Page 185 - Analysis and Design of Machine Elements

Results
Steps Computation Chain Drives 163
Units
7. Calculate chain From Eq. (7.4) v avg = 8 m s −1
speed and decide v = z n p = 21 × 900 × 25.4 = 8ms −1
1 1
lubrication avg 60 × 1000 60 × 1000
method From Table 7.1, we can select spray lubrication
8. Forces acting The effective force is calculated from Eq. (7.15) as F = 1438 N
Q
on the shaft 1000P 1000 × 10
F = = = 1250N
e
v 8
For horizontal transmission select K = 1.15. From Eq. (7.31),
Q
the force acting on the shaft is
F ≈ K F =1.15 × 1250 = 1438 N
Q Q e
9. Contact angle From Eq. (7.3), the contact angle on the small sprocket is = 152.8 ∘
∘
on the small = 180 − z − z 1 p × 57.3 ∘
2
sprocket 1 a
∘ 81 − 21 ∘ ∘
= 180 − × 25.4 × 57.3 = 152.8
1
× 1021
∘
Because this is greater than 120 , it is acceptable.
10. Sprocket Omitted
structure
and dimension
7.5 Drive Layout, Tension and Lubrication
7.5.1 Drive Layout
Due to gravity, the preferred arrangement for a chain drive is with the centreline of
sprockets horizontal and with the tight side on the top to prevent chain pinching
between sprockets (Figure 7.9a). Vertical drives should be avoided. If unavoidable,
the centreline should be inclined (Figure 7.9b). The rotation of sprocket should let
accumulated slack away from the tight strand. Excessive sag on the slack side should be
avoided, especially on drives that are not horizontal. Otherwise, a idler should be used.

7.5.2 Tensioning
Periodic maintenance for adequate tension is important for proper operation and long
life of chain drives. Since wear is inevitable after long service, wear-induced elongation
must be compensated to avoid improper mesh or whip in the slack strand.
Tensioning can be realized either by increasing the centre distance between sprockets
or by using idlers. Adjusting centre distance is often a recommended approach. How-
ever, for a fixed centre distance, chain idlers or a small adjustable idler sprocket should
be provided to remove excessive slack due to wear. Figure 7.10 illustrates the arrange-
ment of idlers for a horizontal and vertical chain drive. The idlers should be located on
the outside of chain on the slack side, close to the driving or driven sprockets to increase
contact angle [9].