Engine Bearing Failures

                           

                           Engine Bearing Failures

Failures on engine bearings , can be broadly classified as

-Manual factors

-Operating conditions

Manual Factors

 1.Foreign particle embedment

APPEARANCE

Foreign particles are embedded in the lining of the bearing. Scratch marks may also be visible on the bearing surface.

DAMAGING ACTION

Dust, dirt, abrasives and/or metallic particles present in the oil supply embed in the soft Babbitt bearing lining, displacing metal and creating a high-spot.

The high-spot may be large enough to make contact with the journal causing a rubbing action that can lead to the eventual breakdown and rupture of the bearing lining. Foreign particles may embed only partially and the protruding portion may come in contact with the journal and cause a grinding wheel action.

POSSIBLE CAUSES

1.    Improper cleaning of the engine and parts prior to assembly.

2.    Road dirt and sand entering the engine through the air-intake manifold or faulty air filtration.

3.    Wear of other engine parts, resulting in small fragments of these parts entering the engine’s oil supply.

4.    Neglected oil filter and/or air filter replacement.

CORRECTIVE ACTION

1.    Install new bearings, being careful to follow proper cleaning procedures.

2.    Grind journal surfaces as deemed necessary.

3.    Recommend that the operator have the oil, air filter, oil filter and crankcase breather-filter replaced as recommended by the manufacturer. 

2.Foreign particles behind the bearing

APPEARANCE

A localized area of wear can be seen on the bearing surface. Also, evidence of foreign particle(s) may be visible on the bearing back or bearing housing directly behind the area of surface wear.

DAMAGING ACTION

Foreign particles between the bearing and its housing prevent the entire area of the bearing back from being in contact with the housing base. As a result, the transfer of heat away from the bearing surface is not uniform causing localized heating of the bearing surface which reduces the life of the bearing.

Also, an uneven distribution of the load causes an abnormally high pressure area on the bearing surface, increasing localized wear on this material.

POSSIBLE CAUSES

Dirt, dust abrasives and/or metallic particles either present in the engine at the time of assembly or created by a burr removal operation can become lodged between the bearing back and bearing housing during engine operation.

CORRECTIVE ACTION

1.    Install new bearings following proper cleaning­ and burr removal procedures for all surfaces.

2.    Check journal surfaces and if excessive wear is discovered, regrind.

3. Wrong Assembly

Engine bearings will not function properly if they are installed wrong. In many cases, wrong assembles will result in premature failure of the bearing.

The following are typical assembly errors most often made in the installation of engine bearings

Position of offset connecting rod reversed   Shims improperly installed

Bearing caps in wrong or reversed position Locating lug not nested

                             Bearing halves reversed   Bearing oil hole not aligned with oil passage  hole

  4. Shifted bearing cap

 

APPEARANCE

Excessive wear areas can be seen near the parting lines on opposite sides of the upper and lower bearing shells.

DAMAGING ACTION

The bearing cap has been shifted, causing one side of each bearing-half to be pushed against the journal at the parting line.

The resulting metal-to-metal contact and excessive pressure cause deterioration of the bearing surface and above normal wear areas.

POSSIBLE CAUSES

1.    Using too large a socket to tighten the bearing cap. In this case, the socket crowds against the cap causing it to shift.

2.    Reversing the position of the bearing cap.

3.    Inadequate dowel pins between bearing cap and housing (if used), allowing the cap to break away and shift.

4.    Improper tightening of cap bolts resulting in a “loose” cap that can shift positions during engine operation.

5.    Enlarged cap bolt holes or stretched cap bolts, permitting greater than normal play in the bolt holes.

CORRECTIVE ACTION

1.    Check journal surfaces for excessive wear and regrind if necessary.

2.    Install the new bearing being careful to use the correct size socket to tighten the cap and the correct size dowel pins (if required).

3.    Proper tightening from side to side as to assure proper seating of the cap.

4.    Check the bearing cap and make sure it‘s in its proper position.

5.    Use new bolts to assure against overplay within the bolt holes.

5.  Excessive pressure during assembly

 

APPEARANCE

Extreme wear areas visible along the bearing surface adjacent to one or both of the parting line.

