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).

                 A schematic diagram of cam operating a valve.

The design process starts with meeting the functional requirements of the product. 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. 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. 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.

Kinematical analysis allows one to satisfy the functional requirements for valve displacements. Dynamic analysis allows one to find forces in the system as a function of time. These forces are needed to continue the design process. The design process continues with meeting the constraints requirements, which in this case are:

1. Sizes of all parts;

2. Sealing between the valve and its seat;

3. Lubrication;

4. Selection of materials;

5. Manufacturing and maintenance;

6. Safety;

7. Assembly, etc.

The forces transmitted through the system during cam rotation allow one to determine the proper sizes of components, and thus to find the overall assembly dimension. The spring force affects the reliability of the valve sealing. If any of the requirements cannot be met with the given assembly design, then another set of

                                   Punch mechanism.

parameters should be chosen, and the kinematic and dynamic analysis repeated for the new version. Thus, kinematic and dynamic analysis is an integral part of the machine design process, which means it uses input from this process and produces output for its continuation.

Is It A Machine, A Mechanism, Or A Structure?

The term machine is usually applied to a complete product. A car is a machine, as is a tractor, a combine, an earthmoving machine, etc. At the same time, each of these machines may have some devices performing specific functions, like a windshield wiper in a car, which are called mechanisms. The schematic diagram of the assembly shown in Figure is another example of a mechanism. In Figure a punch mechanism is shown. In spite of the fact that it shows a complete product, it, nevertheless, is called a mechanism. An internal combustion engine is called neither a machine nor a mechanism. It is clear that there is a historically established terminology and it may not be consistent. What is important, as far as the subject of kinematics and dynamics is concerned, is that the identification of something as a machine or a mechanism has no bearing on the analysis to be done. And thus in the following, the term machine or mechanism in application to a specific device will be used according to the established custom.

The distinction between the machine/mechanism and the structure is more fundamental. The former must have moving parts, since it transforms motion, produces work, or transforms energy. The latter does not have moving parts; its function is purely structural, i.e., to maintain its form and shape under given external loads, like a bridge, a building, or an antenna mast. However, an example of a folding

A skeleton representing the punch mechanism.

chair, or a solar antenna, may be confusing. Before the folding chair can be used as a chair, it must be unfolded. The transformation from a folded to an unfolded state is the transformation of motion. Thus, the folding chair meets two definitions: it is a mechanism during unfolding and a structure when unfolding is completed. Again, the terminology should not affect the understanding of the substance of the matter.