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Summary:
“Applying
vertical tilting pad thrust bearings" With
trilingual summary
In
horizontal-shaft electric motors, the operating principle of the
sleeve or journal bearing is well known, involving formation of a
wedge-shaped oil film between the rotating steel shaft and a
stationary babbitt-lined supporting structure. Re-shaping those
cylindrical components into parallel flat surfaces, separated by a
similar wedge of oil, creates a plate-type or <I>tilting pad<P>
thrust bearing. Such bearings are used in large vertical-shaft motors
when the bearing loading exceeds the capacity of anti-friction
bearings such as the spherical roller type.
To maintain the
oil wedge, the babbitt surface is divided into a series of segments
(pads or shoes) that can tilt slightly. Most bearings are of the
"equalizing" type, in which the pads rest on an interlocking ring of
leveling blocks such that irregularities that depress one pad cause
adjacent pads to rise slightly, equalizing the load carried by each.
Thrust load is applied to the pads by a rigid steel or cast iron
thrust block or runner rotating with the shaft. Runner faces are
precisely machined and polished to form a smooth, flat surface.
Whereas the
allowable load on an anti-friction bearing decreases with increasing
speed (because higher rpm means more rapid fatigue cycling), the
reverse is true for a tilting pad bearing, because higher speed
enhances oil wedge formation.
Fluid friction
within the oil causes tilting pad bearing heat loss to exceed that in
a roller bearing, usually requiring water cooling of the lubricant.
Cooling coils consist of multiple turns of either plain or finned
copper tubing, depending upon the heat loss, temperature limit
(typically between 100 and 120øC), available water temperature, and
the space within the bearing oil sump. Bearing pads are usually
fitted with temperature sensors.
Because
anti-friction thrust bearings will sustain some radial load, they
maintain the vertical centering of the shaft. To perform that
function when a tilting pad bearing is used, the assembly includes a
pilot or guide bearing with similar pads or shoes surrounding the
thrust runner. A second guide bearing (which may be a ball bearing)
keeps the lower end of the shaft equally centered.
From
“Applying
vertical tilting pad thrust bearings" published in Electrical Apparatus
November 2005
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