TORQUE LOADING

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.


Formula
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.

Solid Rivets

In the construction of a metal air frame, permanent joints are made either with rivets or bolts. To securely attach structures together, rivets are cheaper to use, lighter and more rapidly fitted than nuts and bolts, but in the case of power operated machine riveting, more extensive equipment is usually required to make the permanent joints.

Solid rivets have the greatest strength and are therefore preferable to any other type of rivet, but they can only be used where there is access to both sides of the structure.

Rivets are always supplied to the operator with one head already formed and the shank plain to permit insertion into the hole, the opposite end being formed into a head by manual or mechanical means. The size of a rivet is expressed as the diameter and length of its shank; the exception is the countersunk rivet where the length is inclusive of the head

1. SNAP HEAD : for general purposes where strength is required but not a streamline finish.
2. MUSHROOM HEAD: for skin covering to give maximum strength.
3. FLAT HEAD: for internal work where heads are not easily accessible
4. COUNTERSUNK:  for flush finish (90°, 100°, 120° head) in aviation mostly used 100°
5. RAISED COUNTERSUNK : for more streamlined surfaces.
6. UNIVERSAL HEAD
7. 100° COUNTERSUNK TRUNCATED RADIUS HEAD:  

Chobert Rivets

Chobert rivets are made with either snap or countersunk heads.  The action of closing a Chobert rivet is illustrated.  When the mandrel is pulled through the tapered hole the rivet head is formed.  As the mandrel continues pulling through the rivet, the shank expands and fills the hole.

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Sealing pins can be fitted in the rivet bore.  This increases the shear strength of the rivets and prevents moisture entering.

The manufacturer’s rivet part number is in the form of a code giving rivet material and diameter and grip length in ¹/₃₂ in.

Chobert rivets with oversize shanks are available for repair work on aircraft.

The chobert rivet is closed with a steel mandrel which forms part of the riveting tool.  A different sized mandrel is used with each diameter of rivet.  Before use it should be checked for diameter using an approved gauge.  The mandrel must also be inspected for scratches or other damage and must be clean and lightly lubricated.  Mandrels must be replaced when the head diameter has been reduced by 0.002 inch.

Strength of Riveted Joints

The factors that govern the strength of a joint are:

Plate specification             This will be of such a material and gauge as to  successfully withstand tensile and bearing loads.

Rivet Specification             This will be selected to withstand shear loads.  In cases where the

specification of the rivet is not given, use a rivet of the same material as the plate, with a diameter

of 2¹/₂ where T is the thickness of the plate.

Rivet Pitch    This is important as too great a pitch will result in insufficient rivets to take the shear loads and too small a pitch will result in lowering the resistance of the plate to tensile loads.

Types of Rivet Spacing

Single Chain            Used chiefly on attachment and lightly stressed joints.

Multiply Chain         Used on watertight joints and in places of high stress where thick gauge plate is used.

Staggered Riveting            Used as an alternative to multiple chain in watertight joints, circular patches etc.