【正文】 stic[I5lAstIk] deflection[di5flekFEn]. For identical[ai5dentikEl] shapes, the stiffness is proportional[prE5pC:FEnl] to the modulus[5mCdjulEs] of elasticity[IlAs5tIsItI]. A material which deforms less under a given load is more stiff than on which deforms more.8. Gears and ReducersGears[^iE] are vital[5vaitl] factors in machinery[mE5Fi:nEri]. One of the first mechanism[5mekEnizEm] invented using gears was the clocks. In fact, a clock is little more than a train of gears. Considerable[kEn5sidErEbl] study and research have been made on gears in recent years because of their wide use under exacting[i^5zAktiN] conditions. They have to transmit[trAnz5mit] heavier loads and run at higher speeds than ever before. The engineers and the machinists[mE5Fi:nist] all consider gearing the prime element in nearly all classes of machinery.1. Spur[spE:] gearsSpur gears are used to transmit power and rotary[5rEutEri] motion between parallel[5pArElel] shafts. The teeth are cut parallel to the axis of the shaft on which the gears are mounted. The smaller of two gears in mesh[meF] is called the pinion[5pinjEn], and the larger is customarily[`kQstEmErElI。9kQstE`merElI] designated[5dezi^neit] as the gear. In most applications, the pinion is the driving element whereas the gear is the driven element. 2. Helical [5helikEl] gears Helical gears have certain advantages, for example, when connecting parallel shafts they have a higher loadcarrying capacity than spur gear with the same tooth numbers and cut with the same cutter.Helical gears can also be used to connect nonparallel [5nCn5pArElel], nonintersecting [7intE5sekt] shafts at any angle to one another.The maximum [5mAksimEm] gear ratio obtainable with a single pair of gears varies [5vZEri] with the type of gear and the application. The following are approximate maxima [5mAksimE] for the various type for average load conditions: spur, 8。 parallelshaft helical, 10。 straight bevel [5bevEl], 6。 spiral[5spaiErEl] bevel, 8。 hypoid [5hai7pCid], 12。 and worm [wE:m], 80. For lightly loaded, instrument[5instrumEnt], and positioning [pE5ziFEniN] gears, these rations can be exceeded[ik5si:d]. Rations as high as 400 or higher can be obtained with gears that resemble tapered [5teipE] worms meshing with hypoid gear. For heavily loaded gear, the given rations may be so high that a reasonable gear size precludes [pri5klu:d] a satisfactory pinion.Since the ratio in a single pair of gears is the quotient [5kwEuFEnt] of the tooth number, and since there usually are limitations on both the minimum and maximum numbers of teeth on the available [E5veilEbl] gears, it follows that the number of ratios obtainable in a single pair is limited. To enlarge the coverage [5kQvEridV] it is necessary to use multiple[5mQltipl] pairs, or trains. The overall[5EuvErC:l] speed ratio in a train is the product of the ratios in each pair. In certain cases an exact ratio cannot be obtained with gears, but by using two or more parts, the desired ratio can be approximated[E5prCksimeit] to any degree of precision.As a convenience for machine builders and users, packaged[5pAkidV] speed reducers, following an industryaccepted pattern [5pAtEn], are manufactured[7mAnju5fAktFE] in a wide variety of types, configurations [kEn7fi^ju5reiFEn], speed ratios, and capacities。 these consist of a box or housing containing bearings, shafts, lubricant [5lu:brikEnt], and shaft oil seals[si:l]. Speed increasers are usually custom build.All speed reducers when operating continuously bee hot because of friction [5frikFEn] in the teeth, in the lubricant, in the bearings, and in the oil seals. If the heat is generated at a faster rate than it can be dissipated [5dIsIpeItId] to the atmosphere, the lubricant may deteriorate[di5tiEriEreit] and the gear or bearing fail.9. Worm Gear SetsWorm[wE:m] gear sets are widely used because of the many advantages obtained[Eb5tein] by their tooth action and load carrying capacity[kE5pAsiti]. A large speed reduction[ri5dQkFEn] or a high increase of torque[tC:k] can be attained[E5tein] with the worm gear set. Compactness[kEm5pAktnis] of design is easy to obtain with such a bination[7kCmbi5neiFEn]. Worm gear drives are quiet and vibration[vai5breiFEn] free.A worm gear set consists of the worm, which is very similar[5similE] to a screw[skru:], and the worm gear, which is a helical[5helikEl] gear. The shafts[FB:ft] upon which the worm and the gear are mounted are usually at right angels but not in the same plane[plein]. The usual practice is to have the worm drive the worm gear. With same ratios[5reiFiEu], it is possible to have a gear driver the worm. However, with large ratios, it is impossible for the gear to drive the worm.The worm can be made with either righthand or lefthand thread[Wred]. Also, like a screw, it can be made with single, double, triple[5tripl], or quadruple[5kwCdrupl] thread. A singlethread worm advances the worm gear a distance equal[5i:kwEl] to pitch[pitF] for each plete worm rotation. The distance advanced is called the lead[li:d]. Thus, the pitch equals[5i:kwEl] the lead for a single thread. A doublethreaded worm has a lead which is twice the pitch.The geometry[dVi5Cmitri] of a worm is similar to that of a power screw. Rotation of the worm simulates[5simjuleit] a linearly[5liniEli] advancing involute[5invElu:t] rack[rAk]. The geometry of worm gear (sometimes called a worm wheel[wi:l]) is similar to that of a helical gear, except that the teeth are curved to envelop[in5velEp] the worm. Sometimes the worm is modified to envelop the gear. This gives a greater area of contact, but requires extremely[iks5tri:mli] precise[pri5sais] mounting.10. CamsA rotary[5rEutEri] cam[kAm] is part on a machine which mechanically[mi5kAnikEli] changes cylindrical[sI5lIndrIk(E)l] motion to straightline motion. The cam, which is mounted on a shaft, is usually driven by a motor[5mEutE] either applied[E5plai] directly to the cam shaft or connected by