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INTRODUCTIONS

1.1 THE SUBJECT OF KINEMATICS AND DYNAMIC OF MACHINES

This subject is a continuation of statics and dynamics, which is taken by students in their freshman or sophomore years. In kinematics and dynamics of machines and mechanisms, however, the emphasis shifts from studying general concepts with illustrative examples to developing methods and performing analyses of real designs. This shift in emphasis is important, since it entails dealing with complex objects and utilizing different tools to analyze these objects.(Physics Books PDF)

Physics Books PDF

The objective of kinematics is to develop various means of transforming motion to achieve a specific kind needed in applications. For example, an object is to be moved from point A to point B along some path. The first question in solving thi problem is usually:

What kind of a mechanism (if any) can be used to perform this function? And the second question is: How doe one design such a mechanism? The objective of dynamics is analysis of the behavior of a given machine or mechanism when subjected to dynamic forces. For the above example, when the mechanism is already known, then external forces are applied and its motion is studied. The determination of forces induced in machine components by the motion is part of this analysis.(Physics Books PDF)

As a subject, the kinematics and dynamics of machines and mechanisms is disconnected from other subjects (except statics and dynamics) in the Mechanical Engineering curriculum. This absence of links to other subjects may create the false impression that there are no constraints, apart from the kinematic ones, imposed on the design of mechanisms.

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Look again at the problem of moving an object from A to B. In designing a mechanism, the size, shape, and weight of the object all constitute input into the design process. All of these will affect the size of the mechanism. There are other considerations as well, such as, for example, what the allowable speed of approaching point B should be.(Physics Books PDF)

The outcome of this inquiry may affect either the configuration or the type of the mechanism. Within the subject of kinematics and dynamics of machines and mechanisms such requirements cannot be justifiably formulated; they can, however, be posed as a learning exercise.(Physics Books PDF)

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1.2 KINEMATICS AND DYNAMICS AS PART OF THE DESIGN PROCESS

The role of kinematics is to ensure the functionality of the mechanism, while the role of dynamics is to verify the acceptability of induced forces in parts. The functionality and induced forces are subject to various constraints (specifications imposed on the design. Look at the example of a cam operating a valve (Figure 1.1). Ch1Frame Page 1 Friday, June 2, 2000 6:39 PM (Physics Books PDF)

2 Fundamentals of Kinematics and Dynamics of Machines and Mechanisms

The design process starts with meeting the functional requirements of the prod- uct. The basic one in this case is the proper opening, dwelling, and closing of the valve as a function of time. To achieve this objective, a corresponding cam profile producing the needed follower motion should be found.

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Physics Books PDF

The rocker arm, being a lever, serves as a displacement amplifier/reducer. The timing of opening, dwelling and closing is controlled by the speed of the camshaft. The function of the spring is to keep the roller always in contact with the cam.(Physics Books PDF)

Physics Books PDF

To meet this requirement the inertial forces developed during the follower–valve system motion should be known, since the spring force must be larger than these forces at any time. Thus, it follows that the determination of component accelerations needed to find inertial forces is important for the choice of the proper spring stiffness.

 

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