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Unformatted text preview: n the crankshaft.
1. Bending stress ; and 2. Shear stress due to torsional moment on the shaft.
* The values of maximum permissible bearing pressures for different types of engines are given in
Chapter 26, Table 26.3. 1162 n A Textbook of Machine Design Most crankshaft failures are caused by a progressive fracture due to repeated bending or reversed
torsional stresses. Thus the crankshaft is under fatigue loading and, therefore, its design should be
based upon endurance limit. Since the failure of a crankshaft is likely to cause a serious engine
destruction and neither all the forces nor all the stresses acting on the crankshaft can be determined
accurately, therefore a high factor of safety from 3 to 4, based on the endurance limit, is used.
The following table shows the allowable bending and shear stresses for some commonly used
materials for crankshafts :
Table 32.2. Allowable bending and shear stresses.
Material Endurance limit in MPa
Bending Shear Allowable stress in MPa
Bending Shear Chrome nickel
Carbon steel and cast steel 525
124 130 to 175
56 to 75 72.5 to 97
31 to 42 Alloy cast iron 140 140 35 to 47 35...
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- Spring '11