Bearings master

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  • Slide 1 -- Welcome to a continuation of the failure analysis seminar. This section covers main and connecting rod bearing failure analysis.
  • Slide 2 -- In this presentation we will review bearing function, structure, manufacture, installation, operation, and possible bearing problems. We will be following the eight steps of applied failure analysis and we will also apply the principles of management, wear and visual examination covered in earlier sections. Basic lubrication system facts are included as necessary. Being familiar with these things will prepare us to do a better job of bearing failure analysis in the future.
  • Slide 3 -- When using the eight steps of applied failure analysis, we should especially remember to get the payoff with our customers by completing steps 6,7 and 8. We hope the information in this section will help identify root causes of failures and help separate failures caused by abnormal system or loading conditions from failures caused by bearings themselves.  
  • Slide 4 -- Existing Caterpillar publications, such as SEBD 00531, "Engine Bearings and Crankshafts", are also helpful in failure analysis work.  
  • Slide 5 -- Connecting rod and main bearings perform the following five important functions in our engines: 1. Provide a slippery surface on start-ups and at other times when oil film thickness is reduced. 2. Conduct surface heat to the bore. 3. Provide a soft wear surface to embed debris and to allow the bearing to conform to the crankshaft journal profile. 4. Provide load carrying or fatigue strength. 5. Provide replaceable wear surfaces in connecting rods and block bores.
  • Slide 6 -- The bearings first function is lubricity -- protecting itself and its crankshaft journal on start-ups or at other times when there is only a small amount of residual oil between the crankshaft and the bearings. Because the bearing surface is soft and slippery, the hard crankshaft journal slides easily on it, minimizing heat generation. Surface conditions shown here are exaggerated for illustration purposes.
  • 7 -- This is a view of the lead-tin overlay on a new bearing at 400 power magnification. The light colored metal at the bottom is aluminum. Above the aluminum is a very thin layer of copper. The next darker layer is lead-tin overlay. The black above the lead-tin is background material around the bearing.  
  • Slide 8 -- During start-up, generated frictional heat is removed only by conduction through the bearing to the rod or block. Bearings are therefore designed and constructed to provide excellent heat transfer. We call this function "heat conduction".
  • Slide 9 -- Debris is always present in lubrication systems and is removed by good oil filters. When engines are started, or when change intervals are too long, particles can bypass the filters. When debris enters the bearing area, the particles are trapped by the lead-tin overlay, removing them from the system. This bearing function is called "embedability".   The soft lead-tin overlay also moves somewhat with surface loading until the bearing surface "fits" more exactly with the crankshaft journal profile. We call this function "conformability".
  • Slide 10 – If we cut a used bearing in half and look at the cross section with 400 power magnification, we can see that the black particles embed in the lead-tin overlay similar to rocks sinking into mud. The lead-tin, squeezes up around the particles, protecting the crankshaft journal and bearing from abrasive damage.  
  • Slide 11 -- During full load operation, pressures as high as 20,000 psi can be felt by rod bearings. At 1,600 RPM, each rod bearing may feel this pressure 800 times a minute-and this severe cyclic loading can continue for thousands of hours. When trapped oil layers are thin, such as in idling conditions or when heavy loads are applied at very low engine RPM, the bearing must withstand even more severe loading. We call this bearing function "fatigue strength".
  • Slide 12 -- After thousands of hours of operation, normal wear removes some of the bearings' soft surface material, reducing its lubricity and embedability capabilities. Since bearings are inserts, worn bearings can be replaced with new ones, restoring full bearing capabilities. We call this bearing function "replaceability".
  • Slide 13 -- To perform its five functions the bearing must have a composite (many material) structure. This exploded view of a Caterpillar bearing helps us understand its structure.   Most of the bearing is steel that is overlaid with aluminum, copper bond, lead-tin, and a thin tin flash. Each of these layers helps the bearing perform its five functions.   The steel back accounts for about 90 percent of bearing thickness, gives the bearing its structural strength to stay tight in the bore, supports the aluminum load carrying layer, and conducts heat rapidly from the aluminum to the bore.   The aluminum layer accounts for about 10 percent of bearing thickness, is soft enough to provide good embedability, and yet is strong and durable under heavy cyclic loading. It is also a good heat conductor, transferring heat quickly from the bearing surface to the steel back during operation.   The copper bonding layer is about one ten-thousandths of an inch thick, provides good bonding between the aluminum and lead-tin layers, transfers heat quickly from the lead-tin to the aluminum, and provides a soft wearing surface when the lead-tin layer wears away.   The lead-tin layer is about one thousandth of an inch thick (or half as thick as a piece of paper), is very soft, and performs the functions of lubricity, embedability and conformability.   The tin flash is only a few millionths of an inch thick, provides corrosion protection, and gives the bearing a pleasing appearance.
  • Slide 14 -- Thrust washers do not have the lead-tin layer because thrust loading does not require the functions of lubricity and embedability. Thus, thrust bearings have only the steel back, the aluminum layer and the tin flash.
  • Slide 15 -- Bearings are manufactured in three basic stages: bimetallic strip preparation, machining operations, and electroplating processes.
  • Slide 16 -- The steel back is made from a precision thickness low carbon steel sheet in coil form. The sheet is uncoiled and degreased, after which one side is roughened and cleaned in preparation for bonding to aluminum. (Grit blasting and belt sanding are two common roughening methods). The backside must be smooth and free of blemishes, nicks, or gouges to insure complete bore contact.
  • Slide 17 -- The aluminum begins as pig aluminum from which most bearing manufacturers cast their own aluminum alloy and roll it into thin sheet in coil form. The coil is sent to the bimetal bonding line where it is uncoiled, cleaned and roughened before bonding to the steel back.
  • Slide 18 -- In the bonding process, the aluminum and steel sheets are preheated and squeezed together with high-pressure rolls, forming a bimetallic (two metal) strip. No bonding agent is used, the bond being formed mechanically by forcing aluminum into the roughened steel surface at high pressure and temperature.
