Engineering & Materials: Other Engineering Disciplines:Design engineering
Engineering & Materials: Mechanical Engineering:Mechanical engineering
Simple machine
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ny of several elementary machines, one or more of which is found in practically every machine. The group of simple machines usually includes only the lever, wheel and axle, pulley (or block and tackle), inclined plane, wedge, and screw. However, the gear drive and hydraulic press may also be considered as simple machines. The principles of operation and typical applications of simple machines depend on several closely related concepts. See also: Block and tackle; Hydraulic press; Inclined plane; Lever; Screw; Wedge; Wheel and axle
Two conditions for static equilibrium are used in analyzing the action of a simple machine. The first condition is that the sum of forces in any direction through their common point of action is zero. The second condition is that the summation of torques about a common axis of rotation is zero. Corresponding to these two conditions are two ways of measuring work. In machines with translation, work is the product of force and distance. In machines with rotation, work is the product of torque and angle of rotation.
Work is the product of a force and the distance through which it moves. For example, the work done in raising a 10-lb object 15 ft is 150 ft-lb. In this example the work done on the weight goes into increasing the potential energy of the object. Work and energy, both potential and kinetic, have the same units, and in general the purpose of a machine is to convert energy into work. See also: Efficiency; Mechanical advantage; Work
For rotating machines, it is more convenient to consider torque and angular displacements than force and distance. Work is then expressed as the product of the torque and the angle (in radians) through which the object rotates while acted on by the torque. Torque, in turn, is the force exerted at a given radius from an axis of rotation. Thus, a 10-lb force at the end of a 15-ft crank exerts a torque of 150 lb-ft. See also: Torque
Power is the rate of doing work. For example, one horsepower is arbitrarily defined as 550 ft-lb per second, or 33,000 ft-lb per minute. See also: Machine; Power
Richard M. Phelan
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Suggested citation for this article:
Richard M. Phelan, "Simple machine", in AccessScience@McGraw-Hill, http://www.accessscience.com, DOI 10.1036/1097-8542.624600, last modified: April 10, 2000.
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