Machinery's Handbook, 31st Edition
Backlash in Gears 2247 Tooth thickness of fine-pitch spur and helical gears can best be checked on a variable- center-distance fixture using a master gear. When checked in this manner, tooth thickness change = 2 × center distance change × tangent of transverse pressure angle, approximately. Control of Backlash in Assemblies.— Provision is often made for adjusting one gear rel ative to the other, thereby affording complete control over backlash at initial assembly and throughout the life of the gears. Such practice is most common in bevel gearing. It is fairly common in spur and helical gearing when the application permits slight changes between shaft centers. It is practical in worm gearing only for single thread worms with low lead angles. Otherwise faulty contact results. Another method of controlling backlash quite common in bevel gears and less common in spur and helical gears is to match the high and low spots of the runout gears of one to one ratio and mark the engaging teeth at the point where the runout of one gear cancels the runout of the mating gear. Table 3. AGMA Backlash Allowance and Tolerance for Fine-Pitch Spur, Helical and Herringbone Gearing
Tooth Thinning to Obtain Backlash a
Resulting Approximate Backlash (per Mesh) Normal Plane b Inch
Normal Diametral Pitch Range 20 thru 45 46 thru 70 71 thru 90 91 thru 200 20 thru 60 61 thru 120 121 thru 200 20 thru 60 61 thru 120 121 thru 200 20 thru 60 61 thru 120 121 thru 200 20 thru 60 61 thru 120
Allowance, per Gear, Inch
Tolerance, per Gear, Inch 0 to .002 0 to .002 0 to .00175 0 to .00075 0 to .001 0 to .00075 0 to .0005 0 to .0005 0 to .0004 0 to .0003 0 to .00025 0 to .0002 0 to .0001 0 to .00025 0 to .0002
Backlash Designation
.002 .0015 .001 .00075 .00075 .0005 .0005 .00035 .001 .0002 .00025 .0002 .0001
.004 to .008 .003 to .007 .002 to .0055 .0015 to .003 .002 to .004 .0015 to .003 .001 to .002 .001 to .002 .0007 to .0015 .0004 to .001 .0005 to .001 .0004 to .0008 .0002 to .0004 0 to .0005 0 to .0004
A
B
C
D
Zero c
E
121 thru 200 0 to .0002 a These dimensions are shown primarily for the benefit of the gear manufacturer and represent the amount that the thickness of teeth should be reduced in the pinion and gear below the standard calculated value, to provide for backlash in the mesh. In some cases, particularly with pinions involving small numbers of teeth, it may be desirable to provide for total backlash by thinning the teeth in the gear member only by twice the allowance value shown in column (3). In this case both members will have the tolerance shown in column (4). In some cases, particularly in meshes with a small number of teeth, backlash may be achieved by an increase in basic center at distance. In such cases, neither member is reduced by the allowance shown in column (3). 0 to .0001 b These dimensions indicate the approximate backlash that will occur in a mesh in which each of the mating pairs of gears have the teeth thinned by the amount referred to in Note 1, and are meshed on theoretical centers. c Backlash in gear sets can also be achieved by increasing the center distance above nominal and using the teeth at standard tooth thickness. Class E backlash designation infers gear sets operating under these conditions. Backlash in gears is the play between mating tooth surfaces. For purposes of measurement and calculation, backlash is defined as the amount by which a tooth space exceeds the thickness of an engaging tooth. When not otherwise specified, numerical values of backlash are understood to be measured at the tightest point of mesh on the pitch circle in a direction normal to the tooth surface when the gears are mounted in their specified position. Allowance is the basic amount that a tooth is thinned from basic calculated circular tooth thickness to obtain the required backlash class. Tolerance is the total permissible variation in the circular thickness of the teeth.
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