DAMAGING ACTION

Before the bearing cap is assembled, a small portion of the bearing extends just a little beyond the edge of the bearing housing. Thus when the bearing cap is tightened into place, the bearing is forced against the bearing housing. That portion of the bearing which extends beyond the housing is called “crush”.

When there is too much crush, however, the additional compressive force created by the surplus crush that still remains after the bearing is fully seated causes the bearing to bulge inward at the parting faces. This bearing distortion is called “side pinch.”

POSSIBLE CAUSES

1.    The bearing caps were filed down in an attempt to reduce oil clearance.

2.    The bearing caps were assembled too tightly due to excessive torquing.

3.    Not enough shims were utilized (if shims were specified).

CORRECTIVE ACTION

1.    Rework the bearing housing of the engine block if it has been filed down.

2.    Replace the connecting rod if its bearing cap has been filed down.

3.    Check journal surfaces and regrind if necessary.

4.    Install the new bearing and follow proper installation procedures by never filing down bearing caps and using the recommended­ torque wrench setting.

5.    Correct the shim thickness (if applicable).

6.    Check for out-of-roundness of the inside diameter­ of the assembled bearing by means of an  inside micrometer, calipers to assure that any out-of-roundness is within safe limits. The maximum assembled bearing I.D. should always be across the split line.

      6.    Insufficient pressure of assembly

APPEARANCE

Highly polished areas are visible on the bearing back and/or on the edge of the parting line. Areas of pock marks or build-up due to metal transfer between bearing and housing. This is commonly referred to as “fretting”.

DAMAGING ACTION

When a bearing with insufficient crush is assembled in an engine, it is loose and therefore free to work back and forth within its housing.

Because of the loss of radial pressure, there is inadequate contact with the bearing housing, thus impeding heat transfer away from the bearing. As a result, the bearing overheats causing deterioration of the bearing surface.

POSSIBLE CAUSES

1.    Bearing parting faces were filed down in a mistaken attempt to achieve a better fit, thus removing the crush.

2.    Bearing caps were held open by dirt or burrs on the contact surface.

3.    Insufficient tightening during installation (be certain bolt doesn’t bottom in a blind hole).

4.    The housing bore was oversize or the bearing cap was stretched, thus minimizing the pressure.

5.    Too many shims were utilized (if shims are specified).

CORRECTIVE ACTION

1.    Clean mating surfaces of bearing caps and inspect for nicks and burrs prior to assembly.

2.    Check journal surfaces for excessive wear and regrind if necessary.

3.    Check the size and condition of the housing bore and recondition if necessary.

4.    Correct shim thickness (if applicable).

5.    Install new bearings using correct installation procedures (never file bearing parting faces). 

7. Bent or Twisted connecting rod

APPEARANCE

Excessive wear areas can be seen on opposite ends of the upper and lower connecting rod bearing­ shells. The wear is localized on one portion of the bearing surface with little or no wear on the remainder.

DAMAGING ACTION

A bent or twisted connecting rod results in misalignment of the bore, causing the bearing to be cocked so the bearing edge makes metal-to-metal contact with the journal. These metal-to-metal contact areas cause excessive­ wear on the bearing surface.

POSSIBLE CAUSES

Three factors can contribute to connecting rod distortion:

1.    Extreme operating conditions such as “hot rodding” and “lugging.”

2.    Improper reconditioning.

3.    Dropping or abusing the connecting rod prior to assembly.

CORRECTIVE ACTION

1.    Inspect connecting rod and recondition or replace if bent or twisted.

2.    Check journal surfaces for excessive wear and regrind if necessary.

3.    Install new bearing.

4.    Avoid dropping the connecting rod prior to assembly.

5.    Use proper installation techniques.

6.    Check related upper cylinder parts and replace if necessary.

    8.Fillet ride

APPEARANCE

When fillet ride has caused a bearing to fail, areas of excessive wear are visible on the extreme edges of the bearing surface.

DAMAGING ACTION

If the radius of the fillet at the corner where the journal blends into the crank is larger than required, it is possible for the edge of the engine bearing to make metal-to-metal contact and ride on this oversize fillet.

