Spring Inspection and Maintenance

• Inspection and testing of springs follows the maintenance manual for the main assembly.

Spring Inspection Criteria

• Blueing indicates overheating.

• Pitting indicates corrosion.

• Compression strength

• Free length

• Distortion

• Spring compression strength is measured using a spring tester by compressing the spring to a specific height and measuring the force needed to ensure it is within tolerance

• Spring free length is measured against a straight edge in a relaxed state to check for buckling or distortion

• Spring distortion impacts the mechanism it controls

• Always replace a spring with the correct part

• Springs may have colored paint marks to indicate their range in some systems

• Correct operation and maintenance of a spring are essential for a long life; the operating environment determines the failure rate

Spring Failure Risks

• Nicks or scratches concentrate stress, leading to fracture
• Overheating alters the heat treatment and temper of the spring steel, causing loss of tension/compression or fatigue failure
• Corrosion in exposed environments deteriorates the surface, adding stress or fatigue failure; spring steel is vulnerable to corrosion, and protective surface treatments can be damaged

Bearing Maintenance Practices

• Disassembly for access

• Inspection

• Cleaning

• Testing

• Lubrication

• Bearing inspection and testing go together

• Aircraft bearings undergo inspection per maintenance manuals.

• Visual exams finds surface flaws, overheating, lubricant issues, and other problems

• Check bearings to verify inspection needs when disassembling a component

• After bearing removal and cleaning, faults become more visible

Prevalent Bearing Faults

• Broken or cracked races

• Dented or damaged seals

• Cracked or broken separators

• Damage to balls or rollers

• Discoloration from overheating

• Less visible but serious defects include brinelling, burnishing, galling, and spalling

• Brinelling refers to ball or roller spaced indentations in the bearing raceway, similar to Brinell hardness test markings

• Brinelling can be true or false.

• True brinelling: the load on the bearing exceeds the elastic limit of the ring or bearing material

• False brinelling: similar in appearance, but occurs through vibration, not necessarily excessive load

Common Causes of Brinelling

• Incorrect fitting (mounting brinelling)

• Stationary loading (radial brinelling)

• Vibration during transport (shipping brinelling)

• Mounting brinelling results from incorrect fitting or removal: pressure on the unfixed race pushes rolling elements into the raceway shoulder

• Radial brinelling appears as elliptical impressions from excessive shock loadings on stationary bearings and occur in pushbike and motorcycle steering heads

• Vibration brinelling results from vibration during transport, indicated by brownish corrosion deposits around depressions: Wear occurs as the bearing grinds corrosion oxide into the race when it vibrates resulting in impressions

• Brinelling results in measurable dents due to loads exceeding the elastic limits of the steel, deforming it

• False brinelling results from ambient vibration, not excessive loads; bearings are subject to it when not operating

• EHD (elasto-hydro-dynamic) film creates an oil film between rolling elements and raceways during operation

• Proper lubrication at the right speed allows rollers or balls to lift off the raceway slightly and protects the bearing

• When a bearing is stopped, metal-to-metal contact occurs, allowing false brinelling to wear and corrode the pattern

Conditions, Causes, and Solutions

• Brinell marks are indentations at ball/roller frequency caused by any static overload or severe impact, common cause occurs from vibrations felt during transportation, solution is to apply vibration isolation during engine assembly transport

• Brinelling is the plastic deformation of bearing element surfaces due to extreme or repeated shock loads. Common cause is installing a bearing using a hammer, solution is to properly handle and apply pressure to correct raceway

• Another cause of brinelling is pressing bearing by applying force to the non-rotating ring. Solution is correct installation

• Spalling is the cyclic breakdown of surfaces, resulting in fatigue of the metal at localized points

• The metal of the bearing ringway is continually flexing under the pressure from rolling balls causing spalling, which is the flaking of small metal particles that spreads rapidly until failure

Conditions, Common Causes and Solutions

• Scores accompany seizing, the common cause is poor mounting and removing practice, and solution is to improve mounting and removing procedures

• Scores on roller end face and guide rib-cycloidal scores, common cause is discontinuation on the contact surface due to excessive radial load. The solution is to improve operation conditions and correct preload

• Scratches in spinning direction with slippage or poor lubrication as the common cause, improve sealing and lubrication

• Irregular pitting with cracked edges

• Galling occurs when the rolling element of the bearing skids, resulting from over-speeding, overloading, poor lubrication, or lack of clearance

• Metal is transferred due to welding/tearing action at the point of contact

Common Causes and Solutions

• Metal transfers result from metal to metal contact. The cause is skidding form overspeed, overload or lack of clearance so a serviceable lubrication system is needed

• Poor lubrication requires a design capability

• Flaking occurs when pieces of bearing material split off due to rolling fatigue causing rough and coarse texture

Condition, Common Causes and Solutions

• Raceway surface is flaked from rolling fatigue which can be caused early by overload; Find the source of heavy load

