Abstract: The assembly quality of sliding bearings will directly affect the service life of diesel engines. Therefore, when repairing diesel engines, several issues related to the assembly quality of bearings should be paid attention to, such as the tightness of the bearing steel back in the seat hole, the pre tightness of the bearing cover bolt, and the inspection of the height of the bearing seat hole.

Keywords: sliding bearings; Assembly quality; engine

The crankshaft and bearings, as a combination, must withstand the explosive pressure of combustion gases, the inertial force of reciprocating motion of the piston connecting rod group, the centrifugal force of rotating parts, and other forms of additional torque during engine operation. At the same time, the crankshaft inevitably produces torsional and bending vibrations during high-speed rotation. Bearings require high assembly quality due to their complex stress, and their assembly quality will directly affect the service life of diesel engines. Therefore, the following precautions should be taken when repairing diesel engines.

1. The tightness of the bearing steel back in the seat hole

Early damage to bearings due to improper adhesion is inevitable. This tightness can be checked using the following methods.

(1) Check the contact marks between the steel back and the seat hole. Apply a uniform and thin layer of red ink on the surface of the steel back, insert it into the seat hole, press the bearing cover with the specified torque, then remove the bearing cover, take out the bearing, and observe the area of the ink on the surface of the seat hole. Generally speaking, if the application area is more than 80% of the entire area, it can be considered that the tightness is appropriate.

(2) Measure the clearance at the interface of the bearing cover using a thickness gauge. Place the bearing into the seat hole and tighten the bearing cover with the specified torque. Then, loosen the bearing cover bolt (cap) on one end and maintain the original tightness on the other end. Use a thickness gauge to insert and loosen the nut at the interface of the bearing cover on one end, and check the height of the bearing.

(3) Measure the tightness of the pressure fuse. Insert the bearing into the seat hole and place 3A fuses on both sides of the screw holes at the interface between the seat hole and the bearing. Tighten the bearing cover with the specified torque. Remove the bearing cover again, take out the fuse, and measure the thickness of the fuse pressed against the bearing interface and the bearing cover interface with an outer diameter dial gauge of 0-25mm. The thickness difference is the height difference at the bearing interface. The measured height should comply with regulations, with a general range of 0.03-0.05mm for connecting rod bearings and 0.04-0.06mm for main bearings.

2. High deviation inspection and repair

In order to ensure that the bearing has a certain degree of interference fit in its seat hole, the tile should have a certain height when installed in the bearing seat or tile cover. Insufficient height will result in insufficient adhesion between the bearing steel back and the bearing seat hole, leading to wear, and in severe cases, it may cause the outer ring (rolling pad) to move. The excessive interference between the bearing and the seat hole caused by excessive height will result in "jamming" at the interface. Therefore, it must be inspected during assembly. The height deviation of common sliding bearings is 0.1.0-0.18mm. After inspection, if the interference fit is insufficient, the bearing should be replaced; If the interference fit is too large, the flatness of the bearing seat hole and interface, as well as the length of the bearing, should be checked. If the bearing is indeed too long, a file can be used to slightly file and repair the tile mouth.

3. Pre tightening of bearing cover bolts

After tightening the bearing cover bolts, in addition to generating sufficient pre pressure between the bearing steel back and the seat hole, there is still sufficient pre pressure between the interface of the bearing seat when the work is subjected to pulse peak loads, so as not to cause displacement of the bearing cover and weaken the frictional self-locking effect between the steel back and the seat hole. So the tightening torque of the bearing bolts should be ensured to be appropriate. Most bearing cover bolts are made of materials with a yield limit of 90kg/mm2 or 180kg/mm2 or higher.

Insufficient tightening torque during repair can easily cause loosening of the bearing steel back and seat hole. The tightening torque is too high, approaching the yield limit of the bolt material. When subjected to load during operation, the bolt will undergo plastic deformation and be stretched, posing a risk of fracture. So the bolts should be tightened according to the prescribed tightening torque. Attention should be paid to the following points when tightening bearing bolts:

(1) During assembly operations, bolts must be tightened in the prescribed order and method. The tightening sequence of the main bearing covers of each cylinder is generally from the middle to the ends. The tightening sequence for the 6-cylinder series diesel engine is 4-3-5-2-6-1-7. If the specified torque has been reached but the safety pin hole is not aligned, the thickness of the flat pad under the nut should be changed and adjusted. If it is not much different from the pin hole, it can be tightened appropriately, but being too tight is dangerous. At present, there are still a considerable number of repair personnel using torque wrenches without digital components to tighten them by feeling, which will affect the bearing clearance to varying degrees.

(2) The bearing bolts must be carefully inspected before installation, and the bolt threads should be intact and undamaged, without any reverse teeth or severe wear. The guide part of the bolt must have a certain degree of tightness and must not be loose when fitting with the hole. The thread must be cleaned during installation, otherwise the friction torque will increase due to the blockage of the thread, reducing the actual effective pre tightening force.

(3) The thick end of the double headed bolt is fastened to the cylinder block, and the self-locking property is poor compared to the thin end, so it must be tightened in advance. If the nut and double headed bolt are tightened together with the original specified torque, the pre tightening force of the bolt will not meet the requirements.

4. Quality of bearing working surface

The material for the surface of bearings is currently mostly high tin aluminum alloy and copper lead alloy with a third working layer, and the thickness of the alloy layer is relatively thin. If this good working layer is scraped off or key off during repair, it will cause early damage to the bearing due to the destruction of the good performance of the working layer.

The bearing should have good roundness in the seat hole to ensure good oil wedge effect and prevent excessive oil film pressure. The above situation can also occur when the surface roughness is damaged. So when repairing, the following points should be noted:

(1) Bearings cannot be scraped to achieve 75% to 85% contact marks, and this should not be used as a standard to measure the quality of bearing repairs. The densely packed working surface formed by scraping and matching does not actually achieve a good fit with the shaft neck. Due to the uneven protrusion of its working surface and the change in the oil film gap at the protrusion, the pressure and temperature borne by the oil film sharply increase at the protrusion, forming additional high temperature and high pressure, which is an important cause of fatigue fracture or detachment of the bearing alloy. Scratching still has the problem of excessive or insufficient clearance between the bearing and the journal. At present, the average specific pressure of bearings in high-speed diesel engines exceeds 50MPa, and the peak pressure of the oil film can reach about 12 times the average specific pressure. To withstand such high loads and operate for a long time, it is very important to ensure the specified clearance of the bearings. The clearance between the bearing working surface and the shaft neck should meet the requirements without scraping, and the shaft should be able to rotate in the bearing without obvious obstruction. It is considered good if there are no obvious friction contact marks on the bearing working surface.

(2) The clearance between the bearing and the journal should comply with the original factory regulations. If there is no data available, the following data can be used for selection: for copper lead alloy and aluminum alloy bearings, the clearance is 0.06-0.10mm when the journal is 50-70mm; the clearance is 0.085-0.135mm when the journal is 70-90mm; and the clearance is 0.105-0.15mm when the journal is 90-100mm.

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Three-layer metal-polymer self-lubricating bearings:

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