  • Slide 19 -- On the rough machining line the bimetal strip is uncoiled and sheared into half shell bearing blanks. Each bearing blank is then bent into final shape over a forming die that sets bearing free spread. (Free spread means the bearing free diameter is larger at the parting faces than is the bore diameter that requires a slight force to "snap" the bearing into the bore on installation.) Chamfers, grooves, oil holes and tabs are then added at this time.  
  • Slide 20 -- Crush height and wall thickness are the last and most critical finishing machining operations performed. Crush height (the amount the bearing is larger than its bore) is controlled by broaching the half shell parting faces. Wall thickness (bearing thickness) can be controlled by either boring (radial machining) or broaching (side to side machining). Each bearing half is precisely sized and continues processing individually. There is no difference between rod bearing upper and lower half shells and most main bearing upper and lower half shells differ only in oil grooves and oil holes.
  • Slide 21 -- After finish machining, the bearing half shells are ready for application of the lead-tin and tin flash layers. Since lead-tin will not stick directly to aluminum, a very thin bonding layer of copper is first electroplated onto the aluminum. After a water rinse, the lead-tin overlay is electroplated onto the copper bonding layer. After another rinse, an extremely thin layer of tin (called tin flash) is chemically plated on all bearing surfaces and the bearing is ready for inspection and packaging. Since the bearings are made by the half, there is no "matched set", i.e.: an upper half is packaged with a lower half.  
  • Slide 22 -- As we take new bearings out of boxes, we need to remember that they have been carefully made but require proper installation to give good performance. The small lines or marks on bearing surfaces are gauging marks made during quality control wall thickness testing.
  • Slide 23 -- Bearings should be installed in clean, dry bores. This gives maximum heat conduction from the bearing to the bore, avoids hot spots caused by debris under the bearing, and also improves the coefficient of friction between the bearing and the bore to help prevent slippage. It is this frictional fit between the bearing back and the bore that prevents bearing movement and slippage, not the locating tab. The purpose of the tab is to properly align the bearing during installation. After the bearing halves are installed in clean, dry bores, their surfaces should be lubricated before installing crankshafts, caps and retaining bolts.   It is always a good idea to visually check alignment of oil holes during installation. Also, notice that the oil hole in the main bearing is noticeably smaller than the drilled passageway in the block, acting as an orifice to control oil flow to downstream rod bearings.  
  • Slide 24 -- Bearing diameters are slightly larger than bore diameters (crush height), and as bearing caps are installed, the bearing halves contact each other before the bore parting faces make contact. Then, as retaining bolts are tightened, the bearing halves are squeezed into the smaller bore diameter, "crushing" the bearing. This crush is an elastic deformation (temporary physical change similar to compressing a spring) that forces the bearing tightly against the bore and prevents movement. Thus, if the bearing is installed in a clean, dry bore and if bearing crush is proper, the bearing should not move in the bore when loads are applied.   Bearing movement can occur if:   (1) Bearing cap retaining bolts are under torqued (bolts don't exert full clamping force).   (2) Retaining bolts are over torqued (are stretched and weakened).   (3) Debris is left between the bore parting face during bearing installation.   (4) Parting or mating faces are too rough   (5) Long service hours cause bearings to gradually conform to the bore and become loose.   (6) Etc.   These things can lead to fretting corrosion (loose part wear) between the bearing and the bore.  
  • Slide 25 -- After careful assembly with quality parts, cat engines are ready to be put to work.
  • Slide 26 -- As engines begin operation, bearing service life depends on proper lubrication, loading, and temperatures. Let's take a few minutes to review some facts in these critical areas.  
  • Slide 27 -- When engines are started, bearing and crankshaft surfaces are separated only by a small amount of residual oil. Until fresh oil arrives, surface asperities make contact and generate heat. Thus, loading and RPM should be kept to a minimum during start-up to avoid excessive heat generation.  
  • Slide 28 -- During this time the oil pump is also turning, forcing oil rapidly into passageways to the oil cooler and filters. Caterpillar oil pumps supply more oil than is required by the engine, continually bypassing part of their output back to the oil pan. As engine wear occurs and clearances increase, more oil is required by the engine and less oil bypasses at the oil pump. This explains why Caterpillar engines have consistently high oil pressure throughout their lives. Cold, thick oil and system restrictions cause oil pressure to rise. Bypass valves are placed on the oil pump, oil cooler, and oil filters to allow oil to go around these components when restrictions are too great. Thus, high RPM on start-up with cold oil causes bypass valves to open and sends unfiltered oil to bearings.
  • Slide 29 -- Oil filters control not only the size but also the quantity of erosive and abrasive particles in the oil. To do this, oil filters are carefully constructed to work with the oil filter by-pass valve, i.e.: Caterpillar oil filter bypass valve opening pressures are high and the paper has to not only catch harmful debris at high flow rates, but the paper also has to withstand high differential pressures without rupturing as when partially blocked with debris. Since the oil filter is readily accessible and is changed frequently, it represents sales potential. Competition has produced and sold oil filters that fit Caterpillar engines but which may not have necessary construction quality.   Any time we have abrasive bearing wear (often called debris damage), we should always cut the oil filters open and carefully examine them for defects.
  • Slide 30 -- When oil arrives at the bearings, it separates the wearing surfaces and removes excessive heat that has been generated during start-up. During normal operation loads are transferred to bearings by the trapped oil layer and heat generation is tremendously reduced. Only under start-up or other marginal lubrication conditions do bearing surfaces make direct contact with the crankshaft. Notice that the hole in the main bearing upper half shell is acting as the final orifice to limit the amount of oil that flows downstream to the rod bearings.  
  • Slide 31 -- Some people think that system oil pressure creates the oil film thickness at the bearings, but system oil pressure merely supplies oil to the bearing area. It is the rotating crankshaft that builds the oil film thickness as the crankshaft turns faster, it pulls more oil with it and makes the oil film become thicker. Thus, oil film thickness can be decreased by low rotational speed of the crankshaft. Oil dilution, unusually high temperatures, and low oil supply pressures can also decrease it.  
