turbo charger

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1Service Training Malaga

AFA ILion DVD CHAPTER: Turbochargers

Turbochargers

Welcome to a continuation of our failure analysisseminar and a discussion about turbocharger failures. Wewill again be reviewing facts that can guide us to rootcauses of failures.

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In This Presentation• Function• Design• Operation• Normal wear•Why Turbos Fail

CHAPTER: Turbochargers

In this presentation we will discuss some basic factsabout turbocharger function, structure, installation,operation and other problems that will help usunderstand why turbos fail.

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FunctionCHAPTER: Turbochargers

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FunctionCHAPTER: Turbochargers

When turbos fail, it is a temptation to gather factsfrom only the failed turbocharger. We need to rememberto immediately gather basic information about thelubrication, air inlet and exhaust system conditionsprior to failure since these systems are part of theturbocharger's life and more often cause it to fail.Facts about pressures, leaks, restrictions, foreignmaterial, high temperatures, loose connections or recentrepairs should be recorded.

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Turbocharger Functions

• Normalize Air Supply• Boost Air Supply


Turbochargers serve two functions, normalizing airsupply and boosting air supply to engines.

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Normalizing• Compensate for thinner Air•Waste gate valve by-pass turbo boost• Normal Power at high Altitudes• Fuel deration at 2 150 m to avoid over speed• Quieter Exhaust, better combustions and emissions


Normalizing means keeping air supply the same as isnormal for a naturally aspirated engine at sea level.When engines operate at altitudes above sea level, theair becomes less dense, and the turbocharger is neededgather in more of the thinner air. If we don'tnormalize, we must decrease fuel settings as the airbecomes less dense to avoid overfueling the engine.Thus, normalizing allows engines to develop normalhorsepower over abroad range of altitudes.Some turbochargers have what is called a "waste gate"which bypasses turbo boost above a specified pressure.This allows the engine to be operated at variousaltitudes and yet maintain a stable, normalized airsupply.We should be aware that while turbochargers canconcentrate thinner air at higher altitudes to givenormal oxygen supply and normal power, they will spinfaster to do so. Thus, for operation above about 2 150meter we often see fuel deration suggestions to avoidturbocharger over speed.Side benefits of turbocharging include quieter exhaust,better combustion, and cleaner emissions.

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NormalizingFirst commercial use 1940’s in Airplane


Although the idea of turbocharging is an old one,economical metals that could withstand high exhausttemperatures weren't produced until the 1940's. One ofthe first commercial uses of turbocharging was on theairplane engine to enable it to develop full power athigher altitudes.

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Boosting• More Oxygen / Increased Fuel Setting• Higher Power• Better Combustion

- Better Fuel Economy- Cleaner Emissions•Quieter


The second function of a turbocharger is boosting airsupply to give the engine more than normal oxygen. Thisenables increased fuel settings while still providingbetter combustion and quieter exhaust. Improvedcombustion means not only better fuel economy, but alsocleaner exhaust emissions.

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BoostingCHAPTER: Turbochargers

Some customers enjoy the feel of a little extra power.Powered by the Cat, these boats begin to fly when thethrottles are opened.

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DesignCHAPTER: Turbochargers

Schwitzer Turbo

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Design Waste-Gate Turbo


We should be familiar with the structure ofturbochargers, the names of key internal parts, and howthey fit together before doing failure analysis. Let'stake a few minutes and review some basic facts about atypical turbo.When assembled, the cold wheel, the center shaft, andthe hot wheel become one solid piece that turns in free-floating journal bearings. A stationary thrust bearinglocated near the cold wheel controls endplay. Largerturbos have two separate journal bearings while somesmall turbos have a single cartridge style bearing.Thrust washers are positioned on each side of the thrustbearing with a spacer between them.. When the compressorwheel is installed, the retaining nut forces the wheel,the thrust washers and the spacer against the shoulderon the center shaft, making them a part of the rotatingassembly. All bearings ride on a cushion of oil duringturbocharger operation.The turbine back plate, or heat shield, and the airspace behind it serve as insulators to keep high exhausttemperatures from penetrating the center housing. Heatthat is conducted into the center shaft from the hotwheel is removed at the bearing near the hot wheel bylubricating oil. Thus, even though temperatures can be ahigh as 750°C at the turbine wheel, they are normallyunder 150° C at the journal bearing because of thecooling effect of the lubricating oil.The thrust bearing is often considered the most easilydamaged part in a turbo because it withstands full shaftRPM and is therefore more quickly damaged by hostileconditions. The next most easily damaged parts are thefree floating journal bearings. When either the thrustor free floating bearings are damaged, the hot and coldwheels are allowed to move excessively and can makecontact with their housings. High-speed wheel contactimmediately causes major impact damage to wheel bladesand can bend or break center shafts.Rotating parts must be very carefully balanced. Thismeans that both component balance and component stack-upmust be correct. Component balance is the balance ofeach individual part about its centerlines. Componentstack-up relates to the perpendicularity and parallelismof assembled components. Perpendicularity defines thesquareness of surfaces relative to the bore, whileparallelism defines the alignment of component endsurfaces. If these two things are incorrect, when thecompressor wheel nut is tightened the tensile load onthe center shaft will not be axial, bending of the shaftcan occur, and serious unbalance can result. Thus, bothindividual component balance and component stack-up mustbe very carefully controlled. During fieldreconditioning and repair these facts should be kept inmind and much care used when handling and assembling therotating parts.

