Page 320 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
298
last two digits is the bore dimension code indicating the size of bore. For bore sizes of
04 and above, the nominal bore dimension is five times the bore dimension code in
millimetres. For bore sizes of 00, 01, 02 and 03, the nominal bore dimensions are 10,
12, 15 and 17 in millimetres, respectively.
Post-code uses letters and numbers to designate structure, tolerance and material of
bearings. Items in post-codes are in the sequence of internal structure code, seal and
shield structure code, retainer and its material code, special bearing material code, tol-
erance class code, clearance code and so on. Here, we just introduce some of the most
commonly used codes.
Internal structure code indicates the same type of bearings with different structures.
∘
∘
∘
For example, angular contact ball bearings with contact angles of 15 ,25 and 40 use
C, AC and B to indicate the differences and a tapered roller bearing with contact angles
∘
of 27–30 uses B to indicate the differences.
Manufacturing tolerances are extremely crucial for precise operation and have been
standardized. Six tolerance classes are commonly used in the bearing industry to accom-
modate the needs of a wide variety of equipment using rolling contact bearings. From
fine to coarse tolerances, tolerance class codes are P2, P4, P5, P6X, P6 and P0,respec-
tively. P0 is the ordinary tolerance, adequate for most normal applications, and could be
omitted in bearing code.
Generally, internal clearances are designated from the tightest clearance C1 through
to the loosest or largest clearance C5. The normal clearance is 0, which sits between C2
and C3. It is worth noting that if a bearing clearance is not explicitly stated, it is assumed
to have a normal clearance.
As an example, bearing code 6308 represents a medium series deep-groove ball
bearing with the bore of 40 mm, ordinary tolerance and normal clearance. As another
example, bearing code 7211C/P5 indicates a light series angular contact ball bearing
∘
with the bore of 55 mm, contact angle of 15 , class 5 tolerance and normal clearance.

11.2 Working Condition Analysis

11.2.1 Kinematic Analysis

Assume the rotational speed of the inner ring is n , from Figure 11.6, the angular velocity
1
of the inner ring is
1
2 n 1
= (11.1)
1
60
Figure 11.6 Velocity analysis.