Efficient Plant April 2018 - 18

feature | maintenance solutions

Fig. 3. Angular-contact ball bearings

Vertical and
horizontal motors
are different animals,
with different
maintenance issues.
Fig. 4. Spherical roller bearing

thrust classifications do not apply.)
Angular-contact ball bearings
are the most common types of
thrust bearings. A single bearing
is used for normal thrust loads;
two bearings in tandem
(Fig. 3) for 175% of normal
thrust (placing two bearings
in tandem does not double the
thrust capacity); and a stack of
three bearings for 250% of normal
thrust (see table on p. 17). Stacks
of as many as five bearings have been
reported.
Spherical roller bearings (Fig. 4) are
used when even higher thrust loads are
applied, such as in very deep wells where
the weight of the water column, the pump
shaft, the impeller, and the force of moving
the water can be several thousand pounds.
Spherical roller bearings can handle 300%
to 500% of normal thrust load.
A hydrodynamic bearing, also known as
a tilting-pad, plate, or Kingsbury bearing
(Fig. 5), is capable of extremely high thrust
loads and very long life. As with sleeve
bearings in horizontal motors, there is theoretically no wear on the plates because
the load is carried on an oil film that
separates the moving parts.

SPECIAL THRUST CASE
Occasionally, pump designs
will exceed the momentary
up-thrust limits (more common on two-pole [3,000/3,600
rpm] applications), and the
guide bearing cannot accommodate that thrust for the necessary duration. In such cases,
the upper bearings are arranged
face-to-face or back-to-back. Note
that with back-to-back bearings thrust
transmission is in opposite directions (see
arrows in Fig. 6, p. 20). This configuration

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also requires additional parts. A holddown ring, for example, must be located
on the outer race of the upper bearing and
fastened to the upper bracket to prevent the
up-thrust from lifting the bearings from
the bearing bore. A nut or a spiral-lock
snap ring must also be applied to the bearing carrier below the lower bearing inner
race to prevent the up-thrust from pushing
the bearings off the mount.

OIL-LEAK ISSUES
Vertical motors are unique in their ability
to carry external thrust. The thrust bearings that make this possible require care in
assembly and application for optimum service and performance. Unfortunately, those
bearings also put vertical motors at risk
for several lubrication-related problems.
That's because many of them are lubricated
with oil-bath reservoirs, which, in turn,
can develop leaks that may be difficult to
diagnose and correct. Causes of oil leaks
include loose stand tubes (the part that defines the inner boundary of the reservoir),
moving the motor with oil in the reservoir,
over-filling the reservoir, water intrusion,
machine-tool marks, and foaming. The
following tips can help ensure leak-free
performance:
 When disassembling a leaky vertical motor, carefully inspect the space
between the shaft and the inside of the
stand tube. Oil observed in this area
narrows the scope of the investigation,
because the oil must go over the stand
tube to get there. Leaks of this type are
often caused by overfilling the reservoir
or by the sloshing action that results
from moving the motor with oil in the
reservoir. If the motor base is out of
level by more than a degree, the oil-level
indication in the sight glass will be inaccurate and could cause the oil to be above
the stand tube.

APRIL 2018
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