Turbochargers CHAPTER: Turbochargers

Turbochargers CHAPTER: Turbochargers
Welcome to a continuation of our failure analysis seminar and a discussion about turbocharger failures. We will again be reviewing facts that can guide us to root causes of failures.

CHAPTER: Turbochargers

In This Presentation Function Design Operation Normal wear
CHAPTER: Turbochargers In This Presentation Function Design Operation Normal wear Why Turbos Fail In this presentation we will discuss some basic facts about turbocharger function, structure, installation, operation and other problems that will help us understand why turbos fail.

Function

Function CHAPTER: Turbochargers
When turbos fail, it is a temptation to gather facts from only the failed turbocharger. We need to remember to immediately gather basic information about the lubrication, air inlet and exhaust system conditions prior to failure since these systems are part of the turbocharger's life and more often cause it to fail. Facts about pressures, leaks, restrictions, foreign material, high temperatures, loose connections or recent repairs should be recorded.

Normalize Air Supply Boost Air Supply Turbocharger Functions
CHAPTER: Turbochargers Turbocharger Functions Normalize Air Supply Boost Air Supply Turbochargers serve two functions, normalizing air supply and boosting air supply to engines.

Normalizing Compensate for thinner Air
CHAPTER: Turbochargers Normalizing Compensate for thinner Air Waste gate valve by-pass turbo boost Normal Power at high Altitudes Fuel deration at m to avoid over speed Quieter Exhaust, better combustions and emissions Normalizing means keeping air supply the same as is normal for a naturally aspirated engine at sea level. When engines operate at altitudes above sea level, the air becomes less dense, and the turbocharger is needed gather in more of the thinner air. If we don't normalize, we must decrease fuel settings as the air becomes less dense to avoid overfueling the engine. Thus, normalizing allows engines to develop normal horsepower 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 various altitudes and yet maintain a stable, normalized air supply. We should be aware that while turbochargers can concentrate thinner air at higher altitudes to give normal oxygen supply and normal power, they will spin faster to do so. Thus, for operation above about meter we often see fuel deration suggestions to avoid turbocharger over speed. Side benefits of turbocharging include quieter exhaust, better combustion, and cleaner emissions.

Normalizing First commercial use 1940’s in Airplane
CHAPTER: Turbochargers Normalizing First commercial use 1940’s in Airplane Although the idea of turbocharging is an old one, economical metals that could withstand high exhaust temperatures weren't produced until the 1940's. One of the first commercial uses of turbocharging was on the airplane engine to enable it to develop full power at higher altitudes.

Boosting More Oxygen / Increased Fuel Setting Higher Power
CHAPTER: Turbochargers Boosting More Oxygen / Increased Fuel Setting Higher Power Better Combustion - Better Fuel Economy - Cleaner Emissions Quieter The second function of a turbocharger is boosting air supply to give the engine more than normal oxygen. This enables increased fuel settings while still providing better combustion and quieter exhaust. Improved combustion means not only better fuel economy, but also cleaner exhaust emissions.

Boosting CHAPTER: Turbochargers
Some customers enjoy the feel of a little extra power. Powered by the Cat, these boats begin to fly when the throttles are opened.

Design Schwitzer Turbo

Design Waste-Gate Turbo
CHAPTER: Turbochargers Design Waste-Gate Turbo We should be familiar with the structure of turbochargers, the names of key internal parts, and how they fit together before doing failure analysis. Let's take a few minutes and review some basic facts about a typical turbo. When assembled, the cold wheel, the center shaft, and the hot wheel become one solid piece that turns in free-floating journal bearings. A stationary thrust bearing located near the cold wheel controls endplay. Larger turbos have two separate journal bearings while some small turbos have a single cartridge style bearing. Thrust washers are positioned on each side of the thrust bearing with a spacer between them.. When the compressor wheel is installed, the retaining nut forces the wheel, the thrust washers and the spacer against the shoulder on the center shaft, making them a part of the rotating assembly. All bearings ride on a cushion of oil during turbocharger operation. The turbine back plate, or heat shield, and the air space behind it serve as insulators to keep high exhaust temperatures from penetrating the center housing. Heat that is conducted into the center shaft from the hot wheel is removed at the bearing near the hot wheel by lubricating oil. Thus, even though temperatures can be a high as 750°C at the turbine wheel, they are normally under 150° C at the journal bearing because of the cooling effect of the lubricating oil. The thrust bearing is often considered the most easily damaged part in a turbo because it withstands full shaft RPM and is therefore more quickly damaged by hostile conditions. The next most easily damaged parts are the free floating journal bearings. When either the thrust or free floating bearings are damaged, the hot and cold wheels are allowed to move excessively and can make contact with their housings. High-speed wheel contact immediately causes major impact damage to wheel blades and can bend or break center shafts. Rotating parts must be very carefully balanced. This means that both component balance and component stack-up must be correct. Component balance is the balance of each individual part about its centerlines. Component stack-up relates to the perpendicularity and parallelism of assembled components. Perpendicularity defines the squareness of surfaces relative to the bore, while parallelism defines the alignment of component end surfaces. If these two things are incorrect, when the compressor wheel nut is tightened the tensile load on the center shaft will not be axial, bending of the shaft can occur, and serious unbalance can result. Thus, both individual component balance and component stack-up must be very carefully controlled. During field reconditioning and repair these facts should be kept in mind and much care used when handling and assembling the rotating parts.