  • Slide 32 -- Failure of a rod or main bearing usually doesn't cause marginal lubrication to any other bearings unless the engine is used at low RPM. When a bearing begins to fail (not yet spun), oil leakage will be limited by the orifice hole in the main bearing and the oil pump bypass valve will begin to close and send more oil to the engine, thereby maintaining system oil pressure. But at low RPM, the oil pump does not pump much oil, its relief valve is closed, and a failed rod or main bearing may leak enough oil to cause reduced oil supply and other bearing damage. Therefore, it is necessary to obtain facts about engine operation before stating our opinions.
  • Slide 33 -- Bearing loading is much more severe today than it was in earlier diesel engines, demanding greater bearing and crankshaft design and quality.   In the old days, many diesel engines ran at less than 1500 RPM, had peak cylinder pressures less than 1500 PSI, and had generous bearing surface areas.   Today's diesel engines run over 3000 RPM, have peak cylinder pressures over 2000 PSI, and have smaller bearing surface areas.   These demanding conditions mean that crankshaft and bearing quality must be better than ever, and that application, operation and maintenance must be more carefully done.  
  • Slide 34 -- When surface temperatures are allowed to rise above 350 degrees Fahrenheit, bearings can be damaged. Operators must insure low start-up RPM and good lubrication conditions.  
  • Slide 35 -- When application, operation and maintenance are correct, main and rod bearings will develop wear patterns that we call "normal". Normal wear progresses through the thin tin flash, through the lead-tin overlay, through the copper bond, and into the aluminum layer. The back of the bearing normally darkens in irregular patterns with age.
  • Slide 36 -- Because the tin flash layer is very thin (micro-inches}, it readily wears through on high profile points. After the tin flash wears away, the darker lead-tin layer beneath is exposed. This low mileage bearing has normal tin flash wear.  
  • Slide 37 -- Magnification helps us see wear details more clearly. Here we see that the tin flash has worn away on the higher ridges made by finish machining of the aluminum layer. Notice that the machining was radial on this bearing.  
  • Slide 38 -- When we use magnification, normal small surface irregularities may seem too large and cause us to become unnecessarily alarmed. It will take some practice with the magnifying glass to familiarize our minds with the appearance of normal and abnormal surface conditions. Occasional small surface blisters such as we see here are normal in the lead-tin overlay.  
  • Slide 39 -- At high mileage or long hours, the lead-tin in high load areas may either wear off or develop fatigue cracks and break loose, as we see on these 400,000 mile main bearings.  
  • Slide 40 -- Some bearings have lost bits of both lead-tin and copper bond, exposing the aluminum layer beneath. But sufficient lead-tin remains to provide lubricity and embedability and there is no problem.  
  • Slide 41 -- Another of the bearings has lost the lead-tin from the copper bond area, and we can see that the lead-tin loss area is slowly growing wider and wider. This is a type of lead-tin overlay fatigue caused by movement of the lead-tin under heavy load and is normal at high mileage.  
  • Slide 42 -- After thousands of hours the lead-tin layer can wear completely through, exposing the copper bond and aluminum layers. The copper bond layer will usually have a dark color due to oxidation while the aluminum layer will usually have its normal light color. SOS readings will not show high copper levels as wear progresses through the copper bond layer because it is very thin.  
  • Slide 43 -- Only when abnormal, rapid fine abrasive wear occurs will the copper bond have a copper color.  
  • Slide 44 -- When the tin flash, the lead-tin, and the copper bond layers are worn away in high load areas at high mileage, embedability and lubricity efficiency are reduced and it is time to install new bearings. Both color and wear taper are normal on these high mileage rod bearings.  
  • Slide 45 -- Occasionally we see an erratic wear pattern and wonder if there is an alignment or crankshaft journal profile problem. We need to remember that both the tin flash and the lead-tin overlay layers are thin, and that normal dimensional variations can cause them to wear away over large areas. This can give a first impression that misalignment is worse than it really is. Although this set of rod bearings has noticeable variation in its wear pattern, it is a high mileage set with no adhesive wear on the aluminum. We should conclude that there is no objectionable misalignment of parts. Replacement with new bearings is all that is required.  
  • Slide 46 -- Sometimes several different colors are seen as wear progresses through the surface layers of bearings. We need not concern ourselves with whether there are too many layers, for during bearing manufacture either chemical cleaning processes or variations during electroplating can cause minor changes in bearing layers.  
  • Slide 47 -- Bearings can be run after the lead-tin overlay is worn away, as has been done with these two rod bearings. Notice, however, that polishing and fine abrasive cutting have occurred in high load areas because the functions of lubricity and embedability have been reduced. There is little embedded debris and there is no melting present on the surface, telling us that there has never been a lubrication quality or quantity problem. The wear pattern has little taper, indicating good alignment of parts. There is a fair amount of cavitation erosion present, telling us that these are high mileage bearings. This information tells us that everything is normal and that bearing replacement is all that is needed.  
  • Slide 48 -- Cavitation erosion occurs in low load areas of bearings and can work its way through the lead-tin overlay and into the aluminum after thousands of service hours. It is not unusual to find a small pocket of aluminum removed by cavitation erosion on high mileage bearings as we see here. Pieces removed are very small and float easily in the trapped oil film, usually causing very little downstream abrasive wear. Though very small, the removed pieces are much too large to be seen in SOS analysis tests.  
  • Slide 49 -- If bearings are kept in service too long, the aluminum layer may develop fatigue cracks and begin to spall from the steel back. This rod bearing was run almost a million miles and heavy aluminum spalling has occurred.  
  • Slide 50 -- Inspection of the back of a high mileage bearing shows normal, irregular darkened appearance, and the Caterpillar trademark and the part number.  