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Design Lubrication System

LubricatingCoolingCleaning


The lubrication system is also vital to trouble freeturbocharger operation because it performs threeimportant functions: lubricating, cooling and cleaning.Interruptions of oil supply for only a few seconds cancause disastrous results. It is essential thatsufficient quantity of oil continually flows through theturbocharger to provide suspension and stabilization ofthe full floating bearing system and to remove heat.There are many ways that lubricant can be restricted orlost before it reaches the turbocharger. And thelubricant can contain large abrasive particles that canbridge the lubricant film and cause physical damage torotating parts. Thus, not only must adequate lubricantquantity be present, but the lubricant quality must alsobe good. Thus, before inspecting the failed turbo, weshould always gather basic quantity and quality factsabout the lubrication system such as:1. Type and viscosity of oil used2. Oil level on the dipstick3. Oil filter evaluation, including cutting it open andinspecting the paper4. SOS oil sample5. Operators’ comments about lube pressures or otherproblems prior to the failure

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Direct lubrication line to Turbo


All Caterpillar engines have direct lubrication lines toturbochargers to insure that filtered oil arrives assoon as possible. Some engines have a special prioritylubrication valve which sends unfiltered oil to theturbocharger even more quickly. These efforts insurethat fast moving turbo parts are lubricated and cooledas soon as possible.

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Turbochargers receive pressure oil from a central porton top of the center housing. Drilled passagewaysdistribute the oil to the bearings and rotating shaft.Some drilled passageways are small (especially those tothrust bearings) and can be blocked or obstructed byforeign material. Therefore, special care should be usedto insure that no debris is allowed to enter duringhandling or installation. Oil drains from the turbo bygravity force through a port on the bottom of the centerhousing to the engine crankcase.

Hot exhaust gasses enter the hot wheel (at the red areaon the right) at its outer circumference at high speed.The gasses are forced by the blades to change direction900 and exit through the center of the hot wheel,causing the hot wheel to rotate. Since the hot wheel isdirectly connected to the cold wheel, as it turns, thecold wheel also turns. This allows the turbocharger tobeneficially use wasted energy in the exhaust gasses tocompress inlet air for the engine. Any foreign materialentering the exhaust side of the turbocharger willdamage the edges of the blades at their outercircumference.

Incoming air is pulled into the center of the compressorwheel (blue area on the left) and is accelerated andthrown outward into the volute or collector surroundingthe cold wheel. This creates the higher pressure in thecollector which we call "boost" .The collector gives thehigher pressure air to the air inlet piping to theengine. Any foreign material that may enter withincoming air will impact on the leading edges of thecold wheel surrounding the retaining nut.

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This is a Switzer design turbo. Other designs haveslightly different structures, but the ideas we'lldiscuss are generally true of all designs.Parts that spin with the shaft and wheels are shown inblue color. Parts that are stationary are shown in redcolor. The free floating bearings are spun by frictionaldrag and rotate at about one-third the speed of therotating shaft. Lubrication passageways are shown ingreen. Notice that the smallest drilled lube passageway{which is most vulnerable to blockage by foreignmaterial) is the one to the stationary thrust bearing inthis particular turbocharger.

Seal rings are located just behind both the hot and coldwheels to prevent leakage of oil out of, or foreignmaterial into, the turbo. The seal rings fit tightly intheir outer housings and should not rotate -- the centershaft should turn within the seal rings. At low idlethese seals restrict oil leakage into hot and coldhousings, and at full load these seals keep exhaust andabrasive carbon from entering the bearing areas. Sincegravity is the only force draining oil from the turbo,high crankcase pressures can cause elevated pressures atthe seals and force oil to leak past them.

Because the journal bearings are rapidly spinning, anydebris in the oil has a tendency to centrifuge outwardcausing heavier abrasive damage to the outer diameterthan to the inner diameter of the bearings. Foreignmaterial larger than bearing oil holes will be trappedat the outer diameter, will do heavy abrasive cutting,and will eventually become small enough to pass throughthe bearing and exit with oil drain.

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Design Friction Welding

Hollow thermal barrierCentering Pin


Turbo shafts can be welded to turbine wheels with one oftwo processes: electron beam welding or frictionwelding. This acid etched cross-section of an electronbeam weld joint helps us more clearly see the detailsnear the weld. A centering pin is present to keep theparts aligned during welding. Also notice that the shaftand wheel are hollow for a short distance near the weld.This hollow area serves two purposes:1) With friction welding, it allows weld residues(called curl) to escape internally during the weldingprocess {the residue sometimes looks a little likecoarse threads), and2) With friction or inertial welding, it acts like athermal barrier {insulator) during turbo operation toreduce heat transfer from the hot wheel to the centershaft. Since the hot wheel is nonmagnetic, and thecenter shaft is magnetic, we can use a magnet todetermine whether a weld has failed, or whether theshaft has broken, i.e.: if the magnet sticks to the hotwheel side of the fracture, the shaft has broken.Also notice the induction hardening on the center shaftwhere the turbine journal bearing fits.

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Design Induction hardened Shaft


The center shaft is also induction hardened where thecompressor journal bearing fits to provide greaterstrength and wear capability.

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Design Compressor wheel

High Strength Aluminum alloys


Compressor wheels are made from high quality, highstrength aluminum alloys. Special care is taken inprocessing these alloys to prevent stringers andinclusions that could weaken the metal and cause cracksto start. This metal is not designed to withstand hightemperatures and should never be exposed to them.

Compressor wheel blade design can either be straight orback curved. Perhaps the easiest way to notice thedifference is to compare the two. Notice that slope ofthe blades on the bottom wheel is more severe than theslope of the blades on the top wheel. This bottom wheelis a back curve design. When RPM increases, centrifugalforce tries to straighten the back curve blades and bendthem straight. Thus, as RPM increases and then decreasesa cyclic bending load is placed on back curve blades,and this cyclic load from centrifugal force is much moresevere than the cyclic load from compressing air. As wediscussed in the Fracture module, it is cyclic loadsthat cause fatigue fractures. Blades must be designed towithstand these heavy cyclic bending loads as well asthe lighter loading from compressing air.

