Sunday, November 13, 2016

Heat Treatment: Critical Points of Steel

When a piece of steel is heated at a uniform rate, the temperature of the steel will, at first, rise steadily.  When the temperature reaches approx.700°C (a dull red colour) it will remain there for certain time then continue to rise again at a uniform rate.  If the heating is continued further then occurs a second arrest in the rise in temperature.  After this, if the heating is continued, the temperature will continue to rise at approximately the initial rate.

During these periods of arrest, the metal absorbs heat, but instead of raising the temperature, the heat brings about a structural change in the steel.  The temperatures at which these periods occur are called ‘critical’ or ‘arrest’ points.

If the steel is heated to 900°C (a bright reddish yellow colour) then removed from the furnace and observed in a darkened room, it will be seen that, as it cools, it will lose its brilliance.  At the points where it received its checks in heating the metal is seen to glow more brightly and it will seem that the cooling has stopped.  In fact the steel will be seen to take on an extra glow as though it was reheated.  After this the rate of cooling will be normal until the metal is cold to room temperature or becomes equal to environmental temperature. 

The temperature at which the changes start (lower critical point) is the same for all steels and is about 700°C.  At this temperature, the internal structural change is as that the pearlite disappears and the laminae of ferrite and cementite of which it is composed, dissolves and forms the solid solution Austenite.  The finishing point of the transformation is known as the upper critical point this point varies according to the steel carbon content.  


Saturday, September 26, 2015

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.


Tuesday, December 16, 2014

Types Of Landing Gears

The various types of landing gear are dependent on the design and manufacture of the undercarriage units, 

The main type of landing gear for aircraft over 5700 Kg is termed the TRICYCLE UNDERCARRIAGE.

The tricycle undercarriage landing gear comprises two main undercarriage units and one nose undercarriage unit.  However, there are THREE main variations of the tricycle undercarriage as follows:


Standard (Boeing 737)


·         Two main undercarriage units
·         One nose undercarriage unit







Centreline (Airbus A340)


·         Two main undercarriage units
·         One center-line undercarriage unit
·         One nose undercarriage unit


Wing/Body (Boeing 747)


·         Two main (outer) wing mounted, undercarriage units
·         Two main (inner) body mounted, undercarriage units
·        
One nose undercarriage unit