Gear Wear: Overload, Pitting, Cracking

• It is important to understand the types of wear and what to look for when inspecting gears.

Overload Wear

• Overload wear occurs when metal is removed from the tooth surface by the sliding action of a gear's teeth
• Heavy contact squeezes out the lubricant
• Metal forms micro-welds
• Tiny particles become torn free
• This may leave a depression, burrs on the tooth edge, or plastic deformations resembling ripples or ridges caused by the metal being forced out of shape.

Pitting and Spalling

• Pitting and spalling are associated with thin oil films and high temperatures
• Gear teeth show pitting on the teeth or spalling, which is fine cracking
• Spalling is possibly due to faulty hardening, which eventually allows chunks of metal to flake away and chips may damage other teeth.

Cracking and Breakage

• Cracking is usually caused by faulty heat treatment or improper machining during manufacture
• Cracks are often very fine and are not readily apparent until the gears have been used for some time
• Breakage can be caused by one of many faults, from impact loading to manufacturing faults
• Fresh metal over the break area indicates impact overloading, while darker edges indicate fatigue starting from a surface crack.

Interference Wear

• Misalignment of gears or mating of the teeth can cause interference wear
• Heavy contact is caused in small areas, often at teeth tips and roots.

Abrasive Wear

• Abrasive wear includes scratching and scoring and depends on the size of particles carried in the lubricant, which abrade the tooth surface
• Scoring is the heavy gouging of metal from the tooth surface as the temperature rises and the lubricant thins out
• The metal pieces can then damage other teeth.
• The main causes of gear failure or premature wear are excessive backlash and poor lubrication.
• Proper alignment and fit are also important to ensure each tooth absorbs its share of the load.
• Gears corrode when exposed to corrosive conditions
• Often moisture is present in the lubricant along with contaminants, which react to become acidic and corrosive.
• A complete breakdown or lack of lubrication causes the gear to burn.
• Friction causes rapid heating past the temperature limits of the metal, resulting in brittle teeth which are easily broken.
• Normal wear is a polished appearance over the entire surface of the tooth.
• Diagnose gear tooth contact markings with a diagnosis chart
• Backlash is clearance or play between two gears in mesh
• A certain amount is required to ensure lubricant penetration between the teeth and to allow for thermal expansion of the gear train.
• Gear backlash must be established in accordance with the relevant maintenance manual
• Excessive backlash can be caused by worn gear teeth or by improper meshing of teeth or bearings which do not support the gears properly
• Excessive backlash can result in severe impact on the gear teeth from sudden stopping or reversal of load, leads to broken gear teeth, gear bouncing and gear noise.
• Too little backlash will cause excessive loading on the gear teeth, lubricant will be forced from the gear surface and premature failure will result
• Measurable backlash can be detected by hand if one gear is held and the other rocked
• This is usually minimal and a Dial Test Indicator (DTI) is set up to measure it
• A typical gear may have .003- to .004-in. backlash
• The correct backlash is given in the maintenance publications which must be followed
• The meshing pattern of two gears is important and closely related to backlash.
• Backlash and pattern checks are done in conjunction, most critical where directional changes are made through the gears (for instance, right angle changes with bevel gears)
• Adjustable gears that adjust inwards and outwards sets their correct relationship by shimming or adjusting them
• The wear pattern is simply the prints (marks or impressions) that one set of gear teeth leaves on another set when they mate
• Wear patterns may be examined prior to final assembly, during service, overhaul, and after a failure.
• Prior to assembly, the pattern of tooth contact may be established by using Prussian blue, a blue-dyed soft paste smeared lightly on the leading side of driving gear teeth
• Other colored dyes mixed with oil are also sometimes used
• The gears are temporarily assembled and turned, and a pattern of contact is left on the teeth; studying this pattern enables the engineer to set up the correct shimming or assembly relationship
• Normal wear on the teeth mating surfaces may be examined at service intervals for condition or to see if further maintenance is required
• Normal wear leaves a polished surface on the tooth face and early correction of abnormal patterns can prevent further deterioration or failure
• During overhaul, the wear pattern is examined to determine acceptability for further service, and for any indication of problems in the remainder of the assembly
