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Maintenance technicians at a glass fabrication plant recently witnessed
first hand how high temperatures can affect and potentially damage rolling
bearings. Bearings in a fan used to evacuate superheated air during the
glassmaking process began to overheat. Bearing temperatures, which normally
hovered around 170°F (77°C), climbed to 195°F (91°C). While
the fan continued to run, plant technicians consulted with a bearing engineer
to devise a solution. But their efforts came too late: by the time the
meeting ended, the grease inside the bearing had dried up and smoke had
begun to emanate from the bearing, causing shutdown. |
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Failure analysis quickly pinpointed a cause: process temperatures of 1000°F
(538°C) or more produced in the glassmaking process resulted in an
ambient temperature of 220°F (104°C). The plant immediately took
steps to shield fan bearings mechanically from the worst of this heat.
In addition, the "floating" bearing in the fan arrangement was offset in
the housing, providing it with more room to travel axially to accommodate
shaft expansion. |
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Higher-than-normal operating temperatures, whether caused by ambient conditions
or generated within the bearing itself, have the potential to harm rolling
bearings. Normal operating temperatures differ, depending on the application.
Maintenance technicians should be aware of this and know the common causes
of, and remedies for, bearing overheating. |
| Electric Motors |
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The ball bearings used in most electric motors are pre-greased, shielded
ball bearings. Normal motor bearing operating temperatures range from 140°F
(60°C) to 160°F (71°C). |
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Overheating in electric motor bearings is generally lubricant-related.
For example, when relubricating open bearings, users may inadvertently
employ a low-temperature grease which does not provide adequate viscosity
at the normal operating temperature. Or the user may over-grease the bearing,
forcing bearing balls to push through excess grease as they rotate, leading
to a sharp temperature rise. Another cause of overheating is mixing incompatible
greases, which can reduce the consistency of the grease and possibly the
overall viscosity. |
| Fans |
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Commercial fans generally utilize ball and roller bearings mounted in cast
iron or pressed steel housings. Fans are exposed to a wide variety of ambient
conditions, ranging from below-zero temperatures for rooftop fans to extremely
high temperatures for fans used in industrial processes. |
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Normal bearing operating temperatures vary, depending on the environment
and application. The standard grease in most fan bearings remains effective
to an operating temperature of 180°F (82°C). If steady-state operating
temperatures are higher than 180°F (82°C), consider using a grease
with a synthetic base oil. Viscosity in a synthetic oil does not vary as
much with temperature as in a standard mineral oil, and the rate of oxidation
is much slower. For operating temperatures above 200°F (93°C),
a circulating oil system may be needed. These systems pump clean, cool
oil through a bearing arrangement. |
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In hot-gas fans, special measures must be taken to protect bearings from
high temperatures. In virtually all cases, an aluminum disk or flinger
placed on the shaft between the bearing and the fan casing can act as a
heat shield. Often, a blower wheel or compressed air can be used to direct
cooling air across the bearing housing or the shaft. |
| Pumps |
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Depending on the application, normal bearing operating temperatures in
pumps range from 100°F (38°C) to 180°F (82°C), with most
running between 140°F (60°C) and 160°F (71°C). Although
grease is used in some vertical pumps, oil is the preferred lubricant in
the majority of pump applications. Standard bearing oils in pumps remain
effective to approximately 180°F (82°C). If normal operating temperatures
are higher than 180°F (82°C), a synthetic oil should be used; if
temperatures exceed 200°F (93°C), a circulating oil system will
probably be required. |
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As in other bearing applications, higher-than-normal operating temperatures
in pumps can be caused by bearing overlubrication. Overheating can also
be caused by bearing misalignment or ball skidding within the bearing.
Specially designed bearings are available to eliminate ball skidding. Ideally,
bearing temperatures in pumps, especially those in critical applications,
should be regularly monitored. |
| Gear Drives |
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Bearings in gear drives normally operate at 160° (71°C)-180°F
(82°C) and are lubricated with static oil systems. As improved technology
permits reductions in the size of gear drives, there is a growing trend
to transmit more power through a given size drive than ever before. This
practice can cause bearings in gear drives to run hotter and may necessitate
the use of alternative cooling methods. |
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In summary, proper bearing lubrication is the primary concern in all high-temperature
applications. That concern is heightened by the trend of running industrial
equipment at higher speeds than originally intended, further increasing
bearing temperatures. The general rule is to provide the minimum viscosity
required at the expected operating temperature: 100 SUS (20cst) for roller
bearings and 70 SUS (13cst) for ball bearings. In addition, the increased
thermal expansion of the shaft must be accounted for both axially (to ensure
that high thrust loads are not induced) and radially (to ensure that radial
internal clearance is adequate to avoid preload). The solution may also
entail using a grease with a synthetic base oil or converting to a different
lubricant delivery system, such as circulating oil. |
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This article is furnished
courtesy of PTDA
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