Design Lubrication System
CHAPTER: Turbochargers Design Lubrication System Lubricating Cooling Cleaning The lubrication system is also vital to trouble free turbocharger operation because it performs three important functions: lubricating, cooling and cleaning. Interruptions of oil supply for only a few seconds can cause disastrous results. It is essential that sufficient quantity of oil continually flows through the turbocharger to provide suspension and stabilization of the full floating bearing system and to remove heat. There are many ways that lubricant can be restricted or lost before it reaches the turbocharger. And the lubricant can contain large abrasive particles that can bridge the lubricant film and cause physical damage to rotating parts. Thus, not only must adequate lubricant quantity be present, but the lubricant quality must also be good. Thus, before inspecting the failed turbo, we should always gather basic quantity and quality facts about the lubrication system such as: 1. Type and viscosity of oil used 2. Oil level on the dipstick 3. Oil filter evaluation, including cutting it open and inspecting the paper 4. SOS oil sample 5. Operators’ comments about lube pressures or other problems prior to the failure

CHAPTER: Turbochargers Design Lubrication System Direct lubrication line to Turbo All Caterpillar engines have direct lubrication lines to turbochargers to insure that filtered oil arrives as soon as possible. Some engines have a special priority lubrication valve which sends unfiltered oil to the turbocharger even more quickly. These efforts insure that fast moving turbo parts are lubricated and cooled as soon as possible.

CHAPTER: Turbochargers Design Lubrication System Turbochargers receive pressure oil from a central port on top of the center housing. Drilled passageways distribute the oil to the bearings and rotating shaft. Some drilled passageways are small (especially those to thrust bearings) and can be blocked or obstructed by foreign material. Therefore, special care should be used to insure that no debris is allowed to enter during handling or installation. Oil drains from the turbo by gravity force through a port on the bottom of the center housing to the engine crankcase. Hot exhaust gasses enter the hot wheel (at the red area on the right) at its outer circumference at high speed. The gasses are forced by the blades to change direction 900 and exit through the center of the hot wheel, causing the hot wheel to rotate. Since the hot wheel is directly connected to the cold wheel, as it turns, the cold wheel also turns. This allows the turbocharger to beneficially use wasted energy in the exhaust gasses to compress inlet air for the engine. Any foreign material entering the exhaust side of the turbocharger will damage the edges of the blades at their outer circumference. Incoming air is pulled into the center of the compressor wheel (blue area on the left) and is accelerated and thrown outward into the volute or collector surrounding the cold wheel. This creates the higher pressure in the collector which we call "boost" .The collector gives the higher pressure air to the air inlet piping to the engine. Any foreign material that may enter with incoming air will impact on the leading edges of the cold wheel surrounding the retaining nut.

CHAPTER: Turbochargers Design Lubrication System This is a Switzer design turbo. Other designs have slightly different structures, but the ideas we'll discuss are generally true of all designs. Parts that spin with the shaft and wheels are shown in blue color. Parts that are stationary are shown in red color. The free floating bearings are spun by frictional drag and rotate at about one-third the speed of the rotating shaft. Lubrication passageways are shown in green. Notice that the smallest drilled lube passageway {which is most vulnerable to blockage by foreign material) is the one to the stationary thrust bearing in this particular turbocharger. Seal rings are located just behind both the hot and cold wheels to prevent leakage of oil out of, or foreign material into, the turbo. The seal rings fit tightly in their outer housings and should not rotate -- the center shaft should turn within the seal rings. At low idle these seals restrict oil leakage into hot and cold housings, and at full load these seals keep exhaust and abrasive carbon from entering the bearing areas. Since gravity is the only force draining oil from the turbo, high crankcase pressures can cause elevated pressures at the seals and force oil to leak past them. Because the journal bearings are rapidly spinning, any debris in the oil has a tendency to centrifuge outward causing heavier abrasive damage to the outer diameter than to the inner diameter of the bearings. Foreign material larger than bearing oil holes will be trapped at the outer diameter, will do heavy abrasive cutting, and will eventually become small enough to pass through the bearing and exit with oil drain.

Design Friction Welding
CHAPTER: Turbochargers Design Friction Welding Hollow thermal barrier Centering Pin Turbo shafts can be welded to turbine wheels with one of two processes: electron beam welding or friction welding. This acid etched cross-section of an electron beam weld joint helps us more clearly see the details near the weld. A centering pin is present to keep the parts aligned during welding. Also notice that the shaft and 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 welding process {the residue sometimes looks a little like coarse threads), and 2) With friction or inertial welding, it acts like a thermal barrier {insulator) during turbo operation to reduce heat transfer from the hot wheel to the center shaft. Since the hot wheel is nonmagnetic, and the center shaft is magnetic, we can use a magnet to determine whether a weld has failed, or whether the shaft has broken, i.e.: if the magnet sticks to the hot wheel side of the fracture, the shaft has broken. Also notice the induction hardening on the center shaft where the turbine journal bearing fits.

Design Induction hardened Shaft
CHAPTER: Turbochargers Design Induction hardened Shaft The center shaft is also induction hardened where the compressor journal bearing fits to provide greater strength and wear capability.

Design Compressor wheel
CHAPTER: Turbochargers Design Compressor wheel High Strength Aluminum alloys Compressor wheels are made from high quality, high strength aluminum alloys. Special care is taken in processing these alloys to prevent stringers and inclusions that could weaken the metal and cause cracks to start. This metal is not designed to withstand high temperatures and should never be exposed to them. Compressor wheel blade design can either be straight or back curved. Perhaps the easiest way to notice the difference is to compare the two. Notice that slope of the blades on the bottom wheel is more severe than the slope of the blades on the top wheel. This bottom wheel is a back curve design. When RPM increases, centrifugal force tries to straighten the back curve blades and bend them straight. Thus, as RPM increases and then decreases a cyclic bending load is placed on back curve blades, and this cyclic load from centrifugal force is much more severe than the cyclic load from compressing air. As we discussed in the Fracture module, it is cyclic loads that cause fatigue fractures. Blades must be designed to withstand these heavy cyclic bending loads as well as the lighter loading from compressing air. The center shaft hole is drilled on a special machine that calculates precise hole location to give closest wheel balance. More exact balancing is sometimes done by removing stock from the nose near the drilled hole.

CHAPTER: Turbochargers Design Compressor wheel Balancing Notches Balancing is also done on the backside of the cold wheel. We may find several circular notches along the outside diameter as we see here, or

CHAPTER: Turbochargers Design Compressor wheel Radial milling we may find radial milling along the outside diameter as we see here.

