Machinery's Handbook, 31st Edition
2240 Gear Blanks Design of Gear Blanks: The accuracy to which gears can be produced is affected by the design of the blank, so the following points of design should be noted: 1) Gears de- signed with a hole should have the hole large enough that the blank can be adequately supported during machining of the teeth and yet not so large as to cause distortion; 2) Face widths should be wide enough, in proportion to outside diameters, to avoid spring- ing and to permit obtaining flatness in important surfaces; 3) Short bore lengths should be avoided wherever possible. It is feasible, however, to machine relatively thin blanks in stacks, provided the surfaces are flat and parallel to each other; 4) Where gear blanks with hubs are to be designed, attention should be given to the wall sections of the hubs. Too thin a section will not permit proper clamping of the blank during machining operations and may also affect proper mounting of the gear; and 5) Where pinions or gears integral with their shafts are to be designed, deflection of the shaft can be minimized by having the shaft length and shaft diameter well proportioned to the gear or pinion diameter. The foregoing general principles may also be useful when applied to blanks for coarser pitch gears. Specifying Spur and Helical Gear Data on Drawings.— The data that may be shown on drawings of spur and helical gears fall into three groups: The first group consists of data basic to the design of the gear; the second group consists of data used in manufacturing and inspection; and the third group consists of engineering reference data. The accompa- nying table may be used as a checklist for the various data which may be placed on gear drawings and the sequence in which they should appear. Explanation of Terms Used in Gear Specifications: 1) Number of teeth is the number of teeth in 360 deg of gear circumference. In a sector gear, both the actual number of teeth in the sector and the theoretical number of teeth in 360 deg should be given. 2) Diametral pitch is the ratio of the number of teeth in the gear to the number of inches in the standard pitch diameter. It is used in this standard as a nominal specification of tooth size. a) Normal diametral pitch is the diametral pitch in the normal plane. b) Transverse diametral pitch is the diametral pitch in the transverse plane. c) Module is the ratio of the number of teeth in the gear to the number of mm in the standard pitch diameter. d) Normal module is the module measured in the normal plane. e) Transverse module is the module measured in the transverse plane. 3) Pressure angle is the angle between the gear tooth profile and a radial line at the pitch point. It is used in this standard to specify the pressure angle of the basic rack used in defining the gear tooth profile. a) Normal pressure angle is the pressure angle in the normal plane. b) Transverse pressure angle is the pressure angle in the transverse plane. 4) Helix angle is the angle between the pitch helix and an element of the pitch cylinder, unless otherwise specified. a) Hand of helix is the direction in which the teeth twist as they recede from an observer along the axis. A right hand helix twists clockwise and a left hand helix twists counterclockwise. 5) Standard pitch diameter is the diameter of the pitch circle. It equals the number of teeth divided by the transverse diametral pitch. 6) Tooth form may be specified as standard addendum, long addendum, short addendum, modified involute or special. If a modified involute or special tooth form is required, a detailed view should be shown on the drawing. If a special tooth form is specified, roll angles must be supplied (see page 2237 ). 7) Addendum is the radial distance between the standard pitch circle and the outside cir cle. The actual value depends on the specification of outside diameter. 8) Whole depth is the total radial depth of the tooth space. The actual value is dependent on the specification of outside diameter and root diameter.
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