MECHANICAL MOVEMENTS, POW ERS AND APPLIANCES. The primary, fundamental mechanical contrivances are termed the mechanical powers, seven in number and all based on the principles of either the lever or the inclined plane. The common classifi cation is the lever, wheel and axle, cord and pulley, toothed wheels (these four operating on the lever principle), and the inclined plane, wedge and screw (operating on the principle of the inclined plane). What are known as the mechanical movements (sometimes called mechanical motions) include about 750 of the more or less simple and common combinations, covering practically all the fundamental arrange ments of the mechanical powers for machine building and engineering work. They consti tute the groundwork which the machinist or student of engineering must master before he can make much headway in understanding the complex problems which arise in practice.
Taking first the simple lever, it is found to be of three classes : (1) those in which the fulcrum is situated between the power and the weight; (2) those in which the fulcrum is at one end of the lever arm with the weight nearer to it than the power; and (3) those in which the fulcrum is at the end with the power nearer to it than the weight.
In the first, if the weight IV is nearer to the fulcrum, there is a mechanical advantage —illustrated by the crowbar, which on account of the great difference in the length of its arms is advantageously used to overcome great resistance. Scissors and nippers are double levers of this class. If the power P is nearer to the fulcrum, there is a mechanical disadvantage, and if the weight and the power are at an equal distance on either side of the fulcrum, the power is equal to the weight and gives an arrangement similar to the ordinary balance.
The distinction between the gain of power and the loss of velocity, and the reverse of these conditions, as depending upon the position of the fulcrum, is exemplified by the shears used for cutting metal and those used for cut ting cloth, respectively. In the former, short blades with long handles overcome a great resistance slowly; while in the latter, long blades operated by short handles, move quickly.
In the second class there is always a medhan ical advantage. The wheelbarrow is ex ample of the simple lever. The fulcrum is at the centre of the wheel, the weight acts down ward at the centre of gravity of the load and the power is applied at the ends of the handles. A hinged nut-cracker is an example of a double level of this kind.
In the third class there is always tnediani cal disadvantage; but great rapidiV of move ment is obtained. The human forearm is aa a ample of a simple lever of this class. The ful
crum is at the elbow-joint, the weight acts down ward at the hand and the power is applied obliquely by a tendon from the biceps muscle at tached near the elbow. A pair of tongs is an example of a double lever of this class.
The wheel and axle consists of two cylinders of different sizes rigidly connected together and turning about a common axis. The larger cyl inder is called the wheel and the smaller the axle. The power is applied to the end of a rope wound around the wheel and the weight is raised by a rope wound around the axle; see Fig. 4. The diameter of the larger cylin der or wheel being twice that of the smaller cylinder, or axle, a power of one pound at P will balance a weight of two pounds at IF. This is essentially a form of lever, and °the power is to the weight lifted as the radius of the axle is to the radius of the wheel." The principle is applicable to all forms of hoisting machines, steering gear of ships, fusee clock and watch movements, etc. If the axle be fixed and the wheel be loosely mounted to revolve on it, we have the ordinary machinist's loose pulley and a variety of uses suggest them selves. If the axle be fixed to the body of a carriage and the wheel allowed to rest• on level ground, we find that the carriage can be drawn along with slight effort, requiring a push or pull representing only a small fraction of its weight.
The cord and pulley shows the further uses of the wheel as a lever. In the arrangement shown in Fig. 5 the upper points F are fixed, being virtually fulcrums; a downward pull of one pound one foot on the cord P will raise the weight which may weigh nearly two pounds, a half foot; it would balance two pounds but for the loss by friction.
effort on the crank can raise nearly 400 pounds at every turn, and in eight turns he will raise it six feet, the distance of travel of the crank at one turn.
A very common arrangement of pulleys, called stepped pulleys, is shown in Fig. 7, as positioned for driving a lathe. The steps of the pulleys on the lathe are supposed to be 3, 6, 9 and 12 inches diameter, respectively. The belt is shown on the 12-inch power pulley and 3-inch lathe pulley, and obviously, if the power shaft is making 100 revolutions per minute, the lathe-shaft will make 400. If the belt be shifted to the next step,. where the proportions are 9 to 6, the 100 revolutions of the power shaft will give 150 revolutions of the lathe shaft. On the third step 6•to 9 is the propor tion and the lathe will rotate at a speed of 66Y3 revolutions; on the fourth step it is 3 to 12, and the 100 revolutions of the power shaft will give but 25 of the lathe shaft. Sce