Machinery's Handbook, 31st Edition
1654 TORQUE AND TENSION IN FASTENERS The proper torque to use for tightening bolts in sizes up to about 1 ∕ 2 inch (12.7 mm) may also be determined by trial. Test a bolt by measuring the amount of torque required to frac ture it (use bolt, nut, and washers equivalent to those chosen for the real application). Then, use a tightening torque of about 50 to 60 percent of the fracture torque determined by the test. The tension in a bolt tightened using this procedure will be about 60 to 70 percent of the elastic limit (yield strength) of the bolt material. The table that follows can be used to get a rough idea of the torque necessary to properly tension a bolt by using the bolt diameter d and the coefficients b and m from the table; the approximate tightening torque T in ft-lb for the listed fasteners is obtained by solving the equation T = 10 b + m log d . This equation is approximate, for use with unlubricated fasteners as supplied by the mill. See the notes at the end of the table for more details on using the equation. Wrench Torque T = 10 b + m log d for Steel Bolts, Studs, and Cap Screws ( see notes ) Fastener Grade(s) Bolt Diameter d (in.) m b SAE 2, ASTM A307 1 ∕ 4 to 3 2.940 2.533 SAE 3 1 ∕ 4 to 3 3.060 2.775 ASTM A449, A354-BB, SAE 5 1 ∕ 4 to 3 2.965 2.759 ASTM A325 a 1 ∕ 2 to 1 1 ∕ 2 2.922 2.893 ASTM A354-BC 1 ∕ 4 to 5 ∕ 8 3.046 2.837 SAE 6, SAE 7 1 ∕ 4 to 3 3.095 2.948 SAE 8 1 ∕ 4 to 3 3.095 2.983 ASTM A354-BD, ASTM A490 a 3 ∕ 8 to 1 3 ∕ 4 3.092 3.057 Socket Head Cap Screws 1 ∕ 4 to 3 3.096 3.014 a Values for permanent fastenings on steel structures. Usage: Values calculated using the preceding equation are for standard, unplated industrial fasteners as received from the manufacturer; for cadmium-plated cap screws, multiply the torque by 0.9; for cadmium-plated nuts and bolts, multiply the torque by 0.8; for fasteners used with special lubricants, multiply the torque by 0.9; for studs, use cap screw values for equivalent grade. Preload for Bolts in Loaded Joints.— The following recommendations are based on MIL-HDBK-60, a subsection of FED-STD-H28, Screw Thread Standards for Federal Service. Generally, bolt preload in joints should be high enough to maintain joint mem - bers in contact and in compression. Loss of compression in a joint may result in leakage of pressurized fluids past compression gaskets, loosening of fasteners under conditions of cyclic loading, and reduction of fastener fatigue life. The relationship between fastener fatigue life and fastener preload is illustrated by Fig. 1 . An axially loaded bolted joint in which there is no bolt preload is represented by line OAB, that is, the bolt load is equal to the joint load. When joint load varies between P a and P b , the bolt load varies accordingly between P Ba and P Bb . However, if preload P B 1 ′ is applied to the bolt, the joint is compressed and bolt load changes more slowly than the joint load (indicated by line P B 1 ′ A, whose slope is less than line OAB) because some of the load is absorbed as a reduction of compression in the joint. Thus, the axial load applied to the joint varies between P Ba ′ and P Bb ′ as joint load varies between P a and P b . This condition results in a considerable reduction in cyclic bolt-load variation and thereby increases the fatigue life of the fastener. Preload for Bolts in Shear.— In shear-loaded joints, with members that slide, the joint members transmit shear loads to the fasteners in the joint and the preload must be suffi - cient to hold the joint members in contact. In joints that do not slide (i.e., there is no relative motion between joint members), shear loads are transmitted within the joint by frictional forces that mainly result from the preload. Therefore, preload must be great enough for the resulting friction forces to be greater than the applied shear force. With high applied shear loads, the shear stress induced in the fastener during application of the preload must also
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