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1、书名：机械类专业英语ISBN： 9787111429326作者：韩林烨 关雄飞出版社：机械工业出版社本书配有电子课件Evaluation only.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Copyright 2004-2011 Aspose Pty Ltd.Unit 3 Transmission of Gears3.1 Introduction3.2 Advantages and Disadvantages 3.3 Type of Gears 3.4 Gear Failures and Design Crit
2、eria Evaluation only.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Copyright 2004-2011 Aspose Pty Ltd. Gears are the most widely used method of transmitting power from one shaft to another or to a rack which translates in a straight line. Gears have the capacities of transmitting se
3、veral ten thousand kilowatts (kW) of powers and rotating at 150 meters per second (m/s) of pitch-line speed. The maximum pitch-line speed that gears can achieve is approximately 300m/s. The maximum outside diameters may be made above 10 meters (m) and the maximum single transmission ratio may be abo
4、ve 8. Normally, the small gear of the two mating gears is called the pinion and the large one is referred to as the gear, regardless of which is the driver and which is the follower.1 When the pinion is on the driving shaft, the gear pair acts as a speed reducer; when the gear drives, the gear pair
5、is a speed increaser.3.1 IntroductionBackEvaluation only.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Copyright 2004-2011 Aspose Pty Ltd.3.2 Advantages and Disadvantages of Gears 3.2.1Advantages (1)high efficiency(2)high reliability(3)high service life(4)constant transmission ratio
6、(5)compactness(6)having a large range of power and speed transmitted.3.2.2Disadvantages (1)special machine tools are needed to manufacture the gears(2)great quality class of gears is required(3)they cost much(4)the vibration and noise will occur when gears have low precisions(5)gears cannot be used
7、in the case of large center distance.BackEvaluation only.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Copyright 2004-2011 Aspose Pty Ltd.3.3 Type of Gears There are four main types of gears: spur, helical, worm, and bevel (a)Spur : A gear having tooth elements that are straight and
8、 parallel to its axis is known as a spur gear. A spur pair can be used to connect parallel shafts only. Parallel shafts, however, can also be connected by gears of another type. Spur gears are the simplest and least expensive type of gear; also, spur-gear definitions are usually applicable to other
9、types.Evaluation only.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Copyright 2004-2011 Aspose Pty Ltd.(b) Helical: Helical 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 c
10、urved, this angling causes the tooth shape to be a segment of a helix. Helical gears have certain advantages; for example, when connecting parallel shafts they have a higher load-carrying capacity than spur gears with the same tooth numbers and cut with the same cutter. Because of the overlapping ac
11、tion of the teeth, they are smoother in action and can operate at higher pitch-line velocities than spur gears. Helical gears can also be used to connect nonparallel, non-intersecting shafts at any angle to one another. Ninety degrees is the commonest angle at which such gears are used. Evaluation o
12、nly.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Copyright 2004-2011 Aspose Pty Ltd.(c) Worm: Worm gears provide the simplest means of obtaining large ratios in a single pair. They are usually less efficient than parallel-shaft gears, however, because of an additional sliding movem
13、ent along the teeth. Because of their similarity, the efficiency of a worm and gear depends on the same factors as the efficiency of a screw. Single-thread worms of large diameter have small lead angles and low efficiencies. Multiple-thread worms have larger lead angles and higher efficiencies. For
14、lead angles of about 15 degrees and a coefficient of friction less than 0.15, the efficiency ranges from about 55 percent to 95 percent, and the gear can drive the worm. Such units make compact speed increasers; they have been used for driving superchargers on aircraft engines. In self-locking worms
15、, the gear cannot drive the worm, and the efficiency is less than 50 percent.Evaluation only.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Copyright 2004-2011 Aspose Pty Ltd.(d) Bevel: For transmitting rotary motion and torque around corners, bevel gears are commonly used. The conne
16、cted shafts, whose axes would intersect if extended, are usually but not necessarily at right angles to one another. The pitch surfaces of bevel gears are rolling, truncated cones, and the teeth, which must be tapered in both thickness and height, are either straight or curved. Although curved-tooth
17、 bevel gears are called spiral bevel gears, the curve of the teeth is usually a circular arc. The curvature of the teeth results in overlapping tooth action and a smoother transmission of power than with straight teeth. For high speeds and torques, spiral bevel gears are superior to straight bevel g
18、ears in much the same way that helical gears are superior to spur gears for connecting parallel shafts.3BackEvaluation only.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Copyright 2004-2011 Aspose Pty Ltd.3.4 Gear Failures and Design Criteria The common gear failures of gear teeth a
19、re breakage, pitting, abrasive wearing, gluing and ridging. Breakage of gear tooth generally occurs at its base, because a gear tooth is somewhat similar to a cantilever beam. two types of breakage in gearing: fatigue breakage overload breakage. Pitting is a phenomenon that small local fractures are
20、 lost from the surface of the tooth. It is actually the fatigue failure of the tooth surface because of many repetitions of high contact stresses. Prolonged operation on a pit surface will cause vibration and noise. Gears must be safe from failures of teeth by breakage, pitting, gluing, abrasive wea
21、r, and ridging. Design principles are established in accordance with the failures of the gear teeth. In open and semi-open gearing, tooth failures are mainly breakage and abrasive wear. But for abrasive wear resistance there isnt any appropriate design criterion so far, so we take bending strength as design criterion.3Considering the effect of wear on the bending strength, we enlarge the module appropriately. Evaluation only.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Copyright 2004-2011 Aspose Pty Ltd.