Design Free-floating journal bearings
CHAPTER: Turbochargers Design Free-floating journal bearings Copper/tin/lead alloy or from aluminium Lube holes or groove The free-floating journal bearings can be made either from a copper/tin/lead alloy or from aluminum, depending on the turbo design. On older turbos many bearings were completely saturated with lead, while newer bearings have a lower lead content. The lead acts as a lubricant during short periods of marginal lubrication {such as during start-ups). You will also find that some bearings have a thin tin flash over the copper/tin/lead alloy to increase lubricity on start-ups. Bearing inside and outside diameters are carefully controlled to insure correct clearances and oil film thickness. Notice that some of the bearings have oil holes that are chamfered to remove any drilling irregularities and to allow free flow of oil as the bearing is spinning. Other bearings will have oil grooves on the sides.

Design Snap rings Rounded edge towards the bearing
CHAPTER: Turbochargers Design Snap rings Rounded edge towards the bearing Journal bearing retaining snap rings are stamped from high strength steel. The stamping operation gives one side rounded edges, and the other side sharp edges. The smooth, rounded edge should always be installed toward the bearing to minimize abrasive contact.

Design Thrust Bearing Copper/tin/lead alloy or from aluminium
CHAPTER: Turbochargers Design Thrust Bearing Copper/tin/lead alloy or from aluminium Thrust bearings are made from copper/tin/lead and high strength aluminum alloys. Some are tin flashed to improve lubricity on start-ups, but most thrust surfaces bushings have a bronze appearance. Thrust bearings are stationary while adjacent thrust washers turn at full shaft RPM. Because of this, thrust bearings absorb more energy than any other turbo bearing and therefore are more sensitive to marginal lubrication, foreign material and abnormal end loading. Some thrust bearings have drilled oil passageways as we see here to provide direct lubrication to the thrust contact surface. Notice the fine screen that this manufacturer has installed to catch debris that could block the passageway and cause marginal lubrication damage. Bearing stationary Washer full shaft RPM

Design Seal Rings High temperature resistant chrome alloy
CHAPTER: Turbochargers Design Seal Rings High temperature resistant chrome alloy Cold side cast iron Hot side seal rings are made from a high chrome alloy ductile iron that can resist high temperatures. Cold side seal rings are made from cast iron and should never be exposed to high temperatures. Both are carefully made to insure roundness, smooth surface finish, and adequate spring force. These are the things that keep the seal ring from turning in the bore and from leaking. When seal rings are installed, end gap should be about 10 thousandths of an inch (refer to service manuals for exact specifications for a particular turbocharger). End gap

Design Housing Cast aluminum alloy Cast Iron
CHAPTER: Turbochargers Design Housing Cast aluminum alloy Cast Iron Cast ductile iron or nickel alloy Compressor housings are made of a cast aluminum alloy. Bore perpendicularity and parallelism are carefully controlled to insure uniform compressor wheel clearances (usually somewhat less than twenty thousandths of an inch--refer to service manuals for exact specifications for each turbocharger). These housings are designed to withstand the forces of a high-speed compressor wheel separation. Center housings are made from cast iron and are normally not subjected to either high temperatures or high loads. Bore parallelism and perpendicularity are carefully controlled, as well as the inside diameter and surface finish where journal bushings fit. Turbine housings are made of ductile irons or nickel alloyed ductile irons. They must withstand loads of any attachments at temperatures as high as F without creeping (permanently changing size or shape). These housings are also carefully machined to insure bore parallelism and perpendicularity and maintain uniform turbine wheel clearances.

Design Heat Shield Protects center housing from high
CHAPTER: Turbochargers Design Heat Shield Protects center housing from high exhaust temperatures The turbine back plate, or heat shield, acts as an insulator to protect the center housing from high exhaust temperatures. It is made of ductile iron and provides insulation by creating a dead air space between the turbine wheel and the center housing.

Operation Start-up CHAPTER: Turbochargers
When an engine is started, exhaust gasses immediately begin to spin the turbine wheel, center shaft and compressor wheel. Lube flow has not yet arrived and only residual oil is present on the bearings. Thus, asperities are not yet separated, nor is the oil film cushion established that centers the turbo shaft and prevents orbiting shaft motion. We suggest that customers run their engines at low idle for several minutes to establish oil film and stabilize shaft motion before the engine is put to work.

Operation Turbos are matched
CHAPTER: Turbochargers Operation Turbos are matched Turbochargers are carefully matched to each engine to give the engine the inlet air it needs without damaging the turbocharger if the system conditions are within Caterpillar specifications: 1. Inlet air restriction 2. Exhaust restriction 3. Aftercooler restriction 4. Inlet/exhaust temperatures 5. Crankcase pressure

Operation Air Restriction
CHAPTER: Turbochargers Operation Air Restriction Energy stored in rotating component can equal the engine HP Turbochargers are free-spinning components which often spin faster than 80,000 RPM At peak RPM, journal bearing surface speeds can be greater than 30 meter per second, and the energy stored in rotating components can equal engine horsepower. These conditions demand near perfect balance and alignment of all moving parts, as well as proper operating and maintenance environments. Although problems with the turbo itself can cause failures, it is usually simple problems in the turbo's working environment, such as air inlet restriction, that cause most failures. Turbo spin Rpm Surface speed 30m/second

Normal Wear Increased clearance CHAPTER: Turbochargers
After thousands of hours of service, wear occurs and clearances increase. If kept in service too long, the turbocharger will develop center shaft motion, oil leakage and wheel contact. Since outside appearance of worn turbochargers shows little more than the condition of the turbine and compressor wheels, we need to disassemble them and inspect inside parts to see what normal wear looks like. This will also prepare us to identify abnormal wear in the future.

