Gear Wear Pattern Analysis

Chain Maintenance

• To check for elongation, apply a specified load to the chain, measure the length, and calculate the extension using the formula: Extension (%) = (M - (X × P)) × 100 / (X × P)
• Replace the whole chain if elongation is greater than 2%, as it can compromise its strength
• Chain tension is checked by measuring chain deflection under a given load

Chain Tensioning

• Chain tensioning is adjusted using turnbuckles or adjustable sprockets
• The chain should be tightened enough to remove any backlash
• Too much tension causes the chain rollers to bind on the sprocket teeth

Criteria for Chain Rejection

• The chain section is elongated by more than 2%
• Any links are stiff or jammed
• Any part of the chain is kinked or twisted
• Any plates are cracked, worn, or corroded
• Any rollers are cracked, worn, corroded, or jammed

Chain Storage

• Correct storage of chains is important to minimize the possibility of corrosion or kinking of the links
• Chains should be cleaned, dried, and soaked in light lubricating oil before storage
• Chains should be coiled without twisting and checked for kinks or twists

Pulleys and Belt Inspection

• Pulleys should turn freely when the control cables are moved
• If a pulley is out of line, it causes wear to both the pulley and the cable
• Mountings for pulleys should be corrected and the cable carefully examined for wear
• Wear of pulley bearings, bearing bolts, and bushings should be checked for serviceability regularly

Chain Introduction

• Control chains are found in a number of applications in aircraft control systems
• Chains are made up of multiple links and can be joined together to make an endless loop
• Chains can be of a terminating type – a single length with a start and finish
• Chains are manufactured to closer tolerance and have more rigorous inspection criteria

Chain Components

• Chain keepers, similar to cable guards, stop the chain from coming off the sprockets
• Chains have drive and idler sprockets and a method of maintaining the tension of the assembly
• Chain link assembly consists of rollers, bushes, and inner link plates
• Pins are mounted in the outer plates, which are clamped together to join the whole assembly

Chain Sizing

• Chains are sized according to pitch, width, and roller diameter
• Non-reversible chains are joined or connected using end fittings similar to cable terminals
• Chain guards can be fitted to sprockets to prevent the chain from coming adrift when tension is reduced

Chain Inspection

• Chains need to be assessed regularly for wear, corrosion, and distortion, including elongation
• It is always best to clean the chain before checking for wear or damage

Sprockets

• Sprockets are used in many aircraft systems and in a variety of different applications
• Sprockets are usually made from steel, alloy, hard fibre material, or plastic
• Their primary role is to convert rotary motion into linear motion and/or linear motion into rotary motion
• Sprockets act as support for the chain in the form of an idler sprocket

Idler Sprocket

• An idler sprocket is a non-driving sprocket used to support the chain run
• Sprocket teeth are spaced to align with the chain pitch
• The number of teeth on the drive sprocket compared to the number of teeth on the sprocket being driven determines the gearing ratio of the chain system