Chemical Engineering - Apllied Mechanics

Transmissions - Part n4

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TRANSMISSIONS A. Vania Politecnico di Milano, Dept. of Mechanical Engineering , Milan, Italy APPLIED MECHANICS –Prof. Andrea VANIA SPUR GEARS 2 Spur gears or straight -cut gears are the simplest type of gear . They consist of a cylinder or disk with the teeth projecting radially, and although they are not straight -sided in form (they are usually of special form to achieve constant drive ratio, mainly involute), the edge of each tooth is straight and aligned parallel to the axis of rotation . These gears can be meshed together correctly only if they are fitted to parallel shafts . APPLIED MECHANICS –Prof. Andrea VANIA SPUR GEARS 3 APPLIED MECHANICS –Prof. Andrea VANIA HELICAL GEARS 4 Helical or "dry fixed" gears offer a refinement over spur gears . The leading edges of the teeth are not parallel to the axis of rotation, but are set at an angle . Since the gear is curved, this angling causes the tooth shape to be a segment of a helix . Helical gears can be meshed in parallel or crossed orientations . The former refers to when the shafts are parallel to each other ; this is the most common orientation . In the latter, the shafts are non -parallel, and in this configuration the gears are sometimes known as "skew gears" . The angled teeth engage more gradually than do spur gear teeth, causing them to run more smoothly and quietly . With parallel helical gears, each pair of teeth first make contact at a single point at one side of the gear wheel ; a moving curve of contact then grows gradually across the tooth face to a maximum then recedes until the teeth break contact at a single point on the opposite side . In skew gears, teeth suddenly meet at a line contact across their entire width causing stress and noise . Skew gears make a characteristic whine at high speeds . Whereas spur gears are used for low speed applications and those situations where noise control is not a problem, the use of helical gears is indicated when the application involves high speeds, large power transmission, or where noise abatement is important . The speed is considered to be high when the pitch line velocity exceeds 25 m/s . APPLIED MECHANICS –Prof. Andrea VANIA GEARED WHEELS 5 A disadvantage of helical gears is a resultant thrust along the axis of the gear, which needs to be accommodated by appropriate thrust bearings, and a greater degree of sliding friction between the meshing teeth, often addressed with additives in the lubricant . APPLIED MECHANICS –Prof. Andrea VANIA BEVEL GEARS 6 A bevel gear is shaped like a right circular cone with most of its tip cut off . Bevel gears with equal numbers of teeth and shaft axes at 90 degrees are called mitre gears . These gears transmit a torque between shafts whose axes intersect each other . APPLIED MECHANICS –Prof. Andrea VANIA WORM GEARS 7 Worm gears resemble screws. A worm gear is usually meshed with a spur gear or a helical gear, which is called the gear , wheel , or worm wheel . Worm -and -gear sets are a simple and compact way to achieve a high torque, low speed gear ratio . For example, helical gears are normally limited to gear ratios of less than 10 :1 while worm -and -gear sets vary from 10 :1 to 500 :1. A disadvantage is the potential for considerable sliding action, leading to low efficiency . APPLIED MECHANICS –Prof. Andrea VANIA HYPOID GEARS 8 Hypoid gears resemble spiral bevel gears except the shaft axes do not intersect . Skew axes APPLIED MECHANICS –Prof. Andrea VANIA CAR DIFFERENTIAL GEARING 9 APPLIED MECHANICS –Prof. Andrea VANIA CAR DIFFERENTIAL GEARING 10 Rectilinear motion Motion along a corner APPLIED MECHANICS –Prof. Andrea VANIA CAR DIFFERENTIAL GEARING 11