Machine Design

Radial Bearings

Conrad or deep-groove bearings are the most widely used ball bearings. In addition to radial loads, they carry substantial thrust loads at high speeds, in either direction. They require careful alignment between shaft and housing.

The ball complement of a deep-groove bearing is limited to the number of balls that can be packed into the annular space between the rings with the inner ring displaced radially. The inner ring is then snapped into place, and the retainer assembled around the balls. This limits the load-carrying ability of the bearing, since capacity is proportional to the number of balls to the 2/3 power.

Although the majority of deep-groove bearings are made to ABEC-1 specifications, higher-precision bearings are readily available. Unless the application involves high speeds or some other unusual feature, ABEC-1 bearings normally suffice. Additional expense for higher-grade bearings is considerable.

Maximum capacity or filling notch bearings have about 20 to 40% more radial load capacity than deep-groove bearings. The increased capacity comes from additional balls inserted through the filling notch. Except for the filling notch and the extra balls, they are geometrically identical to deep-groove bearings.

But the same filling notch that permits an increased radial load cuts thrust capacity to about one-third that of deep-groove bearings. At higher thrust loads, balls start to contact the notch. For the same reason, tolerance to misalignment is considerably less than that of a deep-groove bearing.

Magneto or counterboredbearings are widely used in small motors. Except for the removal of one shoulder of the outer ring, they are identical to deep-groove bearings. The separable feature permits separate mounting of inner and outer rings. Radial and thrust capacities range from slightly less to about 30% more than deep-groove bearings.

Airframe or aircraft-control bearings transmit heavy radial loads under intermittent oscillation. They are made in a number of different types and styles to meet special needs, but the majority are full-complement cageless bearings.

Self-aligning bearings come in internal and external types. Internal bearings have an outer-ring ball groove ground to a spherical surface. This surface makes the bearing insensitive to misalignment, but increases raceway contact stresses, which cuts load capacity.

Externally self-aligning bearings have a spherical surface on the outside of the outer ring, which matches a concave spherical housing. External bearings have better load capacity than internal, but demand greater radial space.

Double-row, deep-groove bearings embody the same principle of design as do single-row bearings. However, grooves for the two rows of balls may be positioned so that the load lines through the balls converge outwardly or inwardly. Inwardly converging bearings deflect more under moment loading, but can stand more misalignment. Some bearings, particularly the inwardly converging type, have filling notches so extra balls can be added to increase load capacity. These filling notches limit the thrust capacity in one direction.

Double-row bearings can be used where high radial and thrust rigidity is needed and space is limited. They are about 60 to 80% wider than comparable single-row deep-groove bearings and have about 50% more radial capacity.

TAGS: Bearings
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