Page 100 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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4.6 implies that the tensile strength is 400 MPa, and the ratio of yield strength to tensile
strength is 0.6. Nuts are graded to mate with their corresponding class bolts.
The American Society for Testing Materials (ASTM), the Society of Automo-
tive Engineers (SAE) and the Standardization Administration of China (SAC) have
established similar standard specifications for materials and strength levels for
threaded fasteners. Designers can refer to relevant standards or design handbooks
while designing [6, 7, 13].
The allowabletensilestressisdecidedby
s
[ ]= (3.30)
S
where yield strength is selected from Table 3.2, and safety factor S is selected as 4–1.3
s
for M6–M60 carbon steel bolts and 5–2.5 for M6–M60 alloy steel bolts, respectively.
When preloaded bolted connections carry variable loads, safety factors may be twice
the values of those for static loading [7].
The allowable bearing stress is calculated by
s
[ ]= (3.31)
p
S
p
where the safety factor for crushing S is 1.25 for steel and 2.5 for cast iron [8].
p
The allowable shear stress is calculated by [8]
s
[ ]= (3.32)
2.5

3.6.3 Design Criteria
For a tension bolt that is subjected to a preload, or a static axial load or a combination
of both, it must meet the strength requirement by ≤ [ ], as expressed in Eqs. (3.12,
3.15, 3.21), depending on the loading conditions. When a tension bolt carries a fluctu-
ating load, fatigue strength S ≤ [S] should also be guaranteed, as expressed by Eq. (2.30).
Besides, to ensure safe and proper functioning of bolted joints, the preload induced by
bolt tightening must be great enough to prevent joint separation during operation.
The design criterion for a shear bolt is to guarantee crushing strength ≤ [ ]
p
p
expressed by Eq. (3.10), and shear strength ≤ [ ] by Eq. (3.11).
3.6.4 Design Procedure and Guidelines
Although multiply bolted joints usually work together in practical engineering, the fail-
ure of a single fastener in a group can be destructive or even catastrophic. Designers
must select and decide the type, material, property class and size of standard fasteners
to ensure all of them will most adequately suit the application. The following provides
the procedure and guidelines for bolted joint design:

1. Decide the pattern of bolt layout and the number of bolts in the group. Ideally, it is
better to ensure each bolt within the group is uniformly loaded, or to minimize the
maximum load a bolt carries.
2. Analyse operating loads on the bolted joint. Dissolve the loads into simple load con-
ditions; that is, tensile loads, shear loads, torques and moments.