The center shaft hole is drilled on a special machinethat calculates precise hole location to give closestwheel balance. More exact balancing is sometimes done byremoving stock from the nose near the drilled hole.

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Balancing Notches


Balancing is also done on the backside of the coldwheel. We may find several circular notches along theoutside diameter as we see here, or

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Radial milling


we may find radial milling along the outside diameter aswe see here.

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Design Free-floating journal bearings

Copper/tin/lead alloy or from aluminiumLube holes or groove


The free-floating journal bearings can be made eitherfrom a copper/tin/lead alloy or from aluminum, dependingon the turbo design. On older turbos many bearings werecompletely saturated with lead, while newer bearingshave a lower lead content. The lead acts as a lubricantduring short periods of marginal lubrication {such asduring start-ups). You will also find that some bearingshave a thin tin flash over the copper/tin/lead alloy toincrease lubricity on start-ups.Bearing inside and outside diameters are carefullycontrolled to insure correct clearances and oil filmthickness. Notice that some of the bearings have oilholes that are chamfered to remove any drillingirregularities and to allow free flow of oil as thebearing is spinning. Other bearings will have oilgrooves on the sides.

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Design Snap rings

Rounded edge towards the bearing


Journal bearing retaining snap rings are stamped fromhigh strength steel. The stamping operation gives oneside rounded edges, and the other side sharp edges. Thesmooth, rounded edge should always be installed towardthe bearing to minimize abrasive contact.

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Copper/tin/lead alloy or from aluminium

Design Thrust Bearing

Bearing stationaryWasher full shaft RPM


Thrust bearings are made from copper/tin/lead and highstrength aluminum alloys. Some are tin flashed toimprove lubricity on start-ups, but most thrust surfacesbushings have a bronze appearance. Thrust bearings arestationary while adjacent thrust washers turn at fullshaft RPM. Because of this, thrust bearings absorb moreenergy than any other turbo bearing and therefore aremore sensitive to marginal lubrication, foreign materialand abnormal end loading.Some thrust bearings have drilled oil passageways as wesee here to provide direct lubrication to the thrustcontact surface. Notice the fine screen that thismanufacturer has installed to catch debris that couldblock the passageway and cause marginal lubricationdamage.

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Design Seal Rings

High temperature resistant chrome alloyCold side cast iron

End gap


Hot side seal rings are made from a high chrome alloyductile iron that can resist high temperatures. Coldside seal rings are made from cast iron and should neverbe exposed to high temperatures. Both are carefully madeto insure roundness, smooth surface finish, and adequatespring force. These are the things that keep the sealring from turning in the bore and from leaking. Whenseal rings are installed, end gap should be about 10thousandths of an inch (refer to service manuals forexact specifications for a particular turbocharger).

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Design Housing

Cast aluminum alloyCast IronCast ductile iron or nickel alloy


Compressor housings are made of a cast aluminum alloy.Bore perpendicularity and parallelism are carefullycontrolled to insure uniform compressor wheel clearances(usually somewhat less than twenty thousandths of aninch--refer to service manuals for exact specificationsfor each turbocharger). These housings are designed towithstand the forces of a high-speed compressor wheelseparation. Center housings are made from cast iron andare normally not subjected to either high temperaturesor high loads. Bore parallelism and perpendicularity arecarefully controlled, as well as the inside diameter andsurface finish where journal bushings fit.Turbine housings are made of ductile irons or nickelalloyed ductile irons. They must withstand loads of anyattachments at temperatures as high as 14000 F withoutcreeping (permanently changing size or shape). Thesehousings are also carefully machined to insure boreparallelism and perpendicularity and maintain uniformturbine wheel clearances.

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Design Heat Shield

Protects center housing from highexhaust temperatures


The turbine back plate, or heat shield, acts as aninsulator to protect the center housing from highexhaust temperatures. It is made of ductile iron andprovides insulation by creating a dead air space betweenthe turbine wheel and the center housing.

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Operation Start-up


When an engine is started, exhaust gasses immediatelybegin to spin the turbine wheel, center shaft andcompressor wheel. Lube flow has not yet arrived and onlyresidual oil is present on the bearings. Thus,asperities are not yet separated, nor is the oil filmcushion established that centers the turbo shaft andprevents orbiting shaft motion. We suggest thatcustomers run their engines at low idle for severalminutes to establish oil film and stabilize shaft motionbefore the engine is put to work.

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Operation Turbos are matched


Turbochargers are carefully matched to each engine togive the engine the inlet air it needs without damagingthe turbocharger if the system conditions are withinCaterpillar specifications:1. Inlet air restriction2. Exhaust restriction3. Aftercooler restriction4. Inlet/exhaust temperatures5. Crankcase pressure

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Operation Air Restriction

Turbo spin 80 000 RpmSurface speed 30m/second

Energy stored in rotatingcomponent can equal theengine HP


Turbochargers are free-spinning components which oftenspin faster than 80,000 RPM At peak RPM, journal bearingsurface speeds can be greater than 30 meter per second,and the energy stored in rotating components can equalengine horsepower. These conditions demand near perfectbalance and alignment of all moving parts, as well asproper operating and maintenance environments. Althoughproblems with the turbo itself can cause failures, it isusually simple problems in the turbo's workingenvironment, such as air inlet restriction, that causemost failures.

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Normal WearIncreased clearance


After thousands of hours of service, wear occurs andclearances increase. If kept in service too long, theturbocharger will develop center shaft motion, oilleakage and wheel contact. Since outside appearance ofworn turbochargers shows little more than the conditionof the turbine and compressor wheels, we need todisassemble them and inspect inside parts to see whatnormal wear looks like. This will also prepare us toidentify abnormal wear in the future.