  • Slide 51 -- We may find bearing backs with radial black stripes, as we see here. Machining of connecting rods will sometimes leave very slight radial ridges that make these dark stripes on the bearing back. This is a normal condition and is not cause for concern.  
  • Slide 52 -- These bearings are typical of wear and damage caused by abnormal conditions. From left to right they are have impact damage, soft debris abrasion, hard debris abrasion, adhesive wear with oil, lead-tin melting, and adhesive wear without oil.  
  • Slide 53 -- Since each abnormal wear type can have many root causes, we must beware of preconceived ideas. Just because the last customer had an oil leak which caused adhesive wear to many bearings, we can't be sure that the next time we see adhesive wear on many bearings it was also a result of an oil leak.  
  • Slide 54 -- The safest way to proceed in analyzing bearing wear is to use the eight steps of applied failure analysis. This helps us to gather necessary facts, identify wear types, and to follow the road signs to the real root cause of failure.  
  • Slide 55 -- During step 3 of the eight steps, basic application, operation and maintenance background facts should always be gathered. The oil level on the dipstick, the quantity of oil drained from the pan, leaking or damaged hoses, and other physical indicators of hostile environments should be carefully noted. We should also remember to investigate conditions a month or two prior to the failure and look specifically for problems such as unusual loads, high or low temperatures, corrected leaks, or low pressures which could have started a bearing failure.  
  • Slide 56 -- We need to obtain, identify and protect all parts related to failure to do best bearing wear analysis.  
  • Slide 57 -- Oil filters and SOS samples are often as important to the analyst as are bearings.  
  • Slide 58 -- We encourage inspection and identification of bearings during removal from engines. They should be numbered as they are removed from engines by scratching a number into the soft wearing surface near a parting face or in a tab slot. Lower bearing halves can be identified with both a number and an L while upper halves need only a number.  
  • Slide 59 -- When inspection requires transport to another location, bearings should be taped together in sequence and protected from physical and corrosive damage. We should not tape wear surfaces, and bearing tabs should be arranged as they were located in the engine. For example, on v-engines, tab slots alternate from rod to rod and the bearings should be taped together with tab slots alternating. Bearings are often lost, damaged, or thrown away when they are not immediately gathered, identified and protected.  
  • Slide 60 -- Too often bearings are not identified or protected before shipment and suffer secondary damage that further confuses good analysis.  
  • Slide 61 -- It is best if bearings are protected from contamination so that they don’t require cleaning before inspection. But if foreign material has been introduced after removal, the best way to clean the bearing is to wash it with a soft bristle brush and a clean, non-corrosive solvent. Harsh cleaning methods, such as scrubbing with a stiff bristle brush or wiping with a dry shop towel should be avoided.  
  • Slide 62 -- This is a set of dirty, worn bearings before cleaning.  
  • Slide 63 -- This are the same bearings after cleaning in different ways. The bearing on the left has not been cleaned. The middle bearing has been "rubbed" clean with a shop towel. The bearing on the right has been carefully washed clean with a soft bristle brush and clean solvent. Notice the distinct darker color of the center bearing where surface dirt has been rubbed into the soft surface. Any surface blisters or other fragile conditions of wear will also be distorted or destroyed by rubbing the wear surface.  
  • Slide 64 -- Four hundred power magnification of the solvent cleaned surface shows the clean undamaged lead-tin overlay conditions we seek.
  • Slide 65 -- After cleaning unwanted foreign material from bearings, an overall inspection with the unaided eye is the best way to begin visual examination of bearings. Bearing sets should be laid out as located in the engine. Lighting should be good and magnifying glasses should be available to study interesting areas more closely. Looking at all the bearings orients us as to whether we have a system problem or an isolated bearing problem. These bearings clearly say that a lubrication system problem was present that produced marginal lubrication to all bearings.  
  • Slide 66 -- We should next study wear types on individual bearings to discover what particular condition has damaged the bearings. For instance, here we see that adhesive wear has damaged each bearing, indicating that marginal lubrication and high temperatures have been present. We are now led to investigate both quality and quantity of lubricant.  
  • Slide 67 -- We should look at all bearing surfaces during bearing inspection, not just the inside wearing surface. Frequently the top half, bottom half, inside, front, back, parting faces, tab slots and sides each have apart of the wear story we seek. We should take the time to see what each surface is trying to tell us.  
  • Slide 68 -- In addition to identifying wear types and amounts, we should especially look for evidences of unusual heat, misalignment or competitive parts. Heat says hostile environment was present -- identify it. Misalignment says bent part or improper workmanship -- find out which. Competitive parts say be careful to find root cause facts-help customer understand the Cat difference.  
  • Slide 69 -- This connecting rod and bearing set shows extreme high temperature evidence, both in the rainbow temper colors and in physical distortion. The blackened and extruded condition tells us that both parts are results of high temperature exposure. The road sign here tell us to find out why there was no oil supply to cool and lubricate the bearing. We would first inspect the condition of other bearings to see if they have similar damage, and would then follow the road signs to the cause of poor oil supply.  
  • Slide 70 -- Misalignment causes concentration of load on only a part of a wearing surface and results in accelerated off-center wear. Here we see upper and lower connecting rod bearing halves that have been worn on opposite sides after a running twenty minutes on a dynamometer test. The operator reported an unusual thumping noise and stopped the test. Other bearings looked normal. The road sign here tells us to find out why only one bearing was misaligned. Investigation revealed that the connecting rod was bent in a prior failure and was not checked before reuse.  
  • Slide 71 -- Sometimes Caterpillar engines are rebuilt with competitive parts~ that don't provide good service life. Here we see a competitive bearing failing at low hours due to bond separation between the aluminum and steel layers.  
  • Slide 72 -- The back of this failing bearing does have the Caterpillar part number, but there is no Cat trademark. It appears that there is no tin flash to give corrosion protection. Notice also that severe fretting corrosion is present, indicating that the bearing has not been tight in the bore.  
