英语翻译
Gears are direct contact bodies,operating in pairs,that transmit motion and force form one rotating shaft to another,or from 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 transmitted must not depend on frictional contact.As shown in the treatment of direct contact bodies,this requires that the common normal to the surfaces 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 profiles provide both a positive drive and a uniform velocity ratio; i.e.,conjugate action.
Basic relations.The smaller of a gear pair is called the pinion and the larger is the gear.When the pinion is on the 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 have N teeth rotates at n revolutions per minute,the product N*n hat 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 passed through the region of tooth engagement.For conjugate gears of all types,the gear ration 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 the only point at which the tooth profiles have pure rolling contact.Gears on nonparallel,non-intersecting shafts also have pitch circles,but the rolling-pitch0fircle concept is mot valid.Gear type are determined largely by the disposition of the shafts; in addition,certain types are better suited than others for large speed changes.This means that if a specific disposition of the shafts is required,the type of gear will more or less be fixed.On the other hand,if a required speed change demands a certain type,the shaft positions will also be fixed.Spur gears and helical gears.A gear have tooth elements that are straight and parallel to its axis is known as a spur gear.A spur pair can be used to connect parallel shafts only.
If an involute spur pinion were made of rubber and twisted uniformly so that the ends rotated about the axis relative to one another,the elements of the teeth,initially straight and parallel to the axis,would become helices.The pinion the in effect would become a helical gear.Worm and bevel gears.In order to achieve line contact and improve the load carrying capacity of the crossed axis helical gears,the gear can be made to curve partially around the pinion,