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Normal Wear Hot side

Little leak past the seal ring


Little oil leaks past the hot side seal ring, as isevidenced by the fairly dry inside surface of the heatshield. Notice that the heat shield has done itsinsulating job here as shown by the presence of paint onthe hot end of the center housing. This means that thebearings have never been exposed to high temperaturesthat could destroy lubrication or cause metallurgicaldeterioration.

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Normal Wear Turbine Wheel

No discolorationSeal ring with gap separation


The turbine wheel and shaft assembly also show that noabnormal heat has been present. Notice the temper colorsstop just behind the hot wheel and before the journalbearing on the hot side.The chrome alloy hot side seal ring has been exposed tothese higher temperatures, but is not damaged because itis made from chrome alloy ductile iron. Notice that thering has not collapsed as shown by good end gapseparation.

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Normal Wear Bearings

More wear on hot side


These used journal bearings still have some of the tinflash left on the outside, with more wear occurring onthe hot side bearing than on the cold side bearing -- anormal occurrence due to the higher temperatures presentthere. Chamfers on oil holes (inside and outside) and onbearing edges are still present, also indicating thatvery little surface wear has occurred.

There appears to be minor abrasive damage to the hotside bearing. We should notice facts like the small,round indentation on the chamfer of the oil hole andshould suspect that the abrasive damage was caused bysmall, round, hard foreign material such as steel shot.

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Normal Wear Thrust Bearing

Fine abrasive wear


Thrust bearings and washers usually show some fineabrasive wear as a result of fine debris built into theturbo on manufacture or installation. This Air Researchthrust washer shows normal fine abrasive wear.

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Normal Wear Thrust washers


We also may find heavier contact on one side or atvarious points on the surface resulting from deviationsin component perpendicularity or parallelism. TheseSwitzer thrust washers rotate with the center shaft andshow more severe contact on one side than on the other.The temper colors show that metal-to-metal asperitycontact has occurred long enough to generate more than(285° C). Slight discoloration or wear such as thisafter thousands of hours of use should be consideredacceptable.

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• Lack of Lubrication• Abrasive in the Oil• High Exhaust Temperature• Foreign Objects• Turbo Problems

Why Turbos FailCHAPTER: Turbochargers

When others ask: "Why did this turbo fail?" we may feelunsure and be tempted to guess. But if we recognize thatspecific problems in the turbocharger or in its workingenvironment will produce specific failure results, wewill be on our way to finding the real root cause. Weshould begin by gathering facts and reading " roadssigns " from the failed turbo and its workingenvironment. This can lead us to general areas thatcause failures such as the following:

1. Lack of Lubrication

2. Abrasives in the Oil

3. High Exhaust Temperature

4. Foreign Objects

5. Turbocharger Problems.

Because each of these areas may contain many possibleroot causes, we must avoid saying that an area, such aslack of lubricant, is the cause of the failure. We mustcontinue down the trail of road signs until we know whatspecific thing caused the lack of lubrication. Only thenhave we reached our destination, the root cause of thefailure.

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• Temper Colors/ Cooked Oil• Adhesive Wear•Weakened Metal•Wheel Contact with Housing•Wheel separation from Shaft

Lack of Lube Road-signs


For example, lack of lubrication can be caused low oillevel, low oil pressure, wrong oil quality, high oiltemperatures, etc. . Lack of lubrication produces roadsigns such as:1. Temper colors and cooked oil in bearing areas2. Adhesive wear3. Weakened metal4. Hot side seal ring over heating, weakening,collapse, wear and destruction.5. Wheel contact with housings6. Occasional wheel separation from the center shaft.

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Lack of Lubrication Housing Contact


In this failure, wheel contact with housings hasoccurred and oil has been leaking behind the turbinewheel. Excessive shaft motion indicates that bearingsare damaged or worn. We must next look at the insideparts to determine the cause of the bearing damage.

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Temper colors and cooked oil residue

Lack of Lubrication Housing Contact


After disassembly, we see that the thrust bearing andhot side journal bearing surfaces are more severelydamaged than the cold side journal bearing. This isnormal because the thrust bearing encounters full turboRPM and the hot side journal bearing is nearest the highexhaust temperatures. Temper colors and cooked oilresidues tell us that high temperatures have beenpresent. But there is no evidence of abrasive attack. Soit appears that the oil was clean, but that there wasnot enough to lubricate and/or cool the turbo properly.

After some research, it was discovered that the enginewas operated with marginal lubrication from low oillevel until wheel rubbing noise was heard.

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Lack of Lubrication High Temperature


If turbo operation is continued with lack oflubrication, metal to metal contact can generate enoughheat to produce adhesive wear and destroy parts. Noticethat temper colors continue almost to the cold sidebearing, that the hot side seal has been physicallydestroyed, and that oil has been leaking and coking onthe turbine wheel.

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As high temperatures weaken and damage internal parts,excessive shaft motion develops and produces high loadsthat can break the center shaft. Although the casualobserver may suspect that the inertial weld between thehot wheel and center shaft has broken, the carefulanalyst will notice that the fracture faces are not onlyrough and ragged, but that temper colors are present atthe fracture site and heavy wheel to housing contact hasoccurred. This suggests that the shaft was abnormallyhot, was then overloaded, and finally suffered a ductilefracture.

Here, again, the analyst needs to determine the cause oflack of lubrication.

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If we need to verify our opinion that the inertial welddid not break, we can do so by touching a magnet to thefracture face on the hot wheel. If the weld has broken,the magnet will not have much attraction, but if theshaft has broken, the magnet will be strongly attractedto the fracture face. Thus, when the magnet sticks tothe hot wheel fracture face it tells us that theinertial weld is still intact and that it is the steelshaft that has broken.

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Careful visual examination in good lighting also revealsdark temper colors and part of the steel shaft broken bytorsional overload on a 45° angle. These "road signs",even without a magnet check, should tell us that the hotwheel separation is a result, and to inspect thebearings and other conditions that could have been theroot cause.

