TEXT: Machines and Work

Defined in the simplest terms a machine is a device that uses force to accomplish something. More technically, it is a device that transmits and changes force or motion into work. This definition implies that a machine must have moving parts. A machine can be very simple, like a block and tackle to raise a heavy weight, or very complex, like a railroad locomotive or the mechanical systems used for industrial processes.

A machine receives input from an energy source and transforms it into output in the form of mechanical or electrical energy. Machines whose input is a natural source of energy are call­ed prime movers. Natural sources of energy include wind, water, steam, and petroleum. Windmills and waterwheels are prime movers; so are the great turbines driven by water or steam that turn the generators that produce electricity; and so are internal combus­tion engines that use petroleum products as fuel. Electric motors are not prime movers, since an alternating current of electricity which supplies most electrical energy does not exist in nature.

Terms like work, force, and power will be used frequently in this book, so it is necessary to define them precisely. Force is an ef­fort that results of motion or physical change. If you use your muscles to lift a box you are exerting force on that box. The water which strikes the blades of a turbine is exerting force on those blades, thereby setting them into motion.

In a technical sense, work is the combination of the force and the distance through which it was exerted. In the case of the box that you were lifting, work would be the force times the distance you raised the box. Work can be expressed mathematically in the formula: Work = Force x Distance. If you lift a ten-pound box to a table three feet high, you perform thirty foot-pounds of work. Note that the force is measured in terms of the resistance that must be overcome, in this case the weight of the box.

To produce work, a force must act through a distance. If you stand and hold a twenty-pound weight for any length of time, you may get very tired, but you are not doing work in the engineering sense because the force you exerted to hold up the weight was not acting through a distance. However, if you raised the weight, you would be doing work.

Note these two kinds of motion: linear and rotary. Linear mo­tion is movement in a straight line; the technical term for this kind of motion is translation. Reciprocating motion is a linear motion that goes back and forth or up and down in the same path, like the movement of the pistons in a car. Rotary motion is movement in a circular path. To produce rotary motion it is necessary to have torque, a force that can cause a twisting motion called torsion. Torque is the kind of effort that you exert to open a twist-off lid on a jar. In many machines the problem is to change one kind of mo­tion to another. In a car, for example, the motion of the pistons must be converted into rotary motion to make the wheels turn.

The efficiency of a machine is the ratio of the output of work to the input of energy given in terms of a percentage. No machine is 100% efficient because of friction, the resistance to relative motion that is produced by two bodies moving in contact with each other. There are many other reasons why energy is never completely uti­lized; heat is lost into the atmosphere or the full force of a stream of water cannot be brought to bear on a wheel. Friction is a factor in all mechanical devices. In some cases, it is a factor which mechan­ical engineers try to overcome, but in others, such as braking devices, it is a factor that they try to use to advantage.

The ratio between output force and input force is called the mechanical advantage. If a device requires an effort of ten pounds to move a weight of twenty pounds, the mechanical advantage is two. Therefore the mechanical advantage is the resistance divided by the force.

Power is another term used in a special technical sense in speaking of machines. It is the rate or speed at which work is per­formed. If you raise a ten-pound weight a distance of twenty feet in two minutes, you are performing work at a rate of ten pounds x twenty feet X two minutes, or two hundred foot- pounds in two minutes. Since the rate is usually given in units of one minute, this is a rate of 100 foot-pounds in a minute.

In the English-speaking countries, the rate of doing work is usually given in terms of horsepower, often abbreviated hp. You will remember that this expression resulted from the desire of the inventor James Watt to describe the work his steam engines per­formed in terms that his customers could easily understand. After much experimentation, he settled on a rate of 33,000 foot-pounds per minute as one horsepower.

In the metric system power is measured in terms of watts and kilowatts. The watt is the power to do one joule of work per second. The joule is a small unit of work, approximately four foot-pounds. One horsepower is equal to 746 watts. The kilowatt, a more widely used term, equals a thousand watts or approximately horsepower in the English system.