Module 7 (Maintenance Practices) Sub Module 7.11 (Bearings).pdf

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PIA TRAINING CENTRE (PTC) Module 7 - MAINTENANCE PRACTICES Category A/B1 Sub Module 7.11 - Bearings

PTC/CM/B1.1 Basic/M7/03 Rev. 00 7.11 Mar 2014

MODULE 7

Sub Module 7.11

BEARINGS



PTC/CM/B1.1 Basic/M7/03 Rev. 00 7.11 - i Mar 2014

Contents

BEARINGS----------------------------------------------------------------------------- 1

TESTING ------------------------------------------------------------------------------- 1

CLEANING ---------------------------------------------------------------------------- 1

INSPECTION OF BEARINGS ------------------------------------------------------- 2

LUBRICATION ------------------------------------------------------------------------ 3

DEFECTS IN BEARINGS AND THEIR CAUSES ---------------------------------- 9



PTC/CM/B1.1 Basic/M7/03 Rev. 00 7.11 - 1 Mar 2014

BEARINGS When correctly installed and efficiently lubricated, bearings require little but thorough, attention during service, to ensure a long working life. The majority of bearings used in aircraft engineering are of the sealed or shielded type. These bearing are designed to prevent the ingress of foreign matter, which will damage or contaminate the bearing contact surfaces. TESTING Testing a bearing is usually restricted to rotational checks and excessive backlash or free play. Slow rotation of the rolling elements and raceway will highlight any 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 and is normally done by using a DTI. Some wear is usually permitted and will depend on the grade of fit, but any wear that leaves excessive backlash in the system is unsatisfactory. The rate of this wear depends on the speed the bearing is rotated at, with high speed bearings failing quicker than those which rotate slowly or through distances of less than one complete revolution. A bearing that has any indication of a fault should be discarded immediately.

Due to their construction internal inspection of shielded bearings will be restricted. Taper bearings can be dismantled and a thorough inspection of the rolling elements and raceway surfaces can be completed. CLEANING Before a bearing can be checked it must be cleaned thoroughly to remove any dirt and the old lubricating fluid. The manual will dictate the cleaning process including use of any solvent, but a typical method is as follows. Remove any excess grease with clothes and dry compressed air, whilst ensuring that the rolling elements remain stationary or only rotate slowly. Soak the bearing in an approved solvent, such white spirit, to remove the remaining grease. The solvent may be applied as a forced jet if necessary. Dry the bearing by using clean warm and dry compressed air, again avoiding fast rotation of any rolling elements. Lightly lubricate all bearing surfaces with oil to prevent the onset of corrosion.




INSPECTION OF BEARINGS Once clean the bearing should be inspected for signs of failure, some of the more common being: Normal Fatigue Excessive Loading Installation and Misalignment Loose Fitting or Spinning Brinelling Overheating and Lubrication Deficiency Contamination and Corrosion Normal fatigue Normal fatigue failure is often shown as a fracture of the running surface, with subsequent removal of small particles of metal and is commonly called spalling. (Refer Fig. 1) It occurs on both rolling elements and raceways, and is always accompanied by an increase in vibration. Moderately spalled areas show the bearing has reached the end of its normal service life. .

Excessive loads Excessive loading of a bearing is usually the same as normal fatigue, but the rolling elements wear path is usually heavier. There is also increased evidence of overheating with a widespread and deeper fatigue or spalled area. This often causes premature bearing failure. (Refer Fig. 1)

Ball Path

Spalled Area

Fig. 1 Spalled Areas




LUBRICATION Lubricant A lubricant (sometimes referred to as "lube") is a substance (often a liquid) introduced between two moving surfaces to reduce the friction between them, improving efficiency and reducing wear They may also have the function of dissolving or transporting foreign particles and of distributing heat. And the science of lubrication is called tribology History of lubrication Romans were the first to use rag dipped in animal fat to lubricate wagon wheels and other domestic applications. Lubricant composition Typically lubricants contain 90% base oil (most often petroleum fractions, called mineral oils) and less than 10% additives. Vegetable oils, synthetic liquids such as hydrogenated esters, silicones, fluorocarbons and many others are sometimes used as base oils. Additives deliver reduced friction and wear, increased viscosity, improved viscosity index, resistance to corrosion and oxidation, aging or contamination, etc. Adequate lubrication is essential for all types of rolling bearings. The purposes of the lubricant are to lubricate the areas of rubbing contact, e.g. between the rolling elements and the cage, to prevent premature failure. Lubrication will,