Normal Wear Hot side Little leak past the seal ring
CHAPTER: Turbochargers Normal Wear Hot side Little leak past the seal ring Little oil leaks past the hot side seal ring, as is evidenced by the fairly dry inside surface of the heat shield. Notice that the heat shield has done its insulating job here as shown by the presence of paint on the hot end of the center housing. This means that the bearings have never been exposed to high temperatures that could destroy lubrication or cause metallurgical deterioration.

Normal Wear Turbine Wheel
CHAPTER: Turbochargers Normal Wear Turbine Wheel No discoloration Seal ring with gap separation The turbine wheel and shaft assembly also show that no abnormal heat has been present. Notice the temper colors stop just behind the hot wheel and before the journal bearing on the hot side. The chrome alloy hot side seal ring has been exposed to these higher temperatures, but is not damaged because it is made from chrome alloy ductile iron. Notice that the ring has not collapsed as shown by good end gap separation.

Normal Wear Bearings More wear on hot side CHAPTER: Turbochargers
These used journal bearings still have some of the tin flash left on the outside, with more wear occurring on the hot side bearing than on the cold side bearing -- a normal occurrence due to the higher temperatures present there. Chamfers on oil holes (inside and outside) and on bearing edges are still present, also indicating that very little surface wear has occurred. There appears to be minor abrasive damage to the hot side bearing. We should notice facts like the small, round indentation on the chamfer of the oil hole and should suspect that the abrasive damage was caused by small, round, hard foreign material such as steel shot.

Normal Wear Thrust Bearing
CHAPTER: Turbochargers Normal Wear Thrust Bearing Fine abrasive wear Thrust bearings and washers usually show some fine abrasive wear as a result of fine debris built into the turbo on manufacture or installation. This Air Research thrust washer shows normal fine abrasive wear.

Normal Wear Thrust washers
CHAPTER: Turbochargers Normal Wear Thrust washers We also may find heavier contact on one side or at various points on the surface resulting from deviations in component perpendicularity or parallelism. These Switzer thrust washers rotate with the center shaft and show more severe contact on one side than on the other. The temper colors show that metal-to-metal asperity contact has occurred long enough to generate more than (285° C). Slight discoloration or wear such as this after thousands of hours of use should be considered acceptable.

Why Turbos Fail Lack of Lubrication Abrasive in the Oil
CHAPTER: Turbochargers Why Turbos Fail Lack of Lubrication Abrasive in the Oil High Exhaust Temperature Foreign Objects Turbo Problems When others ask: "Why did this turbo fail?" we may feel unsure and be tempted to guess. But if we recognize that specific problems in the turbocharger or in its working environment will produce specific failure results, we will be on our way to finding the real root cause. We should begin by gathering facts and reading " roads signs " from the failed turbo and its working environment. This can lead us to general areas that cause 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 possible root causes, we must avoid saying that an area, such as lack of lubricant, is the cause of the failure. We must continue down the trail of road signs until we know what specific thing caused the lack of lubrication. Only then have we reached our destination, the root cause of the failure.

Lack of Lube Road-signs
CHAPTER: Turbochargers Lack of Lube Road-signs Temper Colors/ Cooked Oil Adhesive Wear Weakened Metal Wheel Contact with Housing Wheel separation from Shaft For example, lack of lubrication can be caused low oil level, low oil pressure, wrong oil quality, high oil temperatures, etc. . Lack of lubrication produces road signs such as: 1. Temper colors and cooked oil in bearing areas 2. Adhesive wear 3. Weakened metal 4. Hot side seal ring over heating, weakening, collapse, wear and destruction. 5. Wheel contact with housings 6. Occasional wheel separation from the center shaft.

Lack of Lubrication Housing Contact
CHAPTER: Turbochargers Lack of Lubrication Housing Contact In this failure, wheel contact with housings has occurred and oil has been leaking behind the turbine wheel. Excessive shaft motion indicates that bearings are damaged or worn. We must next look at the inside parts to determine the cause of the bearing damage.

Lack of Lubrication Housing Contact
CHAPTER: Turbochargers Lack of Lubrication Housing Contact After disassembly, we see that the thrust bearing and hot side journal bearing surfaces are more severely damaged than the cold side journal bearing. This is normal because the thrust bearing encounters full turbo RPM and the hot side journal bearing is nearest the high exhaust temperatures. Temper colors and cooked oil residues tell us that high temperatures have been present. But there is no evidence of abrasive attack. So it appears that the oil was clean, but that there was not enough to lubricate and/or cool the turbo properly. After some research, it was discovered that the engine was operated with marginal lubrication from low oil level until wheel rubbing noise was heard. Temper colors and cooked oil residue

Lack of Lubrication High Temperature
CHAPTER: Turbochargers Lack of Lubrication High Temperature If turbo operation is continued with lack of lubrication, metal to metal contact can generate enough heat to produce adhesive wear and destroy parts. Notice that temper colors continue almost to the cold side bearing, that the hot side seal has been physically destroyed, and that oil has been leaking and coking on the turbine wheel.

CHAPTER: Turbochargers Lack of Lubrication High Temperature As high temperatures weaken and damage internal parts, excessive shaft motion develops and produces high loads that can break the center shaft. Although the casual observer may suspect that the inertial weld between the hot wheel and center shaft has broken, the careful analyst will notice that the fracture faces are not only rough and ragged, but that temper colors are present at the fracture site and heavy wheel to housing contact has occurred. This suggests that the shaft was abnormally hot, was then overloaded, and finally suffered a ductile fracture. Here, again, the analyst needs to determine the cause of lack of lubrication.

CHAPTER: Turbochargers Lack of Lubrication High Temperature If we need to verify our opinion that the inertial weld did not break, we can do so by touching a magnet to the fracture face on the hot wheel. If the weld has broken, the magnet will not have much attraction, but if the shaft has broken, the magnet will be strongly attracted to the fracture face. Thus, when the magnet sticks to the hot wheel fracture face it tells us that the inertial weld is still intact and that it is the steel shaft that has broken.