This metal-to-metal contact between the bearing and fillet causes excessive wear, leading to premature­ bearing fatigue.

POSSIBLE CAUSES

Fillet ride results if excessive fillets are left at the edges of the journal at the time of crankshaft machining.

CORRECTIVE ACTION

1.    Regrinding of  the crankshaft paying particular attention­ to allowable fillet radii. ( Attention to be paid not to reduce fillet radius, since this can weaken the crankshaft­ at its most critical point.- done as per maker’s recommendations and by a qualified personnel/facility)

2.    Installation of new bearings with enlarged chamfers that allow proper fillet clearance.

Operational causes

1.Distorted crankcase

APPEARANCE

A wear pattern is visible on the upper or lower halves of the complete set of main bearings. The degree of wear varies from bearing to bearing depending upon the nature of the distortion. The center bearing usually shows the greatest wear.

DAMAGING ACTION

A distorted crankcase imposes excessive loads on the bearings, with the point of greatest load being at the point of greatest distortion. These excessive bearing loads cause excessive bearing wear. Also, oil clearance is reduced and metal-to-metal contact is possible at the point of greatest distortion.

POSSIBLE CAUSES

Alternating periods of engine heating and cooling during operation is a prime cause of crankcase distortion. As the engine heats the crankcase expands, and as it cools, the crankcase contracts. This repetitive expanding and contracting causes the crankcase to distort over time in some situations.

Distortion may also be caused by:

1.    Extreme operating conditions (for example “overheating” and “lugging”)

2.    Improper tightening procedure for cylinder head bolts, particularly with overhead valve V-8 engines

CORRECTIVE ACTION

1.    Determine if distortion exists by use of Prussian blue or visual methods.

2.    Align bore the housing (if applicable).

3.    Install new bearings

2.Bent crankshaft 

APPEARANCE

A wear pattern is visible on the upper and lower halves of the complete set of main bearings. The degree of wear varies from bearing to bearing depending upon the nature of the distortion. The center bearing usually shows the greatest wear.

DAMAGING ACTION

A distorted crankshaft subjects the main bearings to excessive loads, with the greatest load being at the point of greatest distortion. The result is excessive bearing wear. Also, the oil clearance spaces between journals and bearings are reduced, making it possible for metal-to-metal contact to occur at the point of greatest distortion.

POSSIBLE CAUSES

A crankshaft is usually distorted due to extreme operating conditions, such as “over-speeding” and “lugging”. It may also be caused by improper handling prior to installation.

CORRECTIVE ACTION

1.    Determine if distortion exists by means of Prussian blue or visual methods.

2.    Install a new or reconditioned crankshaft.

3.    Install new bearings.

3.Out-of-round bore

APPEARANCE

Localized excessive wear areas are visible near the parting line on both top and bottom shells.

DAMAGING ACTION

Oil clearance near the parting line is decreased to such an extent that metal-to-metal contact between bearing and journal takes place, resulting in areas of above-normal wear.

Also, improper seating between the bearing back and the housing bore may be present which hinders proper heat transfer causing localized heating of the bearing surface and thus reducing fatigue endurance.

POSSIBLE CAUSES

Alternating loading and flexing of the connecting rod can cause the bearing housing to become elongated. And because replacement bearing shells, when installed, tend to conform to the shape of the bearing housing, this can result in an out-of-round bearing surface.

CORRECTIVE ACTION

1.    Check the ovality of bearing housings before installing the new bearings. If they are found to be out-of-range, recondition the bearing housings (or replace connecting rod).

2.    Check the journal surfaces for excessive wear and regrind if necessary.

3.    Install new bearings.

4. Out-of-shape journal

APPEARANCE

In general, if a bearing has failed because of an out-of-shape journal, an uneven wear pattern is visible on the bearing surface. Specifically, however, these wear areas can be in any one of three patterns: Photo A above shows the wear pattern caused by a tapered journal. Photo B shows the wear pattern caused by an hour-glass shaped journal. Photo C shows the pattern of a barrel shaped journal.