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Rapid Shutdown



There any many other possible ways that lack oflubrication can occur. For instance, this turbo failedwhen the operator repeatedly went from load conditionsto engine shutdown without cooling down the turbo. Afterthis continual abuse, the hot and cold wheels beganrubbing the housings and boost dropped off.

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Cooked oil residual



Closer inspection of the hot side journal bushing showsthat there is a dark layer of cooked residual oil on thebearing. And beneath that layer of cooked oil we can seefine abrasive cutting caused by prior hot shutdown andstart-ups on earlier cooked oil layers. So hot shutdownsnot only remove residual oil and create dry starts, butthey also create small abrasive cooked oil particlesthat cut the bearing and shaft the next time the engineis started. This rapidly wears out the softer bushing,producing shaft motion and wheel contact.

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Quench dots/rings



One good way to identify that hot shutdown has been doneis to remove the journal bushings and look for quenchdots or quench rings, where residual oil has draineddown into bearing oil holes or over bearing edges andhas tried to cool the superheated center shaft. Somecooling occurs in these areas and leaves thecharacteristic quench dots and rings.

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Bushing stuck to shaft



Continued hot shutdown operation will create more andmore bearing wear until bearing diameters aresignificantly reduced. This hot side bushing has alsostuck to the shaft as a result of inside gumming andcooking of residual oil. This caused the bearing to turnat full shaft RPM which damages the lube oil film andcreates marginal lubrication.

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Turbine wheel separation



Heat seldom travels to the cold side journal bushing aswe see evidenced here by the discolored center shaft andquench dots.Quench dots and rings can also be produced if a turbinewheel separates at the inertial weld during full loadoperation and the center shaft stops turning. But therewill be no abnormal wear on journal or thrust bushings.Thus, when we have wheel separation during operation, wemight expect to find a quench dot or ring on the hotside and normal appearance on the cold side.

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The report on the cause of failure on this turbochargerwas "The shaft broke causing the turbo to fail". Let'sreview the different parts more closely and see if wereach the same conclusion.

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Adhesive wear andThermal cracking


When journal bearings are allowed to get hotter thannormal, it is possible that some of the lead in thebearings can leach out and decrease clearances betweenthe bearing and the housing bore and/or the shaft. Thismay cause the bearing to stick on both the housing andshaft and totally stop rotation.The hot bearing here is firmly stuck in the housing borein fact, as the serviceman tried to remove it he brokethe center housing. The bearing itself shows adhesivewear on the outside and also has thermal cracking.

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Discoloration


High-speed rotation with excessive shaft motion putsextreme and unusual cyclic loading on rotating parts.Coupled with abnormally high temperatures, the centershaft can bend and allow the wheels to contact thehousings. This puts more cyclic loading on the shaft andit may soon break, as we see here.

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Ductile FractureDiscoloration


Inspection of the other half of the fracture revealsmore clearly the high temperature that was present, andthat plastic flow preceded the ductile fracture. Noticethat the plastic flow at the fracture face isaccompanied by a variety of dark temper colors. Thisdamage had to be done before the shaft broke becauseafter the shaft breaks the wheel stops turning and thereis no way for heat to be transmitted to the shaft.

Also, the back of the compressor wheel shows rubbing onone side indicating that the shaft had weakened and bentbefore it broke.

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Lack of Lubrication

WHY


But everything we have just discussed resulted from lackof lubrication. Was the field analysis of this failure,"The shaft broke and caused the turbo to fail",accurate? Definitely, this diagnosis is wrong. The jobthat should have been done was to find the cause of theshaft breaking, and all results point to lack oflubrication. Why was there lack of lubrication? Therepairing manager should have studied the road signs andthen followed them to the particular cause of lack oflubrication in this failure.

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Abrasive in the Oil Road-signs

• Scratches, Cuts, Grooves• Little Heat Damage• Rapid Wear• Embedded Debris


Abrasive material in the oil can damage bearings, causeexcessive shaft motion and lead to total failure.Contamination can result from dirty assembly, dirtymaintenance, exhaust leakage into the turbo, extendedoil change intervals, oil filter problems, etc.

Road signs that abrasives have been present in the oilinclude:

1. Scratches, cuts or grooves in rotating parts

2. Little heat build-up

3. Rapid wear

4. Embedded debris in bearings

5. Excessive bearing wear and center shaft motion

6. Hot and cold wheel contact with their housings

7. Seal rings leaking, collapsed, worn, missing

8. Occasional wheel separation from the center shaft.

All of the above are road signs of results that tell uswhich direction in which to search for the root cause.

Since the external appearance of failed turbos issimilar, it is the internal parts we must carefullyinspect to obtain the road signs we seek.

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Abrasive in the Oil Extended Oilchange Intervals

Excessive shaft motion


For instance, as we inspect the internal parts of thisfailed turbo we find that much oil has been leaking pastthe hot side seal, covering the heat shield with sludgeand oily residues. This indicates that there has beenexcessive shaft motion and/or that the hot side seal wasdamaged and leaking. The journal bearing unit is coatedwith oil sludge and varnish. There is no adhesive wear,but there is discoloration of the journal bearingassembly. These things indicate that there wassufficient oil to keep parts cool, but that the qualityof the oil needs investigation. We need informationabout oil type, oil viscosity, filter condition, andchange intervals.Investigation of these things led to discovery that theoperator had not changed oil for several months, but hadjust added oil periodically. This allowed unfiltered oilto circulate and caused this accelerated wear.

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Abrasive in the OilCHAPTER: Turbochargers

The internal parts of this failed turbo do not lookdirty--there is no evidence of high temperature oradhesive wear and some oil has been leaking past the hotside seal, indicating shaft motion or seal damage. Thereis noticeably more wear on the hot bearing than on thecold bearing, telling us to study that worn area moreclosely.