CHAPTER: Turbochargers Lack of Lubrication High Temperature Careful visual examination in good lighting also reveals dark temper colors and part of the steel shaft broken by torsional overload on a 45° angle. These "road signs", even without a magnet check, should tell us that the hot wheel separation is a result, and to inspect the bearings and other conditions that could have been the root cause.

CHAPTER: Turbochargers Lack of Lubrication High Temperature Rapid Shutdown There any many other possible ways that lack of lubrication can occur. For instance, this turbo failed when the operator repeatedly went from load conditions to engine shutdown without cooling down the turbo. After this continual abuse, the hot and cold wheels began rubbing the housings and boost dropped off.

CHAPTER: Turbochargers Lack of Lubrication High Temperature Cooked oil residual Closer inspection of the hot side journal bushing shows that there is a dark layer of cooked residual oil on the bearing. And beneath that layer of cooked oil we can see fine abrasive cutting caused by prior hot shutdown and start-ups on earlier cooked oil layers. So hot shutdowns not only remove residual oil and create dry starts, but they also create small abrasive cooked oil particles that cut the bearing and shaft the next time the engine is started. This rapidly wears out the softer bushing, producing shaft motion and wheel contact.

CHAPTER: Turbochargers Lack of Lubrication High Temperature Quench dots/rings One good way to identify that hot shutdown has been done is to remove the journal bushings and look for quench dots or quench rings, where residual oil has drained down into bearing oil holes or over bearing edges and has tried to cool the superheated center shaft. Some cooling occurs in these areas and leaves the characteristic quench dots and rings.

CHAPTER: Turbochargers Lack of Lubrication High Temperature Bushing stuck to shaft Continued hot shutdown operation will create more and more bearing wear until bearing diameters are significantly reduced. This hot side bushing has also stuck to the shaft as a result of inside gumming and cooking of residual oil. This caused the bearing to turn at full shaft RPM which damages the lube oil film and creates marginal lubrication.

CHAPTER: Turbochargers Lack of Lubrication High Temperature Turbine wheel separation Heat seldom travels to the cold side journal bushing as we see evidenced here by the discolored center shaft and quench dots. Quench dots and rings can also be produced if a turbine wheel separates at the inertial weld during full load operation and the center shaft stops turning. But there will be no abnormal wear on journal or thrust bushings. Thus, when we have wheel separation during operation, we might expect to find a quench dot or ring on the hot side and normal appearance on the cold side.

CHAPTER: Turbochargers Lack of Lubrication High Temperature The report on the cause of failure on this turbocharger was "The shaft broke causing the turbo to fail". Let's review the different parts more closely and see if we reach the same conclusion.

CHAPTER: Turbochargers Lack of Lubrication High Temperature Adhesive wear and Thermal cracking When journal bearings are allowed to get hotter than normal, it is possible that some of the lead in the bearings can leach out and decrease clearances between the bearing and the housing bore and/or the shaft. This may cause the bearing to stick on both the housing and shaft and totally stop rotation. The hot bearing here is firmly stuck in the housing bore in fact, as the serviceman tried to remove it he broke the center housing. The bearing itself shows adhesive wear on the outside and also has thermal cracking.

CHAPTER: Turbochargers Lack of Lubrication High Temperature Discoloration High-speed rotation with excessive shaft motion puts extreme and unusual cyclic loading on rotating parts. Coupled with abnormally high temperatures, the center shaft can bend and allow the wheels to contact the housings. This puts more cyclic loading on the shaft and it may soon break, as we see here.

CHAPTER: Turbochargers Lack of Lubrication High Temperature Ductile Fracture Discoloration Inspection of the other half of the fracture reveals more clearly the high temperature that was present, and that plastic flow preceded the ductile fracture. Notice that the plastic flow at the fracture face is accompanied by a variety of dark temper colors. This damage had to be done before the shaft broke because after the shaft breaks the wheel stops turning and there is no way for heat to be transmitted to the shaft. Also, the back of the compressor wheel shows rubbing on one side indicating that the shaft had weakened and bent before it broke.

WHY Lack of Lubrication CHAPTER: Turbochargers
But everything we have just discussed resulted from lack of lubrication. Was the field analysis of this failure, "The shaft broke and caused the turbo to fail", accurate? Definitely, this diagnosis is wrong. The job that should have been done was to find the cause of the shaft breaking, and all results point to lack of lubrication. Why was there lack of lubrication? The repairing manager should have studied the road signs and then followed them to the particular cause of lack of lubrication in this failure.

Abrasive in the Oil Road-signs
CHAPTER: Turbochargers Abrasive in the Oil Road-signs Scratches, Cuts, Grooves Little Heat Damage Rapid Wear Embedded Debris Abrasive material in the oil can damage bearings, cause excessive shaft motion and lead to total failure. Contamination can result from dirty assembly, dirty maintenance, exhaust leakage into the turbo, extended oil change intervals, oil filter problems, etc. Road signs that abrasives have been present in the oil include: 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 us which direction in which to search for the root cause. Since the external appearance of failed turbos is similar, it is the internal parts we must carefully inspect to obtain the road signs we seek.

Abrasive in the Oil Excessive shaft motion
CHAPTER: Turbochargers Abrasive in the Oil Extended Oilchange Intervals Excessive shaft motion For instance, as we inspect the internal parts of this failed turbo we find that much oil has been leaking past the hot side seal, covering the heat shield with sludge and oily residues. This indicates that there has been excessive shaft motion and/or that the hot side seal was damaged and leaking. The journal bearing unit is coated with oil sludge and varnish. There is no adhesive wear, but there is discoloration of the journal bearing assembly. These things indicate that there was sufficient oil to keep parts cool, but that the quality of the oil needs investigation. We need information about oil type, oil viscosity, filter condition, and change intervals. Investigation of these things led to discovery that the operator had not changed oil for several months, but had just added oil periodically. This allowed unfiltered oil to circulate and caused this accelerated wear.

