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中型載重汽車動力性能分析和減速器設計畢業(yè)設計-資料下載頁

2025-06-28 23:42本頁面
  

【正文】 ng。 worm and worm gear, and the presence of vibration.2. Rolling Contact Bearings Bearings are used to hold shafts that must rotate relative to supports that are fixedfor instance, the housing of a motor, gearbox, or transmission. A wide variety of bearings exist, which are classified into two broad groupsrolling contact and journal. Each particular type is appropriate for a different set of installation and operating conditions. At this point we discuss only rolling contact bearing. Rolling contact bearing typically prise an inner race。 an outer race。 rolling elements in the form of balls, cylinders, or cones。 and a separator that prevents the rolling elements from rubbing against one another. They are ubiquitous in machine design, and are found in applications as diverse as bicycle wheels, robotic joints, and automobile transmissions. A sample installation of a rolling contact bearing is illustrated in ( Figure 6 ). The shaft and the bearing 39。s inner race rotate together, while the out race and the transmission 39。s housing are stationary. When a shaft is supported in this way, the bearing 39。s outer race will fit tightly into a matching circular recess that is formed in the housing. As the shaft turns and transmits power, the bearing could be subjected to forces directed either along the shaft (call a thrust force) or perpendicular to it (a radial force). An engineer will make a decision on the type of bearing to be used in a machine depending on whether thrust force, radial forces, or some bination of the two act on the bearing. Thus, engineers apply the properties of force systems when they analyse select bearing and other machine ponents. Pillow block bearing can be used to support a power transmission shaftI. BALL Bearings The most mon type of rolling element bearing is the ball bearing, which incorporates hardened, precisionground steel spheres. ( Figure 7 ) depicts the major elements of a standard ball bearing: the inner race, outer race, balls, and separator. The inner and outer races are the bearing 39。s connections to shaft and housing. The separator keeps the balls evenly spaced around the bearing 39。s perimeter and prevents them from contacting one another. Otherwise, if the bearing used at high speed or subjected to large forces, friction could cause it to overheat and bee damage. In some case, the gap between the inner and outer races will be sealed by a rubber or plastic ring to keep grease in the bearing and debris out of it. Ball bearing In principle, the balls press against the inner and outer races of the bearing at single points. The force that each ball transfer between the inner and outer races is concentrated on those surfaces in an intense and relatively sharp manner. If the forces between the rolling elements and races were instead spread over a larger area, we might expect the bearing to last longer. Rolling contact bearings that incorporate cylindrical rollers or tapered cones in place of spherical balls are one solution for more evenly distributing forces, and we discuss those types of bearings next. II. Straight Roller Bearings While ball bearings may be inexpensive, they have a relatively modest capacity for carrying forces because of the point contacts present between the balls and races. Cylindrical rollers can also be used in rolling element bearing in order to better distribute forces over the bearing 39。s races. ( Figure 8 ) illustrates the of straight roller bearing. In ( Figure 8 ), the centerlines of the rollers and the race are parallel to one another, and a separator is again used to space rollers around the bearing 39。s circumference. Straight roller bearings are used in machinery when large radial forces must be carried, but they are not well suited for supporting thrust loads. Straight Roller BearingIII. Tapered Roller Bearings Instead off having spheres or cylinders roll on the race, a tapered roller bearing uses rollers that are shaped like truncated cones, as seen in ( Figure 9 ). The conical rollers all have the same taper angle and their centerlines intersect at a single point on the center line of the shaft, located some distance away from the bearing itself. Separators in the bearings of ( Figure 9 ) prevent friction from rollertoroller contact and better distribute force around the bearing. As was the case for straight roller bearings, the rollers of a tapered roller bearing contact the outer race and the inner race along lines, rather than points, in order to lower wear. The lifetime of straight and tapered roller bearings is generally further improved by giving the rollers a slight crown or barrel shape, a feature again intended to smooth out the transfer of forces between the races. A type of tapered roller bearing that is widely used in automotive front wheel, differentials, and machine tool spindles. Although they may not be as familiar to you as ball bearings, tapered roller bearings are quite mon in mechanical engineering. They are intended for applications where both thrust and radial forces are presentthe wheel bearings in an automobile are a prime example. Wheel bearings must support both the weight of the vehicle ( which is radial force between the bearing and the wheel 39。s shaft ) and the cornering force generated during turns ( which is a thrust force ). Tapered roller bearings are regarded as having an intermediate cost, a high capacity for radial force, and a moderate capacity for thrust forces.IV. Thrust Roller BearingsWhile straight roller bearings are well suited for supporting loads that are directed mostly radically, and tapered roller bearings can support a bination of radial and thrust forces, thrust roller bearings carry loads that are directed primarily along a shaft 39。s center line. One type of thrust bearing is show in ( Figure 10 ). The rolling elements in this type of bearing are cylinders having a s
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