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Ebook Machine elements life and design: Part 2

(BQ) Part 2 book "Machine elements life and design" has contents: Gear design, housings, bolted connections (BCs), connection of units, strength of metal parts, calculations for strength, finale. | 9563_C008.fm Page 215 Wednesday, July 25, 2007 4:21 PM 8 Gear Design All gears have one common element: their teeth. In all other respects, the shapes and structures of gears may differ very much. Figure 8.1 shows several typical shapes of gears: pinion shafts (Figure 8.1a and Figure 8.1b), gears mounted on shafts (Figure 8.1c to Figure 8.1f), sun wheels of planetary gears (Figure 8.1g and Figure 8.1h), planet wheels (Figure 8.1i and Figure 8.1j), and ring gears (Figure 8.1k and Figure 8.1l). In this chapter, we will mainly discuss simple gearings. 8.1 GEAR AND SHAFT: INTEGRATE OR SEPARATE? When the gear must be rigidly connected to the shaft, the designer is often unsure about what is preferable: to make the gear as an integral part of the shaft or to make it apart and connect it to the shaft by some means. Sometimes, the split design is impracticable because the diameters of the shaft and the pinion are very similar (Figure 8.2a). The diameter of the pinion can even be less than that of the shaft (Figure 8.2b). It may be associated with the need to increase the bending rigidity of the shaft or with unification of its bearings (when gears with different ratios are produced in the same housing). Sometimes the split design is dictated by assembly needs or by manufacturing limitations (for example, when the shaft is too long to be installed on an available gear-cutting machine). Perhaps, the gear teeth must be case-hardened but this can’t be done along with the shaft on the existing equipment or it requires some additional equipment to decrease the warpage of the shaft. When there are no such constraining conditions, economic considerations prevail in making design decisions. Yet the split design may result in increased weight, lesser accuracy because of tolerance summation (also called stack-up) and decreased reliability. Figure 8.3 shows some design options for forged gears: those made together with the shaft (Figure 8.3a) and separate from the shaft (Figure 8.3b