1、外文文献原文Helical，Worm and Bevel GearsIn the force analysis of spur gars, the forces are assumed to act in a single plain. In this lesson we shall study gears in which the forces have three dimensions. The reason for this, in the case of helical gears, is that the teeth are not parallel to the axis of r
2、otation. And in the case of bevel gears, the rotational axes are not parallel to each other. There are other reasons, as we shall learn.Helical gears are used to transmit motion between parallel shafts. The helix angle is the same on each gear, but one gear must have a righthand helix and the other
3、a lefthand helix. The shape of the tooth is an involute helicoids. If a piece of paper cut in the shape of a parallclogram is wrapped around a cylinder, the angular edge of the paper becomes a helix. If we unwind this paper, each point on the angular edge generates an involute curve. The surface obt
4、ained when every point on the edge generates an involute is called an involute helicoids.The initial contact of spurgear teeth is a line extending all the way across the face of the tooth. The initial contact of helical gear teeth is a point,which changes into a line as the teeth come into more enga
5、gement. In spur gears the line of contact is parallel to the axis of the rotation; in helical gears, the line is diagonal across the face of the tooth.It is this gradual engagement of the teeth and the smooth transfer of load from one tooth to another ,which give helical gears the ability to transmi
6、t heavy loads at high speeds. Helical gears subject the shaft bearings to both radial and thrust loads. When the thrust loads become high or are objectionable for other reasons, it may be desirable to use double helical gears. A double helical gear（herringbone）is equivalent to two helical gears of o
7、pposite hand, mounted side by side on the same shaft. They develop opposite thrust reaction and thus cancel out the thrust load. When two or more single helical gears are mounted on the same shaft, the hand of the gears should be selected so as to produce the minimum thrust load.Crossedhelical, or s
8、piral, gears are those in which the shaft centerlines are neither parallel nor intersecting. The teeth of crossed-helical gears have point contact with each other, which changes to line contact as the gears wear in. For this reason they will carry out very small loads and are mainly for instrumental
9、 applications, and are definitely not recommended for use in the transmission of power. There is no difference between a crossed helical gear and a helical gear until they are mounted in mesh with each other. They are manufactured in the same way. A pair of meshed crossed helical gears usually have
10、the same hand; that is, a right-hand driver goes with a right hand driven. In the design of crossed-helical gears, the minimum sliding velocity is obtained when the helix angle are equal. However, when the helix angle are not equal, the gear with the larger helix angle should he used as the driver i
11、f both gears have the same hand.Worm gears are similar to crossed helical gears. The pinion or worm has a small number of teeth, usually one to four, and since they completely wrap around the pitch cylinder they are called threads. Its mating gear is called a worm gear, which is not a true helical g
12、ear. A worm and worm gear are used to provide a high angular-velocity reduction between nonintersecting shafts which are usually at right angle. The worm gear is not a helical gear because its face is made concave to fit the curvature, nature of the worm in order to provide line contact instead of p
13、oint contact. However, a disadvantage of worm gearing is the high sliding velocities across the teeth, the same as with crossed helical gears. Worn gearing are either single or double enveloping. A single enveloping gearing is one in which the gear wraps around or partially encloses the worm, A gear
14、ing in which each element partially encloses the other is, of course, a double enveloping worm gearing. The important difference between the two is that area contact exists between the teeth of double enveloping gears while only line contact between those of single-enveloping gears. The worm and wor
15、m gear of a set have the same hand of helix as for crossed helical gears, but the helix angles are usually quite different. The helix angle on the worm is generally quite large, and that on the gear very small. Because of this, it is usual to specify the lead angle on the worm, which is the compleme
16、nt of the worm helix angle, and the helix angle on the gear; the two angles are equal for a 9O deg. shaft angle.When gears are to be used to transmit motion between intersecting shafts, some form of bevel gear is required. Although bevel gears are usually made for a shaft angle of 9O deg., they may
17、be produced for almost any shaft angle. The teeth may be east, milled, or generated. Only the generated teeth may be classed as accurate. In a typical bevel gear mounting, one of the gear is often mounted outboard of the bearing. This means that shaft deflection can be more pronounced and have a gre