DAMAGING ACTION

An out-of-shape journal imposes an uneven distribution of the load on the bearing surface, increasing heat generated and thus accelerating bearing wear. An out-of-shape journal also affects the bearing’s oil clearance, making it insufficient in some areas and excessive in others, thereby upsetting the proper functioning of the lubrication system.

POSSIBLE CAUSES

If the journal is tapered there are two possible causes:

1.     Uneven wear of the journal during operation (misaligned rod).

2.     Improper machining of the journal at some previous time.

If the journal is hour-glass or barrel shaped, this is usually the result of improper machining or polishing.

CORRECTIVE ACTION

Regrinding the crankshaft can best remedy out-of-shape journal problems. Then install new bearings in accordance with proper installation procedures.

5.Bearing failures due to metal-to-metal contact

Mixed lubrication is one of the main causes of engine bearing failures. Metal-to-metal contact may appear in the following forms of bearing wear:

•      Accelerated wear: is when the bearing is not overheated and only a shiny appearance of the bearing surface is observed.

•      Wiping or heavy wear: this appears in the form of overheating and partial melting of the overlay

•         Severe wear or Hot Short: it results in torn surfaces, severe overheating, melted overlay and lining material

  POSSIBLE CAUSES

1. Insufficient oil supply in passages ,insufficient clearance, etc (oil starvation)

2. Breaking the oil film (due to oil contamination or bearing material fatigue)

3. Misalignment (e.g. out-of-shape grinding, distorted connecting rod)

4. Poor journal surface finish

5. Foreign particles embedded in the bearing surface

6. Low viscosity oil

7. Grinding chatter marks (waviness).

CORRECTIVE ACTION

1.Check oil supply system

2.Change bearing material (tri-metal instead of bi-metal)

3.Solve overloading problem

4.Correct deficient machining, fix/replace distorted parts

5.Determine origin of particles, improve cleaning procedures prior to assembly.

6 .Identify/address source of oil dilution, use higher viscosity oil

                        Hot short

APPEARANCE

Bearing surface wiped and torn, blackened from heat, with patches of lining material torn cleanly from steel backing.

POSSIBLE CAUSES

1.    Breakdown of lubrication and resulting high friction elevates operating temperature.

2.    Lead in bearing material melts and allows shaft to tear away patches of bearing lining.

3.    Lack of lubrication.

4.    Wiping.

5.    Dirt contamination.

6.    Concentrated loading (misalignment, etc.).

CORRECTIVE ACTION

A hot short is a catastrophic failure that results from one of the conditions already covered in detail. To properly correct this it must first be determined which specific condition lead to the hot short. For further details please see:

1.    Foreign Particles in Lining 

2.    Foreign Particles on Bearing Back  

3.    Out-of-Round Bore  

4.    Excessive pressure on bearing halves

5.    Bent or Twisted Connecting Rod  

6.    Shifted Bearing Cap  

7.    Distorted Crankcase  

8.    Bent Crankshaft  

9.    Oil Starvation 

10. Wrong assemblies

6.Bearing failures due to fatigue

Bearing material fatigue is the second cause of bearing failure.

Fatigue cracks. The fatigue cracks form on the surface and propagate inside the lining reaching the steel  back. The cracks then progress along the bond line between the lining and the steel. Pieces of the lining flake out from the steel back resulting in oil contamination and eventual bearing failure. 

 

Fatigue of a copper based intermediate layer. Fatigue of a copper based lining starts from a fatigue of the overlay. The overlay flakes out from the copper lining resulting in an interruption of the oil film, breaking the hydrodynamic lubrication regime. The load localizes at the contact area causing formation of small cracks on the lining surface.The cracks then propagate throughout the lining thickness, meeting the steel back surface and continuing to advance along the steel-copper boundary. As a result, parts of the intermediate layer detach from the steel surface.

The table below shows factors that cause fatigue and methods of preventing bearing failures due to fatigue.

  

Causes :

1.Wrong selection of bearing material

2.Wrong timing leading to pulsating loads

3.Oil starvation, due to excessive ovality.

4.Degradation of oil.

