
started a fire, resulting in the total loss
of the combine harvester. The shaft is off
center and the rollers are missing.
Roller bearings are similar to ball bearings with the exception that the rollers are
cylindrical in nature with cages of similar
construction. It should be noted that the
cage in Figure 5 was not found, suggesting that it may have been a plastic cage
that was consumed in the fire.
Typically when a bearing fails and overheats, the metallic cage debris remains as
shown in Figure 6, a ball bearing cage, and
Figure 7, a roller bearing cage. When the
claims analyst is faced with a bearing failure
loss, the existence of cage remains may offer
a clue as to a deficiency in machine design.
The absence of cage debris may suggest that
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