A sliding bearing is installed between the crankshaft neck and the large end of the connecting rod in a diesel engine, which is called the connecting rod bearing shell due to its shape resembling a tile. There is a clearance fit between the connecting rod crankshaft neck and the connecting rod bearing shell, so that when the crankshaft rotates, the oil can form an oil film on the friction surface, thereby reducing the friction and wear of the crankshaft neck. The inner surface of the bearing shell is cast with a layer of wear-resistant alloy layer. During use, faults such as melting, peeling, scratching, and excessive wear of the alloy layer often occur, which disrupts the normal fitting relationship between the journal and the bearing shell. In severe cases, it can lead to burning and holding of the bearing shell, and sometimes even cause crankshaft fracture and accidents. The damaged bushing need to be replaced. Now, we will analyze the methods for replacing, installing, and repairing the connecting rod bushing.
1.Common defects in connecting rod bushing
(1) The bearing alloy is melted. It refers to the phenomenon where the crankshaft bushing of a diesel engine are damaged or subjected to high temperatures, resulting in sintering (jamming) between the bushing and the crankshaft journal, causing the engine to be unable to operate normally. Burning tiles is one of the common faults in diesel engines. Before burning the tiles, there are the following signs: black smoke from the engine, a decrease in oil pressure, a noticeable sense of difficulty, a rapid decrease in engine speed, white oil smoke from the crankcase ventilation hole, and a dry friction sound of "chirping". In this situation, the engine should be immediately shut down and turned off, and the crankshaft should be shaken to avoid bearing damage. Although diesel engines that have already suffered from tile burning can be repaired, their normal technical condition is difficult to restore.
(2) The bearing alloy has fallen off. During operation, the bushing are subjected to alternating loads, which can cause fatigue cracks or even detachment on the alloy surface.
(3) Scratches on the bearing alloy. Scratches on the bearing caused by metal abrasive particles in the lubricating oil or burrs on the edge of the oil hole in the journal can accelerate wear between the journal and the bearing.
(4) Alloy wear. Wear of the journal and bushing is inevitable, leading to an increase in the gap between the two. When the gap reaches a certain level, a large amount of oil will leak from it, causing low oil pressure and accelerating their wear. In severe cases, bearing shell noise may also occur.
2.Replacement and assembly of connecting rod bearings
The clearance between the diesel engine connecting rod bearing shell and the connecting rod journal must comply with regulations. Excessive clearance makes it easy for lubricating oil to leak out, making it difficult to establish an oil film, resulting in poor lubrication and increased wear; Knocking on the journal and bushing can damage the crankshaft or connecting rod bushing. If the gap is too small, the amount of lubricating oil entering will be reduced, the wear will intensify, and it may even cause biting and burning accidents. If it is found that the clearance between the bushing has exceeded the usage standard, a new bearing shell needs to be replaced. At present, there are two main types of thin-walled bushing commonly used in small high-speed diesel engines: one type has a wall thickness of 3-6 mm and a wear reducing alloy layer thickness of 0.5-1.5 mm; Another type is a wall thickness of 0 9-3 mm, the thickness of the wear-resistant alloy layer is 0.25-0.75 mm. Thin walled tiles are generally not repaired, but replaced. When replacing, attention should be paid to the following tasks:
(1) The connecting rod bushing should be replaced as a complete set, and the size should correspond to the size of the connecting rod journal. The upper and lower bearings of the connecting rod bearing can be interchanged. The connecting rod and connecting rod cap are machined in pairs and cannot be replaced. The connecting rod cap is positioned on the connecting rod by the locating lip, and the locating lip grooves on each connecting rod are on the same side.
(2) Install the upper and lower bearings separately in the connecting rod big end and connecting rod cap, and ensure that the positioning lip on the bearing is accurately embedded in the corresponding positioning groove.
(3) Take an equivalent piece of soft metal or wire and place it between the bearing and the journal. When placing it, be sure to be perpendicular to the horizontal line between the bearing and the journal. Tighten the bearing cover to a certain torque, and then take out the metal sheet or wire that is inserted to measure its thickness. At this point, the thickness is the gap between the bushing.
