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Causes of Crankshaft Fracture and Repair After Wear

Pulished on Jan. 22, 2021



Here is the Crankshaft Supplier's introduction to the cause of crankshaft fracture and repair problems after wear


Crankshaft fracture is a serious mechanical failure of the engine. The fracture generally occurs at the corners of the crank pin and the connection between the main journal and the crank arm or the stress concentration locations such as the journal oil hole.


(1) Due to improper selection of engine oil, or careless cleaning and replacement of the "three filters" by individual users, the engine oil has deteriorated after long-term use; serious overload and overhang have caused the engine to run overloaded for a long time and cause a burning accident. The crankshaft was severely worn due to the engine burning. The engine crankshaft is repaired by replacement, that is, a new crankshaft is purchased and the damaged crankshaft is sent to the manufacturer for repair and spare. After the crankshaft wear problem occurred in the vehicle, some users, out of cost and time considerations, found some small local factories for repair and processing, and the severely worn crankshaft was surfacing, processed, and ground after overall heat treatment. Due to repair methods and process problems, the connection fillet between the crank pin and the main journal and the crank arm has changed, resulting in local stress concentration; because the crankshaft is forged with precision 45 steel, the metallographic weaving of the crank has also occurred. Variety. The above two are the main causes of crankshaft fracture.


(2) After the engine is repaired, the loading period has not passed the running-in period, that is, overload and overload, and the long-term overload operation of the engine causes the crankshaft load to exceed the allowable limit.


(3) Surfacing welding was used in the repair of the crankshaft, which destroyed the dynamic balance of the crankshaft, and did not perform a balance check. The unbalanced amount exceeded the standard, which caused the engine to vibrate greatly and caused the crankshaft to break.


(4) Due to poor road conditions and severe overloading of the vehicle, the engine often runs within the critical speed of torsional vibration, and the failure of the shock absorber will also cause the crankshaft torsional vibration fatigue damage and fracture.


Crankshafts

Crankshafts


How to repair the crankshaft after it is worn?


Generally speaking, the journal diameter is below 80mm, and the roundness and cylindricity error exceeds 0.025mm; or the journal diameter is above 80mm. Crankshafts with roundness and cylindricity errors exceeding 0.0400 should be ground to the specified size, or vibrated surfacing, chrome-plated, iron-plated, and then ground to the specified size or repaired.


1. Grinding of crankshaft


The grinding of the crankshaft journal is carried out on the basis of crankshaft correction. In addition to the dimensional accuracy and surface roughness of the journal surface, the grinding of the crankshaft must meet the requirements of the geometric tolerance: when grinding the crankshaft, the coaxiality of the axis of the main journal and the connecting rod journal must be guaranteed And the parallelism between the two axis lines to limit the crank radius error. And to ensure the accuracy of the angle between the connecting rod journals. Grinding of the crankshaft is usually carried out on a dedicated crankshaft grinder.


2. Grinding of connecting rod journal


Due to the uneven wear of the connecting rod journal, two grinding methods are produced: eccentric grinding and concentric grinding.


The concentric grinding method is to keep the axial position of the connecting rod journal unchanged after grinding, that is, the crank radius and distribution angle remain unchanged. When grinding a diesel engine crankshaft, the concentric method is often used to keep the crank radius unchanged and the compression ratio of the diesel engine unchanged, but the amount of grinding is large each time. Currently, during the service life of the car, the number of overhauls is reduced, and the concentric method can ensure the engine performance remains unchanged.


The eccentric grinding method is to position and grind according to the worn connecting rod journal surface. At this time, the centerline position and crank radius of the journal are changed. Generally, the crank radius after grinding is larger than the original crank radius, which increases the compression ratio and changes the cylinders unevenly. At the same time, the mass center of the entire crankshaft is not on the centerline of the crankshaft main journal, which causes the crankshaft to be unbalanced and causes additional operation during operation. Dynamic load. Therefore, when grinding the connecting rod journals, the increase in crank radius should be minimized to ensure that the coaxiality error of the shaft journals of the connecting rod journals in the same position is not greater than ±0.10mm, so as to ensure the balance of the crankshaft during operation.


How to repair the severely worn crankshaft?


If the engine crankshaft is severely worn and the grinding method cannot be repaired or the effect is poor, plasma spraying can be used to repair it.


(1) Surface treatment of the journal before spraying


① According to the wear of the journal, round it on the crankshaft grinder, and the diameter is generally reduced by 0.50-1.00mm.


②Use copper to shield and protect the adjacent journals to be sprayed.


③Use a napping machine for napping the surface to be coated. Use nickel strips as electrodes, and melt nickel on the surface of the journal under 6-9V, 200-300A alternating current.


(2) Spraying


Clamp the crankshaft on the rotatable worktable and adjust the distance between the spray gun and the workpiece (about 100mm). Choose nickel-clad aluminum (Ni/AL) as the base material, and a mixture of wear-resistant alloy cast iron (NT) and nickel-clad aluminum as the working layer material; the thickness of the bottom layer is generally about 0.20 mm, and the thickness of the working layer is determined according to needs.


During the spraying process, the temperature of the sprayed journal should generally be controlled at 150~170℃. After spraying, the crankshaft shall be kept in an oven at 150-180℃ for 2 hours and cooled with the oven to reduce the stress between the sprayed layer and the journal.


(3) Treatment after spraying


After spraying, check whether the sprayed layer and the journal base are tightly bonded, if not tight enough, remove the respray. If the inspection is qualified, the crankshaft can be ground. Due to the high hardness of the plasma sprayed coating, a softer tin carbide grinding wheel is generally used for grinding, and the feed rate during grinding should be smaller (0.05-0.10mm) to avoid cracking the coating. In addition, after grinding, be sure to grind the oil passage holes with sand bars to prevent burrs from scratching the tiles. After cleaning, the crankshaft is immersed in 80-100℃ lubricating oil and boiled for 8-10h. After the lubricating oil fully penetrates into the coating, it can be loaded into the car.


I hope that the above sharing will be helpful to everyone. In addition, the company also provides Gear Shafts and drive shafts. Please feel free to contact us if necessary


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