Page 142 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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Nomenclature n rotational speed of a pulley or
sheave, rpm
A cross sectional area of belt, mm 2 P transmitted power, kW
a centre distance, mm P 0 basic power rating of a single
b datum width of V-belt groove, mm standard V-belt, kW
d
b pitch width, mm ΔP basic power rating increment of a
p 0
D sheave or pulley diameter, mm single standard V-belt, kW
d datum diameter, mm P design power, kW
d ca
d pitch diameter, mm P actual power rating of a single
p r
E equivalent elastic modulus of belt, V-belt, kW
MPa Q radial force acting on the pulley or
F initial tension, N sheave, N
0
F tight tension, N q mass of belt per unit length, kg m −1
1
F slack tension, N r pulley radius, mm
2
F centrifugal tension, N v belt speed, m s −1
c
F effective tension, N v linear speed of the small pulley or
e 1
F critical or maximum effective sheave, m s −1
ec
tension, N z number of belt
∘
F frictional force, N contact angle, or rad
f
F force acting on the pulley or slip ratio
Q
sheave shaft, N wedge angle, ∘
f coefficient of friction tensile stress in tight side, MPa
1
f v equivalent coefficient of friction of 2 tensile stress in slack side, MPa
V-belt bending stress in a belt, MPa
b
h belt height, mm centrifugal stress in a belt, MPa
c
i speed ratio maximum stress in a belt, MPa
max
K service factor [ ] allowable stress of a belt, MPa
A
K correction factor for belt length
L
K correction factor for contact angle Subscripts

L belt datum length, mm 1 driving pulley or sheave
d
N normal force, N 2 driven pulley or sheave
6.1 Introduction
6.1.1 Applications, Characteristics and Structures
A belt drive represents one of the major types of flexible drive. They are extensively used
in automotive or industrial drives to reduce a higher rotational speed of one part to a
lower value over a considerable distance.
Figure 6.1 shows a power transmission system consisting of a belt drive, a gear drive
and a chain drive. The power required by the driven machine is provided by an elec-
tric motor. Since the electric motor usually operates at a high speed and delivers a low
torque, which is not appropriate for the driven machine, a power transmission system
is employed. In a power transmission system, belt drives are usually used at the first
stage from the power source. Gear reducers, together with other drives with properly
designed speed ratios, are used between the belt drive and the driven machine to satisfy
the speed requirement of the driven machine.