Machinery's Handbook, 31st Edition
Backlash in Gears 2243 lubricated with relatively light oil, but if there is insufficient clearance for an oil film, and particularly if oil trapped at the root of the teeth cannot escape, heat and excessive tooth loading will occur. Heat is a factor because gears may operate warmer, and, therefore, expand more, than the housings. The heat may result from oil churning or from frictional losses between the teeth, at bearings or oil seals, or from external causes. Moreover, for the same temperature rise, the material of the gears—for example, bronze and aluminum—may expand more than the material of the housings, usually steel or cast iron. The higher the helix angle or spiral angle, the more transverse backlash is required for a given normal backlash. The transverse backlash is equal to the normal backlash divided by the cosine of the helix angle. In designs employing normal pressure angles higher than 20 degrees, special consider ation must be given to backlash, because more backlash is required on the pitch circles to obtain a given amount of backlash in a direction normal to the tooth profiles. Errors in boring the gear housings, both in center distance and alignment, are of extreme importance in determining allowance to obtain the backlash desired. The same is true in the mounting of the gears, which is affected by the type and adjustment of bearings, and similar factors. Other influences in backlash specification are heat treatment subsequent to cutting the teeth, lapping operations, need for recutting, and reduction of tooth thickness through normal wear. Minimum backlash is necessary for timing, indexing, gun-sighting, and certain instru ment gear trains. If the operating speed is very low and the necessary precautions are taken in the manufacture of such gear trains, the backlash may be held to extremely small limits. However, the specification of “zero backlash,” so commonly stipulated for gears of this nature, usually involves special and expensive techniques, and is difficult to obtain. Table 1. AGMA Recommended Backlash Range for Coarse-Pitch Spur, Helical, and Herringbone Gearing Center Normal Diametral Pitches
0.5-1.99
2-3.49
3.5-5.99
6-9.99
10-19.99
Distance (Inches)
Backlash, Normal Plane, Inches a
Up to 5
… … … …
… … …
… …
…
.005-.015 .010-.020 .010-.020
Over 5 to 10 Over 10 to 20 Over 20 to 30 Over 30 to 40 Over 40 to 50 Over 50 to 80 Over 80 to 100 Over 100 to 120
.010-.020 .015-.025 .020-.030 .025-.035 .030-.040
.020-.030 .025-.030 .030-.040 .035-.050 .040-.060
.030-.040 .035-.045 .040-.055 .045-.065 .050-.080
… … … … …
.040-.060 .050-.070 .060-.080 .070-.095
… … …
… …
.080-.110 … a Suggested backlash, on nominal centers, measured after rotating to the point of closest engagement. For helical and herringbone gears, divide above values by the cosine of the helix angle to obtain the transverse backlash. … The above backlash tolerances contain allowance for gear expansion due to differential in the operating temperature of the gearing and their supporting structure. The values may be used where the operating temperatures are up to 70 deg F higher than the ambient temperature. For most gearing applications the recommended backlash ranges will provide proper running clearance between engaging teeth of mating gears. Deviation below the minimum or above the maximum values shown, which do not affect operational use of the gearing, should not be cause for rejection. Definite backlash tolerances on coarse-pitch gearing are to be considered binding on the gear manufacturer only when agreed upon in writing. Some applications may require less backlash than shown in the above table. In such cases the amount and tolerance should be by agreement between manufacturer and purchaser.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online