Page 180 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
158
Therefore, the required rating of a chain is
K K z
A
P ≥ P (7.24)
0
K p
where K is service factor, considering the inherent characteristics of power sources and
A
driven machines, operating conditions and duration of service. Service factors slightly
higher thanthelowestvaluesinthe recommendedrangeinTable 2.1can be selected for
a given application.
−1
For low speed, that is, v < 0.6 m s , or heavily loaded chain drives, the failure mode
may be static breakage. The design is then followed strength criteria, that is,
QN
S = ≥ 4 ∼ 8 (7.25)
ca
K F
A 1
where
N – the number of strands;
Q – limiting tensile load of a single-strand chain, N, which can be found in design
handbooks or manufacturer’s catalogues;
F – tensile force on the tight side of chain, N.
1
7.4.4 Design Procedure and Guidelines
This section introduces a systematic step-by-step procedure and suggestions for the
design of medium- to high-speed chain drives. The design of low speed chain can refer

to relevant design handbooks [9] following relevant design criteria.

7.4.4.1 Tentatively Select the Number of Sprocket Teeth z and Speed Ratio i
The number of teeth of sprockets, z, should be within a proper range. When the size of
the driving sprocket is constant, small sprocket teeth number z leads to a large pitch p
1
and consequently increases the undesirable effect of chordal action, noise and dynamic
force. It will also increase pitch angle , resulting in the increase of wear.
On the other hand, increasing small sprocket teeth number z will consequently
1
increase large sprocket teeth number z , resulting in a large chain drive and will make a
2
chain more likely to shift outwards on the sprocket teeth, as illustrated in Figure 7.8.

p+Δp Figure 7.8 Chain engagement
before and after wear.

p
D+ΔD
φ D