18、ater effect on the contact of the teeth. Another difficulty, which occurs in predicting the stress in bevel gear teeth, is the fact that the teeth are tapered.Straight bevel gears are easy to design and simple to manufacture and give very good results in service if they are mounted accurately and po
19、sitively. As in the case of spur gears, however, they become noisy at higher values of the pitch-line velocity. In these eases it is often good design practice to go to he spiral bevel gear, which is the bevel counterpart of the helical gear, as in the case of helical gears, spiral bevel gears give
20、a much smoother tooth action than straight bevel gears, and hence are useful where high speed are encountered. It is frequently desirable, as in the case of automotive differential applications, to have gearing similar to bevel gears but with the shaft offset. Such gears are called hypoid gears beca
21、use their pitch surfaces are hyperboloids of revolution. The tooth action between such gears is a combination of rolling and sliding along a straight line and has much in common with that of worm gearsSAND CASTINGMost metal casting are made by pouring molten metal into a prepared cavity and allowing
22、 it to solidify. The process dates from antiquity. The largest bronze statue in existence to-day is the great Sun Buddha in Nara, Japan. Cast in the eighth century, it weighs 551 tons(500 metric tons) and is more than 71 ft (21m) high. Artisans of the Shang Dynasty in China ( 1766 - 1222B. C. ) crea
23、ted art works of bronze with delicate filigree as sophisticated as anything that is designed and produced today.There are many casting processes available today, mid selecting the best one to produced particular part depends on several basic factors, such as cost, size. production rate. finish, tole
24、rance, section thickness, physical-mechanical properties, intricacy of design mach inability, and weld ability.Sand casting. the oldest and still the most widely used casting process. will be presented in more detail than the other processes since many of the concepts carry over into those processes
25、 as well.Green Sand Green sand generally consists of silica sand and additives coated by rubbing the sand grains together with clay uniformly wetted with water. More stable and refractory sands have been developed, such as fused silica, zircon, and mullets, which replace lower-cost silica and have o
26、nly 2% linear expansion at ferrous metal temperatures. Also, relatively un-stable water and clay bonds are being replaced with synthetic resins, which are much mores table at elevated temperatures.Green sand molding is used to produce a wide variety of castings in sizes of less than around to as lar
27、ge as several tons. This versatile process is applicable to both ferrous and nonferrous materials.Green sand can be used to produce intricate molds since it provides for rapid collapsibility: that is, the mold is much less resistant to the contraction of the casting as it solidifies than are other m
28、olding processes. This results in less stress and strain in the casting.The sand is rammed or compacted around the pattern high a variety of methods, including hand or pneumatic-tool ramming, jolting (abrupt mechanical shaking), squeezing (com-pressing the top and bottom mold surfaces), and driving
29、the sand into the mold at high velocities (sad slinging). Sand slings are usually resented for use in making very large casting where great volumes of sand are handled.For smaller casting, a two-part metal box or flask referred to as a cope and drag issued. First the pattern is positioned on a mold
30、board. and the drag or lower half of the flask is positioned over it. Parting powder is sprinkled on the paten and the box is filled with sand. A jolt squeeze machine quicky compacts the sand. The flask is then turned over and again parting powder is dusted on it. The cope is then positioned on the
31、top half of the flask and is filled with sand, and the two-part mold with the patter board sandwiched in between is squeezed.PatternsPatterns for sand casting have traditionally been made of wood or metal. However, it has been found that wood patterns change as much as 3% due to heat and moisture. T
32、his factor alone would put many casting out of acceptable tolerance for more exacting specifications. Now, patterns are often made from epoxies and from cold-setting rubber with stabilizing inserts. Patterns of simple design, with one or more flat surface, can be molded in one piece, provided that t
33、hey can be withdrawn without disturbing the compacted sand. Other patterns may be split into two or more parts to facilitate their removal from the sand when using two-part flasks. The pattern must be tapered to permit easy removal from the sand. The taper is referred to as draft. When a part does n
34、ot have some natural draft, it must be added. A more recent innovation in patterns for sand casting has been to make them out of foamed polystyrene that is vaporized by the molten metal. This type of casting, known as the full-mold process, does not require pattern draft.Spruces, Runners, and Gates.
