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2025-03-24 00:34本頁(yè)面
【正文】 r dynamics. Also, frictional and drag forces are magnified which, in extreme cases , may lead to operational problems. Epicyclic Gearing:Normally, a gear axis remains fixed and only the gears rotates. But in an epicyclic gear train, various gears axes rotate about one anther to provide specialized output motions. With suitable clutchse and brakes, an epicyclic train serves as the plaary 蘭州交通大學(xué)畢業(yè)設(shè)計(jì)（論文） 4 gear monly found in automatic transmissions. Epicyclic trains may use spur or helical gears, external or internal, or bevel gears. In transmissions, the epicyclic (or plaary) gears usually have multiple plas to increase load capacity. In most cases, improved kinematic accuracy in a gearset decreases gear mesh excitation and results in lower drive noise. Gearset accuracy can be increased by modifying the tooth involute profile, by substituting higher quality gearing with tighter manufacturing tolerances, and by improving tooth surface finish. However, if gear mesh excitation generaters resonance somewhere in the drive system, nothing short of a “perfect” gearset will substantially reduce vibration and noise. Tooth profiles are modified to avoid interferences which can result from deflections in the gears, shafts, and housing as teeth engage and disendgage. If these tooth interferences are not pensated for by profile modifications, gears load capacity can be seriously reduced. In addition, the drive will be noisier because tooth interferences generate high dynamic loads. Interferences typically are eliminated by reliving the tooth tip, the tooth flank, or both. Such profile modifications are especially important for highload , highspeed drives. The graph of sound pressure levelvs tip relief illustrates how tooth profile modifications can affect overall drive noise. If the tip relief is less than this optimum value, drive noise increases because of greater tooth interference。 a greater amount of tip relief also increase noise because the contact ratio is decreased. Tighter manufacturing tolerances also produce quietier gears. Tolerances for such parameters as profile error, pitch AGMA quality level. For instance, the graph depicting SPL vs both speed and gear quality shows how noise decreases example, noise is reduced significantly by an increase in accuracy from an AGMA Qn 11 quality to an AGNA Qn 15 quality. However, for most mercial drive applications, it is doubtful that the resulting substantial cost increase for such an accuracy improvement can be justified simply on the basis of reduced drive noise. Previously, it was mentioned that gears must have adequate clearance when loaded to prevent tooth interference during the course of meshing. Tip and flank relief are mon profile modifications that co
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