19-01-2013, 11:53 AM
Introduction to Belt drives
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Instructional Objectives:
At the end of this lesson, the students should be able to understand:
• Uses and advantages of belt drives
• Types of belt drives and their nomenclature
• Relationship between belt tensions
• Some commonly used design parameters
Flexible Machine Elements
Belt drives are called flexible machine elements. Flexible machine elements are used for a large number of industrial applications, some of them are as follows.
1. Used in conveying systems
Transportation of coal, mineral ores etc. over a long distance
2. Used for transmission of power.
Mainly used for running of various industrial appliances using prime movers like electric motors, I.C. Engine etc.
3. Replacement of rigid type power transmission system.
A gear drive may be replaced by a belt transmission system
Flexible machine elements has got an inherent advantage that, it can absorb a good amount of shock and vibration. It can take care of some degree of misalignment between the driven and the driver machines and long distance power transmission, in comparison to other transmission systems, is possible. For all the above reasons flexible machine elements are widely used in industrial application.
Although we have some other flexible drives like rope drive, roller chain drives etc. we will only discuss about belt drives.
Typical belt drives
Two types of belt drives, an open belt drive, (Fig. 13.1.1) and a crossed belt drive (Fig. 13.1.2) are shown. In both the drives, a belt is wrapped around the pulleys. Let us consider the smaller pulley to be the driving pulley. This pulley will transmit motion to the belt and the motion of the belt in turn will give a rotation to the larger driven pulley. In open belt drive system the rotation of both the pulleys is in the same direction, whereas, for crossed belt drive system, opposite direction of rotation is observed.
Belt tensions
The belt drives primarily operate on the friction principle. i.e. the friction between the belt and the pulley is responsible for transmitting power from one pulley to the other. In other words the driving pulley will give a motion to the belt and the motion of the belt will be transmitted to the driven pulley. Due to the presence of friction between the pulley and the belt surfaces, tensions on both the sides of the belt are not equal. So it is important that one has to identify the higher tension side and the lower tension side, which is shown in Fig. 13.1.3.
Elastic Creep and Initial Tension
Presence of friction between pulley and belt causes differential tension in the belt. This differential tension causes the belt to elongate or contract and create a relative motion between the belt and the pulley surface. This relative motion between the belt and the pulley surface is created due to the phenomena known as elastic creep.
The belt always has an initial tension when installed over the pulleys. This initial tension is same throughout the belt length when there is no motion. During rotation of the drive, tight side tension is higher than the initial tension and slack side tension is lower than the initial tension. When the belt enters the driving pulley it is elongated and while it leaves the pulley it contracts. Hence, the driving pulley receives a larger length of belt than it delivers. The average belt velocity on the driving pulley is slightly lower than the speed of the pulley surface. On the other hand, driven pulley receives a shorter belt length than it delivers. The average belt velocity on the driven pulley is slightly higher than the speed of the pulley surface.