PULLEY.
Toothed wheels, now commonly called gear wheels, illustrate another type of wheel lever age. Spur gears are shown in Fig. 8; here the smaller gear-wheel rotates twice to cause one revolution of the large wheel; thus a small wheel secures increased power at reduced speed.
In Fig. 9, the large wheel, being the driver, turns both the small wheels in the same direc tion, but oppositely to itself, and gives the shafts of the small wheels two revolutions to one of the large wheel. In Fig, 10 are shown bevel gears, which coact at right angles. It is apparent that almost infinite combinations of such gears are possible. See WHEEL GEARING.
The inclined plane is simply a rigid fixed In Fig. 6 a movable pulley is shown in com bination with the wheel a and axle b and a crank R is added. If the crank is one foot long, the wheel one foot in diameter and the axle six inches in diameter, one turn of the crank may draw un the right hand cord about three feet, and lengthen the left hand cord about 18 inches, resulting in raising the pulley and attached weight W 9 inches. As the crank handle has traveled about six feet, or eight times as far as the weight is raised, the leverage is 8 to 1, and a boy who can put 50 pounds of plane inclined to the horizon at a* angle, and upon which a weight tends to slide down by gravity, or up the slope of which a weight may be pushed or rolled by a force usually horizon tal in its action. In Fig. 11 the sliding weight would fall back if the power was withdrawn; it also shows power applied horizontally to move a rolling weight up the plane. In this latter case a continuing power of less than a pound will push a pound roller up the incline because the power has a distance greater than the height overcome in which to exert itself.
The inclined plane principle is used in roll ing barrels up on a wagon, in loading logs on a sled, in the operation of a mountain railway and in the cam, which is described later. The employment of the principle does not reduce the work of lifting to the top of the plane, but extends the time of the lift, so that a smaller force used for a longer time does the work.
The screw is essentially an inclined plane wound spirally around a cylinder, in the form of an advancing groove, known as the screw thread. A cylinder having such grooves cut interiorly like the inside of a nut is termed a female screw. The screw shown in Fig. 12 has two grooves spiraling around and is therefore double-threaded. There being eight threads to The wedge (Fig. 11a) is a double plane and gives power at the expense of speed. It is sometimes considered as a combination of two inclined planes placed base to base. The me chanical advantage increases as the angle of the wedge decreases — "the power being equal to twice the resistance into the sine of the angle of the wedge.° The relation, however, the inch, the pitch of this screw may be re garded as one-eighth of an inch, the distance between threads; or as one-fourth of an inch, the distance between one spiral and the next turn of the same spiral. The principle of the screw is used in the screw propeller (q.v.), and the lifting jack or jackscrew. See SCREW.
A mechanical movement is some simple com bination of the elementary powers described above. Perhaps the simplest are the familiar crank (g.v.), cam (q.v.) and cylinder and pis ton. (See STEAM AND STEAM-ENGINES). A has but little practical value since the resistance due to the friction is very great. The principle is used in many forms of cutting tools such as the knife, the chisel, axe and plane. For work ing in comparatively soft materials, the angle is kept small and the edge sharp; but for harder materials the angle is increased. Metal planing tools have very large angles, ranging from 60 degrees to 80 degrees. When the wedge is em ployed for cleaving wood, the cohesion and friction combined produces so great a resistance that the force is applied in the form of a blow from a heavy body, and the resulting strain is equal to the force of the blow multiplied by the length of the wedge divided by its width. See WEDGE.