Abrasive in the Oil CHAPTER: Turbochargers
The internal parts of this failed turbo do not look dirty--there is no evidence of high temperature or adhesive wear and some oil has been leaking past the hot side seal, indicating shaft motion or seal damage. There is noticeably more wear on the hot bearing than on the cold bearing, telling us to study that worn area more closely.

Abrasive in the Oil Exhaust carbon cause fine abrasive cutting
CHAPTER: Turbochargers Abrasive in the Oil Missing Hot side Seal Exhaust carbon cause fine abrasive cutting Close inspection with good lighting reveals that the hot side seal has been displaced from its wear groove and has not been functioning. During operation exhaust carbon has been introduced into the hot side journal bearing, has done very fine abrasive cutting, and was then flushed to the pan by clean oil that fed the turbo. The cold side journal bearing was not damaged because no oil contamination occurred there. These things tell us gather facts about the assembly of the turbocharger, particularly about the installation of the hot wheel and shaft assembly where the seal ring must be carefully slipped into the center housing seal ring bore. The turbo builder apparently had been too rough when installing the wheel and shaft assembly into the center housing and had displaced the seal ring from its groove. After a few months of service the fine abrasive wear allowed excessive shaft motion, oil leakage and wheel contact with the housings.

As we carefully inspect several failed turbos we'll notice that the road signs become familiar and that our minds more quickly grasp their meaning. Here we see that heat has been removed at the hot side bushing, indicating that normal oil quantity has always been present. The softer journal bushings have abrasive cutting the hard (about Rockwell C55) center shaft shows only minor scratching. All parts are clean with no varnish or sludge build up. Neither wheel has had significant housing contact.

The hot side journal bearing feels greater temperatures and more stress than does the cold side journal bearing, and closer inspection of that area is always a good idea. Now we can see clearly that the seal ring end gap is gone -- the ring is collapsed. With no high temperatures present, we should suspect that excessive shaft motion has done this damage. When the ring collapses, it no longer can stay tight in the bore and begins to rotate with the shaft. This will rapidly wear out both the ring and the groove in the center shaft. Eventually the ring will wear thin, break, do more abrasive damage, and flush through the drain port to the oil pan. This closer view also allows us to see the scratching on the shaft more clearly.

Closer inspection of the journal bearings in good light shows that heaviest abrasive wear has occurred on the outside surfaces, partially closing the oil holes on the hot side bearing. The cold side bushing is in better condition because of the cooler temperatures in that area. The inside surfaces of both bearings show much less wear because heavier contaminants will move outward due to centrifugal force present when the bearings spin. The deep abrasive grooves tell us that the size of the contaminants was large. These road signs tell us that either another failure was in progress within the engine and self generated large pieces of foreign material, or that the foreign material was introduced during turbo build, installation, or during engine maintenance. We need to gather facts in these areas as we search for the root cause of the failure.

Abrasive in the Oil New turbo failed after 3 weeks of service
CHAPTER: Turbochargers Abrasive in the Oil New turbo failed after 3 weeks of service This thrust washer came from a truck turbocharger which failed just three weeks after the customer had purchased the complete turbo and installed it himself. He returned to the selling dealership and complained that the new turbo he installed was performing worse than the old "tired" one he replaced. The service manager had a serviceman remove and disassemble the turbocharger and found the turbo rotating parts completely worn out. The serviceman said that this thrust bearing was typical of the wear he found on all parts, and he thinks it is a debris failure. Pretend that you are the service manager who has the unhappy customer waiting for you in the lounge. What are you going to do next? (Students should want to study the thrust bearing more closely to identify the type of wear and read the road signs.

Abrasive in the Oil CHAPTER: Turbochargers 1. 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 of wear? (Abrasive) What should we do now that we know abrasive damage has failed the turbocharger? (Find the cause of the abrasive damage, i.e.: identify the abrasive material and where it came from. That will determine who is responsible for the repair bill. ) Help students organize their thinking by listing characteristics of debris on the blackboard or flipchart, i.e.: 1. Debris size = pin head 2. Debris shape = spherical 3. Debris uniformity = same size 4. Debris color = black, blue, grey, silver. 5. Magnetic = yes Ask students what debris fits these characteristics. (steel shot) Ask students who is most likely to put steel shot into the turbocharger, the customer or the factory. (The factory) Now ask students what should be done with the customer. (They should respond that an apology should be extended for his inconveniences and another new part offered on parts 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, because the parts tell us what has happened to them. Failed parts are really nice guys, because not only will they tell us what questions and answers to use with our customers, but they also let us take all the credit for figuring out the cause of the failure. Is there a better friend in failure analysis?

High Exhaust Temp. Road-signs
CHAPTER: Turbochargers High Exhaust Temp Road-signs Much Heat Damage - Cooked Oil - Oxidation - Temper Colors Much Bearing Wear High exhaust temperature can force heat to penetrate the center housing of the turbocharger and damage rotating parts. It also causes parts such as the turbine housing and center housing to oxidize and distort. Road signs of high exhaust temperature include: 1. Much heat damage a. Cooked or carburized oil b. Oxidation/scaling of metal parts c. Temper colors d. Turbine seal ring collapsed 2. Worn bearings 3. Wheel contact with housings 4. 4. Occasional wheel separation from the center shaft

High Exhaust Temp. Open Turbo and look inside CHAPTER: Turbochargers
Visual inspection of the outside of this failed turbo should tell us that severe high temperature operation has occurred. That condition might still exist on the engine, or it may have existed earlier, was recognized, and was corrected. It is also possible that the turbocharger has been rebuilt and the damaged center housing reused after a prior engine problem was corrected. If we inspect the inside parts of the turbo we will have the answers to some of these questions.

High Exhaust Temp. CHAPTER: Turbochargers
Many times the inside parts show evidence of severe overheating, but not the bearing contact surfaces. This means that the engine either now has, or very recently had, a high exhaust temperature problem, and we need to gather more facts about possible causes of high exhaust temperatures. The turbo is clearly a result--we must seek the root cause.