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Abrasive in the OilExhaust carbon cause fine abrasive cutting

Missing Hot side Seal


Close inspection with good lighting reveals that the hotside seal has been displaced from its wear groove andhas not been functioning. During operation exhaustcarbon has been introduced into the hot side journalbearing, has done very fine abrasive cutting, and wasthen flushed to the pan by clean oil that fed the turbo.The cold side journal bearing was not damaged because nooil contamination occurred there. These things tell usgather facts about the assembly of the turbocharger,particularly about the installation of the hot wheel andshaft assembly where the seal ring must be carefullyslipped into the center housing seal ring bore.The turbo builder apparently had been too rough wheninstalling the wheel and shaft assembly into the centerhousing and had displaced the seal ring from its groove.After a few months of service the fine abrasive wearallowed excessive shaft motion, oil leakage and wheelcontact with the housings.

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As we carefully inspect several failed turbos we'llnotice that the road signs become familiar and that ourminds more quickly grasp their meaning. Here we see thatheat has been removed at the hot side bushing,indicating that normal oil quantity has always beenpresent. The softer journal bushings have abrasivecutting the hard (about Rockwell C55) center shaft showsonly minor scratching. All parts are clean with novarnish or sludge build up. Neither wheel has hadsignificant housing contact.

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The hot side journal bearing feels greater temperaturesand more stress than does the cold side journal bearing,and closer inspection of that area is always a goodidea.Now we can see clearly that the seal ring end gap isgone -- the ring is collapsed. With no high temperaturespresent, we should suspect that excessive shaft motionhas done this damage. When the ring collapses, it nolonger can stay tight in the bore and begins to rotatewith the shaft. This will rapidly wear out both the ringand the groove in the center shaft. Eventually the ringwill wear thin, break, do more abrasive damage, andflush through the drain port to the oil pan. This closerview also allows us to see the scratching on the shaftmore clearly.

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Closer inspection of the journal bearings in good lightshows that heaviest abrasive wear has occurred on theoutside surfaces, partially closing the oil holes on thehot side bearing. The cold side bushing is in bettercondition because of the cooler temperatures in thatarea. The inside surfaces of both bearings show muchless wear because heavier contaminants will move outwarddue to centrifugal force present when the bearings spin.The deep abrasive grooves tell us that the size of thecontaminants was large.

These road signs tell us that either another failure wasin progress within the engine and self generated largepieces of foreign material, or that the foreign materialwas introduced during turbo build, installation, orduring engine maintenance. We need to gather facts inthese areas as we search for the root cause of thefailure.

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Abrasive in the OilNew turbo failed after 3 weeks of service


This thrust washer came from a truck turbocharger whichfailed just three weeks after the customer had purchasedthe complete turbo and installed it himself. He returnedto the selling dealership and complained that the newturbo he installed was performing worse than the old"tired" one he replaced. The service manager had aserviceman remove and disassemble the turbocharger andfound the turbo rotating parts completely worn out. Theserviceman said that this thrust bearing was typical ofthe wear he found on all parts, and he thinks it is adebris failure.

Pretend that you are the service manager who has theunhappy customer waiting for you in the lounge. What areyou going to do next? (Students should want to study thethrust bearing more closely to identify the type of wearand read the road signs.

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Abrasive in the Oil1. Debris size = pin head

2. Debris shape = spherical

3. Debris uniformity = same size

4. Debris color = black, blue, grey, silver.

5. Magnetic = yes


Closer inspection with good lighting shows what type ofwear? (Abrasive) What should we do now that we knowabrasive damage has failed the turbocharger? (Find thecause of the abrasive damage, i.e.: identify theabrasive material and where it came from. That willdetermine who is responsible for the repair bill. )Help students organize their thinking by listingcharacteristics of debris on the blackboard orflipchart, i.e.:1. Debris size = pin head2. Debris shape = spherical3. Debris uniformity = same size4. Debris color = black, blue, grey, silver.5. Magnetic = yesAsk students what debris fits these characteristics.(steel shot)Ask students who is most likely to put steel shot intothe turbocharger, the customer or the factory. (Thefactory)Now ask students what should be done with the customer.(They should respond that an apology should be extendedfor his inconveniences and another new part offered onparts warrants.) Point out that if we just let the parts“talk” to us as we do careful visual inspections,failure analysis becomes easier and accurate, becausethe parts tell us what has happened to them. Failedparts are really nice guys, because not only will theytell us what questions and answers to use with ourcustomers, but they also let us take all the credit forfiguring out the cause of the failure. Is there a betterfriend in failure analysis?

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High Exhaust Temp. Road-signs

• Much Heat Damage- Cooked Oil- Oxidation- Temper Colors• Much Bearing Wear


High exhaust temperature can force heat to penetrate thecenter housing of the turbocharger and damage rotatingparts. It also causes parts such as the turbine housingand center housing to oxidize and distort.Road signs of high exhaust temperature include:1. Much heat damagea. Cooked or carburized oilb. Oxidation/scaling of metal partsc. Temper colorsd. Turbine seal ring collapsed2. Worn bearings3. Wheel contact with housings4. 4. Occasional wheel separation from the center shaft

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High Exhaust Temp.Open Turbo and look inside


Visual inspection of the outside of this failed turboshould tell us that severe high temperature operationhas occurred. That condition might still exist on theengine, or it may have existed earlier, was recognized,and was corrected. It is also possible that theturbocharger has been rebuilt and the damaged centerhousing reused after a prior engine problem wascorrected. If we inspect the inside parts of the turbowe will have the answers to some of these questions.

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High Exhaust Temp.CHAPTER: Turbochargers

Many times the inside parts show evidence of severeoverheating, but not the bearing contact surfaces. Thismeans that the engine either now has, or very recentlyhad, a high exhaust temperature problem, and we need togather more facts about possible causes of high exhausttemperatures. The turbo is clearly a result--we mustseek the root cause.

