Bearing Defect Identification PDF

Bearing Defect Identification Bearing Maintenance Maintenance practices on bearings include the disassembly for access, inspection, cleaning, testing and lubrication of bearings. The inspection and testing of bearings go hand in hand. All aircraft bearings must be inspected in accordance with approp...

Bearing Defect Identification Bearing Maintenance Maintenance practices on bearings include the disassembly for access, inspection, cleaning, testing and lubrication of bearings. The inspection and testing of bearings go hand in hand. All aircraft bearings must be inspected in accordance with appropriate maintenance manuals. Visual examinations will detect many problems with bearings, including surface flaws, over-heating, lack of lubricant and other problems, some of which indicate problems requiring correction in the higher assembly. After disassembling a component to the stage that the bearings are to be removed, you should check that the bearing does in fact require inspection. Some bearings are rejected automatically, as is normally the case with split-shell plain bearings. After the bearing has been removed and cleaned and is ready for inspection, obviously some faults will be more apparent than others. The more prevalent fault include: Broken or cracked races Dented or damaged seals Cracked or broken separators Damage to the balls or rollers Discoloration due to overheating. In addition, some less-visible but serious defects common to bearings are well known and their causes understood. These particular defects will each be briefly discussed, and include: Brinelling Burnishing Galling Spalling. Ball Bearing graphic by PlusMinus. Licence GFDL CC-by-SA. Ball bearing 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 406 of 498 Brinelling Brinelling refers to the indentations in the bearing raceway at ball or roller spaced intervals. The name results from the similarity to a Brinell hardness test marking. Brinelling presents itself in two ways; true brinelling and false brinelling. True brinelling indicates that the load on the bearing was greater than the elastic limit of the ring or bearing material. False brinelling looks similar to true brinelling, but occurs through vibration, not necessarily excessive load. The three most common causes of brinelling are: Incorrect fitting (mounting brinelling) Stationary loading (radial brinelling) Vibration during transport (shipping brinelling). Bearing defects - Brinelling examples Mounting Brinelling Mounting brinelling is caused by incorrect fitting or removal methods. If pressure is applied to the unfixed race of the bearing with the removal/fitting tool, the rolling elements are pushed into the raceway shoulder; the indentation often extends into the race. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 407 of 498 Radial Brinelling Radial brinelling appears as elliptical impressions at ball spaced intervals in the races caused by excessive shock loadings applied to the bearing while it is stationary. Their appearance is similar to mounting brinelling, but positioned in the centre of the race. You can commonly find these in pushbike and motorcycle steering heads. Shipping or Vibration Brinelling Vibration brinelling is similar to the previous impressions, but results from vibration during transport and can be identified by brownish corrosion deposits around the depressions. The impression is a result of wear, as the bearing grinds the corrosion oxide into the race when it vibrates. Brinelling is caused when a load is applied to a ball bearing that exceeds the elastic limits of the steel and the raceways are permanently deformed. It creates measurable dents at each ball location, similar to the deformation caused by a Brinell Hardness Tester. This type of damage can occur quite easily if proper care is not taken. High-energy impacts (from hammers and smash-ups), improper bearing handling and incorrect spindle assembly can all damage bearings. As previously mentioned, false brinelling is not related to excessive loads. False brinelling is caused by ambient vibration. Even a brand-new bearing, sealed in a box on a shelf, is subject to false brinelling if it is exposed to environmental vibrations for an extended period. When a bearing is not operating, it is subject to false brinelling in the box or in the machine. When a bearing is operating, there is an oil film between the rolling elements and the raceways. Bearing gurus talk about EHD (elasto-hydro-dynamic) film. Most people can relate to hydroplaning. When you reach a certain speed on a wet road, your tyres actually lift off the road (not good). But when a bearing operates with the proper lubrication and at the right speed, the balls or rollers lift off the raceway slightly (good). This extremely thin film protects and lubricates the bearing while it is running. When the bearing is stopped, there is no EHD film and there is metal-to-metal contact. That is when false brinelling can quietly