Machinery's Handbook, 31st Edition
Transmission Roller Chains 2621 Sprocket Classes.— The American National Standard ANSI/ASME B29.1-2011 (R2016) provides for two classes of sprockets designated as Commercial and Precision. The selec- tion of either is a matter of drive application judgment. The usual moderate to slow speed commercial drive is adequately served by Commercial sprockets. Where extreme high speed in combination with high load is involved, or where the drive involves fixed centers, critical timing, or register problems, or close clearance with outside interference, then the use of Precision sprockets may be more appropriate. As a general guide, drives requiring Type A or Type B lubrication (see page 2638) would be served by Commercial sprockets. Drives requiring Type C lubrication may require Precision sprockets; the manufacturer should be consulted. Keys, Keyways, and Set Screws.— To secure sprockets to the shaft, both keys and set screws should be used. The key is used to prevent rotation of the sprocket on the shaft. Keys should be fitted carefully in the shaft and sprocket keyways to eliminate all backlash, especially on the fluctuating loads. A set screw should be located over a flat key to secure it against longitudinal displacement. Where a set screw is to be used with a parallel key, the following sizes are recommended by the American Chain Association. For a sprocket bore and shaft diameter in the range of: 1 ⁄ 2 through 7 ⁄ 8 inch, a 1 ⁄ 4 -inch set screw 15 ⁄ 16 through 1 3 ⁄ 4 inches, a 3 ⁄ 8 -inch set screw 1 13 ⁄ 16 through 2 1 ⁄ 4 inches, a 1 ⁄ 2 -inch set screw 2 5 ⁄ 16 through 3 1 ⁄ 4 inches, a 5 ⁄ 8 -inch set screw 3 3 ⁄ 8 through 4 1 ⁄ 2 inches, a 3 ⁄ 4 -inch set screw 4 3 ⁄ 4 through 5 1 ⁄ 2 inches, a 7 ⁄ 8 -inch set screw 5 3 ⁄ 4 through 7 3 ⁄ 8 inches, a 1-inch set screw 7 1 ⁄ 2 through 12 1 ⁄ 2 inches, a 1 1 ⁄ 4 -inch set screw Sprocket Diameters.— The various diameters of roller chain sprockets are shown in Fig. 2 . These are defined as follows. Pitch Diameter: The pitch diameter is the diameter of the pitch circle that passes through the centers of the link pins as the chain is wrapped on the sprocket.
Bore
Caliper diameter
Fig. 2. Sprocket Diameters Because the chain pitch is measured on a straight line between the centers of adjacent pins, the chain pitch lines form a series of chords of the sprocket pitch circle. Sprocket pitch diameters for one-inch pitch and for 9 to 108 teeth are given in Table 4. For lower (5 to 8) or higher (109 to 200) numbers of teeth use the following formula in which P = pitch, N = number of teeth: Pitch Diameter = P ÷ sin (180 ° ÷ N ). Bottom Diameter: The bottom diameter is the diameter of a circle tangent to the curve (called the seating curve) at the bottom of the tooth gap. It equals the pitch diameter minus the diameter of the roller.
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