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"Sleeve
bearings: modern use of an old technology"
With
trilingual summary
The
sleeve bearing (often called a sliding or journal bearing) is one of the oldest
known machinery components. Until the 1930's, it was preferred for use in all
electric motors. It remains the most reliable bearing for large machines, most
often as a steel cylinder lined with a tin-based alloy as the bearing surface
and split into two halves for easy repair or replacement.
Unlike
the ball or roller ("anti-friction") bearing, a sleeve bearing offers
indefinitely long life if properly lubricated and cooled. The lubricant is oil,
which forms a thin pressurized film under the rotating shaft journal to prevent
metal-to-metal contact. Shaft-riding rings carry oil up from a sump below the
bearing; large, high-speed bearings require an external pressure system.
Proper
clearance--the space between shaft and bearing liner where the oil film
develops--is crucial to bearing performance. Radial clearance typically ranges
from 0.008 to 0.012 millimeter per centimeter of shaft diameter,
Friction
does occur within the oil film. The accompanying power loss, significantly
greater than in an anti-friction bearing of comparable size, depends upon shaft
speed, bearing load, and oil viscosity. Pressure lubrication, to cool the oil
externally, is required for bearings when journal surface speed exceeds
approximately 10 meters per second.
Motor
manufacturer recommendations for oil viscosity and chemistry vary widely,
particularly with motor speed and ambient temperature. Also variable are
suggested oil change intervals, often between six months and one year, depending
upon oil condition.
Modern
bearings have length-to-diameter ratios of 1.0 or less, rendering them less
susceptible to stress concentrations resulting from misalignment or natural
shaft deflection. The ratio was often 1.4 or more in earlier designs.
Sleeve
bearings lack the high load-carrying capacity of the anti-friction type. But if
too lightly loaded, or with excessive clearance, sleeve bearings are subject to
oil whirl, in which the shaft centerline oscillates, leading to destructive
vibration.
Repair of
a damaged or worn bearing, though not difficult, can be time-consuming.
Nevertheless, sleeve bearings remain the customary choice for motors above 750
kW, and even lower at 3,000-3,600 RPM.
From “Sleeve
bearings: modern use of an old technology," published in the
Electrical Apparatus February 2008 issue
. Visit our online webstore to
order
copies. © 2008
Barks Publications, Inc. All Rights Reserved.
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