Reduce friction and wear Direct metallic contact between the bearing rings, rolling elements and cage, which are the basic components of a bearing, is prevented by an oil film, which reduces the friction and wear in the contact areas. Extend of fatigue life The rolling fatigue life of bearings depends greatly upon the viscosity and film thickness between the rolling contact surfaces. A heavy film thickness prolongs fatigue life, while insufficient film thickness shortens it. Dissipate frictional heat and cooling Circulating lubrication may be used to carry away frictional heat or heat transferred from the outside to prevent the bearing from overheating and the oil from deteriorating. Others Adequate lubrication also helps to prevent foreign material from entering the bearings and guards against corrosion and rusting. For low rotational speeds, or for oscillating functions such as are found in a number of airframe applications grease is a suitable lubricant; at higher rotational speeds grease would generate excessive temperatures because of churning, and oil is more suitable. Because of the variety of uses to which rolling bearings are put, and the varying requirements of different locations, it is important that only those lubricants recommended in the approved Maintenance Manual should be used.




A typical bearing




Bearings fitted in engines and gearboxes are generally lubricated by oil spray, splash, mist, drip feed, or controlled level oil bath, and loss of lubricant is prevented by the use of oil retaining devices such as labyrinth seals, felt or rubber washers, and oil throwers. Most bearings used in airframe and External applications are shielded or sealed to prevent the entry of dirt or fluids which could adversely affect bearing life and are usually packed with anti-freeze grease because of the low temperatures encountered; these bearings cannot normally be re-greased, and must be replaced if it is evident that the lubricant has been washed out, or otherwise lost through failure of the seals or bearing wear.

Ball Bearing Grease nipples are provided for some open bearings so that the grease may be replenished at specified intervals, or when grease is lost through the use of solvents, paint strippers, detergents or de-icing fluid. Nipples should be wiped clean before applying the grease gun, to prevent the entry of dirt into the bearing. Grease forced into the bearing will displace the old grease, and any surplus exuding from the bearing should be wiped off with a clean lint-free cloth.

Wheel bearings are normally tapered roller bearings, and should be re-packed with the correct grease when refitting the wheel. Ball and roller bearings, if properly lubricated and installed, have a long life and require little attention. Bearing failures may have serious results, however, and aircraft Maintenance Manuals and approved Maintenance Schedules include inspections and where applicable, lubrication instructions for all types of rolling bearings.




Types of lubrication Four distinct forms of lubrication may be identified:

Hydrodynamic (Film) Boundary Hydrostatic Solid-film

Hydrodynamic lubrication means that the load carrying surfaces of the bearing are separated by a relatively thick film of lubricant, so as to prevent metal-to-metal contact. Hydrodynamic lubrication does not depend upon the introduction of the lubricant under pressure, though that may occur; but it does require the existence of an adequate supply at all times. Hydrodynamic lubrication is also called full-film or fluid, lubrication. In this type of lubrication a measurable quantity of oil is retained between the mating surfaces. In this form of lubrication the oil comprises three distinct layers, with the two outer layers of oil clinging to their respective surfaces. The central layer consists of molecules of oil, which are continuously being torn apart from each other or sheared as the mating parts move.

The thinner the oil, then generally the greater the ease with which shearing can take place. In this type of lubrication viscosity is the important property, because it is the friction between the oil layers, which reduces the mechanical friction.




Boundary lubrication; In this situation the oil film between the mating parts are only a few molecules thick. Viscosity is not the important factor; the important factor is Oiliness. This is the ability of oil to cling together (cohesiveness) and stick to the surfaces (adhesiveness).

Effect on coefficient of friction

Hydrostatic lubrication is obtained by introducing the lubricant, which is air into the load bearing area at a pressure high enough to separate the surfaces with a relatively thick film of lubricant. So unlike hydrodynamic lubrication, this kind of lubrication does not require motion of one surface relative to another.




Solid film lubrication is the use of suitable grease for lubrication purposes. When parts are operated at extreme temperatures, a lubricant such as graphite or molybdenum disulfide must be used because the ordinary mineral greases are not satisfactory.




DEFECTS IN BEARINGS AND THEIR CAUSES In general, if rolling bearings are used correctly, they will survive to their predicted fatigue life. Bearings, however, often fail prematurely due to avoidable mistakes. The causes of, this premature failure include improper mounting, mishandling, poor lubrication, entry of foreign matter or abnormal heat generation. For example, one cause of premature failure is rib scoring which is due to insufficient lubrication, use of improper lubricant, faulty lubrication system, entry of foreign matter, bearing mounting error, excessive deflection of the shaft or some combination of these. Examples of bearing damage and their causes are presented in the following sections. Please consult these sections when trying to determine the cause of bearing damage. Flaking Damage condition 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. Causes Excessive load; Poor mounting (misalignment); Moment load; Entry of foreign debris, water penetration; Poor lubrication, improper lubricant; Unsuitable bearing clearance; Improper precision for shaft or housing, unevenness in housing rigidity, large shaft bending; Progression from rust, corrosion pits, smearing, dents (brinelling).