  • Slide 73 -- Caterpillar bearings are marked with not only part numbers, but also with trademark, supplier identification, and a date code to identify the date the part was manufactured. The date code on this bearing is 5NEOA.  
  • Slide 74 -- The date code is called the NUMERAL KOD and describes the manufacturing month and year for bearings. The translation of the code is:   NUMERAL KOD 0123456 789   For example, the previous bearing had date code 5NEOA. The 5 defines the aluminum alloy used. The NEOA describes the month and year of bearing manufacture. The N = 0 and the E = 3, so the month is the third month, or March. The 0 = 8 and the A = 5, so the year is 1985. This bearing was made in March 1985. We can use this manufacturing date information to verify whether or not bearings are original or have been replaced, or whether bearings are old or new.  
  • Slide 75 -- Most abnormal bearing wear results from adhesion, abrasion, cavitation erosion, fretting corrosion, or impact damage. There are several root causes that could produce each of these types of wear. It is also possible for bearings themselves to have quality problems that cause failures. If we become familiar with key facts in these basic areas, we will recognize the road signs more quickly in future bearing analysis work.  
  • Slide 76 -- Adhesive bearing wear results when surface temperatures become high enough to cause the lead-tin or aluminum layers to melt and adhere to the crankshaft. There are many, many root caused which result in adhesive wear such as: (1) No oil in sump   (2) Low oil level that allows oil pump to lose suction. (a) Tilt operation (b) Wrong dipstick, tube or oil pan (c) Leaks (d) Oil consumption (e) Under fill (extra parts present such as lubrifiner)   (3) Wrong oil viscosity   (4) Wrong oil type   (5) Improper oil additives   (6) Fuel dilution   (7) Coolant in oil   (8) Over speed   (9) Misalignment   (10) Excessive clearances   (11) By-pass valve malfunction   (12) Etc.  
  • Slide 77 -- Adhesive bearing wear starts with polishing of the tin flash. If temperatures rise, the lead-tin will soon melt and "smear". This set of 3208 rod bearings is typical of bearings operated with interrupted oil supply. Notice that there is more damage present on odd numbered bearings than on even numbered bearings. There are two reasons why the center of the bearing gets hotter and melts before the edges do:   (1) Under low oil supply conditions, there is less oil at the center of bearings than at the edges because the oil hole in the crankshaft collects oil from the bearing center. This depletes the trapped oil film at the center and causes greater frictional contact and heat generation than at the edges.   (2) As the journal center gets hotter, it swells and gets larger in diameter, further increasing the center frictional contact and heat.  
  • Slide 78 -- If we study the oil flow in atypical 3208 engine, we notice that even numbered rod bearing oil passageways are drilled into the slanting odd numbered passageways. As the crankshaft rotates, centrifugal force pushes material in the slanting oil passageways out the intersecting passageways to even numbered rod bearings.   This means that if only a little oil is in the rotating crankshaft, centrifugal force will make it move to the even numbered bearings. Thus the odd numbered rod bearings on 3208 engines will receive less oil and show more damage than even numbered rod bearings when little oil is present.   In the same way, if debris is present in 3208 lubrication systems, centrifugal force will move it to the even numbered rod bearings, giving even numbered bearings more severe debris damage than odd numbered bearings.   Other engines will have different drilled passageway locations, but centrifugal force will still cause heavier materials to the exit first. If we study the physical layout of the passageways, we can establish which bearings will be most affected by lack of oil or by presence of debris.  
  • Slide 79 -- Sometimes it is not the quantity of oil, but the quality of oil that causes adhesive wear. For instance, this customer operated his engine several days with a sizeable coolant leak until the engine stopped. One rod bearing had totally seized and others were in varying states of adhesive wear. Main bearings had not seized. All bearings were darkened as seen here. We can see that this adhesive wear road sign is similar to the no oil road sign, but with coolant dilution we have the additional road sign of darkened lead-tin overlay.  
  • Slide 80 -- Or consider this set of rod bearings that were operated with 20 percent fuel dilution until a knocking noise was heard from the engine. All rod bearings had developed adhesive wear and had spun. Two bearings yet have some evidence remaining on its wear surface.  
  • Slide 81 -- Closer inspection of these bearings shows that some lead-tin overlay remains near the edges, but that it is worn away at both the center and extreme edges where temperatures have been higher and adhesive wear has occurred. Notice also that the color of the bearing is different than either the no oil adhesive wear or the coolant dilution adhesive wear we just reviewed. Each particular root cause produces its own particular set of road signs.  
  • Slide 82 -- Adhesive wear can progress very quickly through the aluminum and into the steel bearing back. Bearing crush will usually keep the bearing from spinning during meltdown of the aluminum while the engine is operating. However, as adhesive wear progresses into the steel back, the force of seizure will become greater than the crush force and the bearing will stick to the crankshaft and spin in the bore.   These bearings failed when an operator drained the oil during an oil change and forgot to add new oil before restarting the engine. It was operated only a few minutes under light load to do this damage. Notice that there has been NO oil in the oil hole or groove in the upper half of the bearing.   Other clues were:   1. The oil pan was full of oil correct quantity).   2. The oil was very clean (good quality).   3. The oil filters were Caterpillar filters and they looked new (good quality).   4. The oil filters had no oil or debris inside-looked dry and new (bad quantity, good quality).   5. There was no debris in the oil pan (good quality).  
  • Slide 83 -- When bearings are operated to failure without lubrication, temperatures will pass 15000 F (430° C) and the steel back will soften and extrude as we see here. We can usually analyze these failures because the cause of lack of lubrication is often still present at the time of failure.  
  • Slide 84 -- Occasionally we find that only one bearing has spun and others look normal, as we see here. These bearing failures are often difficult to analyze because (1), the cause of the failure may have occurred months or hundreds of hours earlier, and because (2), when the bearing spins, it destroys the facts which could have guided us to the root cause. We are left with circumstantial facts at the customer's site, or in the failed iron, to help define the most probable root cause.  
