How To Repair A Worn Crankshaft?

Generally speaking, the journal diameter is less than 80mm, and the roundness and cylindricity errors exceed 0.025mm; Or the journal diameter is greater than 80 mm. Crankshafts with roundness and cylindricity errors exceeding 0.0400 should be ground to the specified dimensions or ground to the specified dimensions or repaired after vibration surfacing, chromium plating, and iron plating.

Grinding of crankshaft

The grinding of the crankshaft journal is carried out on the basis of crankshaft correction. In addition to meeting the technical requirements for the dimensional accuracy and surface roughness of the journal, the grinding of the crankshaft must also meet the requirements for geometric tolerances. When grinding the crankshaft, it is necessary to ensure the coaxiality of the main journal and connecting rod journal axes and the parallelism of the two shafts, and to limit the radial error of the crank. Ensure the accuracy of the included angle between the position of the connecting rod journal. Grinding of the crankshaft is usually carried out on a dedicated crankshaft grinder.

Grinding of connecting rod journal

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

The concentric grinding method is to keep the axis position of the connecting rod journal unchanged after grinding, that is, the crank radius and distribution angle remain unchanged. When grinding the crankshaft of a diesel engine, the concentric method is often used to keep the radius of the crankshaft unchanged and the compression ratio of the diesel engine unchanged, but the amount of grinding is large each time. At present, reducing the number of major repairs during the service life of automobiles can ensure that the engine performance remains unchanged by using the concentric method.

The eccentric grinding method performs positioning grinding based on the worn surface of the connecting rod journal. At this time, the center line position of the journal and the crank radius have changed. Generally, the crank radius after grinding is larger than the original crank radius, increasing the compression ratio, and causing uneven changes in each cylinder. At the same time, the center of mass of the entire crankshaft is not on the centerline of the crankshaft journal, resulting in imbalance and additional dynamic load during operation of the crankshaft. Therefore, when grinding the connecting rod journal, the increase in the crank radius should be minimized, and the coaxiality error of the same connecting rod journal axis should not be greater than 0.10 mm to ensure the balance of the crankshaft during operation.

How to repair the severely worn crankshaft?

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

(1) Surface treatment of sprayed front axle journal

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

② Shield and protect adjacent journals of the shaft to be coated with copper.

③ Use a roughening machine to roughen the surface to be coated. Using a nickel rod as an electrode, melt nickel on the surface of the journal under an alternating current of 6 to 9 V and 200 to 300 A.

(2) Spraying

Clamp the crankshaft on a rotatable workbench and adjust the distance between the spray gun and the workpiece (about 100mm). Nickel clad aluminum (Ni/Al) was selected as the base material, and a mixture of wear-resistant alloy cast iron (NT) and nickel clad aluminum was selected as the working layer material. The thickness of the bottom layer is generally around 0.20mm, and the thickness of the working layer depends on needs.

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

(3) Treatment after spraying

After spraying, check whether the sprayed coating is tightly bonded to the journal substrate. If not, remove and repaint. If qualified, the crankshaft can be ground. Due to the high hardness of plasma sprayed coatings, soft tin carbide grinding wheels are generally used for grinding, and the feed speed during grinding should be small (0.05-0.10mm) to avoid coating cracking. In addition, the polished oil hole must be polished with a sand strip to prevent burrs from scratching the ceramic tile. After cleaning, immerse the crankshaft in lubricating oil at 80-100 ℃ and boil for 8-10 hours. After the lubricating oil has fully penetrated into the coating, it can be loaded for use.