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Foreign Object Road-signs

•Wheel Blade Damage• Bent Center Shaft• Normal Bearing Wear•Wheel Separation


When foreign objects enter a turbocharger it isimmediately and seriously damaged. And unbalance can bemore destructive than the physical distortion done bythe foreign material.Road signs of foreign object damage include:1. Bent and torn wheel blades and usually blades aredamaged)a. At the inside diameter of compressor wheel bladesb. At the outside diameter of turbine wheel blades2. Bent center shaft3. Normal wear and color of bearings (except that wearmay be misaligned if the center shaft is bent)4. Occasional wheel separation from shaft if foreignmaterial was large.

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Foreign Object Wheel Blade Damage


Fairly large pieces I of foreign material have damaged Ithe hot wheel of this turbocharger. Both the hot andcold .I wheels were making light contact with theirhousings, indicating that there is excessive shaftendplay.

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Uniform Bending


Closer inspection of the turbine wheel with goodlighting shows the uniform bending and chewing of allblades at the outside diameter where exhaust gassesenter the wheel. When a foreign object enters,centrifugal force keeps it at the outer diameter whereit grinds and becomes smaller until the force of exhaustgasses can over, come centrifugal force, carry it to thecenter of the wheel, and out into the exhaust piping.

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“Fresh Damage”


When foreign objects enter the compressor wheel, theblades at the inside diameter are bent and chewed.Centrifugal force assists inlet airflow in moving thedebris through the wheel and into the inlet piping tothe engine. We know the damage is fresh on this wheelbecause of the bright appearance of the fractures on theblades.

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Old Damage


Small foreign objects damaged this compressor wheel.Again, the damage is confined the leading edges of theblades. We know this damage occurred some time beforeoperation was stopped because of the dull lookingfractures on the blades, and because of the depositscoming away from the ragged leading edges.

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Turbocharger Problems• Design/Materials• Manufacturing


While most turbocharger failures are caused byenvironmental problems, some occur because of problemswith the turbocharger itself. We can group theseproblems into to general areas: (1) design or materials,and (2) manufacturing.

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Design/Materials•Wheel Burst• Blade Fatigue•Casting Inclusion•Weak Wheel


Errors in design or materials can cause compressorwheels to fracture at high speeds. These failures arenicknamed "wheel burst" because of the massive damagedone when the wheel separates at high speed. Wheelcastings can have inclusions that create localweaknesses and lead to fractures. Or it is possible thatthose who designed the turbo wheel underestimated normalcyclic loads it would have to carry.

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Wheel BurstCHAPTER: Turbochargers

When asked "What caused this compressor wheel to fail?"most people respond that foreign material is the rootcause. The next most popular answer is "The retainingnut backed off".The correct answer is " I don' t have enough facts yetto answer that question, but as soon as I've inspectedthe wheel and talked to the operator I'll give you myopinion".We should especially inspect the fracture faces.

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Now the fracture face clearly shows a semicircular areathat is smoother and brighter at the lower right side ofthe center shaft hole. This is a fatigue crack. Noticethat the surrounding areas are rough and dark and woody,characteristics of a final ductile fracture. Even thoughfracture features are less distinct in castings than inforgings, the fracture general characteristics willstill be observable.What load produces fatigue fractures? (Cyclic) Whatproduced the cyclic load that caused this wheel tobreak? (Varying turbo RPM) Was there a stressconcentrator present at the fatigue crack initiationsite? (None observable) Because the wheel breaks atmaximum load condition, and maximum loading occurs athigh RPM, the compressor wheel seems to explode into thehousing at final fracture. Thus, this failure isnicknamed "wheel burst". One of the functionalcharacteristics of the compressor and turbine housingsis to be able to withstand the forces of wheel burst andhigh energy wheel sections.Is this a design or workmanship problem that Caterpillarshould pay on warranty or policy? Not necessarily -- weshould first ask the Double check question "Is there anyway the other party could have caused this failure?"Since turbocharger RPM created the centrifugal force andthe cyclic load, we need to be sure that no inlet orexhaust restrictions, fuel settings, lug operation, latetiming, etc., were present which could have caused theturbo RPM to increase above specification. Caterpillarshould be questioned only when all these answers areno.Occasionally a supplier will change the design ormaterial of the compressor wheel that will cause wheelburst. Your recognition and timely communication of suchproblems will enable factory corrections to be made morequickly.

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If we are lucky, we may find a fatigue crack by closevisual inspection during rebuild or repair. A carefulserviceman discovered this crack before serious damagewas done. Notice that the crack has progressed welloutward from the center.

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The crack has progressed to the outside surface and isgrowing more rapidly in both directions. If put back inservice, this wheel would have soon suffered wheelburst.

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Blade FatigueCHAPTER: Turbochargers

At times only a blade will be missing from a compressoror turbine wheel, with minor damage to a few otherblades. When asked "What do you think caused thiscompressor wheel to fail?" the number one answer isagain "Foreign material damage". Is that the correctanswer? (No, because leading edges of the blades at theinside diameter are not damaged as they would be ifforeign material had struck them.)The correct answer will again be "I haven't inspectedthe wheel yet or gathered enough facts, but as soon as Ido so I will give you my opinion".

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Blade FatigueCHAPTER: Turbochargers

Closer inspection with good lighting shows that thefracture face of the missing blade is smoother andflatter near the compressor nut, and gets rougher andwoodier toward the outside diameter. The fact that onlyone blade is missing and that others show little damageshould verify that we have a fatigue fracture of a bladewhich itself acted as foreign material and did minorsecondary damage as it quickly left the compressor wheelwith inlet air flow.

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Too often when blades fatigue fracture customers aretold that they have allowed foreign material to entertheir turbocharger because we have seen blade damagecaused by foreign material entry. We need to guardagainst preconceived ideas and remember to let the factsbuild our opinions.

