Maintenance Technology October 2015 - (Page 36)
7 Ways to Avoid
ROLLING BEARINGS ARE manufactured with
extreme accuracy. For them to perform at their
best, great attention must be paid to the mounting
and maintenance process. If not, misalignment
may occur, leading to bigger problems down the road, including
bearing seizure, pump failure, and costly downtime.
Over time, a well-aligned bearing will exhibit a running path
down the center of the inner and outer rings, while a misaligned
bearing will exhibit uneven running paths. The technical experts
at bearing manufacturer NSK Americas, Ann Arbor, MI, offer
these helpful tips for preventing and alleviating bearing misalignment in your pump applications.
-Jane Alexander, Managing Editor
Typical running paths
of deep-groove ball bearings
Inner ring rotation
with radial load
Inner ring rotation
with moment load
Inner ring rotation
with axial load
The diagram shows two typical misalignment situations,
compared with a properly aligned bearing. Correctly
aligned bearings will exhibit a running path down the
center of the inner and outer rings.
1. Shaft and housing: The shoulders of the shaft
or housing that contact the face of a bearing must be
orthogonal to the shaft center line. The fillets should not
contact the bearing chamfer, while the supporting shoulder
diameter still must be large enough to fully support the face
of the bearing.
2. Clean installation: During installation, all mating
surfaces should be cleaned and all shaft and shoulderabutting surface edges should be free of burrs. Bearing
mounting methods will vary depending on the bearing
type and the type of fit.
3. Proper fit: Most bearings require a tight fit, either with
the housing or, most commonly, with the shaft. Depending
on the type of fitting surface, different mounting techniques
are required. Typically, bearings with cylindrical bores are
mounted by pressing through the inner ring onto the shaft
(press fit) or heating them to expand their diameter (shrink
fit), while bearings with tapered bores can be mounted
directly on tapered shafts or on cylindrical shafts using
tapered sleeves. Always apply a light film of oil on the fitting
surfaces first to prevent scoring. When pressing a bearing
into a housing, push on the outer ring of the bearing. When
pressing onto a shaft, push on the inner ring.
4. Nylon cages: If you use ball bearings where some
degree of misalignment is unavoidable, consider those
with nylon cages. Their flexibility allows them to endure
misalignment better than steel cages.
5. Internal clearance: Increasing a bearing's internal
clearance will increase its tolerance for misalignment and
help mitigate alignment issues.
6. Ball bearings: Self-aligning ball bearings may also be a
good alternative. They have a spherical outer ring raceway
that allows the axis of the inner ring, balls, and cage to
deflect around the bearing center.
7. Follow directions carefully: Rigorously following
catalog-recommended assembly tolerances and installation
processes is the best way to prevent bearing misalignment.
Remember that reducing or eliminating misalignment is
critical for longer bearing and system life. If misalignment
cannot be completely avoided in an application, additional
calculations are required to determine the effect this condition will have on the life of the bearings. MT
For more information from NSK experts, including assistance in calculating the impact of
unavoidable misalignment on bearing life and other application analyses, visit nskamericas.com.
Table of Contents for the Digital Edition of Maintenance Technology October 2015
For on the Floor
Crushing Limestone with Reliability
Choose Reliability or Cost Control
Get To The Root of the Cause
Select the Right Safety Logic System
Simple Purchasing Practices Incite Lubrication Failure
CMMS Upgrade Tips
Maintenance Technology October 2015