Learning Objective: Recognize the importance of the proper torquing of fasteners and the required torquing procedures.

Fastener fatigue failure accounts for the majority of all fastener problems. Fatigue breaks are caused by insufficient tightening and the lack of proper preload or clamping force. This results in movement between the parts of the assembly and bending back and forth or cyclic stressing of the fastener. Eventually, cracks will progress to the point where the fastener can no longer support its designed load. At this point the fastener fails with varying consequences.

For the nut to properly load the bolt and prevent premature failure, a designated amount of torque must be applied. Proper torque reduces the possibility of the fastener loosening while in service. The correct torque to apply when you are tightening an assembly is based on many variables. The fastener is subjected to two stresses when it is tightened. These stresses are torsion and tension. Tension is the desired stress, while torsion is the undesirable stress caused by friction. A large percentage of applied torque is used to overcome this friction, so that only tension remains after tightening. Proper tension reduces the possibility of fluid leaks.

Table 3-4.Recommended Torque Values (Inch-Pounds)

The recommended torque values provided in table 3-4 have been established for average dry, cadmium- plated nuts for both the fine and coarse thread series of nuts. Thread surface variations such as paint, lubrication, hardening, plating, and thread distortion may alter these values considerably. The torque values must be followed unless the MIM or structual repair manual for the specific aircraft requires a specific torque for a given nut. Torque values vary slightly with manufacturers. When the torque values are included in a

Figure 3-12.Torque wrenches.

technical manual, these values take precedence over the standard torque values provided in the of nuts, bolts, and screws used in aircraft construction. You should use this manual when specific torque values are not provided as a part of the removal/replacement instructions.

To obtain values in foot-pounds, divide inch-pound values by 12. Do not lubricate nuts or bolts except for corrosion-resistant instructed to do so, steel parts or where specifically Always tighten by rotating the nut first if possible. When space considerations make it necessary to tighten the fastener by rotating the bolt head, approach the high side of the indicated torque range. Do not exceed the maximum allowable torque value. Maximum torque ranges should be used only when materials and surfaces being joined are of sufficient thickness, area, and strength to resist breaking, warping, or other damage. 

For corrosion-resisting steel nuts, use the torque values given for shear-type nuts. The use of any type of drive-end extension on a torque wrench changes the dial reading required to obtain the actual values indicated in the torque range tables. See figure 3-12.