Machinery's Handbook, 31st Edition
Backlash in Gears 2245 Bevel and Hypoid Gears: Table 2 gives similar backlash range values for bevel and hyp oid gears. These are values based upon average conditions for general purpose gearing, but may require modification to meet specific needs. Backlash on bevel and hypoid gears can be controlled to some extent by axial adjustment of the gears during assembly. However, due to the fact that actual adjustment of a bevel or hypoid gear in its mounting will alter the amount of backlash, it is imperative that the amount of backlash cut into the gears during manufacture is not excessive. Bevel and hyp oid gears must always be capable of operation without interference when adjusted for zero backlash. This requirement is imposed by the fact that a failure of the axial thrust bearing might permit the gears to operate under this condition. Therefore, bevel and hypoid gears should never be designed to operate with normal backlash in excess of 0.080/ P where P is diametral pitch. Fine-Pitch Gears: Table 3 gives similar backlash range values for fine-pitch spur, helical and herringbone gearing. Providing Backlash.— In order to obtain the amount of backlash desired, it is necessary to decrease tooth thicknesses. However, because of manufacturing and assembling inaccuracies not only in the gears but also in other parts, the allowances made on tooth thickness almost always must exceed the desired amount of backlash. Since the amounts of these allowances depend on the closeness of control exercised on all manufacturing operations, no general recommendations for them can be given. It is customary to make half the allowance for backlash on the tooth thickness of each gear of a pair, although there are exceptions. For example, on pinions having very low numbers of teeth it is desirable to provide all the allowance on the mating gear, so as not to weaken the pinion teeth. In worm gearing, ordinary practice is to provide all of the allow ance on the worm which is usually made of a material stronger than that of the worm gear. In some instances the backlash allowance is provided in the cutter, and the cutter is then operated at the standard tooth depth. In still other cases, backlash is obtained by setting the distance between two tools for cutting the two sides of the teeth, as in straight bevel gears, or by taking side cuts, or by changing the center distance between the gears in their mountings. In spur and helical gearing, backlash allowance is usually obtained by sinking the cutter deeper into the blank than the standard depth. The accompanying table gives the excess depth of cut for various pressure angles. Excess Depth of Cut E to Provide Backlash Allowance Distribution of Backlash Pressure Angle φ , Degrees 14 1 ∕ 2 17 1 ∕ 2 20 25 30 Excess Depth of Cut E to Obtain Circular Backlash B a All on One Gear 1.93 B 1.59 B 1.37 B 1.07 B 0.87 B One-half on Each Gear 0.97 B 0.79 B 0.69 B 0.54 B 0.43 B Excess Depth of Cut E to Obtain Backlash B b Normal to Tooth Profile b All on One Gear 2.00 B b 1.66 B b 1.46 B b 1.18 B b 1.99 B b One-half on Each Gear 1.00 B b 0.83 B b 0.73 B b 0.59 B b 0.50 B b a Circular backlash is the amount by which the width of a tooth space is greater than the thickness of the engaging tooth on the pitch circles. As described in pages 2242 and 2246 this is what is meant by backlash unless otherwise specified. b Backlash measured normal to the tooth profile by inserting a feeler gage between meshing teeth; to convert to circular backlash, B = B b ÷ cos φ . Control of Backlash Allowances in Production.— Measurement of the tooth thickness of gears is perhaps the simplest way of controlling backlash allowances in production. There are several ways in which this may be done including: 1) chordal thickness mea surements as described on page 2224 ; 2) caliper measurements over two or more teeth as described on page 2315 ; and 3) measurements over wires.
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