2. Read the text and give titles to paragrahps. Translate the text. Types of Gears

A). This is a gear. It is a set of tooth­ed wheels working together in a machine. The gears may be different types: adjusting gear, bevel gear, cone friction gear, clutch gear, feed gear.

Fig. 23. Rack and pinion

Gears are used in machines to trans­mit motion and power by means of positive, con­tact between toothed wheels. Gears are a form of wheel-and-axle. Gears may be classified according to their shaft (axle) relationship. Three such types are parallel, intersecting, and skewed shafts. Spur and helical gears are commonly used with parallel shafts, bevel gears with intersecting shafts, and helical and worm gears with skewed shafts. One of the most com­mon examples of gearing in a machine is the transmission of an automobile. Gears are a vital part of such household machines as the hand-held drill, the electric saw, the hedge-clipper, and the snowblower. Gears are also classified according to the shape oftheir teeth.The major types are spur, helical, bevel, hypoid, and worm and wheel gears

B). Spur Gears. The form of spur gear teeth resembles a mathematical curve – usually the involute, sometimes the cycloid. Only a portion of the tooth follows the involute form because the lower part of the tooth (flank) must provide adequate clearance. In ordinary spur gears, the teeth are cut parallel to the working shaft. This means that there is no side thrust on the shaft. It also means that only a limited number of teeth can be in meshing contact at a given time. One special configur­ation of spur gears is called a planetary gear train (see illustration). It has been used in auto­mobiles and propeller-driven aircraft.

Fig. 24. a.spur gears engaging b. spiral bevel gears engaging

C). Helical Gears. Helical gears are usually used for large power transmission. Such gears are cut in the form of helices and thus lie at an angle to the shaft. Because gear­ing cut at an angle causes a side thrust on the shaft, it is customary to cut such gears double, so that each series of teeth offsets the side thrust of the other.

Bevel Gears. The driving pinion and ring gear are spiral bevel gears –that is, each gear tooth follows the curve of a spiral. The idler and side gears have straight teeth. Bevel gears are frequently used to drive vertical pumps or process machinery with horizontal drivers.

D). Worm and Wheel Gears is another type of gearing used for nonintersecting shafts or extensions of shafts and make use of worm and wheel gear. The driving gear is a rotating, spiral (worm) that meshes with the teeth of the wheel. Large speed reductions are possible with this arrangement. Gear teeth of a cycloidal profile engage in a rolling action which minimizes friction; however, pinion and gear shafts of cycloidal gears must be very accurately aligned and spaced.

E). Planetary gears consist of three sets of gears, a pinion (sun gear) meshing with two or more planet gears which in turn rotate within a stationary internal gear, that is forming part of the housing. The planet gears are connected to one shaft and the sun gear to the other shaft. Planetary-gear systems are compact, achieve a large reduction in a limited space, and result in input and output shafts in the same center line.