Of the different types of gears in use the principal ones are the spur (Fig. 149), the bevel (Fig.
150), and the worm (Fig. 151) gears. Spur gears are wheels with the teeth or cogs arranged round the outer or inner surfaces of the rim, in the direction of radii from the center, and then-action may be regarded as that of two cylinders, rolling one upon the other. Bevel gears are wheels the teeth of which are placed upon the outer periphery (circumference) in a direction converging to the apex of a circle and their action is similar to that of two cones rolling upon each other. When two bevel wheels of the same diameter work together at an angle of 45°, they are called miter wheels (Fig. 152). The teeth are called teeth when they are of the same piece as the body of the wheel, and cogs when they are of separate material. Wheels in whose rims cogs are inserted are called mortise wheels.
Fig. 149. - Spur Gear.
Fig. 150. - Bevel Gear.