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Casting InclusionCHAPTER: Turbochargers

This turbine wheel has separated from the center shaftat high RPM and has much secondary impact damage. Didthe inertial weld fail, or did the shaft break for otherreasons? For example, a blade is broken off on the leftside. Did it have a fatigue fracture, come off, jam thewheel or bend the center shaft, and finally cause theshaft to break? We need to remember that when a turbinewheel loses a blade, centrifugal force will throw it tothe outer diameter against exhaust gas flow where itcannot escape until it becomes small enough to exit withexhaust gas force. Thus, turbine wheel blade fatiguefracture will cause much worse secondary damage thanwill compressor wheel blade fatigue fracture.

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Checking the inertial weld area with a magnet is a goodway to begin to answer questions. The magnet sticks tothe fracture face, indicating that the weld did notfail, but that the shaft broke. Inspection of thefracture face of the missing blade is the next area ofinterest.

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Close inspection with good lighting reveals a smallfatigue crack near the center of the fracture. It issurrounded by the rougher ductile final fracture.

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Even closer inspection of the initiation site at thebottom of the fracture shows what appears to be acasting flaw that initiated the fatigue crack.Our opinion of the cause of failure might be: "Theturbine wheel had a casting flaw which caused fatiguefracture of a blade". The blade did severe secondarydamage that led to wheel separation and total turbofailure.

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Manufacturing•Weak Inertial/Friction Weld• Bent Shafts• Rough Bores• Misdrilled Oil Holes• Balancing Errors


Errors in manufacturing include weak inertial welds,bent shafts, rough bearing bores, misdrilled oil holes,and balancing errors.

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Weak Inertial/Friction WeldCHAPTER: Turbochargers

We've mentioned inertial weld failure several times andshould take a few minutes to review facts about them.The weld location is between the hot side seal ring andthe hot wheel. Both the wheel and the shaft have a holein the center, 1 and after they are welded together ahollow cavity is present. This hollow area helpsinsulate the hot wheel from the center shaft and slowsthe conduction of heat from the exhaust into the bearingareas. We may sometimes see what looks like coarsethreads inside the hole in the shaft after a weldbreaks. This material is actually inertial weld residuecalled "curl".An error in the welding process caused this shaft tostop spinning much too soon, and no weld was made, andin just a few hours of operation the wheel separatedfrom the shaft.

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This inertial weld was also weak and after severalthousand hours allowed the wheel to separate from thecenter shaft. Notice how sharp and clean the faces are,and that there are no temper colors present. If we holda magnet to the hot wheel side of the fracture we willfind that there is weak attraction. If we hold themagnet to the shaft side, we will find that there isstrong attraction. These things tell us that the weldhas failed.And because the wheel shows contact with the housing andheat shield, we know that the turbine wheel continued tospin after the weld broke. Some have asked: "How longwill the hot wheel turn after it separated from theshaft?"

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The answer to that is: "There is sufficient exhaust gasforce to spin the broken wheel until it machines itselfsmall enough to go through the turbine housing and intothe exhaust piping."

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Bent ShaftUse T bar when tightening Compressor nut


If care is not used when assembling turbochargers, thecenter shaft can be bent slightly on the compressor sidewhere shaft diameters change. Either rough handling oroff center loading when tightening the compressor wheelretaining nut can bend the center shaft.Even though the shaft may not cause the wheels to rubthe housings when we turn them by hand after assembly,they will be off balance and at operational RPM willbend themselves further. Wheels then make contact withhousings, bearings do not have adequate oil films due tomisalignment, and shaft motion becomes excessive.The safest way to tighten the compressor wheel retainingnut is to use a T-handle torque wrench.

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Bent ShaftCHAPTER: Turbochargers

If too much metal is removed from the nose of thecompressor wheel during balancing, there may not beenough left to withstand the compressive load of thewheel retaining nut, especially if the nut face is cupshaped and hits the wheel at its outside diameter. Thiscan cause off-center loading of the center shaft, bendthe center shaft, and cause wheel contact with thehousing. Notice here that the blades are worn on theoutside diameter on one side of the wheel and on theinside diameter on the other side, indicating that theshaft was rotating bent.

The wheel retaining nut is missing, but if the shaftbreaks and the wheel hits the housing and stops spinningabruptly, the inertial energy of the nut can cause it toremove itself. Notice that the retaining nut madecontact more heavily at the outside diameter of the nutface, indicating that the nut face was not flat, but wascup-shaped. This puts additional load near the edge,adding more stress to any balancing recessed area.

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Misdrilled Oil HolesCHAPTER: Turbochargers

If a journal bearing wears only on the inside diameter,or on the outside diameter, it is usually a result ofthe other side sticking and full RPM transferring to theworn side. Sticking can result from hot shutdowns, orwhen bores are too rough, too small, or out of round.When the bearing does not turn freely in the centerhousing, accelerated wear such as we , see here canoccur.

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Misdrilled Oil HolesCHAPTER: Turbochargers

Misdrilled passageways are a turbocharger root cause ofmarginal lubrication failures. Here we see that the oilsupply passageway has been drilled off-center thatrestricts oil flow to the bearings. Failures may notoccur for several months after installation becausepartial oil supply is present continuously. Those who donot follow road signs to the off-center passageway mayreuse such apart and have exactly the same failureagain.

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Missed Oil HolesCHAPTER: Turbochargers

Slide 101 -- Or we may find lack of lubrication failurescaused by omission of a drilled passageway. This Switzerthrust bearing has no cross-drilled hole connecting theupper oil slot with the center thrust area. Without goodoil supply to clean, lubricate and cool the thrustsurface, the bearing was soon damaged and center shaftendplay became excessive. This led to wheel contact andturbocharger failure.

turbo charger

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