attack your bearings. The combination of metal-to-metal contact and vibration creates a wear and corrosion pattern that mimics brinelling. © Aviation Australia Bearing Defects - Brinelling 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 408 of 498 Spalling Spalling is the breakdown of surfaces due to cyclic (rhythmically repetitive) loading, resulting in fatigue of the metal at localised points. It appears as irregular pitting with cracked edges and progresses with the loss of large areas of metal from balls or raceway surfaces. The metal of the bearing ringway is continually flexing under the extremely high pressure of the balls rolling in the ringway. This flexing eventually causes spalling, which is the flaking of small metal particles from the ringway. This condition spread rapidly until failure. Spalling defect on a bearing © Aviation Australia Bearing Defects - Spalling 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 409 of 498 Galling Galling occurs when the rolling element of the bearing skids. With metal-to-metal contact on its race, metal is transferred from, or to, either surface as a result of the welding/tearing action at the point of contact. The skidding could result from over-speeding, overloading, poor lubrication or lack of clearance. Galling wear on a bearing © Aviation Australia Bearing Defects - Galling 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 410 of 498 Flaking Flaking occurs when small pieces of bearing material are split off from the smooth surface of the raceway or rolling elements due to rolling fatigue, thereby creating regions having rough and coarse texture. Bearing flaking © Aviation Australia Bearing Defects - Flaking 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 411 of 498 Wear Dirt and other hard particles such as metal and chips from abrasive wheels can get into the bearing ringways and be squeezed between the balls and ringway. This causes roughness of the ring and ball, and eventually failure. If the particles are the consistency of a very fine dust, they will act like a lapping compound and accelerate wear of the rings and ball. Wear on a bearing © Aviation Australia Bearing Defects - Wear 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 412 of 498 Rust and Corrosion Bearing rust and corrosion are pits on the surface of rings and rolling elements and may occur at the rolling element pitch on the rings or over the entire bearing surfaces. Rust and corrosion can be caused by water in lubricant © Aviation Australia Bearing Defects - Rust and Corrosion 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 413 of 498 Electrical Pitting Applies to the roller bearings inner ring. The raceway surface is corrugated by electric pitting. Tapered roller bearing rollers may get electric pitting at middle of rolling contact surfaces. Electrical pitting © Aviation Australia Bearing Defects - Electrical Pitting 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 414 of 498 Burnishing Burnishing is smoothing of the original surface roughness in the bearing races (microscopic roughness), as a result of being mechanically rolled over during normal running. The area may be discoloured, but there is virtually no measurable wear. Burnishing may occur during periods of marginal lubrication. A ball carves a channel through a flat plate (microscopic channel, this effect is exaggerated in diagram) Cracking Sometimes a bearing can get a cracked raceway surface in the circumferential direction, this is caused by angular contact with a slipping ball bearing in the outer race. Slipping occurs due to poor lubrication. Cracking 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 415 of 498 © Aviation Australia Bearing Defects - Cracking Rolling Path Skewing Rolling path skewing © Aviation Australia Bearing Defects - Rolling Path Skewing 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 416 of 498 Damage to Retainers Retainer damage © Aviation Australia Bearing Defects - Retainer Damage 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 417 of 498 Peeling Peeling © Aviation Australia Bearing Defects - Peeling Fretting and Fretting Corrosion Fretting 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 418 of 498 Fretting corrosion © Aviation Australia Bearing Defects - Fretting and Fretting Corrosion 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 419 of 498 Smearing Smearing © Aviation Australia Bearing Defects - Smearing 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 420 of 498 Speckles and Discolouration Speckles and discolouration © Aviation Australia Bearing Defects - Speckles and Discolouration 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 421 of 498 Bearing Inspection and Testing Inspecting Plain Bearings Split-shell bearings are normally replaced in accordance with appropriate manuals whenever disturbed; the journals are inspected for any flaws. Visual inspection of the shells often reveals problems developing in the higher assembly; always refer