35、Access to the mold cavity for entry of the molten metal is provided by sprees, runners, and gates, as shown in Fig. 7 I. A pouring basin can be carved in the sand at the top of the spree, or a pour box, which provides a large opening, may be laid over the spree to facilitate pouring. After the metal
36、 is poured, it cools most rapidly in the sand mold. Thus the outer surface forms a shell that permits the still molten metal near the center to flow toward it. As a result, the last portion of the casting to freeze will be deficient in metal and, in the absence oaf supplemental metal-feed source, wi
37、ll result in some form of shrinkage.2 This shrinkage may take the form of gross shrinkage (large cavities) or the more subtle micro shrinkage ( finely dispersed porosity). These porous spots can be avoided by the use of risers, as shown in Fig.7-1, which provide molten metal to make up for shrinkage
38、 losses.CoresCores are placed in molds wherever it is necessary to preserve the space it occupies in the mold as a void in the resulting castings. As sown in Fig.7-1, the core will be put in place after the pastern is removed. To ensure its proper location, the pattern has extensions known as core p
39、rints that leave cavities in the mold into which the core is seated. Sometimes the core may be molded integrally with the green sand and is then referred to as a green-sand core. Generally, the core is made of sand bonded with core oil, some organic bonding materials, and water. These materials are
40、thoroughly blended and placed in a mold or core box. After forming, they are removed and baked at 350to 450F ( 177to 232C). Cores that consist of two or more parts are pasted together after baking.CO2 CoresCO2 cores are made by ramming up moist sand in a core box. Sodium silicate is used as a binder
41、 which is quickly hardened by blowing CO2 gas over it. The C02 system has the advantage of making the cores immediately available.Pouring the MetalSeveral types of containers are used to move the molten metal from the furnace to the pouring area. Large castings of the floor-and-pit type are poured
42、with a ladle that has a plug in the button, or, as it is called, a bottom-pouring ladle. It is also employed in mechanized operations where the molds are moved along a line and each is poured as it is momentarily stopped beneath the large bottom-pour ladle.ladles used for pouring ferrous metals are
43、lined with a high alumina-content refractory. After long use and oxidation, it can be broken out and replaced. Ladles used in handling ferrous metals most be preheated with gas flames to approximately 2600 to 2700F ( 1427 to 1482C) before filling. Once the ladle is filled, it is used constantly unti
44、l it has been emptied.For nonferrous metals, simple clay-graphite crucibles are used. While they are quite susceptible to breakage, they are very resistant to the metal and will hold up a long time under normal condition. They usually do not require preheating, although care must he taken to avoid m
45、oisture pickup. For this reason they are sometimes baked out to assure dryness.The pouring process must he carefully controlled, since the temperature of the melt greatly affects the degree of liquid contraction before solidification, the rate of solidification, which in turn affects the around of c
46、olumnar growth present at the mold wall, the extent and nature of the dendrite growth, the degree of alloy burnout, and the feeding characteristics of the rise ring system.Finishing OperationsAfter the castings have solidified and cooled somewhat. they are placed on a shakeout table or grating on wh
47、ich the sand mold is broken up, leaving the casting free to be picked out. The casting is then taken to the finishing room where the gates and risers are removed. Small gates and risers may he broken off with a hammer if the material is bride. Larger ones requiem sawing, cutting with a roach, or she
48、aring. Unwanted metal protrusions such as fins, bosses, and small portions of gates and risers need to be smoothed off to blend with the surface. Most of this work is done with a heavy-duty grinder and the process is known as snagging or snag grinding. On large castings it is easier to move the grin
49、der than the work, so swing-type grinders are used. Smaller castings are brought to stand or bench-type grinders. Hans and pneumatic chisels are also used to trim castings. A more recent method of removing excess metal from famous castings is with a carbon air torch. This consists of a carbon rod and high-amperage current with a stream of compressed air blowing at the base of it. This oxidizes and removes the metal as soon as it is molten, In many foundries this method has replaced nearly all chipping
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