Page 143 - Analysis and Design of Machine Elements

Figure 6.1 A typical power transmission Belt Drives 121
system. Electric motor

Belt drive

Gear reducer

Chain drive Coupling

Driven machine V

Since belt drives are used to transmit motion over comparatively long distances, the
toleranceonthe centre distance is notascriticalcomparedwithgeardrives, andthe
layout of driving and driven shafts has considerable flexibility. Although a belt may slip
when it overloads, it provides a measure of safety for the driver and driven machines. Belt
drives also have advantages of smooth operation, simple structure and shock absorption.
In many cases, their use simplifies machine design and substantially reduces manufac-
turing and maintenance costs [1].
However, except for timing belts, the speed ratio of a belt drive is inaccurate due to
inevitable elastic creep between belt and pulleys. Belt slippage may also shorten belt life.
Besides, the power and torque transmission capacities are limited by the coefficient of
friction and interfacial pressure between belt and pulleys [2]. To ensure proper function-
ing, a belt must be mounted on pulleys with proper initial tension, and belt-tensioning
devices are essential to maintain the initial tension in belt drives.
A typical belt drive consists of a driving pulley 1, a driven pulley 3 and a continuous
belt 2 (Figure 6.2). A belt is a flexible band passing around two wheels that transmits
motion and power from one to another. Wheels with a flat profile are called pulleys,
while wheels with grooves are termed sheaves [3]. When a belt drive is used for speed
reduction, which is the usual case in industry, the small driving pulley is mounted on
the high-speed shaft while the large driven pulley is connected to the driven machine
shaft. The speed reduction will cause proportional increase of torque. Since belts are
virtually endless loops, machines must have provision to allow belt adjustment and
replacement.
Figure 6.2 shows two typical layouts of belt drives, that is, an open-belt drive
(Figure 6.2a) and cross-belt drive (Figure 6.2b). An open-belt drive is used when
the driven pulley is to be rotated in the same direction as the driving pulley. A cross-belt
drive is adopted when the driven pulley is to be rotated in the opposite direction to the
driving pulley.

6.1.2 Types of Belts
Because of cost and efficiency, several types and sizes of belts are available for different
load levels. There are primarily four types of belt configurations, that is, flat, V-, round