  • Slide 85 -- All that the failed bearing tells us is that it had lubrication and cooling when the engine was last stopped. These failures are nicknamed "unexplained single bearing failures" and will continue to be our most difficult analysis challenge.  
  • Slide 86 -- Abrasive wear occurs when foreign particles bridge oil film thickness, or when damage to wear surfaces causes irregularities that bridge the oil film. Bearing surfaces are cut and gouged if hard debris has entered, or scuffed and embedded if soft debris has entered. Because lubricants rapidly remove generated heat, surfaces remain near normal temperatures during initial abrasive damage. When surfaces become so rough that oil films cannot separate them, frictional heat is generated and adhesive wear can begin. There are many possible root causes of abrasive wear which include:   1. Foreign material left in oil passageways during assembly   2. Unfiltered lubricant with carbon particles and self generated debris   3. Rough wear surfaces   4. Servicing debris   5. Electrical discharge pitting or arc welding damage   6 .Etc.   It has been estimated that over half of field bearing failures result from abrasive damage, with improper oil filtration the leading cause of abrasives in the oil.  
  • Slide 87 -- These are common abrasive particles taken from a filter in a lubrication system. Regardless of care used during engine assembly, operation and maintenance, these small bits of paint, dirt and carbon will find their way into the oil. Using the Caterpillar oil filter is the safest way to keep this material out of lubrication oil.
  • Slide 88 -- Abrasive damage caused by large, hard debris is easiest to recognize and identify. Large steel chips left in the crankshaft at assembly damaged this 3600 rod bearing from a 200 hour factory test engine. When the engine was started, oil flow pushed the chips into the bearing, severely gouging the center of the bearing and embedded hard debris randomly. All other bearings were extremely clean and undamaged in any way.  
  • Slide 89 -- Closer examination of the lower (unloaded) half shows that the steel chips did v-grooved machining, and that many of the chips embedded in the soft lead-tin and aluminum. Because most embedded chips have worked loose and escaped, we know that the abrasive cutting occurred much earlier in the bearings life and that many hours of operation followed in order to remove the embedded chips. Some heat has been present, as seen by the polished lead-tin overlay near the damaged areas.  
  • Slide 90 -- Closer examination of the upper {loaded} half of this bearing shows that the roughened surface has already caused secondary adhesive wear. Notice also how the high surface temperatures have oxidized and darkened the lead-tin overlay on both sides of the center. Continued operation {especially during start-ups before oil arrives) allows the roughened surface to generate more and more heat, eventually leading to adhesive wear into the steel back. The end result could be another "unexplained bearing failure". But in this case we know the damage was caused by debris left in the crankshaft.  
  • Slide 91 -- This bearing has less severe abrasive damage and is typical of damage caused when filters are defective or change intervals are extended.  
  • Slide 92 -- Bearings that are blackened by excessive carbon embedment will begin to lose their functions of lubricity and embedability. On start-ups more friction will be present and more heat will be generated. The bearings will slowly deteriorate until one develops secondary adhesive wear and spins.  
  • Slide 93 -- Closer inspection of the worst main bearing shows that the surface has been fully embedded, and it appears that parts of the fully embedded lead-tin overlay have been pulled loose on start-ups by interlocking of crankshaft and bearing surface asperities. Then, like a scouring pad, the loosened material has caused abrasive cutting before oil supply arrived to flush it from the bearing area. This damage resembles the carbon cutting sometimes seen above the top ring on pistons after hundreds of hours of operation.  
  • Slide 94 -- If we cut one of these abnormally embedded bearings in half and use high power magnification, we can inspect the condition of the lead-tin overlay. Notice in this actual cross-section of the bearing that the bright aluminum layer makes up only about 10% of the bearing thickness. It is difficult to see the lead-tin overlay because it is so thin.  
  • Slide 95 -- With 400 power magnification we see that on this bearing, much of the embedded lead-tin overlay has already been torn away. With each start-up, more of the overlay can be damaged and removed. Eventually I the damaged bearing can begin I secondary adhesive wear and can I spin.  
  • Slide 96 -- When we find bearings that look black, or that are black with lead-tin overlay removed by fine abrasive cutting, as we see here, we should begin gathering facts about possible ways excessive carbon could have been present in the oil.  
  • Side 97 -- When main bearings are worn and polished to a chrome-like appearance (with perhaps one totally failed), and when rod bearings are in relatively good condition, we should suspect that electric discharge arcing has occurred. The careless analyst may jump to the preconceived idea that adhesive wear has occurred without doing necessary close inspection of the bearing and crankshaft wear surfaces.  
  • Slide 98 -- Closer inspection of the bearing surface reveals fine abrasive cuts and small healed spots that look like the "measles".  
  • Slide 99 -- The crankshaft has damage only to main bearing journals while rod journals are in normal condition.  
  • Slide 100 -- Closer inspection of a damaged main bearing journal shows a frosted appearance except around the oil hole. The rod journals show some wavy discoloration caused by acceptable grinding chatter.
  • Slide 101 -- The back of a damaged main bearing has irregular black oxide deposits in one or two areas. Within the black areas are a few small pits. Identification marks verify that this is a Caterpillar bearing.   This abrasive wear is caused by small electric arc discharge from the crankshaft, through the main bearing, and into the block. The source of the voltage build-up is usually an ungrounded alternator, caused by a missing or broken ground strap between the engine and the vehicle chassis. As electric current flowed from the alternator it could not flow easily back because the ground strap was missing. The next easiest path was through the crankshaft and main bearings, pitting both the bearing and crankshaft surfaces. Bearing pits healed over, while crankshaft sharp edged pits cut the soft lead-tin and aluminum bearing layers. The generated soft debris was so small that it floated in the oil through the downstream rod bearings without causing damage.  
  • Slide 102 -- Operator facts which reveal electric discharge pitting are a loss of about 10 psi oil pressure at full load RPM, or a knocking noise at low idle RPM. If the operator continues to use the damaged engine, secondary adhesive wear can develop and cause the bearing to spin, as we see here.  