(4) When installing connecting rod bolts, a comprehensive inspection should be conducted to confirm that the bolts are qualified before use.
3.Repair
The wear and damage of lead copper alloy bearings are generally irreparable, but for defects in white alloy bearings, such as cracks and peeling, corresponding repair methods can be taken according to the degree of damage:
(1) Welding repair. When repairing with hydrogen oxygen welding, attention should be paid to the following issues: ① The material of the welding rod should be consistent with the chemical composition of the bearing alloy. ② Determine the welding repair area and use a scraper to scrape around it Heat the bearing shell to 160~180 ℃ before welding. ④ When welding, the repaired bearing should be rotated appropriately to ensure that the repaired area is at the lowest horizontal position The welding repair sequence should jump from the middle to both sides. ⑥ After welding repair, use a scraper to scrape it flat.
(2) Boring of connecting rod bushing. It is boring with a tile boring machine. Before boring, the bending and twisting of the connecting rod should be inspected and corrected. When boring, it is necessary to find the center, and the amount of boring should not be too large at a time to prevent bearing pulling. The center distance between the large and small ends of the connecting rod must be strictly controlled within the specified range. After boring, the roundness and cylindricity should not exceed 0.02 mm, and the thickness of the alloy layer should meet the requirements. And the parallelism between the centerline of the connecting rod bushing and the bearing shell shall not exceed 0.05 mm within a length of 100 mm.
(3) Repair and scraping of connecting rod bushing. The specific method steps are as follows: ① Place the crankshaft on a dedicated bracket. ② Apply a thin layer of red lead oil or other colored paint on the surface of the connecting rod journal, and then install the connecting rod in the correct position and direction on the journal. Gradually tighten the connecting rod bolts evenly until there is slight resistance to rotating the connecting rod Rotate the connecting rod several times, then loosen the bolt, remove the connecting rod, and observe the contact between the surface of the bearing and the journal. If the color dots are uneven and located at both ends, use a scraper to scrape them off When scraping, hold the connecting rod or shaft cover with your left hand (or clamp it onto a vice), hold the scraper flat with your right hand, and use the force of your wrist to scrape from the outside to the inside. When repairing, the direction of scraping should be constantly changed. The direction of the first scraping should be at 30 ° with the axis of the bearing, and the direction of the second scraping should be at 30 ° with the axis, but facing the other side. Apply appropriate force during scraping, as excessive force can cause ripples. Scrape large and leave small, scrape heavy and leave light, lift and drop the knife accurately, lightly, and steadily. After scraping a few times, when the gap is slightly larger, use your hand to shake the connecting rod, and use its own gravity and centrifugal force generated by the rod body to conduct grinding to ensure that the bearing is scraped and matched in a round and straight manner. After scraping, garbage bins should be scraped at both ends of the bearing After the bearing is scraped and assembled, it is required that the contact points should be evenly distributed, and the contact area with the shaft neck should not be less than 75%. The roundness and cylindricity of the bearing should not exceed 0.02-0.03 mm, and the fitting clearance should be within the assembly clearance range specified in the instruction manual. In actual maintenance, for old bearings with minimal wear and clearance within the required range, a triangular scraper can be used to repair surface scratches and corrosion. The amount of scraping should be as small as possible to ensure the correct fit between the shaft and the bearing.
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MG-TEX Steel with PTFE Fiber Fabric Bearings:
This new material uses the PTFE fibre fabric overlay on steel backing, the fabric is with high load capacity and much longer operating life comparing with conventional 3-layer bushes.The metal matrix provides the bearing with excellent load-bearing performance and can transfer the heat generated during the operation of the bearing in time, while PTFE woven material is designed to be used in completely dry friction conditions. It has a lower friction coefficient and excellent wear resistance. Compared with traditional bearings, in addition to high load performance, it can completely eliminate oil and eliminate process maintenance, and it slides smoothly. There will be no "sticky" phenomenon.
