The purpose of torque loading is to make sure the correct and efficient clamping together of two surfaces. This prevents over-stressing, distortion, and shearing of bolts, studs, nuts etc. The majority of bolts, nuts and unions on aircraft are subject to a standard torque loading. Special bolts are subjected to torque loading, which are specified in the aircraft maintenance manual. There will be a range for the torque value in most cases an Torque load values are normally determined by friction, type of thread, material, lubrication and finish of the surfaces of the fasteners.
Torque = Force X Length
Under tightening of screw fasteners may result in lack of firmness between the separate parts of the assembly causing fretting corrosion due to relative movement , and early failure of a component may occur through fatigue or mechanical breakdown. Conversely, over-tightening is likely to cause immediate failure of the bolts, distortion of one or more parts of the assembly leading to eventual failure, or to stress corrosion, or cracking this will result by the high stressed areas cause by over torque. Due to the varying effects of friction under different conditions of assembly it is important that torque be applied in accordance with the manufacturer's instructions. The pre-load applied to a fastener at a specified lubricated torque would be considerably higher than if the same torque were applied dry.
Before even start to use torque wrench make sure All torque wrenches that are used on aircraft are regularly inspected, tested and calibrated by a facility equipped to do so.if the serviceable tag indicates as expired. DO NOT USE THE TOOL.
How to do it :
- Clean, and lubricate the threads(if instructed on AMM) and mating surfaces of nut, bolt and washer.
- Tighten the nut to half the specified torque value.
- Loosen the nut then finally re-tighten to the specified torque value.
- When the torque-loaded fastener is to be secured by means of a split pin or lock wire, tighten first to the low side of the torque range. If necessary, tighten the fastener so that the next slot aligns with the hole, ensuring that the maximum torque is not exceeded. If the maximum torque is reached and the slot in the nut does not line up with the hole in the bolt, the nut and/or washer must be changed.