  • Slide 103 -- This bearing has heavy debris embedment and abrasive wear. We may immediately want to follow this road sign, say that the oil was contaminated with debris, and begin searching for filtration or maintenance facts. We need to remember to inspect all surfaces and obtain all road signs during inspection of each part before proceeding to other fact areas.  
  • Slide 104 -- In particular, the back of this bearing has unusual information for us. There are some heavy pits surrounded by temper colors, indicating high temperatures have been present at the heavy pits.   Our minds are now led in anew direction, such as possible electric arc discharge that would produce the road signs seen on the front and back of this bearing. It was found that machine welding repair had been done with the ground strap attached to the block of the engine, causing high current flow through the easiest path, the main bearings.  
  • Slide 105 -- Cavitation erosion is normal on rod and main bearings after long hours of service. Trapped vapors in the lubricating oil will form bubbles in low pressure areas. When the bubbles enter a high pressure area in the trapped oil film they collapse, sending a high velocity jet of oil inward. The speed of the oil jet can be supersonic, exerting tremendous force when it strikes a small area on the bearing surface. The lead-tin overlay is soft, and after repeated implosions small areas develop fatigue cracks. Soon small bits of lead-tin break free and float downstream in the trapped oil film, doing little abrasive damage as they exit.   Severe cavitation erosion means that trapped vapor bubbles were either too big, too numerous, or both. Following are some possible sources of abnormal vapor bubbles and resulting cavitation erosion:   1. Oil pump inlet restrictions that cause cavitation.   2. Oil pump bypass valve sends oil {and trapped vapor bubbles) back to the oil pump inlet instead of to the oil pan, allowing the bubbles to recirculate.   3. Wrong oil viscosity or extended oil change intervals that causes the oil to be too thick and increases oil cavitation.   4. High oil level that allows the crankshaft to dip in the oil and introduce air bubbles.   5. Low oil level that allows pickup of air at the suction bell.   6. Oil suction side air leaks that allow air bubbles to enter.  
  • Slide 106 -- Cavitation erosion has begun to remove the aluminum layer on this bearing, and some downstream abrasive damage has already been done. Many other bearings in this set had similar damage. The erosion has been severe enough to dislodge larger bits of aluminum. Notice the three pieces that have loosened and are ready break free and move downstream.   Cavitation erosion worse than this can cause secondary abrasive wear and shorten bearing service life. The cause of the erosion should be determined and appropriate corrective action taken. If erosion cause is due to severe, but necessary, application or operation, then corrective action may be bearing change out at appropriate hours.  
  • Slide 107 -- Because trapped oil film pressures rise ahead of high load areas, cavitation erosion damage occurs in unloaded areas. The damage is noticeable and can alarm customers, but seldom causes failures.  
  • Slide 108 -- Fretting corrosion occurs when bearings become loose in their bore. This can be the result of:   1.     Bearing size too small 2.     Bore too big 3.     Bolt or nut torque too loose 4.     Loading too severe 5.     Bore tapered or has irregular profile 6.     Debris between parting faces at bearing assembly   It is not the fretting corrosion on the original bearing that does the most damage. The replacement bearing is often held off the bore by fretting corrosion build-up from the original bearing that causes hot spots and adhesive wear to the replacement bearing.  
  • Slide 109 -- Consider this set of 3208 main bearings that operated only a few hours after engine rebuild. Number one main bearing has spun in the block bore, ruining the crankshaft and requiring line boring of the block. We can tell from looking at the failed bearing that it was being cooled with oil when it failed, but other road signs on the bearing have been destroyed when it spun. We will need to look for circumstantial facts on the remaining bearings to get some ideas on what could have caused the number one main bearing to fail.   Notice that several main bearings appear to have hot spots, especially near parting faces. In particular, the second bearing from the right deserves closer inspection.  
  • Slide 110 -- As we move the damaged bearing to the side, we see that there is considerable fretting corrosion build-up on the bore in that area. This is a fretting corrosion deposit from the previous bearing which was loose in the bore. The deposit is over 4 thousandths of an inch high and has caused the new bearing to make contact with the crankshaft and develop a hot spot in just a few hours operation.  
  • Slide 111 -- As we inspect the back of the new bearing, we see that abroad area of the steel back never made contact with the bearing bore, reducing heat transfer in that area. Because many of these bearings have serious fretting corrosion deposits under them, it is probable that the failed bearing also had considerable fretting corrosion present that could have caused it to seize and spin.   As old bearings are removed, the careful serviceman inspects the back of each for evidence of fretting corrosion, and when he finds it, he takes appropriate corrective action before installing new bearings.  
  • Slide 112 -- Impact damage occurs when bearings are released from their bores during operation. This damage has its own distinct road signs. The bearings will be physically deformed and may have secondary abrasive damage caused after the bearing was allowed to move.  
  • Slide 113 -- There is generally no adhesive wear and wearing surfaces are often in their normal condition.  
  • Slide 114 -- Connecting rod bearing backs will usually have impact marks where the parting face of the loose rod has been driven down against it, denting and rotating the bearing slightly. Several such impact marks are usually seen, telling us that the bearing was repeatedly struck and physically rotated out of the bore.  
  • Slide 115 -- As we look for root causes of bearing failures, we don't want to forget the possibilities that bearings themselves may be the root cause. However, we need to remember that even when bearings have quality problems they will generally last into midlife and will not cause low hour failures.   Bearings which are assembled improperly or are used in severe load or hostile temperature and system environments account for most bearing failures. Too often we are not careful enough in gathering facts in these areas before blaming the bearing itself.   We must look for facts and road signs to identify the cause of the problem, and let those facts tell us if it is the bearing itself, or if it is a hostile environment that caused the failure.  
  • Slide 116 -- The lead-tin overlay on new bearings may have occasional small blisters such as we see in this magnified view of a low hour bearing. This condition can occur during the electroplating process when the bonding agent and the lead-tin layers are applied.  