• The examination of the wear pattern after the failure of the gear or assembly can determine servicing procedures to prevent future failures and give clues to its cause
• Each tooth of a helical bevel gear has a heel and toe, a coasting side and a drive side
• The heel is the larger-width outer end of the bevel gear, and the toe is the smaller inner end
• The coasting side of the tooth is concave and is not under pressure during normal drive; the drive side is convex and under pressure during normal operation.
• Pulleys are used in aircraft control systems to change the direction of a cable
• Pulleys are designated by numbers AN210, AN219, AN229 and AN221
• Aircraft pulleys are made of metal and compressed phenolic sheet, Teflon fluorocarbon resins, and other plastic materials.
• Pulley bearings are usually of the sealed type and require no lubrication
• The pulley is bonded to the bearing so that the bearing cannot be removed
• When replacing a pulley, the technician must make sure to install the correct type which is determined from the manufacturer's instructions or by the number on the pulley.
• Pulleys should be inspected in accordance with the manufacturer's instructions
• Replace worn or cracked pulleys
• Pulleys should turn freely when the control cables are moved
• Misaligned pulleys cause wear to the pulley and cable
• Correct pulley mountings, examine cables, and pulleys regularly.
• All pulleys must be equipped with guards to prevent the cable jumping out of the groove when the cable is slack.
• Control chains are found in aircraft control systems where there is a short distance between two hardware items (two torque tubes, for example) or the system requires a relatively large amount of force to operate and is not practical for a small cable run to be installed.
• The cockpit pedestal area is a common place to find short chain assemblies.
• Aircraft control chains are made up of multiple links and can be joined together to make an endless loop or can be a terminating type, a single length with a start and finish.
• Aircraft chains are manufactured to closer tolerance and have more rigorous inspection criteria.
• Chain keepers, similar to cable guards, stop the chain from coming off the sprockets
• Parts of chain assembly also include drive and idler sprockets and a method of maintaining the tension of the assembly
• Aircraft control chains are usually of the roller type
• Rollers are free to rotate around bushes which hold the inner link plates together
• Pins are mounted in the outer plates, which are clamped together to join the whole assembly
• Chains are sized according to pitch, width and roller diameter
• Non-reversible chains (i.e. not a continuous loop) are joined or connected using end fittings similar to cable terminals, and can only be fitted in one direction.
• Chain guards can be fitted to sprockets to prevent the chain from coming adrift when tension is reduced
• Stop pieces on the chain guard and non-interchangeable end connectors are used to prevent incorrect installation of non-reversible chains.
• Chains need to be assessed regularly for wear, corrosion and distortion, including elongation
• Clean the chain before checking for wear or damage and place it on a flat surface to examine its condition.
• Measure chain elongation using the formula:
  • Extension% = (M-(XxP)x100)/XxP (M = measured length, P = pitch, X = number of links)
• Replace the whole chain if elongation is greater than 2%, as elongation can compromise its strength
• Chain tension is checked by measuring chain deflection under a given load
• Chain tensioning is adjusted using turnbuckles or with adjustable sprockets such as the one shown above
• The chain should be tightened enough to remove any backlash, but excess tension causes the chain rollers to bind on the sprocket teeth
• Tension is checked using a spring balance or similar is attached to the chain at a specified point to measure the deflection from the normal chain run profile, tension must be increased when the deflection is too great.
• Criteria for rejection of a chain include 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, or any rollers are cracked, worn, corroded or jammed.
• Correct chain storage minimises corrosion or kinking
• Chains should be cleaned in solvent, dried, soaked in light lubricating oil, and coiled without twisting.
• Check for kinks or twists by freely suspending the chain and sighting along its length
• No twist is allowed
• Sprockets are used in many aircraft systems and in a variety of different applications
• Sprockets (depending on their application) 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
• They act as support for the chain in the form of an idler sprocket for long chain systems
• An idler sprocket is a non-driving sprocket used to support the chain run
• Sprocket teeth are spaced to align with the chain pitch
• Gearing ratio of the chain system is determined by the number of teeth on the drive sprocket compared to the number of teeth on the sprocket being driven
• Chain tension and load have a direct impact on chain and sprocket service life as does dirt and debris
