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Construction of Crankshaft for Diesel Generator Set

Dec. 29, 2018

Construction of diesel generator crankshaft


The diesel generator crankshaft is mainly composed of a main journal, a connecting rod journal (crank pin), a crank arm, a counterweight, a front end (free end), and a rear end (power output end).



1. Spindle neck and connecting rod journal

The main journal and the connecting rod journal of the diesel engine are cylinders with high dimensional accuracy and low roughness, which are connected to the crank arm with a large arc radius. The main journal is used to support the crankshaft, and the crankshaft rotates at a high speed around the center of the main journal. The main journal is mostly solid, while the crankshaft main journal of the ductile iron and the connecting rod journal are mostly hollow. The advantage is that the rotation quality can be reduced, thereby reducing the centrifugal force, and at the same time, it can be used as a cavity for lubricating oil centrifugal filtration. The main journal and the connecting rod journal are pressure-lubricated, and the lubricating oil is pressed into the connecting rod journal cavity through the inclined oil passage in the crank arm, and under the action of the rotating centrifugal force, the medium-density metal wear debris in the oil and Other impurities are directed to the outer wall of the cavity, and the clean inner oil flows through the oil pipe to the connecting rod journal and the bearing friction surface.


2. Crank arm

The function of the crank arm is to connect the main journal and the connecting rod journal. It is usually made elliptical or circular, and its thickness and width should be sufficient for the crankshaft to have sufficient rigidity and strength.


3. Balance weight

As shown in Fig. 1, the balance weight is usually placed on the crank arm on the side opposite to the journal of the connecting rod, and the switch is mostly fan-shaped. The function of the balance weight is to balance the weight of the connecting rod journal and the crank arm, the centrifugal force and its torque to reduce the load on the main bearing and increase the smoothness of the operation.


4. Front end of the crankshaft


The crankshaft is formed into a cylindrical shape with a shoulder, as shown in Fig. 2, and is equipped with a timing gear, a slinger, an oil seal, a pulley and a thrust piece, respectively. Some medium and small power diesel engines have a starting claw at the front end of the crankshaft, and some high-speed diesel engines have a power output device at the front end of the crankshaft.


As shown in Fig. 3, the rear end of the crankshaft is generally provided with an oil seal, a return oil groove, a rear flange and the like. The rear end of the crankshaft extends out of the body to deliver the power of the diesel engine to the transmission of the associated implement. The rear end is equipped with a flywheel, which is matched by a spline or a flange, and then fastened with bolts. Since the flywheel is large and heavy, there are certain requirements for the fastening of the bolt.


Causes, inspection and repair of cracks and breaks in crankshaft of diesel generator sets


The causes, inspection and repair methods for the crack and break of the crankshaft of the diesel generator set are as follows:




1. When grinding the journal, there is no fixed fillet at the joint between the journal and the crank arm (generally, the fillet of the journal is required to be between 1 and 3 mm), causing stress concentration and breaking the crankshaft.


2. The clearance of the bearing is too large or the alloy is detached, causing an increase in the impact load.


3. Fatigue damage occurs after the crankshaft has been working for a long time.


4. The crankshaft is often operated at critical speeds.


5. The cylinder block is deformed and the crankshaft bearing housing is not correct. When the crankshaft bearing is repaired, the crankshaft bearing housing holes are not on one axis.


6. The lubricating oil passage is not smooth, and the crankshaft is in a semi-dry friction state, causing the crankshaft to break.


7. Poor crankshaft material or defective manufacturing.


8. The balance of the crankshaft is destroyed, and the crankshaft is subjected to a large inertial impact, which causes the crankshaft to fatigue and break.


Crankshaft cracks occur mostly at the end of the connecting rod journal or at the junction of the crank arm and the crankshaft journal. The inspection method is as follows:


1. Magnetic flaw detection


Check with a magnetic flaw detector, first magnetize the crankshaft with a magnetic flaw detector, then use iron powder to sprinkle on the part to be inspected, and gently tap the crankshaft with a small hammer. If there is a crack, it will be found in the middle of the accumulation of iron powder. There are clear crack lines.


2. Hammering


First remove the oil adhering to the surface of the crankshaft, then dip the entire crankshaft with kerosene or diesel, then remove the crankshaft to wipe it clean, and finally support the two ends of the crankshaft on the wooden frame, and gently tap each crankshaft with a small hammer. arm. For example, if a "squeaky" coherent sharp metal sound is emitted, it means that the crankshaft has no crack; if the sound "wave" is inconsistent and short metal sound, it means that the crankshaft has cracks. Then, in the vicinity of the crack, look at it with a magnifying glass or a magnifying glass. If it is found that the oil spills out or becomes a black line, it is the crack.


3. Plastering method


After the crankshaft is washed and smeared with kerosene or diesel oil, a layer of talc powder is evenly coated on the surface of the crankshaft, and then the crank arm is tapped with a small hammer. If there is a crack in the crankshaft, the oil will seep out and the talc powder on the surface of the crankshaft becomes Yellow-brown, you can find the crack.


4. Lime milk method


Wash the crankshaft in hot oil (oil) for about 2 hours, let the oil enter the crack, remove it and dry it, then spray the lime emulsion onto the crankshaft with a spray gun to dry it (the lime emulsion is a mixture of clean chalk and alcohol). At this time, the chalk absorbs the oil stored in the crack to become dark, showing a crack shape.


The repair of crankshaft cracks and breaks can be repaired by the method of “welding repair”. The main points of the process are as follows:


1. First, the crankshaft is placed in alkaline water to be washed and cleaned to remove oil stains, and then a chisel is used to cut a "U" shaped groove along the surface of the crack. It is better to not see cracks in the groove depth. The bottom of the groove has a circular arc shape, and the width of the groove needs to be determined according to the depth, length and shape of the crack. Then correct it so that the bending of the crankshaft does not exceed the specified range. Finally, the crankshaft is mounted on an exclusive welding frame, or mounted on the cylinder block, and an iron lining is placed between the crankshaft and the welding frame or the cylinder block. Then tighten the bearing cap with bolts to avoid bending and deformation of the crankshaft during welding. If the fractured crankshaft is welded, the center seam should be found according to the original trace of the crankshaft fracture. The welding should be spotted on both sides of the fracture joint by electric welding, and then welded on both sides of the crack without spot welding.


2. Use a gas welding flame to heat the weld to 350~450 °C, and then use a low carbon steel electrode with a diameter of 3~4mm for electric welding. When welding, the method of butt welding is used. After each layer is welded, the welding slag is removed and the next layer is welded.


3. After welding, first weld the flat, drill the oil passage, check the weld for cracks, and whether the crankshaft is deformed.


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