Peeling Damage condition Dull or cloudy spots appear on surface along with light wear. From such dull spots, tiny cracks are generated downward to a depth of 5-10 m. Small particles fall off and minor flaking occurs widely. Causes

Unsuitable lubricant

Entry of debris into lubrication

Rough surface due to poor lubrication

Surface roughness of mating rolling parts. Scoring Damage condition Scoring is surface damage due to accumulated small seizures caused by sliding under improper lubrication or severe operating conditions. Linear damage appears circumferentially on the raceway and roller surfaces. Cyclical shaped damage on the roller ends and scoring on the rib surface contacting roller ends also occur.




Causes

Poor lubrication

Particles are caught in the surface

Inclination of inner and outer rings

Shaft bending

Poor precision of the shaft and housing Smearing Damage condition Smearing is surface damage, which occurs from a collection of small seizures between bearing components caused by oil film rupture and sliding. Surface roughening occurs along with melting.

High speed and light load

Sudden acceleration/deceleration

Improper lubricant

Entry of water

Fracture Damage condition Fracture refers to small pieces, which were broken off due to excessive load or shock load acting locally on a roller corner or rib of a raceway ring. Causes

Impact during mounting

Excessive load

Poor handling such as dropping Cracks Damage condition Cracks in the raceway ring and rolling elements. Continued use under this condition leads to larger cracks or fractures. Possible causes

Excessive interference

Excessive load, shock load

Progression of flaking




Heat generation and fretting caused by contact between

mounting parts and raceway ring

Heat generation due to creep

Poor taper angle of tapered shaft

Poor cylindricality of shaft

Interference with bearing chamfer due to a large shaft corner radius

Cage damage Damage condition Cage damage includes:

Cage deformation, fracture and wear

Fracture of cage pillars

Deformation of side face

Wear of pocket surface

Wear of guide surface

Causes

Poor mounting (Bearing misalignment)

Poor handling

Large moment load

Shock and large vibration

Excessive rotation speed, sudden acceleration and deceleration

Poor lubrication

Temperature rise Denting Damage condition possible causes When debris such as small metallic particles is caught in the rolling contact zone, denting occurs on the raceway surface or rolling element surface. Denting can occur at the rolling element pitch interval if there is a shock during the mounting (brinell dents).




Causes

Debris such as metallic particles are caught in the surface

Excessive load

Shock during transport or mounting Pitting Damage condition Pitting has a dull luster and appears on the rolling element surface or raceway surface. Causes

Debris becomes caught in the lubricant

Exposure to moisture in Poor lubrication Wear Damage condition Wear is surface deterioration due to sliding friction at the surface of the raceway, rolling elements, roller end faces, rib face, cage pockets, etc.

Causes

Entry of debris

Progression from rust and electrical arcing

Poor lubrication

Sliding due to irregular motion of rolling elements Fretting Damage condition Wear occurs due to repeated sliding between the two surfaces. Fretting occurs at fitting surface and also at contact area between raceway ring and rolling elements. Fretting corrosion is another term used to describe the reddish brown or black worn particles. Causes

Poor lubrication

Vibration with a small amplitude

Insufficient interference




False brinelling Damage condition Among the different types of fretting, false brinelling is the occurrence of hollow spots that resemble brinell dents and are due to wear caused by vibration and swaying at the contact points between the rolling elements and raceway. Causes

Oscillation and vibration of a stationary bearing during

such times as transporting

Oscillating motion with a small amplitude

Poor lubrication Creep Damage condition Creep is the phenomenon in bearings where relative slippage occurs between fitting surfaces and thereby creates a clearance between them surface. Creep causes a shiny appearance, occasionally with scoring or wear.

Causes

Insufficient interference or loose fit

Insufficient sleeve tightening Seizure Damage condition When sudden overheating occurs during rotation, the bearing becomes discolored. Then, the raceway rings, rolling elements, and cage will soften, melt and deform as damage accumulates. Causes

Poor lubrication

Excessive load (Excessive preload)

Excessive rotational speed

Excessively small internal clearance

Entry of water and debris

Poor precision of shaft and housing Excessive shaft bending

Module 7 (Maintenance Practices) Sub Module 7.11 (Bearings).pdf

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