• A worn sprocket will produce tight and loose spots as the chain rolls over it
• Sprocket wear is usually not uniform
• Once wear starts in one place on the chain or sprocket, the rate of wear will accelerate
• Drive belts are used in a number of applications (for example, in helicopter drive systems)
• Belt drives carry a lesser load than chain drives, but do not require lubrication, and are more shock resistant, quieter, and smoother during operation
• All belts are a variation of two types Vee belts and Non-slip toothed belts.
• Belt drives can come in single or multiple belt and pulley systems
• Bottom of Belts Crack due to belt slipping due to heat build-up (friction) and gradual hardening of the under-cord reinforcing fibres.
• Top of Tie Band Fray or Damage due to obstruction on the sheave interfering with normal belt operation.
• Top of Tie Band Blister or Perforation by foreign material accumulating between the belts of the tie band.
• Belts cut on the Bottom from running over sheave and coming off, or being forced over sheave flange during installation without proper slack
• Belts riding outside sheave grooves can cause progressive failure resulting in separation
• Separation is caused by improper belt tension, misalignment of sheaves and/or foreign object struck belt forcing it from its normal path
• Screw jacks convert rotational movement into linear movement, giving a high mechanical advantage
• Also called screw jack actuators or linear actuators
• Three basic types of screw jack actuators exist The worm-and-peg mechanism, The worm-and-nut mechanism, and The recirculating ball mechanism.
• The worm and peg mechanism is not used much because excessive backlash often results from localised wear on the peg and worm screw.
• The worm and nut mechanism type is often used on aircraft, with Acme or square threads and activated by a shaft turning to drive the nut, or by turning the nut to drive the shaft
• The recirculating ball type is a low-friction version of the worm-and-nut mechanism, sometimes known as a ball screw actuator
• The closed pathway for the ball bearings allows them to continually circulate reducing friction when the mechanism is actuated, and is used particularly for the flap drive system in aircraft
• Follow relvant manufacturer's maintenance manual for inspection procedures, checking screw shaft for wear,ball nut for end float, and ball nut lubrication level,, as well as the screw jack full travel and stop limits
• Push-pull rods have many systems applications, such as aileron and trim tab mechanisms, requiring at least three adjustable push-pull rods
• Push-pull rods and their adjustable end-fittings create a type of rigid linkage that eliminates the problem of varying tension in a control system, which permit transferring compression or tension
• Push-pull rods are usually made of seamless aluminum alloy tubing, and are a fixed length or include at least one adjustable end
• Adjustable rod ends are screwed onto threaded ends and locked with a check nut
• An adjustable push-pull rod may have a left-hand threaded end and a right-hand threaded end for adjusting the rod when altering system rigging
• Loosening the check nuts enables the rod to be turned without disturbing the rod eye ends
• Check push-pull rods for rod straightness, wear in the eye ends, and security of the check nuts and end fittings including corrosion, scratches, dents, chafing, and buckling.
• A check or witness hole installed on the adjustable end is used to determine whether the adjustable end is screwed onto the threaded portion sufficiently
• One method uses a piece of lockwire or a thin split-pin on the witness hole
• the adjustable eye end is said to be 'in safety' when inserted through the witness hole (screwed onto the threaded portion sufficiently)
• No more than three threads should be exposed after the eye end is tightly screwed onto the threaded portion.
• Control rod bearings are self-aligning, subject to both radial and axial loads, and inspected for play in both these planes.
• Acceptable wear limits are established in the maintenance manual for a particular aircraft
• Control rod bearings require movement in two planes
• A bearing must align itself to a changing axis
• Rotational movement in two planes must not be mistaken for axial and radial wear
• Bell cranks transmit force or motion, and permit a change in direction of that force or motion.
• Bell cranks pivot on bearings mounted on a shaft
• Although the push-pull rods adjust for enabling correct system rigging, there is no adjustment possible to the bell crank
• The torque arm receives the linear motion input from the control system and causes the torque tube to rotate, converting linear motion to rotary motion.
• The torque tube connects directly to the control surface and causes it to move
• Torque tubes and torque arms are usually of fixed length, diameter and angle and are not adjustable
• The adjustable push-pull rods connect the rudder balance lever to the rudder balance tab.