• Surface after flaking is very rough from excessive load due to improper handling, and should examine operating conditions and larger capacity bearings

• Additionally flaking can be initiated by poor shaft or housing accuracy, installation error, or ingress of foreign objects. In this case increase oil viscosity and eliminate errors

• Wear occurs when dirt and hard particles get into the bearing ringways causing roughness of the ring and ball

Common Causes and Solutions for Wear

• The Surface is worn and dimensions are reduced from ingress of solid foreign objects so ensure you have an optimal lubrication system

• The Surface mostly roughened and scored from poor lubrication to increase sealing efficiency

• Skewing of rollers; eliminate misalignment

• Rust and corrosion are pits on the surface of rings and rolling elements happening on rolling element pitch or over bearing surfaces

Condition, Common Causes and Solutions

• Rusting or corrosion occurs from water or corrosive material and requires improvements in sealing, periodic maintenance etc.

• The surface is sometimes rusted at equal spacing with poor packaging and storing conditions or excessive handling; This requires careful handling of bearings

• Electrical pitting applies to roller bearings inner ring where the raceway surface is corrugated by electric pitting. Tapered roller bearing rollers may get electric pitting at middle of rolling contact surfaces

Condition, Common Causes and Solutions

• The surface is speckled visually from electric current generating sparks which fuse the raceway surface. To fix this avoid electric currents

• Burnishing is smoothing the original surface roughness in the bearing races (microscopic roughness), as a result of being mechanically rolled over during normal running

• Burnishing area may be discolored and virtually no measureable wear can occur from periods of marginal lubrication

• Cracking occurs when a bearing gets a cracked raceway surface in the circumferential direction

• This is caused by angular contact with a slipping ball bearing in the outer race due to poor lubrication

Condition, Common Causes and Solutions

• Splits, and cracks in bearing rings and rolling elements form from excessive loads requiring examination and improvement

• Cracks can also from from overheating by creep and correction requires prevention

• Loose fits must be corrected

• Roll path skewing occurs when rolling elements stray or skew along the raceway

Condition, Common Causes and Solutions

• Rolling element contact path strays due to deformation or tilt which needs better accuracy
• Poor rigidity requires shaft and housing improvement
• Deflection needs clearance

Component Retainer Damages, Common Causes and Solutions

• Breaking from excessive loads or highspeeds needs improvements
• Wear of pockets and guide resulting from poor lubrication demands system improvement
• Loosening from installation or vibration needs improved housing and shaft

Peeling Details

• Peeling is a cluster of minor spalls occurring in roller bearings
• Tends to occur if the surface of the opposite part is rough or lubrication characteristics are poor
• Can require changes to dust quality or lubricants

Fretting Details

• Fretting surfaces that wear producing red rust-colored particles that are contained within pockets within them
• These occur from bearing angles that vibrate and drive out lubricants which leads to corrosion

Condition, Common Causes and Solutions

• Red rusts occur from contact elements and vibration which can be fixed with suitable packaging

• Dents called false brinelling also form from those vibration or poor lubrication during transportation requiring improved fits

• Smearing occurs when components adhere and has connections to roughness, use optimum lubricants

Surface Speckles and Discoloration Causes and Solutions

• Surfaces will be contaminated with debris that can be improved with filtration

• discoloration is also related to this same surface

• Split-shell bearings are normally replaced with appropriate versions

• Bushings may be reused if they don't have any severe flaws and are of the right dimensions

• Bearings may be magnetized and must have any ferrous metals removed before using

• Heavy grease, carbon or oil is removed in a bath

Demagnetizing Procedure

• When bearings are taken apart, disassembled and passed through de-magnetizer

• Large bearings should have less than 5Gs of residual magnetism, while smaller ones can have up to but no more than 2Gs

• A gaussmeter is used to measure magnetism

Bearing Cleaning Protocol

• Bearings are cleaned of preservatives in a solvent bath
• Grease and carbon are removed in a soak bath (except carbon filled tanks)
• Ultrasonic cleaning is used for final cleaning
• After having completed, the bearing is rinsed and treated with a preservative

Bearing Inspection Protocol

• Inspection takes place in climate controlled rooms

• Grease should be prevented form building

• Use high magnification for detailed inspection

• Some tests determine specific start and running torque

• Testing typically consists in rotation checks for rotation and excessive play

• Bearings with faults should be disposed of immediately

• Modern motors often use lubricant systems with specific schedules

Lubrication Notes

• Dirty lubricant should be avoided as it can introduce debris

• To little too much lubricant should be avoided

• There are several different application methods and the lubricant reduces abrasion, transfers heat, prolongs service life and prevents corrosion

• Use the correct grease as bearings may be pre-packaged

Other Lubrication Notes

• Use grease nipple to grease zerks and limit pressure
• Clean out lubricant before applying and ensure to not introduce debris
• Under-lubricated is worse over
• Splash lubricant is the basic of lubrication applications