STANDARD TORQUE LIMIT APPLICATION

TM 55-1520-240-23-1 1-13 STANDARD TORQUE LIMIT APPLICATION 1-13 This task provides standard torque limits for threaded fasteners, hose and tube coupling nuts, bulkhead fittings, and pipe thread fittings, and the minimum breakaway torque values for determining reusability of self-locking nuts. These standard torque values apply only when special torque values are not specified in procedures. General instructions for installation and fit of threaded fasteners are as follows: 1. Thread shall not be in bearing when thickness of sheet or fitting is 3/32 inch or less. If thickness is more than 3/32 inch, a maximum of two threads in bearing is permissible. 2. In shear applications, thread shall not be in bearing regardless of material thickness. 3. Washers are used for the following purposes: a. To compensate for differences in bolt grip length and material thickness due to manufacturing tolerances, protective coating, and other surface variances. b. To distribute bearing load over a greater area to prevent damage to material under a bolt head or nut. c. To protect the material surface when a bolt or nut is tightened. d. Prevent galling of aluminum or other soft material when bolt or nut is tightened. e. To insulate dissimilar metals to prevent corrosion. The washer material should be similar to the material on which it rests rather than the bolt or nut material. This insures that it corrosion occurs, if will be between the bolt and washer, which can be replaced. 4. When nut-bolt assemblies are installed, the nut shall not engage the first incomplete thread next to the bolt shank. 5. Nuts are properly installed when all threads are engaged and the bolt chamfer extends thru the nut. When flat-end bolts are used, the threaded end must extend at least 1/32 inch thru the nut. 6. Threads shall be clean and dry before installation. If threads are lubricated, torque limits are reduced by 30 percent. 7. The tightening sequence in multiple fitting installation is as follows: a. Finger tighten all bolts or nuts. b. Snug up opposite bolts or nuts all around. c. Tighten opposite bolts or nuts all around to proper torque. Do not torque adjacent bolts or nuts in sequence. 8. All-metal self-locking nuts shall be replaced with new identical parts at each installation. If new nuts are not available, all metal self-locking nuts may be reused. The reused nuts must meet the required minimum friction torque. See "Friction Torque in Inch-Pounds for Threaded Fasteners" in this task. 1-12