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1850 ANSI BUTTRESS THREADS Löwenherz Thread Diameter Approximate Diameter Pitch, No. of Threads Millimeters Inches Millimeters per Inch Millimeters Inches Pitch, Millimeters American National Standard Buttress Inch Screw Threads Approximate No. of Threads per Inch 1.0 0.0394 0.25 101.6 9.0 0.3543 1.30 19.5 1.2 0.0472 0.25 101.6 10.0 0.3937 1.40 18.1 1.4 0.0551 0.30 84.7 12.0 0.4724 1.60 15.9 1.7 0.0669 0.35 72.6 14.0 0.5512 1.80 14.1 2.0 0.0787 0.40 63.5 16.0 0.6299 2.00 12.7 2.3 0.0905 0.40 63.5 18.0 0.7087 2.20 11.5 2.6 0.1024 0.45 56.4 20.0 0.7874 2.40 10.6 3.0 0.1181 0.50 50.8 22.0 0.8661 2.80 9.1 3.5 0.1378 0.60 42.3 24.0 0.9450 2.80 9.1 4.0 0.1575 0.70 36.3 26.0 1.0236 3.20 7.9 4.5 0.1772 0.75 33.9 28.0 1.1024 3.20 7.9 5.0 0.1968 0.80 31.7 30.0 1.1811 3.60 7.1 5.5 0.2165 0.90 28.2 32.0 1.2599 3.60 7.1 6.0 0.2362 1.00 25.4 36.0 1.4173 4.00 6.4 7.0 0.2756 1.10 23.1 40.0 1.5748 4.40 5.7 8.0 0.3150 1.20 21.1 … … … … The buttress form of thread has certain advantages in applications involving exceptionally high stresses along the thread axis in one direction only. As the thrust side (load flank) of the standard buttress thread is made very nearly perpendicular to the thread axis, the radial component of the thrust is reduced to a minimum. On account of the small radial thrust, the buttress form of thread is particularly applicable when tubular members 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 B1.9-1973 (R1992), manufacture by milling, grinding, rolling, or other suitable 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 Figs. 2a and 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 d) 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 B1.9-1973 (R1992) Preferred Nominal Major Diameters, Inches Machinery's Handbook 27th Edition Threads per Inch a Preferred Nominal Major Diameters, Inches Threads per Inch a 0.5, 0.625, 0.75 (20, 16, 12) 4.5, 5, 5.5, 6 12, 10, 8, (6, 5, 4), 3 0.875, 1.0 (16, 12, 10) 7, 8, 9, 10 10, 8, 6, (5, 4, 3), 2.5, 2 1.25, 1.375, 1.5 16, (12, 10, 8), 6 11, 12, 14, 16 10, 8, 6, 5, (4, 3, 2.5), 2, 1.5, 1.25 1.75, 2, 2.25, 2.5 16, 12, (10, 8, 6), 5, 4 18, 20, 22, 24 8, 6, 5, 4, (3, 2.5, 2), 1.5, 1.25, 1 2.75, 3, 3.5, 4 16, 12, 10, (8, 6, 5), 4 a Preferred threads per inch are in parentheses. Copyright 2004, Industrial Press, Inc., New York, NY
Thds. a per Inch ANSI BUTTRESS THREADS 1851 Table 2. American National Standard Inch Buttress Screw Threads— Basic Dimensions ANSI B1.9-1973 (R1992) Pitch, p Machinery's Handbook 27th Edition Basic Height of Thread, h = 0.6p Height of Sharp-V Thread,H = 0.89064p Crest Truncation, f = 0.14532p Height of Thread, h s or h n = 0.66271p Max. Root Truncation, b s = 0.0826p Max. Root Radius, c r = 0.0714p 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. Buttress Thread Tolerances.—Tolerances from basic size on external threads are applied in a minus direction and on internal threads in a plus direction. Pitch Diameter Tolerances: The following formula is used for determining the pitch diameter product tolerance for Class 2 (standard grade) external or internal threads: where D=basic major diameter of external thread (assuming no allowance) L e =length of engagement p=pitch of thread When the length of engagement is taken as 10p, the formula reduces to Width of Flat at Crest, F = 0.16316p 20 0.0500 0.0300 0.0445 0.0073 0.0331 0.0041 0.0036 0.0082 16 0.0625 0.0375 0.0557 0.0091 0.0414 0.0052 0.0045 0.0102 12 0.0833 0.0500 0.0742 0.0121 0.0552 0.0069 0.0059 0.0136 10 0.1000 0.0600 0.0891 0.0145 0.0663 0.0083 0.0071 0.0163 8 0.1250 0.0750 0.1113 0.0182 0.0828 0.0103 0.0089 0.0204 6 0.1667 0.1000 0.1484 0.0242 0.1105 0.0138 0.0119 0.0271 5 0.2000 0.1200 0.1781 0.0291 0.1325 0.0165 0.0143 0.0326 4 0.2500 0.1500 0.2227 0.0363 0.1657 0.0207 0.0179 0.0408 3 0.3333 0.2000 0.2969 0.0484 0.2209 0.0275 0.0238 0.0543 21 ⁄ 2 0.4000 0.2400 0.3563 0.0581 0.2651 0.0330 0.0286 0.0653 2 0.5000 0.3000 0.4453 0.0727 0.3314 0.0413 0.0357 0.0816 11 ⁄ 2 0.6667 0.4000 0.5938 0.0969 0.4418 0.0551 0.0476 0.1088 11 ⁄ 4 0.8000 0.4800 0.7125 0.1163 0.5302 0.0661 0.0572 0.1305 1 1.0000 0.6000 0.8906 0.1453 0.6627 0.0826 0.0714 0.1632 PD tolerance = 0.002 3 D + 0.00278 Le + 0.00854 p 0.002 3 D + 0.0173 p It is to be noted that this formula relates specifically to Class 2 (standard grade) PD tolerances. Class 3 (precision grade) PD tolerances are two-thirds of Class 2 PD tolerances. Pitch diameter tolerances based on this latter formula, for various diameter pitch combinations, are given in Table 4. Functional Size: Deviations in lead and flank angle of product threads increase the functional size of an external thread and decrease the functional size of an internal thread by the cumulative effect of the diameter equivalents of these deviations. The functional size of all buttress product threads shall not exceed the maximum-material limit. Tolerances on Major Diameter of External Thread and Minor Diameter of Internal Thread: Unless otherwise specified, these tolerances should be the same as the pitch diameter tolerance for the class used. Tolerances on Minor Diameter of External Thread and Major Diameter of Internal Thread: It will be sufficient in most instances to state only the maximum minor diameter of the external thread and the minimum major diameter of the internal thread without any tol- Copyright 2004, Industrial Press, Inc., New York, NY
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