Page 318 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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Angular Contact Bearings, 70000, 30000
Angular contact ball bearings (70000, Figure 11.3e) carry both radial loads and unidi-
rectional axial loads, while tapered roller bearings (30000, Figure 11.3f) are designed
to take substantial radial loads, axial loads and their combinations compared with
angular contact ball bearings. Both bearings generate an induced thrust load even
though no external axial load is present. To counteract axial loads, these bearings are
usually employed in pairs, mounted in opposition either face to face or back to back.
Tapered roller bearings are separable and highly suited for combined radial and axial
loads.
Thrust Ball Bearings, 50000
Thrust ball bearings carry axial loads acting in one direction (51000, Figure 11.3c) or in
both directions (52000). Several types of thrust bearings are commercially available.
Shaft speeds must be low, otherwise the centrifugal force on the bearing becomes
unacceptable.
Self-aligning bearings, 10000, 20000, 29000
Self-aligning bearings are made to compensate for an appreciable misalignment
between shaft and housing due to shaft deflections, mounting inaccuracies or
other causes commonly encountered in assembly or in service. A double-row of
self-aligning ball bearings (10000) consists of a double row of balls running on a
circumferential inner raceway. The outer raceway is spherically ground to tolerate
substantial angular misalignment of shaft. A double row of self-aligning roller
bearings (20000, Figure 11.4) has a similar structure, but with a greater radial load
carrying capacity compared to a self-aligning ball bearing. Self-aligning thrust

bearings (29000, Figure 11.3d) are a better choice when both self-aligning capability
and combined radial and axial load carrying ability are required.
Extensive descriptions of many types of bearings are available in manufacturers’ cat-
alogues or references [6, 7] (http://www.machinedesign.com).

11.1.3.3 Bearing Type Selection
While selecting bearing types, load carrying capacity, speed limitation, misalignment
capability, space allowance and cost are the factors that need to be considered and
weighted.
The load magnitude and direction a bearing will take is the main consideration in
bearing selection. Generally, roller bearings have a higher load carrying capacity than
comparably sized ball bearings. If loads are mainly radial loads, deep-groove ball bear-
ings (60000), cylindrical roller bearings (N) and needle bearings (NA) can be selected,
with an increasing radial load carrying capacity. If loads are mainly axial loads, thrust
bearings are selected. Thrust ball bearings (50000) are used for small axial loads, while
thrust roller bearings (29000) are used for great axial loads. If both radial and axial
loads are to be taken, deep-groove ball bearings (60000), angular contact ball bearings
with small contact angles (70000A, 70000AC), or tapered roller bearings (30000) can be
selected for relatively small axial loads, while angular contact ball bearings with a large
contact angle (70000B), tapered roller bearings with a large contact angle (30000B) or
the combination of radial and thrust bearings are preferred for greater axial loads.
Most catalogues list limiting speeds for each bearing. The limitation is for linear sur-
face speed rather than rotating speed; hence, small bearings can operate at higher rotat-
ing speeds than large bearings [4]. Exceeding these limits may result in excessively high
operating temperatures due to friction. Generally, point contact in ball bearings leads