to the maintenance manual for guidance. Bushes may be re-used if they pass visual inspection and, most importantly, dimensional checks. Bushes and plain bearings are measured with Precision Measuring Equipment (PME) such as callipers, internal micrometers or plug gauges, and returned to service if within limits. New bushes may require reaming or boring to size before final assembly. Plain bearing Inspecting Rolling Element Bearings After the bearing is removed from its housing, no attempt should be made to clean or remove grease from it. Cleaning takes place in a clean and tidy facility separated from workshops and inspection areas. The area must be ventilated, although temperature and humidity control is not required. Fire equipment must be available and Personal Protective Equipment (PPE) supplied and used. No eating or drinking is permitted in the cleaning area. Prior to cleaning, the bearing must be de-magnetised. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 422 of 498 De‑magnetising Bearings may become magnetised in service and attract ferrous (iron/steel) particles to the balls, rollers or races. These particles are not always removed by the cleaning processes, therefore the bearings must be de-magnetised at the latest prior to final cleaning. Hand De-magnetising Procedure De-magnetising involves passing the bearing, disassembled if possible, through the coil of a demagnetiser at not more than 12 ft per second, and rotating it by 90° as it passes through and back again. The bearing is then withdrawn a good distance from the coil before it is switched off and tested with the Gaussmeter. For large bearings, 5 Gs of residual magnetism is acceptable, and a maximum of 2 Gs for bearings under 2 in. in diameter. De-magnetiser 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 423 of 498 Gaussmeter A gaussmeter is used to assess the effectiveness of the de-magnetising coil, and accurately measures magnetism. It is calibrated in gauss (Gs), a unit measure of magnetic density. The gaussmeter is also known as a magnetometer and is used to measure the strength and, in some cases, the direction of magnetic fields. Consider a small slab of semiconductor material (Hall element). An electrical current is passed from one end of the slab to the other, and the voltage at each edge of the slab is the same when no magnetic field is present. If a magnetic field is now applied through the top to bottom surfaces of the slab, a voltage appears across the sides of the slab, which is directly proportional to the magnetic flux density (a linear result should be achieved). A simple magnetic sensor can be constructed using a rectangular sheet of gold foil, four wires and a source of electric current. The wires are attached to the midpoints of each side of the foil. The current source is connected to the two wires attached to the shorter sides of the foil, and the other two wires are attached to a sensitive voltmeter. In the absence of a magnetic field, there will be no reading on the voltmeter, but as the intensity of a magnetic field passing through the foil increases, so will the reading on the voltmeter. Gaussmeter 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 424 of 498 Bearing Cleaning Not all cleaning processes are adequate or correct for all bearings. The correct procedure for specific bearings described in publications must be referred to. In general, however, the following is a minimum cleaning procedure. The bearing should be cleaned of any preservative oil in a solvent bath approved for bearing cleaning purposes. Cleaning Baths Heavy grease, carbon or oil is removed in a soak bath with chemicals to suit the requirements. The tank is often agitated, with the exception of a carbon removing compound filled tank. These have a water seal on the compound surface and should therefore undergo minimal disturbance. Cleaning bath Ultrasonic baths are used as a final cleaning method in which high-frequency sound is passed through a solvent bath to vibrate loose any last particles of contaminant. When the bearing has completed its cleaning process, it is rinsed and then coated in light preservative oil if it is not to be immediately inspected. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 425 of 498 Ultrasonic bath Bearing Inspection Inspection after cleaning should take place in a clean room with controlled temperature and humidity. When inspecting dry bearings, approved gloves and tools must be used to prevent surface corrosion and bearing damage. Visual inspection is carried out using the unaided eye or low magnification. Visual bearing inspection aids 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 426 of 498 Detailed inspection or fault analysis is carried out using high magnification. High magnification inspection aids Specialised tools such as radius scribers are used to determine acceptability of bearing surface defects. Radius scribers Some bearings may require ‘start’ and ‘running’ torque checks, while others have a noise limit which is checked with audiometric equipment. Start torque is a measure of the power required to start the bearing rotating. Running torque measures the energy used to overcome resistance and keep the bearing rotating at a predetermined speed. