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Note for Machine Dynamics - MD by Dr Ramesh Chandra Mohapatra

  • Machine Dynamics - MD
  • Note
  • Biju Patnaik University of Technology Rourkela Odisha - BPUT
  • Mechanical Engineering
  • B.Tech
  • 8 Topics
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  • Uploaded 8 months ago
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BIJU PATNAIK UNIVERSITY OF TECHNOLOGY, ROURKELA B TECH MECHANICAL ENGINEERING 4TH SEMESTER MACHINE DYNAMICS MODULE-II (Introduction, Thermodynamic Analysis of cycles, Fuels, Carburetion) Prepared/Uploaded by: Dr. Ramesh Chandra Mohapatra, HOD ME Dept, Government College of Engineering, Keonjhar

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GOVERNORS INTRODUCTION: Flywheel which minimizes fluctuattions of speed within the cycle but it cannot minimize fluctions due to load variation. This means flywheel doesnot exercise any control over mean speed of the engine. To minimize fluctuations in the mean speed which may occur due to load variation, governor is used.  The governor has no influence over cyclic speed fluctuations but it controls the mean speed over a long period during which the load on the engine may vary. Definition: Governor is a device which is used to maintain a mean speed of rotation of the crank shaft over long period during which the load on the engine may vary.  The function of the governor is to increase the supply of working fluid going to the prime-mover when the load on the prime mover increases and to decrease the supply when the load decreases, so as to keep the speed of the prime mover almost constant at different loads. Example: When the load on an engine increases, its speed decreases, therefore it becomes necessary to increase the supply of working fluid. On the other hand, when the load on the engine decreases, its speed increases and hence less working fluid is required. OBJECTIVES: After studying this unit, you should be able to      Classify governors Analyse different type of governors Know characteristics of governors Know stability of spring controlled governors Compare different types of governors TYPES OF GOVERNORS: The governors may broadly be classified as 1. Centrifugal governors 2. Inertia governors

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The centrifugal governors may further be classified as Centrifugal Governors The centrifugal governors are based on the balancing of centrifugal force on the rotating balls by an equal and opposite radial force, known as controlling force. It consists of two balls of equal mass, which are attached to the arms as shown in the fig. above. These balls are known as governor balls or fly balls. The balls revolve with a spindle, which is driven by the engine

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through bevel gears. The upper ends of the arms are pivoted to the spindle, so that the balls may rise up or fall down as they revolve about the vertical axis. The sleeve revolves with the spindle; but can slide up and down. The balls and the sleeve rises when the spindle speed increases and falls when the speed decreases. In order to limit the travel of the sleeve in the upward and downward direction, two stops S, S are provided on the spindle. The sleeve is connected by a bell crank lever to a throttle valve. The supply of working fluid decreases when the sleeve rises and increases when it falls. Working: When the load on the engine decreases, the speed of the engine increases. As the spindle of the governor is driven by the engine, hence the speed of the spindle also increases. This will increase the centrifugal force on the governor balls and the balls will move outwards. Due to the outward movement of the balls the sleeve will rise upwards. The upward movement of the sleeve will operate a throttle valve at the other end of the bell crank lever to reduce the supply of the working fluid by reducing the throttle valve opening. Similarly when the load on the engine increases, the speed of the engine decreases. Also the speed of the spindle of the governor decreases. Hence the centrifugal force on the governor balls will also decrease. The balls of the governor will move inwards and hence the sleeve will move downwards. The downward movement of the sleeve will increase the supply of the working fluid by increasing the opening of the throttle valve and thus the engine speed is increased. Inertia Governors: This works on a different principle. The governor balls are arranged so that the inertia forces caused by angular acceleration or retardation of the governor shaft tend to alter their positions. The amount of displacement of the balls is controlled by springs and the governor mechanism to alter the supply of energy to the engine. The advantage of this type of governor is that the positions of the balls are affected by the rate of change of speed of the governor shaft. Consequently, a more rapid response to a change of load is obtained, since the action of the governor is due to acceleration and not to a finite change of speed. The advantage is offset, however, by the practical difficulty of arranging for a complete balance of the revolving parts of the governor. For this reason centrifugal governors are more frequently used. WATT GOVERNOR One of the simplest forms of centrifugal governor is known as Watt’s governor. It consists of a pair of balls attached to the rotating spindle with the help of arm and sleeve. In this governor, as the speed of spindle fluctuates, the centrifugal force tries to flyout or flyin the balls which results

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