Remedies: 1.Change bearing / type in consultation  with maker 2.Check and adjust timing and other causes

7.Cavitation erosion of the overlay

Cavitation erosion is another type of engine bearing failure differing from both fatigue and metal-to-metal

Contact. Cavitation occurs when the load applied to the bearing fluctuates at high frequency (high RPM).

The oil pressure instantly falls causing formation of bubbles (cavities) due to fast evaporation.

When the pressure rises the cavitation bubbles collapse at high velocity.

Such collapse results in impact pressure, which can erode the bearing material.

Soft lead based overlays of tri-metal bearings are prone to the cavitation erosion.

Therefore replacement of tri-metal bearings with babbitt overlay with bi-metal material or with high

strength tri-metal bearings (e.g. GP) will prevent the failures due to cavitation.

Cavitation Damage on Outside Dia of collar

Thrust Shoe Cavitataion- Towards Outer Dia meter

Thrust Shoe Cavitataion- Damage in Babbitt Face

8.Corrosion

  

APPEARANCE

Bearing surface darkened, spongy, etched by chemical attack.

DAMAGING ACTION

This is usually the result of contamination of the oil from either the fuel system or internal engine leaks. This condition is further pronounced when there is poor routine maintenance

POSSIBLE CAUSES

1.    Acids in oil.

2.    Excessive operating temperature.

3.    Excessive blow-by.

4.    Coolant contamination of oil.

5.    Use of high sulfur fuel.

6.    Excessive oil change interval.

CORRECTIVE ACTION

1.    Identify and correct source of contamination.

2.    Install new bearings using correct installation procedures.

3.    Use a better quality fuel, if possible.

4.    Oil should be changed at recommended intervals using the proper grade and rating. In some applications oil analysis may be needed to determine the optimum oil change intervals. 

9.Electrical Pitting

Electrical pitting appears as rounded pits in the bearing lining. The pits may appear frosted or they may be blackened due to oil deposits. It is not unusual for them to be small and difficult to observe with the unaided eye. A clearly defined boundary exists between the pitted and unpitted regions, with the pitting usually occurring where the oil film is thinnest.

Sources : Electrical pitting can be electrostatic or electromagnetic in origin. Although both sources result in pitting damage, they differ in origin and destructive capabilities

Mechanism of pitting progress : As pitting progresses, the individual pits lose their characteristic appearance as they begin to overlap. Pits located near the boundary should still be intact. The debris that enters the oil begins abrasion damage. Once the bearing surface becomes incapable of supporting an oil film, the bearing will wipe. The bearing may recover an oil film and continue to operate, and pitting will begin again. This process may occur several times before the inevitable catastrophic bearing failure.

Electrical pitting damage is caused by intermittent arcing between the stationary and rotating machine components. Because of the small film thicknesses relative to other machine clearances, the arcing commonly occurs through the bearings. Although the rotating and other stationary members can also be affected, the most severe pitting occurs in the soft Babbitt.

Electrostatic shaft current (direct current) is the milder of the two. Damage progresses slowly, and it always occurs at the location with the lowest resistance to ground. It can be attributed to charged lubricant, charged drive belts or impinging particles.

Control mechanism:

    This shaft current can be eliminated with grounding brushes or straps. Bearing isolation is also recommended

    Electromagnetic shaft current (alternating current) is stronger and more severe than electrostatic current. It is produced by the magnetization of rotating and/or stationary components.This type of current will not always occur at the location of lowest resistance. Because the current is stronger, bearing damage is often accompanied by journal, collar or runner damage.

 Electromagnetic currents are best eliminated by demagnetizing the affected component.Grounding brushes or straps may or may not be helpful. The bearings should also be isolated.

T   The lubricating oil must be filtered or replaced. Pitting damage often blackens the oil and fills it with debris. In addition to filtering or replacing the oil, the entire bearing assembly, oil reservoir and piping should be flushed and cleaned. The original bearing finish should also be restored. Journal shoes typically must be replaced, but if the correction leaves the bearing within design tolerance, the bearing may be reused. The condition of the rotating journal, collar or runner surfaces must also be evaluated. It must be restored to original condition, either by lapping, hand stoning or replacement.