What Are The Causes Of Crankshaft Bearing Damage?

Reasons for wear of crankshaft journal

In the working process of automobile engine, there is high friction between crankshaft journal and bearing. Especially when the engine is running at low speed or starting, dry friction is generated due to the difficulty of lubricating oil film establishment, resulting in bearing wear.

Reasons for crankshaft journal and bearing damage

The radial clearance between the crankshaft journal and the bearing is very small, which can make the crankshaft free rotation without opening and radial runout. If the radial clearance between the journal and the bearing changes, it often causes early wear and destruction of the crankshaft journal and shaft, so that the engine working vibration increases, noise increases, fuel consumption increases, and accidents increase. Crankshaft journal and bearing damage in the form of scratch, wear, fatigue spalling, corrosion, burning and so on.

• 1. Fatigue spalling：the crankshaft is constantly subjected to periodic gas force, alternating inertia force and rotating centrifugal force, which act on different parts of the crankshaft respectively, accelerating the wear of the crankshaft; Bearing under the action of long-term mutation load, impact load, alloy fatigue, and then crack, and to the depth of the expansion, so that the alloy spalling.
• 2. Corrosion: oil failure caused by its acid value is too large, it will have a corrosive effect on crankshaft and bearing. Especially when the bearing is exposed to copper, the corrosion rate is faster because of the lack of surface layer and nickel protective layer that are resistant to corrosion. The honeycomb structure produced by corrosion will greatly reduce the bearing strength
• 3.Burning-out: when the crankshaft journal and bearing between the oil, because of the low pressure oil oil duct plug, oil viscosity, the intense friction occurs between journal and bearing, the journal and the bearing temperature rise rapidly, to make the journal burns, serious causes the bearing metal melt, adhesion on the shaft, the crankshaft stuck even rupture.
• 4. Scratch: metal dust, lubricating oil filtration poor or foreign granular impurities with the flow of oil into the journal and bearing surface between these hard particles will make the alloy surface scratch, serious can be in the alloy surface and journal surface scrape out many ring grooves.
• 5. Defects of connecting rod geometry: when the geometric shape of connecting rod appears saddle shape, drum shape, not coaxial and not straight, the normal clearance between journal and bearing will change. If the clearance is too large, the contact radian between the journal and the bearing becomes smaller, which increases the pressure load of the oil film and aggravates the fatigue of the bearing. Too small clearance will limit the flow of oil, it is difficult to form an oil film, can not take away the friction heat generated, increase the tendency of overheating deformation and “holding tile”.
• 6. Improper assembly of connecting rod: improper torsion of connecting rod bolt, assembly error of connecting rod bearing cover and assembly error of connecting rod bearing will cause damage to crankshaft in different degrees and in different forms. Crankshaft processing error Crankshaft geometry, such as journal step, cone, drum and saddle errors, will make the crankshaft journal and bearing clearance changes, reduce the thickness of oil film or prevent the formation of oil film, resulting in direct metal contact and abnormal bearing wear.

Crankshaft journal surface wear

crankshaft journal is the main working surface of the crankshaft, its wear is inevitable, the general connecting rod journal wear is mostly on the crankshaft center line side, and the main shaft journal wear is near the connecting rod journal side. Mainly related to the stress in the working process of crankshaft. If the connecting rod is bent and the cylinder sleeve is deflected, the eccentric wear of crankshaft journal will be aggravated. If the mechanical impurities in the oil are too much, due to the inclination of the oil passage of the spindle journal and the connecting rod journal, under the action of the centrifugal force of the crankshaft rotation, the abrasive is biased in the oil passage away from the rotating center of the one side, and concentrated on the one side of the connecting rod journal, resulting in larger wear of the connecting rod journal on the side. In addition, the deviation and deformation of the installation position of parts will also make the force on both sides uneven, and the larger side of the force is larger.

Crankshaft journal surface wear is not serious, can be used to restore the journal grinding. After crankshaft grinding, cylindricity error, surface roughness and fillet radius at both ends of crankshaft should meet the requirements; The parallelism error of the journal axis of each connecting rod to the journal axis and crank radius should meet the original factory regulations, and the allocation Angle deviation of the journal axis of each connecting rod to the center plane of the keyway of the crankshaft timing gear should meet the manufacturer’s regulations. When worn journal size exceeds repair size, crankshaft must be replaced.

Crankshaft journal burn and abrasion

defects such as burn and abrasion appear on the surface of crankshaft journal. The reason is poor lubrication, insufficient cooling, or crankshaft mechanical load is too large, resulting in high thermal load on the surface of the journal and bearing bush. In addition, the severe friction causes the surface temperature to rise further, and in serious cases, it leads to the burning of friction reduction alloy of bearing bush, and the burning failure occurs. Excessive temperature also causes the surface of the journal to oxidize and burn blue. Journal surface slight scratches, abrasive cloth can be used after grinding, and then with felt or canvas and coated with oil and a little chrome oxide paste for polishing. After cleaning the crankshaft, blow through the oil passage.

Crack

crankshaft crack is mainly caused by stress concentration. First, the processing size of fillet between journal and crank (fillet radius) is too small, which is easy to produce stress concentration. The other is the stress concentration at the junction between the crank and the journal due to the “sharp” change in shape of the transition position. Thirdly, due to technical reasons, the fatigue strength of the rounded corners of the journal decreases when the surface of the journal is quenched at high frequency. In addition, when vibration and torsion occur in crankshaft work, crankshaft cracks increase, and when radial cracks develop seriously, it may lead to crankshaft fracture.

Transverse cracks are not allowed on crankshaft journal surface. For the axial crack, if its depth is within the repair size of crankshaft journal, it can be ground off, otherwise it should be scrapped.

Crankshaft bending, distortion and fracture:

crankshaft bending and distortion deformation, often due to the crankshaft is caused by too large impact load. If the diesel engine deflagration is caused by combustion fault in use, the crankshaft will be deformed by great impact load; Start too fierce, throttle beyond obstacles, will make the crankshaft in a short time by a big impact. When the assembly clearance of bearing bush is too large, the gas explosive force constantly impacts the crankshaft when the piston connecting rod moves up and down, which causes the deformation of the crankshaft due to periodic vibration. After the assembly of engine flywheel, there is no dynamic balance test, and the balance of flywheel is poor in operation, resulting in crankshaft vibration, which will also cause crankshaft deformation. Crankshaft jitter is easy to cause diesel engine vibration and crankshaft fracture, a major crash.

Crankshaft also has some other damage forms, such as starter screw hole damage, crankshaft front and rear oil seal journal wear, crankshaft rear flange fixed flywheel bolt hole wear, flange disc middle bearing hole wear and so on.