  • Slide 117 -- As the bearing is used, the top of the blisters are worn away, leaving a dark hole in the overlay. This appearance may lead us to suspect corrosive attack, but oil analysis results should verify acceptable oil TBN number (TBN number is the quantity of acid neutralizing additive remaining in the oil).   Though no failures have been specifically attributed to presence of blisters, they do worry the analyst and should be considered if an unexplained bearing failure has occurred.  
  • Slide 118 -- Occasionally we may find a bearing on which the lead-tin overlay has begun to flake away from the copper bonding layer. If this happens on a large scale, it is called lead-tin overlay "sheeting".  
  • Slide 119 -- Closer inspection shows that the lead-tin is clearly breaking loose from the copper bond layer. At times bearing manufacturers can have process problems that can cause weak lead-tin overlay bond.   But since high temperatures can also stress bonds, we would want to carefully look for facts verifying that normal temperatures had been present at the bearing surfaces before accusing the bearing of having a bonding problem.   If any such hostile condition has been present, bond separation could be a result of high temperatures and not the root cause.  
  • Slide 120 -- We very seldom find total separation of the lead-tin overlay from the aluminum layer as we see here. This bearing problem did not cause a failure, but the effectiveness of the lead-tin overlay has been lessened.  
  • Slide 121 -- Closer inspection helps us see that improper cleaning after electroplating of the aluminum has left a residue on the aluminum layer and weakened the bond between the copper and aluminum layers. With normal loads the weakly bonded lead-tin layer separated from the aluminum. Pieces of lead-tin have broken free and caused downstream soft debris damage.  
  • Slide 122 -- The aluminum layer can separate from the steel back as a result of poor bond strength, or it can be caused to separate by severe loading or high temperatures.   These bearings came from an engine in a severe load application that caused high bearing stress. These are high hour bearings as can be seen from the cavitation erosion and wear through of the lead-tin overlay. The bearings show considerable taper in the lead-tin overlay wear, suggesting possible misaligned loading of the bearing and a need to gather facts about the crankshaft journal profile and possible field regrinding.   The bearing on the far right has more severe damage than do the others, and deserves closer inspection.  
  • Slide 123 -- We can now see that the aluminum layer has already separated from the steel back in the load area, and that fatigue fracture of the loose aluminum is occurring, sending small pieces of aluminum steadily downstream into the bearing. But the bearing has not failed or caused another to fail.  
  • Slide 124 -- As bond separation progresses, more and more of the aluminum layer will break away, cause downstream abrasive wear, and deposit in the oil pan or oil filters. When we find the aluminum layer separating from the steel back, and (1) no abnormal loading or extended service has occurred, or (2) no adhesive wear is present (the lead-tin and aluminum layers have no melting or smearing), we can correctly suspect weak bond of the aluminum to the steel.  
  • Slide 125 -- Closer inspection helps us see the side-to-side roughening marks on the steel back and the small fatigue cracks in the highly loaded surrounding lead-tin surface. There has been no high temperature present, and only soft abrasive damage has been done as pieces of aluminum have broken loose and travelled through the bearing.  
  • Slide 126 -- Eventually the entire aluminum layer will separate if a bearing with bond failure is kept in service. Notice on this bearing that the load is being carried by the small high spots of remaining aluminum. If kept in service any longer this bearing would have made steel-to-steel contact and generated enough heat to surface weld (adhere) to the crankshaft and spin the bearing.  
  • Slide 127 -- This is another case of aluminum bond separation often blamed on bearing quality, but notice that the bond separation has occurred only at the center where the bearing surface has been very hot. When surface temperatures rise above 700° F (190° C) and the steel back remains cool because of good heat conduction to the bore, the bond between the aluminum and steel layers is stressed by thermal expansion of the aluminum layer. When we see adhesive wear and aluminum bond separation present at the same time, we should suspect that the bond separation is a result of high temperatures. The "road sign" here tells us to look for the cause of high surface temperature.  
  • Slide 128 -- When it is determined that a part has caused a failure, we should always ask ourselves: "Is the part that caused the failure a Caterpillar part or a competitive part?" Competitors with competitive parts can rebuild Caterpillar engines. We need to always check failed parts for Caterpillar trademark and identification.  
  • Slide 129 -- Caterpillar bearings are not only carefully designed and manufactured, but they are also carefully checked after manufacture to assure quality construction. Few of the bearing problems we just reviewed have caused failures. Therefore we can say with confidence that Caterpillar bearings seldom cause failures. Bearing failures most often are the result of workmanship, application, operation, or maintenance problems.  
  • Slide 130 -- As we finish any failure analysis situation, we need to remember to use the "double-check" question, "Is there any way the other party could have caused this failure?" before stating our opinion. For instance, when we find adhesive wear on bearings, we should remember to write down possible ways that the factory could have caused marginal or no lubrication (such as oil pump failure, wrong oil dipstick or wrong oil pan) before stating that the customer must have run the engine low on oil.  
  • Slide 131 -- We hope that this presentation has conveyed important facts about bearing function, structure, installation, operation, failure analysis and possible bearing problems. We also hope it has shown how the 8 steps of applied failure analysis can help us efficiently understand bearing failures.  
  • Slide 132 -- If we follow preconceived ideas,  
  • Slide 133 -- or guess at root causes,  
  • Slide 134 -- we can expect to see fewer and fewer customers at our door.  
  • Slide 135 -- And we NEED happy, successful customers to achieve our goal to sell product, parts and service.  
  • Slide 136 -- So, after all the hard work of organizing the problem, gathering the facts, thinking with the facts, and figuring out the most probable root cause, let's remember to GET THE PAYOFF! by communicating, making professional repairs, and following up with the customer.  
  • Slide 137 -- This will help insure that each customer feels good about both his Cat product and his Cat product support and will keep him coming back to buy our product, parts and service.  
  • Slide 138 -- Quiz
  • Bearings master

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