机械类期刊.doc

Machine Parts Gears Gears are direct contact bodies, operating in pairs， that transmit motion and force form one rotating shaft to another, or form a shaft to a slide (rack), by means of successively engaging projections called teeth. Tooth profiles. The contacting surfaces of gear teeth must be aligned in such a way that the drive is positive； i.e.，the load transmit must not depend on frictional contact. As shown in the treatment of direct contact bodies, this requires that the common normal to the surface not to pass through the pivotal axis of either the driver or the follower. As it is known as direct contact bodies,，cycloidal and involute profile provide both a positive drive and a uniform velocity ratio; i.e., conjugate action. Basic relations. The smaller of a gear pairs is called the pinion and the larger is the gear. When the pinion is on driving shaft the pair acts as a speed reducer; when the gear drives, the pair is a speed increaser. Gears are more frequently used to reduce speed than to increase it. If a gear having N teeth rotates at n revolutions per minute, the product N﹡n has the dimension “teeth per minute”. This product must be the same for both members of a mating pair if each tooth acquires a partner form the mating gear as it passes through the region of tooth engagement. For conjugate gears of all types, the gear ratio and the speed ratio are both given by the ratio of the number of teeth on the gear to the number of teeth on the pinion. If a gear has 100 teeth and a mating pinion has 20, the ratio is 100/20=5.thus the pinion rotates five times as fast as the gear, regardless of the speed of the gear, their point of tangency is called the pitch point, and since it lies on the line of centers, it is only point at which the tooth profiles have pure rolling contact. Gears on nonparallel, non-intersecting shaft also have pitch circles, but the rolling-pitch- circle concept is not valid. Gear types are determined largely by the disposition o