Inspection and maintenance of crankshaft

The function of the crankshaft
Convert the air pressure transmitted from the piston rod assembly into torque and output it to the outside world. It is also used to drive the valve train and other auxiliary devices of the engine.
Working conditions: withstand gas pressure, inertia force, and moment of inertia. Affected by alternating loads.
Crankshaft materials: medium carbon steel (steam), alloy cast iron (diesel), and ductile iron.
Classification of crankshafts:
Integers (regular)
Combination type (commonly used in small gasoline engines with connecting rods and engines with rolling bearings as the main crankshaft bearings).
The composition of the crankshaft:
)1) Main neck - used to support the crankshaft.
Full support: The number of crankshaft journals is one more than the number of cylinders. Most use diesel and gasoline engines with good strength and rigidity, reduced wear and tear.
Incomplete support: The number of crankshaft main journals must not exceed the number of cylinders. Overloading shortens the overall length of the crankshaft.
Bending: It consists of a connecting rod journal and two end cranks and main shaft journals.
Common damage to crankshaft
Damage to the surface of the journal, burning, bending and deformation of the crankshaft, cracks, and even fractures.
Wear pattern of crankshaft
1. The radial wear of the main journal and connecting rod journal is elliptical, and the maximum wear positions correspond to each other. The maximum wear of each main journal is close to the side of the connecting rod journal; In addition, the maximum wear of the connecting rod journal is also near the main journal.
Practice has shown that the wear of the connecting rod journal is more severe than that of the main shaft journal. This is because the journal load of the connecting rod is high and the lubrication conditions are poor.
3. The wear on the inner side of the connecting rod neck is the greatest, forming an oval shape.
Cracks and fracture patterns of crankshaft
1. Location: Most crankshaft cracks appear in the transition fillet and oil hole between the crank arm and the journal. The former has more transverse cracks, which can lead to crankshaft fracture in severe cases, and should be carefully inspected. The latter has many axial cracks that extend axially along the sharp edge of the inclined oil hole.
2. Reason:
The transverse and axial cracks of the crankshaft are mainly caused by stress concentration.
The cracks in the journal oil hole are mainly caused by uneven stress distribution in the oil hole due to the inclined arrangement of the oil passage.
The causes of crankshaft bending and twisting
(1) The main bearing clearance is too large, and the engine operates under shock conditions such as detonation and overload, causing excessive crankshaft vibration.
2) A few cylinders are not working or unbalanced.
3) The looseness of each main bearing cover is inconsistent, and the crankshaft is subjected to uneven force.
4) The concentricity deviation of the main housing hole in the cylinder block.
5) Improper operation resulted in a fierce start when pulling a trailer.
6) The crankshaft is stored improperly and has no support for a long time.
Inspection of crankshaft damage
When wearing, use an outer micrometer.
2. Crack inspection
(1) Oil seepage method: immerse the cleaned crankshaft in kerosene, take it out and wipe it, sprinkle white powder on the surface, and use a hammer to axially strike the non working surface of the crankshaft. The white powder has obvious crack like oil stains, where there are cracks.
Magnetic testing method uses a flaw detector to magnetize the parts and spray magnetic powder on areas where cracks may occur. When the magnetic field lines pass through the edge of the crack, the magnetic powder is adsorbed on the crack, indicating the location and size of the crack.
Bend the dial gauge for inspection.
Repair of crankshaft
When the roundness and cylindricity error of the crankshaft journal exceeds 0.01-0.0125mm, use a dedicated crankshaft grinder for grinding.
The radial runout of the crankshaft is generally below 0.04~0.06 millimeters, and it needs to be corrected if it exceeds 0.10 millimeters.
Method: Cold pressure calibration method and detonation method.
Cracks: Magnetic testing method, oil immersion tapping method.
1) Repair crankshaft journal dimensions:
Generally, the crankshaft master cylinder neck and connecting rod cylinder neck have standard and reduced sizes of 2-4, and there are corresponding sizes of bearings. Some engines are equipped with 0.05mm maintenance bearings. A few crankshafts do not have repair dimensions.
Please control the actual repair size below the maximum possible journal size after grinding.
2) Crankshaft grinding
The technical requirements and key points for crankshaft journal grinding are as follows.
1) The grinding size of the journal should be b
If the crack occurs in an unstressed part or does not propagate, the crack can be repaired. When the crankshaft crack is located in the stressed area, such as the crankshaft arm or journal, please replace the crankshaft with a new one.
2. Compensation for crankshaft bending
(1) Surface explosion method: This method is suitable for crankshafts with bending amounts below 0.30-0.50 millimeters. It can be carried out using a spherical hammer and an air hammer.
Introduction to cold pressure compensation crank method
1) Cold pressure compensation can be achieved through a press. Place the V-shaped blocks on the journals at both ends of the crankshaft (with the copper sheet placed in contact with the journals) and place them on a pressure table.
2) Rotate the crankshaft, bend it upwards, and align the pressure head with the middle main journal.
3) Press the dial indicator on the spindle neck to make contact between the needle rod and the lower surface of the spindle neck, and adjust the dial to zero the needle.
4) Apply pressure to the most curved convex surface of the crankshaft, gradually increasing the pressure. The bending amount is 10-15 times the bending amount of the crankshaft, and the pressure should be maintained for 1.5-2 minutes.
5) Perform aging heat treatment to eliminate internal stress generated during cold pressing (heat up to 300-500, maintain insulation for 0.5-1h).