Machinery's Handbook, 31st Edition
Internal Gearing
2249
Internal Gearing Internal Spur Gears.— An internal gear may be proportioned like a standard spur gear turned “outside in” or with addendum and dedendum in reverse positions; however, to avoid interference or improve the tooth form and action, the internal diameter of the gear should be increased and the outside diameter of the mating pinion is also made larger than the size based upon standard or conventional tooth proportions. The extent of these en- largements will be illustrated by means of examples given in the following table, Rules for Internal Gears – 20-degree Full-Depth Teeth . The 20-degree involute full-depth tooth form is recommended for internal gears; the 20-degree stub tooth and the 14 1 ∕ 2 -degree full-depth tooth are also used. Methods of Cutting Internal Gears.— Internal spur gears are cut by methods similar in principle to those employed for external spur gears. They may be cut by one of the following methods: 1) By a generating process, as when using a Fellows gear shaper; 2) by using a formed cutter and milling the teeth; 3) by plan ing, using a machine of the template or form-copying type (especially applicable to gears of large pitch); and 4) by using a formed tool that reproduces its shape and is given a plan ing action either on a slotting or a planing type of machine. Internal gears frequently have a web at one side that limits the amount of clearance space at the ends of the teeth. Such gears may be cut readily on a gear shaper. The most practical method of cutting very large internal gears is on a planer of the form-copying type. A regular spur gear planer is equipped with a special tool holder for locating the tool in the position required for cutting internal teeth. Formed Cutters for Internal Gears.— When formed cutters are used, a special cutter usu- ally is desirable, because the tooth spaces of an internal gear are not the same shape as the tooth spaces of external gearing having the same pitch and number of teeth. This differ ence is because an internal gear is a spur gear “turned outside in.” According to one rule, the standard No. 1 cutter for external gearing may be used for internal gears of 4 diametral pitch and finer, when there are 60 or more teeth. This No. 1 cutter, as applied to external gearing, is intended for all gears having from 135 teeth to a rack. The finer the pitch and the larger the number of teeth, the better the results obtained with a No. 1 cutter. The standard No. 1 cutter is considered satisfactory for jobbing work, and usually when the number of gears to be cut does not warrant obtaining a special cutter, although the use of the No. 1 cutter is not practicable when the number of teeth in the pinion is large in proportion to the number of teeth in the internal gear. Arc Thickness of Internal Gear Tooth.— Rule: If internal diameter of an internal gear is enlarged as determined by Rules 1 and 2 for Internal Diameters (see Rules for Internal Gears – 20-degree Full-Depth Teeth ), the arc tooth thickness at the pitch circle equals 1.3888 divided by the diametral pitch, assuming a pressure angle of 20 degrees. Arc Thickness of Pinion Tooth.— Rule: If the pinion for an internal gear is larger than con- ventional size (see Outside Diameter of Pinion for Internal Gear, under Rules for Internal Gears – 20-degree Full-Depth Teeth ), then the arc tooth thickness on the pitch circle equals 1.7528 divided by the diametral pitch, assuming a pressure angle of 20 degrees. Note: For chordal thickness and chordal addendum, see rules and formulas for spur gears. Relative Sizes of Internal Gear and Pinion.— If a pinion is too large or too near the size of its mating internal gear, serious interference or modification of the tooth shape may occur. Rule: For internal gears having a 20-degree pressure angle and full-depth teeth, the dif ference between the numbers of teeth in gear and pinion should not be less than 12. For teeth of stub form, the smallest difference should be 7 or 8 teeth. For a pressure angle of 14 1 ∕ 2 degrees, the difference in tooth numbers should not be less than 15.
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