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Machines employ power to achieve desired forces and movement (motion). A machine has a power source and actuators that generate forces and movement, and a system of mechanisms that shape the actuator input to achieve a specific application of output forces and movement. Modern machines often include computers and sensors that monitor performance and plan movement, and are called mechanical systems. The meaning of the word "machine" is traced by the Oxford English Dictionary〔Oxford English Dictionary〕 to an independently functioning structure and by Merriam-Webster Dictionary〔Merriam-Webster Dictionary (Definition of machine )〕 to something that has been constructed. This includes human design into the meaning of machine. The adjective "mechanical" refers to skill in the practical application of an art or science, as well as relating to or caused by movement, physical forces, properties or agents such as is dealt with by mechanics.〔 Similarly Merriam-Webster Dictionary〔Merriam-Webster Dictionary (Definition of mechanical )〕 defines "mechanical" as relating to machinery or tools. Power flow through a machine provides a way to understand the performance of devices ranging from levers and gear trains to automobiles and robotic systems. The German mechanician Franz Reuleaux〔Reuleaux, F., 1876 (''The Kinematics of Machinery,'' ) (trans. and annotated by A. B. W. Kennedy), reprinted by Dover, New York (1963)〕 wrote "a machine is a combination of resistant bodies so arranged that by their means the mechanical forces of nature can be compelled to do work accompanied by certain determinate motion." Notice that forces and motion combine to define power. More recently, Uicker et al.〔J. J. Uicker, G. R. Pennock, and J. E. Shigley, 2003, ''Theory of Machines and Mechanisms,'' Oxford University Press, New York.〕 stated that a machine is "a device for applying power or changing its direction." And McCarthy and Soh〔J. M. McCarthy and G. S. Soh, 2010, (''Geometric Design of Linkages,'' ) Springer, New York.〕 describe a machine as a system that "generally consists of a power source and a mechanism for the controlled use of this power." == Simple machines == The idea that a machine can be decomposed into simple movable elements led Archimedes to define the lever, pulley and screw as simple machines. By the time of the Renaissance this list increased to include the wheel and axle, wedge and inclined plane. The modern approach to characterizing machines focusses on the components that allow movement, known as joints. Wedge (hand axe): Perhaps the first example of a device designed to manage power is the hand axe, also see biface and Olorgesailie. A hand axe is made by chipping stone, generally flint, to form a bifacial edge, or wedge. A wedge is a simple machine that transforms lateral force and movement of the tool into a transverse splitting force and movement of the workpiece. The available power is limited by the effort of the person using the tool, but because power is the product of force and movement, the wedge amplifies the force by reducing the movement. This amplification, or mechanical advantage is the ratio of the input speed to output speed. For a wedge this is given by 1/tanα, where α is the tip angle. The faces of a wedge are modeled as straight lines to form a sliding or prismatic joint. Lever: The lever is another important and simple device for managing power. This is a body that pivots on a fulcrum. Because the velocity of a point farther from the pivot is greater than the velocity of a point near the pivot, forces applied far from the pivot are amplified near the pivot by the associated decrease in speed. If ''a'' is the distance from the pivot to the point where the input force is applied and ''b'' is the distance to the point where the output force is applied, then ''a/b'' is the mechanical advantage of the lever. The fulcrum of a lever is modeled as a hinged or revolute joint. Wheel: The wheel is clearly an important early machine, such as the chariot. A wheel uses the law of the lever to reduce the force needed to overcome friction when pulling a load. To see this notice that the friction associated with pulling a load on the ground is approximately the same as the friction in a simple bearing that supports the load on the axle of a wheel. However, the wheel forms a lever that magnifies the pulling force so that it overcomes the frictional resistance in the bearing. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Machine (mechanical)」の詳細全文を読む スポンサード リンク
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