Machinery's Handbook, 31st Edition
BUTTRESS THREADS 2079 are screwed together. Examples of actual applications are the breech assemblies of large guns, airplane propeller hubs, and columns for hydraulic presses. 7 ° /45 ° Buttress Thread Form.— In selecting the form of thread recommended as standard, ANSI/ASME B1.9-1973 (R2017), manufacture by milling, grinding, rolling, or other suit able means, has been taken into consideration. All dimensions are in inches. Form of Thread: The form of the buttress thread is shown in the accompanying Fig. 2a and Fig. 2b , and has the following characteristics: a) A load flank angle, measured in an axial plane, of 7 degrees from the normal to the axis. b) A clearance flank angle, measured in an axial plane, of 45 degrees from the normal to the axis. c) Equal truncations at the crests of the external and internal threads such that the basic height of thread engagement (assuming no allowance) is equal to 0.6 of the pitch b) Equal radii, at the roots of the external and internal basic thread forms tangential to the load flank and the clearance flank. (There is, in practice, almost no chance that the thread forms will be achieved strictly as basically specified, that is, as true radii.) When specified, equal flat roots of the external and internal thread may be supplied. Table 1. American National Standard Diameter—Pitch Combinations for 7 °/ 45 ° Buttress Threads ANSI/ASME B1.9-1973 (R2017)
Preferred Nominal Major Diameters, Inches 0.5, 0.625, 0.75 0.875, 1.0 1.25, 1.375, 1.5 2.75, 3, 3.5, 4
Preferred Nominal Major Diameters, Inches
Threads per Inch a
Threads per Inch a
(20, 16, 12) (16, 12, 10) 16, (12, 10, 8), 6
4.5, 5, 5.5, 6 7, 8, 9, 10 11, 12, 14, 16
12, 10, 8, (6, 5, 4), 3 10, 8, 6, (5, 4, 3), 2.5, 2 10, 8, 6, 5, (4, 3, 2.5), 2, 1.5, 1.25 8, 6, 5, 4, (3, 2.5, 2), 1.5, 1.25, 1
1.75, 2, 2.25, 2.5 16, 12, (10, 8, 6), 5, 4 18, 20, 22, 24
16, 12, 10, (8, 6, 5), 4 a Preferred threads per inch are in parentheses. Table 2. American National Standard Inch Buttress Screw Threads— Basic Dimensions ANSI/ASME B1.9-1973 (R2017)
Max. Root Trunca tion, b s = 0.0826 p
Max. Root Radius, c r = 0.0714 p
Crest Truncation, f = 0.14532 p
Height of Thread, h s or h n = 0.66271 p
Width of Flat at Crest, F = 0.16316 p
Basic Height of Thread, h = 0.6 p
Height of Sharp-V Thread, H = 0.89064 p
Thds. a per Inch
Pitch, p 0.0500 0.0625 0.0833 0.1000 0.1250 0.1667 0.2000 0.2500 0.3333 0.4000 0.5000 0.6667 0.8000 1.0000
20 16 12 10
0.0300 0.0375 0.0500 0.0600 0.0750 0.1000 0.1200 0.1500 0.2000 0.2400 0.3000 0.4000 0.4800 0.6000
0.0445 0.0557 0.0742 0.0891 0.1113 0.1484 0.1781 0.2227 0.2969 0.3563 0.4453 0.5938 0.7125 0.8906
0.0073 0.0091 0.0121 0.0145 0.0182 0.0242 0.0291 0.0363 0.0484 0.0581 0.0727 0.0969 0.1163 0.1453
0.0331 0.0414 0.0552 0.0663 0.0828 0.1105 0.1325 0.1657 0.2209 0.2651 0.3314 0.4418 0.5302 0.6627
0.0041 0.0052 0.0069 0.0083 0.0103 0.0138 0.0165 0.0207 0.0275 0.0330 0.0413 0.0551 0.0661 0.0826
0.0036 0.0045 0.0059 0.0071 0.0089 0.0119 0.0143 0.0179 0.0238 0.0286 0.0357 0.0476 0.0572 0.0714
0.0082 0.0102 0.0136 0.0163 0.0204 0.0271 0.0326 0.0408 0.0543 0.0653 0.0816 0.1088 0.1305 0.1632
8 6 5 4 3
2 1 ∕ 2
2
1 1 ∕ 2 1 1 ∕ 4
1
a All other dimensions are in inches. b Minimum root truncation is one-half of maximum. c Minimum root radius is one-half of maximum.
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