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Page 3 Installing the gearsBefore assembling a pair of bevel gears, look them over, observe all markings, and read any tags which may be attached. The mounting distance is indicated as "M.D.." followed by the dimension. Backlash is indicated as "lash" followed by the amount. It is strongly recommended that gauges be used to set the pinion at its correct mounting distance and that the gear be then set to have the requisite backlash as measured by a dial indicator, and as explained in following paragraphs. This is most important in the case of ratios 2 to 1 and higher, because a change in the axial location of the pinion affects the tooth bearing much more rapidly than a comparable change in the gear. In some cases, where direct measurements in mounting is difficult, the larger bevel gears have their teeth cut to run best with the back cone surfaces flush. These gears can be satisfactorily assembled by setting them to have the back cone surfaces flush and with the proper backlash. The back cone surfaces of hypoid gears are never tangent and these gears must always be set by measurement. The tooth bearing should always be checked before putting in the lubricant and running under load. The mounting is correct only when the proper tooth bearing and required amount of backlash are obtained. To check the gears, paint the teeth with a mixture of marking compound and a few drops of light oil, and run the gears under a light load. The tooth bearing will then show plainly. Figure 3 shows a pair of gears with the correct tooth bearing.
Marked teethBevel gears of exactly even or multiple ratios, which have been lapped, have certain teeth marked "X". These gears should always be assembled with the marked teeth together as shown in figure 4. This should also be done whenever such gears are put together after having been disengaged.
Figure 4. Bevel gear markings. The mating of marked teeth (X), the backlash (Lash) and the mounting distance (M.D.) are all indicated to facilitate correct assembly.. Locating the pinionThe position for mounting the pinion should be determined by measuring from the shaft of the mating gear. If the gear shaft does not extend in front of the pinion, a dummy shaft which accurately fits in its place should be used. The distance from the locating surface on the pinion to the gear shaft will be equal to the mounting distance minus the radius of the dummy or gear shaft. The measurement may be made by means of micrometer gauges, gauge blocks, or by special gauges prepared for that purpose. It is often more convenient to measure to the front of the mounted pinion rather than to its back. In this way the mounting distance can be readily checked, after the pinion is in place. Convenient points for measuring are shown in figure 5. A gauge, such as that shown in Figure 8, is recommended for setting hypoid pinions. The mounting gauge is generally made shorter than the nominal mounting distance, in order to provide space for using feelers when assembling pinions whose mounting distance is less than the nominal. For example, the gauge in Figure 8 is indicated to be 0.030" shorter. The pinion can be located in correct position by a collar ground to the required thickness or by shims, see Figure 5, and is then fastened in place.
Figure 5. Measurements for locating the pinion. Measuring or gauging as shown above is the recommended method of locating the pinion. Locating the gearAfter the pinion is in place, the correct position of the gear may be accurately determined by moving the gear along its shaft and into mesh with the pinion until the specified backlash is obtained, as previously explained. It is then locked into place.
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