Page 162 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
140
Steps Computation Results Units
6. Confirm initial From Eq. (6.30) )
(
tension, F 0 P ca 2.5 2
F = 500
0 zv K a − 1 + qv
8.25 ( 2.5 ) 2
= 500 × − 1 + 0.105 × 7.59 = 139 N
7 × 7.59 0.92
F = 139 N
0
7. Compute the force From Eq. (6.31)
acting on the sheave 1 150.9 ∘
shaft, F F = 2zF sin 2 = 2 × 7 × 139 × sin = 1884 N
0
Q
Q 2
F = 1884 N
Q
Select solid sheave
for driving sheave,
8. Structural design D = 100 mm
1
of V-belt sheaves D = 355 mm and spoke sheave
for driven sheave
2
6.5 Installation and Maintenance
At installation, the two sheaves are first moved closer to facilitate belt assembly. The cen-
tre distance is then increased to a designed value to generate initial tension in the belt.
A proper amount of initial tension must be well maintained. An excessive initial tension
shortens belt life and overloads bearings and shafts, while insufficient initial tension may
cause slippage and generate heat and wear, which may also reduce belt life. Besides, belts
stretch permanently and loose initial tension after a period of service. Therefore, a belt
drive must have provisions to allow for belt adjustment and replacement.
When the centre distance of a belt drive is fixed, a grooved idler is preferably used
on the inside of slack strand close to the large sheave to take up slack (Figure 6.13).
If the centre distance is adjustable, belt tension can be maintained by regular manual
tensioning or by the weight of pivoted, overhung motor [9]. Manufacturers’ catalogues
and design handbooks give details for proper belt-tensioning procedures.

Figure 6.13 Belt tensioning by an idler pulley.