Page 182 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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Therefore, a small pitch chain is preferred for a light load and high-speed chain drive,
whereas a large pitch chain is better for a heavy load and low-speed chain drive. Gen-
erally, to transmit large power at a high speed, a multiply stranded chain with a small
pitch is recommended. From the aspect of cost, a small pitch, multiply stranded chain is
suitable for small centre distances and large speed ratio transmission while a large pitch,
single-stranded chain is for large centre distance and small speed ratio transmission [2].
A multiply stranded chain needs less radial but more axial space than a single-stranded
chain does.
7.4.4.4 Determine the Centre Distance Between the Sprocket Shafts, a and Chain
Length, L
p
A short centre distance provides a compact design and allows for a shorter, less expen-
sive chain. However, a shorter chain also means more articulation and thus more wear
and short life. On the other hand, large centre distance easily causes whip and surge in
the slack side [5]. Therefore, the initially selected centre distance should be within 30–50
pitches, that is, a = (30–50)p, and the maximum value is no greater than 80p,thatis,
0
a max = 80p.
The length of chain is generally measured in pitches, expressed by the number of links
L . The initial value L canbecalculatedby[2]
p
p0
2a 0 z + z 2 ( z − z 1 ) 2 p
2
1
L p0 = + + (7.28)
p 2 2 a 0
Since the length of chain must be an integer multiple of pitch and an even number of
links is recommended to avoid an offset link, the initial value L should be then rounded
p0
to a whole number L .
p
The theoretical centre distance for a given chain length, expressed in pitches is calcu-
lated by
√
( ) ( ) 2 ⎤
⎡
p z + z 2 z + z 2 ( z − z 1 ) 2
1
1
2
a = ⎢ L − + L − − 8 ⎥ (7.29)
p
p
4 ⎢ 2 2 2 ⎥
⎣ ⎦
The theoretical centre distance assumes no sag on either the tight or slack side of
the chain. However, the actual centre distance should be adjustable to accommodate
variation in chain length due to sag and wear. The allowance is usually selected as
Δa = (0.002–0.004)a. Therefore, the actual centre distance is
′
a = a −Δa (7.30)
7.4.4.5 Select an Appropriate Lubrication According to the Speed of Chain
Lubrication method is selected from Table 7.1 according to the speed of chain calculated
by Eq. (7.4).
7.4.4.6 Forces Acting on the Shaft
The force acting on the shaft F is calculated approximately by
Q
F ≈ K F (7.31)
Q e
Q