Foreign Object Road-signs
CHAPTER: Turbochargers Foreign Object Road-signs Wheel Blade Damage Bent Center Shaft Normal Bearing Wear Wheel Separation When foreign objects enter a turbocharger it is immediately and seriously damaged. And unbalance can be more destructive than the physical distortion done by the foreign material. Road signs of foreign object damage include: 1. Bent and torn wheel blades and usually blades are damaged) a. At the inside diameter of compressor wheel blades b. At the outside diameter of turbine wheel blades 2. Bent center shaft 3. Normal wear and color of bearings (except that wear may be misaligned if the center shaft is bent) 4. Occasional wheel separation from shaft if foreign material was large.

Foreign Object Wheel Blade Damage
CHAPTER: Turbochargers Foreign Object Wheel Blade Damage Fairly large pieces I of foreign material have damaged I the hot wheel of this turbocharger. Both the hot and cold .I wheels were making light contact with their housings, indicating that there is excessive shaft endplay.

CHAPTER: Turbochargers Foreign Object Wheel Blade Damage Uniform Bending Closer inspection of the turbine wheel with good lighting shows the uniform bending and chewing of all blades at the outside diameter where exhaust gasses enter the wheel. When a foreign object enters, centrifugal force keeps it at the outer diameter where it grinds and becomes smaller until the force of exhaust gasses can over, come centrifugal force, carry it to the center of the wheel, and out into the exhaust piping.

CHAPTER: Turbochargers Foreign Object Wheel Blade Damage “Fresh Damage” When foreign objects enter the compressor wheel, the blades at the inside diameter are bent and chewed. Centrifugal force assists inlet airflow in moving the debris through the wheel and into the inlet piping to the engine. We know the damage is fresh on this wheel because of the bright appearance of the fractures on the blades.

CHAPTER: Turbochargers Foreign Object Wheel Blade Damage Old Damage Small foreign objects damaged this compressor wheel. Again, the damage is confined the leading edges of the blades. We know this damage occurred some time before operation was stopped because of the dull looking fractures on the blades, and because of the deposits coming away from the ragged leading edges.

Turbocharger Problems
CHAPTER: Turbochargers Turbocharger Problems Design/Materials Manufacturing While most turbocharger failures are caused by environmental problems, some occur because of problems with the turbocharger itself. We can group these problems into to general areas: (1) design or materials, and (2) manufacturing.

Design/Materials Wheel Burst Blade Fatigue Casting Inclusion
CHAPTER: Turbochargers Design/Materials Wheel Burst Blade Fatigue Casting Inclusion Weak Wheel Errors in design or materials can cause compressor wheels to fracture at high speeds. These failures are nicknamed "wheel burst" because of the massive damage done when the wheel separates at high speed. Wheel castings can have inclusions that create local weaknesses and lead to fractures. Or it is possible that those who designed the turbo wheel underestimated normal cyclic loads it would have to carry.

Wheel Burst CHAPTER: Turbochargers
When asked "What caused this compressor wheel to fail?" most people respond that foreign material is the root cause. The next most popular answer is "The retaining nut backed off". The correct answer is " I don' t have enough facts yet to answer that question, but as soon as I've inspected the wheel and talked to the operator I'll give you my opinion". We should especially inspect the fracture faces.

Now the fracture face clearly shows a semicircular area that is smoother and brighter at the lower right side of the center shaft hole. This is a fatigue crack. Notice that the surrounding areas are rough and dark and woody, characteristics of a final ductile fracture. Even though fracture features are less distinct in castings than in forgings, the fracture general characteristics will still be observable. What load produces fatigue fractures? (Cyclic) What produced the cyclic load that caused this wheel to break? (Varying turbo RPM) Was there a stress concentrator present at the fatigue crack initiation site? (None observable) Because the wheel breaks at maximum load condition, and maximum loading occurs at high RPM, the compressor wheel seems to explode into the housing at final fracture. Thus, this failure is nicknamed "wheel burst". One of the functional characteristics of the compressor and turbine housings is to be able to withstand the forces of wheel burst and high energy wheel sections. Is this a design or workmanship problem that Caterpillar should pay on warranty or policy? Not necessarily -- we should first ask the Double check question "Is there any way the other party could have caused this failure?" Since turbocharger RPM created the centrifugal force and the cyclic load, we need to be sure that no inlet or exhaust restrictions, fuel settings, lug operation, late timing, etc., were present which could have caused the turbo RPM to increase above specification. Caterpillar should be questioned only when all these answers are no. Occasionally a supplier will change the design or material of the compressor wheel that will cause wheel burst. Your recognition and timely communication of such problems will enable factory corrections to be made more quickly.

If we are lucky, we may find a fatigue crack by close visual inspection during rebuild or repair. A careful serviceman discovered this crack before serious damage was done. Notice that the crack has progressed well outward from the center.

The crack has progressed to the outside surface and is growing more rapidly in both directions. If put back in service, this wheel would have soon suffered wheel burst.

Blade Fatigue CHAPTER: Turbochargers
At times only a blade will be missing from a compressor or turbine wheel, with minor damage to a few other blades. When asked "What do you think caused this compressor wheel to fail?" the number one answer is again "Foreign material damage". Is that the correct answer? (No, because leading edges of the blades at the inside diameter are not damaged as they would be if foreign material had struck them.) The correct answer will again be "I haven't inspected the wheel yet or gathered enough facts, but as soon as I do so I will give you my opinion".

Blade Fatigue CHAPTER: Turbochargers
Closer inspection with good lighting shows that the fracture face of the missing blade is smoother and flatter near the compressor nut, and gets rougher and woodier toward the outside diameter. The fact that only one blade is missing and that others show little damage should verify that we have a fatigue fracture of a blade which itself acted as foreign material and did minor secondary damage as it quickly left the compressor wheel with inlet air flow.