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 427 of 498 Testing Bearings Testing a bearing is usually restricted to rotational checks for excessive backlash or free play. Slow rotation of the rolling elements and raceway will highlight roughness due to damage, corrosion or wear. A serviceable bearing should have a smooth actuation. Free play should be tested in both a radial and axial direction. Some wear is usually permitted and will depend on the grade of fit. Any wear that leaves excessive backlash in the system is unsatisfactory. A bearing which has any indication of a fault or defect should be discarded immediately. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 428 of 498 Lubrication Introduction to Lubrication Modern motor designs usually provide a generous supply of lubricant in tight bearing housings. Lubrication on a scheduled basis, in conformance with the manufacturer's recommendations, provides optimum bearing life. Thoroughly clean the lubrication equipment and fittings before lubricating. Dirt introduced into the bearings during lubrication probably causes more bearing failures than the lack of lubrication. Too much grease can over-pack bearings and cause them to run hot, shortening their life. Excessive lubricant can find its way inside the component, where it collects dirt and causes insulation deterioration. Lubricant Selection The purpose of lubricating the bearing is to cover the rolling and sliding contact surfaces with a thin oil film to avoid direct metal-to-metal contact. Effective lubrication of the rolling-element bearing has the following effects: Reduces friction and abrasion Transports heat generated by friction Prolongs service life Prevents rust (corrosion) Keeps foreign objects (or contamination) away from rolling elements and raceways. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 429 of 498 Grease All bearings require some form of lubrication to ensure long life and prevent failure. Many bearings require grease which is specified by the bearing manufacturer and may be a high-temperature, multi‑purpose or low-temperature grease, depending on operating conditions. Some bearings are sealed with their own grease lubricant and require no further lubrication during their service. Others require servicing at intervals determined by the bearing manufacturer; note that the intervals quoted are the minimum requirement. If the bearing is operated in dust, sand or saltwater environments, more frequent servicing should be undertaken to remove contaminants and therefore limit undue wear. Methods of grease lubrication vary from hand-packing the bearing to applying with a grease nipple, which you may see referred to as a grease ‘zerk’. To avoid damaging seals, only manual grease guns should be used to service bearings, limiting the pressure the seal is subjected to. Grease reservoirs Greasing Bearings Bearings may be pre-packed with lubricant prior to fitting, either by machine or by hand. It is important to use only the correct type and quantity of grease specified in the publications. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 430 of 498 Hand Lubrication Hand lubrication is not normally recommended, but is acceptable if mechanical pressure systems are not available. A quantity of grease is taken in the palm of one hand and the bearing is scraped through it, forcing grease through the assembly. A continuous film of grease must coat all surfaces. Hand greasing Unless otherwise stated, the internal voids of a bearing are packed to one third of what would constitute a full pack of the specified grease (a full pack being all the internal space filled). The cleanliness and condition of the grease are important. Every precaution must be taken to avoid contamination. Some precautions which should be observed are: Wipe the container off before opening. Replace lids when not in use. Use stainless steel spatulas for grease transfer. Ensure shelf life is current. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 431 of 498 Mechanical Greasing Mechanical greasing Under-lubrication Insufficient lubrication causes premature bearing failure due to the lubricant being rapidly expended; operating temperatures rising and internal wear. Over-lubrication Churning of the grease results in raised bearing temperatures, leading to lubricant breakdown and bearing failure. Excess grease can be thrown from the bearing contaminating other components. