Abstract: The assembly quality of sliding bearings will directly affect the service life of diesel engines. Therefore, when repairing diesel engines, attention should be paid to several issues related to the assembly quality of bearings, such as the tightness of the bearing steel back in the seat hole, the pre tightness of the bearing cover bolts, and the inspection of the bearing's protrusion in the seat hole.
Keywords: sliding bearing; Assembly quality; engine
The crankshaft and bearings, as a mating pair, 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. Due to the complex force acting on bearings, there is a high demand for assembly quality, which directly affects the service life of diesel engines. Therefore, the following precautions should be taken when repairing diesel engines.
1. Tightness of the bearing steel back in the seat hole
Early damage to bearings due to improper tightness 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, install it into the seat hole, press the bearing cover tightly according to the specified torque, then remove the bearing cover, take out the bearing, and observe the application area of the oil 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 press the bearing cover tightly according to 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 bearing cover interface on one end, and check the bearing's protrusion.
(3) Measure the tightness of the fuse. Install 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 according to the specified torque. Remove the bearing cover again, take out the fuse, and measure the thickness of the fuse at the bearing interface and the bearing cover interface using a 0-25mm outer diameter percentage. The thickness difference is the protrusion at the bearing interface. The measured height shall meet the requirements. The general connecting rod bearing is 0.03~0.05mm, and the main bearing is 0.04~0.06mm.
2. High protrusion inspection and repair
In order to ensure a certain degree of interference in the bearing seat hole, the tiles should be installed in the bearing seat or cover with a certain degree of protrusion. Insufficient protrusion will cause insufficient adhesion between the bearing steel back and the bearing seat hole, leading to wear and tear, and in severe cases, the phenomenon of outer ring (rolling pad) may occur. Excessive height may cause the interference between the bearing and the seat hole to be too large, resulting in "jamming" at the interface. Therefore, inspection is necessary during assembly. The protrusion of common sliding bearings ranges from 0.1.0 to 0.18mm. After inspection, if the interference is insufficient, the bearing should be replaced; If the interference is too large, the flatness of the bearing seat hole and interface, as well as the bearing length, 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 preload between the bearing steel back and the seat hole, there is still sufficient preload between the interface of the bearing seat when working under pulse peak load, preventing displacement of the bearing cover and weakening of the frictional self-locking effect between the steel back and the seat hole. Therefore, 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 above.
Insufficient tightening torque during repair can easily cause loosening of the bearing steel back and seat hole. The tightening torque is too large, 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 specified tightening torque. The following points should be noted when tightening bearing bolts:
(1) During assembly operations, bolts must be tightened in the prescribed sequence and method. The tightening sequence of each cylinder's main bearing cover is generally from the middle to both ends. The tightening sequence for 6-cylinder series diesel engines 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 washer under the nut should be changed and adjusted. If the difference with the pin hole is not significant, it can be tightened appropriately, but being too tight is dangerous. At present, there are still many repair personnel who use torque wrenches without numbers 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 chamfering or severe wear. The guide part of the bolt must have a certain degree of tightness in conjunction with the hole and must not be loose. The thread must be clean 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 buckle end of the double head bolt is fastened to the cylinder block, and its self-locking performance is poor due to the thin buckle, so it must be tightened in advance. If the nut and double head bolt are tightened as a whole according to the original specified torque, the pre tightening force of the bolt will not meet the requirements.
4. Quality of bearing working surface
The materials used for the surface of bearings are currently mostly high tin aluminum alloy and copper lead alloy with a third working layer, with a relatively thin thickness of the alloy layer. If this good working layer is scraped or keyed off during repair, it will cause early damage to the bearing due to damaging 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 if 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 should not be used as a standard to measure the quality of bearing repair. The densely dotted working surface formed by scraping cannot actually achieve a good fit with the journal. Because its working surface bulges unevenly, and because of the change of oil intermembrane space at the bulge, the pressure and temperature borne by the oil film rise sharply at the bulge, forming additional high temperature and pressure, which is an important reason for bearing alloy fatigue cracking or falling off. There is also a problem of excessive or small clearance between the bearing and the journal during scraping. 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 its specified clearance. The clearance between the bearing working surface and the journal should meet the requirements without scraping, and the shaft should be able to rotate without obvious obstruction. The bearing working surface should have no obvious friction contact marks, which is considered good.
(2) The fit 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, when the journal is 50-70mm, the clearance is 0.06-0.10mm; When the journal is 70-90mm, the clearance is 0.085-0.135mm; When the journal is between 90 and 100mm, the clearance is between 0.105 and 0.15mm.
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E20 is bimetallic bearing material, based on steel backing, and a layer of copper sinter, which is composed of special copper powder with solid lubricants (major ingredient is graphite) dispersed, acts as wear resistant surface and processed through oil-impregnating treatment.
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