Too often when blades fatigue fracture customers are told that they have allowed foreign material to enter their turbocharger because we have seen blade damage caused by foreign material entry. We need to guard against preconceived ideas and remember to let the facts build our opinions.

Casting Inclusion CHAPTER: Turbochargers
This turbine wheel has separated from the center shaft at high RPM and has much secondary impact damage. Did the inertial weld fail, or did the shaft break for other reasons? For example, a blade is broken off on the left side. Did it have a fatigue fracture, come off, jam the wheel or bend the center shaft, and finally cause the shaft to break? We need to remember that when a turbine wheel loses a blade, centrifugal force will throw it to the outer diameter against exhaust gas flow where it cannot escape until it becomes small enough to exit with exhaust gas force. Thus, turbine wheel blade fatigue fracture will cause much worse secondary damage than will compressor wheel blade fatigue fracture.

Checking the inertial weld area with a magnet is a good way to begin to answer questions. The magnet sticks to the fracture face, indicating that the weld did not fail, but that the shaft broke. Inspection of the fracture face of the missing blade is the next area of interest.

Close inspection with good lighting reveals a small fatigue crack near the center of the fracture. It is surrounded by the rougher ductile final fracture.

Even closer inspection of the initiation site at the bottom of the fracture shows what appears to be a casting flaw that initiated the fatigue crack. Our opinion of the cause of failure might be: "The turbine wheel had a casting flaw which caused fatigue fracture of a blade". The blade did severe secondary damage that led to wheel separation and total turbo failure.

Manufacturing Weak Inertial/Friction Weld Bent Shafts Rough Bores
CHAPTER: Turbochargers 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.

Weak Inertial/Friction Weld
CHAPTER: Turbochargers Weak Inertial/Friction Weld We've mentioned inertial weld failure several times and should take a few minutes to review facts about them. The weld location is between the hot side seal ring and the hot wheel. Both the wheel and the shaft have a hole in the center, 1 and after they are welded together a hollow cavity is present. This hollow area helps insulate the hot wheel from the center shaft and slows the conduction of heat from the exhaust into the bearing areas. We may sometimes see what looks like coarse threads inside the hole in the shaft after a weld breaks. This material is actually inertial weld residue called "curl". An error in the welding process caused this shaft to stop spinning much too soon, and no weld was made, and in just a few hours of operation the wheel separated from the shaft.

CHAPTER: Turbochargers Weak Inertial/Friction Weld This inertial weld was also weak and after several thousand hours allowed the wheel to separate from the center shaft. Notice how sharp and clean the faces are, and that there are no temper colors present. If we hold a magnet to the hot wheel side of the fracture we will find that there is weak attraction. If we hold the magnet to the shaft side, we will find that there is strong attraction. These things tell us that the weld has failed. And because the wheel shows contact with the housing and heat shield, we know that the turbine wheel continued to spin after the weld broke. Some have asked: "How long will the hot wheel turn after it separated from the shaft?"

CHAPTER: Turbochargers Weak Inertial/Friction Weld The answer to that is: "There is sufficient exhaust gas force to spin the broken wheel until it machines itself small enough to go through the turbine housing and into the exhaust piping."

Bent Shaft Use T bar when tightening Compressor nut
CHAPTER: Turbochargers Bent Shaft Use T bar when tightening Compressor nut If care is not used when assembling turbochargers, the center shaft can be bent slightly on the compressor side where shaft diameters change. Either rough handling or off center loading when tightening the compressor wheel retaining nut can bend the center shaft. Even though the shaft may not cause the wheels to rub the housings when we turn them by hand after assembly, they will be off balance and at operational RPM will bend themselves further. Wheels then make contact with housings, bearings do not have adequate oil films due to misalignment, and shaft motion becomes excessive. The safest way to tighten the compressor wheel retaining nut is to use a T-handle torque wrench.

Bent Shaft CHAPTER: Turbochargers
If too much metal is removed from the nose of the compressor wheel during balancing, there may not be enough left to withstand the compressive load of the wheel retaining nut, especially if the nut face is cup shaped and hits the wheel at its outside diameter. This can cause off-center loading of the center shaft, bend the center shaft, and cause wheel contact with the housing. Notice here that the blades are worn on the outside diameter on one side of the wheel and on the inside diameter on the other side, indicating that the shaft was rotating bent. The wheel retaining nut is missing, but if the shaft breaks and the wheel hits the housing and stops spinning abruptly, the inertial energy of the nut can cause it to remove itself. Notice that the retaining nut made contact more heavily at the outside diameter of the nut face, indicating that the nut face was not flat, but was cup-shaped. This puts additional load near the edge, adding more stress to any balancing recessed area.

Misdrilled Oil Holes CHAPTER: Turbochargers
If a journal bearing wears only on the inside diameter, or on the outside diameter, it is usually a result of the other side sticking and full RPM transferring to the worn side. Sticking can result from hot shutdowns, or when bores are too rough, too small, or out of round. When the bearing does not turn freely in the center housing, accelerated wear such as we , see here can occur.

Misdrilled Oil Holes CHAPTER: Turbochargers
Misdrilled passageways are a turbocharger root cause of marginal lubrication failures. Here we see that the oil supply passageway has been drilled off-center that restricts oil flow to the bearings. Failures may not occur for several months after installation because partial oil supply is present continuously. Those who do not follow road signs to the off-center passageway may reuse such apart and have exactly the same failure again.

Missed Oil Holes CHAPTER: Turbochargers
Slide Or we may find lack of lubrication failures caused by omission of a drilled passageway. This Switzer thrust bearing has no cross-drilled hole connecting the upper oil slot with the center thrust area. Without good oil supply to clean, lubricate and cool the thrust surface, the bearing was soon damaged and center shaft endplay became excessive. This led to wheel contact and turbocharger failure.

Turbochargers CHAPTER: Turbochargers

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