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 432 of 498 Oil Of those bearings not lubricated by grease, most are lubricated by splash, spray or pressure-fed oil. Pressure systems more commonly service plain bearings, particularly those in piston engines. Balland-roller bearings in gearboxes, for instance, receive splashed or sprayed oil. Splash systems require a certain level of oil to be maintained in the assembly and rely on movement of components to distribute oil as droplets or mist. Oil spray systems use pressure systems and plumbing to deliver oil to spray nozzles or jets, which direct the oil at the bearing. To ensure correct and adequate oil for the bearings, servicing must include regular checking and replenishment of oil. Checking the level is usually performed by using dipsticks, checking sight gauges, or visually checking tank or sump levels. Lubrication charts are part of the maintenance manuals and specify the type of oil to use, the frequency, and the method of checking and replenishing. Oil lubrication system In theory, lubricating fluids fill all surface irregularities, providing oil films which slide against each other and prevent metal-to-metal contact. The primary purpose of a lubricant is to reduce friction between moving parts. As long as the film remains unbroken, metallic friction is replaced by internal fluid friction. The heated oil is then carried away to be cooled and re-used. In addition to reducing friction, oil acts as a cushion between metal parts. As oil circulates through the engine, it collects foreign matter and deposits it into the oil filtration system. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 433 of 498 Oil Lubrication Methods Splash Lubrication The most common method of supplying lubricant to the gears and bearings is splash lubrication. The gear case is partially filled with oil to a level at which one or more gears is dipping, carrying the oil up, and splashing it over the gears and bearings. Pressure Feed Lubrication A lube system containing a pump to lift the oil to the top of the gearbox pressure feeds the oil into the gear set mesh and lubricates the bearings. An integral system uses the gearbox sump as the reservoir, while a central system uses an external sump. Special Plain Bearings Oil-impregnated plain bearings, such as sintered bearings or bushes, require re-lubrication if they have been cleaned with solvents or machined by reaming, boring or grinding. The only recommended re-oiling procedure is by vacuum impregnation, although oil soaking can restore some lubricant. Some bearings receive a ‘dry film’ lubricant during assembly and are identified as such by the manufacturer. These and others, such as Teflon-impregnated bushes, require no further lubricant during service. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 434 of 498 Storage of Bearings Introduction to Storage of Bearings Bearings require a lubricant or preservative coating prior to installation or storage. Bearings lubricated by grease while in service are packed with that grease before short-term storage, and those working in oil are protected with a preservative coating compatible with the operating oil. Oil-lubricated bearings can be stored for 30–90 days with a coating of mineral or synthetic oil. For indefinite storage, they are preserved in accordance with instructions. Many small bearings may be expected to operate for their life on their initial oil lubricant. Short-Term Storage Bearings are preserved according to instructions. Bearings returning to supply or for overhaul, or being prepared for local short-term storage, require lubrication followed by an ‘intimate’ wrapping, then sealing into a heat-sealable plastic bag. The package is placed in a container for storage in a supply area; the container should conform closely to the package size to restrict movement. The container must be identified with part numbers and stock numbers, name and quantity of bearings, type of preservation, date and place of packing. Local storage must be in a cool, dry area. The packing must not be disturbed simply to identify the bearings. Use of stocks must be on a first in, first out basis to prevent accumulation of old stocks. 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 435 of 498 Long-Term Storage Bearings are preserved according to instructions in hot or cold dip preservative for long-term storage. Correctly coated and packed, they may be stored in an uncontrolled environment. The actual packaging, however, must take place in clean room facilities as per the following general procedure: Coat the bearing in preservative. Wrap it in waxed paper. Seal it in an evacuated plastic bag. Pack it in an appropriate container, closely conforming to package size to restrict movement. Clearly label the container with the correct part number, serial number, name, date and method of packing. Store the package in local storage in a safe, cool, clean and dry area. The packing must not be disturbed simply to identify the bearings. Use of stocks must be on a first in, first out basis to prevent accumulation of old stocks. Ideally, long-term storage occurs in controlled environment rooms. Bearing storage procedures 2023-11-24 B-07b Maintenance Practices CASA Part 66 - Training Materials Only Page 436 of 498