CHAINS AND THEIR ACCESSORIES – Aircraft

This article was written according to aircraft standards but the principles of Chains remain the same but if applying the knowledge to low power chains on Bicycles, Then you can make your own risk economic based reduced standards. If working on aircraft, always refer to the Maintenance manual.

The purpose of chains is to transfer power from one sprocket to another transfer kinetic(motion) energy. To accomplish this task you need at least an assembly of chain(s)(manufactured to SBAC standards like BS228:1994 or ISO606-1982), sprockets(toothed wheels) and possibly other accessories like interplaner blocks for changing the direction of the chains.

We will start with the main unit called the chain. This is an assembly of:-

  • outer plates
  • inner plates
  • Rollers
  • Bearing pins
  • Bushes

The pitch of the chain is taken from centre of one roller to the next one i.e. the distance between the bearing pins.

Maintenance

Chains riveted links are not allowed to be broken down and re-riveted and only accepted from an approved manufacturer, with the correct packaging and transportation precautions taken.

It is permitted that a bolted joint may be disassembled and re-assembled, but, it is worth while noting these next few points.

1) SBAC states that all nut and bolted chain assemblies must be peened with the exception of the 8mm variety which must be split pinned

2) All nuts used on the chains must be locknuts(this nut is normally part of the outer plate)

3) All attachments must be either riveted or bolted

4) SBAC have standardised four sizes of chains by pitch size

BUT

British Standards have laid down the proof load as one third of the minimum breaking load (mbl)

5) Continuous(endless) chains must consist of an even amount of gaps between rollers(pitches)

6) The following table is the four classifications of chains and their statistics.

PITCH — MBL(lbs) — PROOF LOAD(lbs) — BS
8mm ——— 800 ————- 267 ——————— 1
0.375″ ——- 1900 ———— 634 ——————— 2
0.5″ ———– 1800 ———— 600 ——————— 4
0.5″ ———– 3500 ———— 1166 ——————- 6

7) A non-reversible chain is a chain that may only fit a certain way round which can be achieved by correct unsymmetrical positioning by the outer plates fitted to the chain.

8) Irreversibility can be achieved by

  • Non-reversible chains
  • Non-interchangeable end fittings
  • Correct positioning of sprockets
  • Guards and shrouds

9) When storing a chain it must be well soaked in the approved oil, laid flat on its side on top of greaseproof paper and coiled firmly (not tight but not too loose).

GENERAL INSPECTION

This unit is used as a high strength power transfer device. When inspecting the chain make sure to check the proceedure in the maintenance manual which should at minimum account for the Following

  • Wear on the sprocket ( See spec in aircraft manual)
  • Wear on the rollers ( See spec. in aircraft manual)
  • Twisting (This is cause for rejection)
  • Stiffness ( This can be determined by drawing the chain around the fingers on your hand or maybe a substitute of the same sort of shape and texture [ You do not want to damage the chain by scraping the links on an object used in inspection] and checking for smooth articulation of the links. If stiffness is detected, the chain may first be cleaned and re-checked but if this is not a solution, the rollers may be lightly TAPPED preferably with a small pin hammer. if this does not fix it the chain is the scrapped.
  • Deformities, Cracks or corrosion ( These is cause for rejection)
  • Overheating (Bluing)
  • Elongation (Maximum of 2%)( To check for this the chain must be cleaned and dried with compressed air. Lay the chain flat and straight and apply a tensile load/ force [see table below]. Now measure the distance between all the pin centres and apply the following formula:

Percentage of Elongation (Method 1)

Length Of Chain with Force applied x (No. Of Pitches x Pitch measurement)
———————————————————————
No. Of Pitches x Pitch Measurement

Note: All measurements are in inches

Tensile load required on Chain (Size is British Standard)
Size – Load (lbs)
1 —- 12
2 —- 16
4 —- 28
6 —- 28

Percentage of Elongation (Method 2)

Measured Length
——————————— x100
Original Length