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The main fan is one of the core equipment in a catalytic cracking unit; it supplies primary air to the regenerator of the reaction system in order to provide the oxygen needed for coking； On the other hand, it also plays a key role in maintaining the regenerator pressure and ensuring thermal balance, oxygen balance, pressure balance, and fluidization. Therefore, improving the quality of unit maintenance and ensuring the safe, stable, long-term, full-capacity, and optimal operation of the units is key to guaranteeing the steady operation of the facility. The main fan used in our factory is a MCL524-6 centrifugal compressor produced by Shenyang Blower Factory; this unit does not have a backup unit, and its maintenance cycle is 2 years. Through efforts in recent years, we have accumulated some experience in the maintenance of turbines. Here is a brief introduction to some issues that should be noted during maintenance. 1 Data measurement before maintenance 1.1 Coupling concentricity: Adjust the axial and radial concentricity of the coupling that connects the motor to the gearbox. Mount the alignment table stands on the motor side and the gearbox side respectively, with the dial indicator installed on the bracket on the motor side. Due to the large shaft play in the motor, the shaft should be turned several times evenly so that the reading on the shaft turning return gauge is 0 each time before the value can be recorded. Align the main fan and gearbox for coaxiality. 1.2 Sealing clearance: Use a small diagonal feeler gauge to measure the side clearance of the labyrinth seals at locations such as the impeller mouth ring and balance drum. Take 2 values for each maze seal, add them together, and compare the result with the standard value; if it falls outside the specified range, it should be replaced. Gently tap on the labyrinth seal to check for any damage. 1.3 Radial and axial runout of the rotor: Place the dial indicator at the area to be measured, rotate the rotor evenly, and record the maximum and minimum values corresponding to 180° respectively. Considering the axial play of the rotor, multiple rotations of the shaft should be performed for measurement. Place two dial indicators at the shaft diameters near the end bearing shells (the bearing shell covers cannot be removed). Place the brackets designed to lift the shaft diameter at both ends of the shaft, and secure the shaft with steel cables. Slowly tighten the bolts on the crossbeams of the brackets to lift the shaft upward, while observing and recording the readings from the dial indicators. The measurement of gear radial and axial runout is the same. 1.4 Axial play of the host rotor: Remove the thrust bearings at the front and rear of the thrust disc, and use a dial indicator to measure the axial end face of the rotor. Move it axially in both forward and backward directions until the internal components of the rotor come into contact with the casing components. Measure the total rotor play S; its value should equal the sum of the plays on the left and right sides between the rotor and the stator. Install the working-side bearing shell, and measure the shaft movement of the rotor from the working side to the exhaust end, denoted as S1. The forward movement of the rotor from its working position is S2 = S – S1; the alignment of the rotor is determined using these three values. Compare S1 and S2, and adjust the thrust bearing shims if necessary. The rotor should be rotated to take multiple measurements. 1.5 Main fan bearing preload: The method for measuring the bearing preload is the same as that used for measuring the gear bearing preload. Lead wires with a diameter of approximately 0.1–0.2 mm are placed on the back of the bearing shell and on the mid-surface of the bearing housing, respectively. Then the bearing cover is placed on top, and the bolts are tightened evenly. Finally, the bearing cover is removed, and a micrometer is used to measure the thickness of the lead wires; the difference between the measurement values for the lead wire on the back of the bearing shell and that on the mid-surface of the bearing housing represents the tightening force. The standard value for tension is generally 0.03–0.05 mm. After the measurement is complete, remove the lower bearing shell. When removing it, apply lubricant to the bearing shell; use a wooden stick to lift the shaft, and gently tap it with a copper rod to rotate it axially upward so that it can be taken out and placed in the designated location. 1.6 Gear backlash and clearance: Insert a wire between the two gears, rotate the driving gear in the direction of rotation of the motor, and the wire will move out in that same direction. Measure the thickness of the wire compressed by the top and sides of the gear; the thickness of the wire at the top of the gear represents the top clearance value, while the sum of the wire thicknesses at the sides of the gear represents the side clearance value. A dial indicator can also be used to measure the side play of gears. Place the dial indicator vertically on the teeth of the pinion, then use an adjustable wrench to tighten the square head at the end of the pinion that is not connected to the shaft. Rotate the pinion using impact force, ensuring that the gear remains stationary. At this point, the reading on the dial indicator is the side clearance value. 1.7 Gear parallelism and inclination: Use a gauge block placed on the horizontal mid-plane of the housing to measure the dimensions at both ends; taking into account the deviation in shaft diameter, the parallelism of the two shafts of the gear is determined. Using the horizontal midplane as a reference, the inclination can be determined by measuring each shaft diameter with a dial indicator; alternatively, a combined level can be used to make measurements at the shaft diameters of the large and small gears. 1.8 Inspection of tooth surface contact patch coloring: Before coloring, the lubricating oil on the gears should be wiped clean. The coloring powder should be applied gently and in a thin layer; it must be applied to the meshing surfaces of the driving gear. Then gently rotate the driving wheel to check the gear mesh marks. The position of the contact mark should not be biased toward the crest of the concave tooth. 2 Inspection and maintenance of the main fans 2.1 Coloring and lapping of the thin-walled bearing shells of the gears After measuring the side clearances of each gear’s bearing shell, those with smaller clearances need to be lapped. The grinding direction follows the axis rotation direction; the grinding on the side where the axis turns inward should be more thorough, while the grinding on the side where the axis turns outward should be less thorough. After the lapping is complete, when lifting the gear back to its position over the bearing shell, it is necessary to ensure that both sides of the gear shaft are level and directly above the bearing shell; then, use a hand chain hoist to lower the gear shaft slowly. During the lowering process, hold both ends of the shaft firmly to prevent the gears from tilting and colliding. After the gear is in place, apply a thin layer of coloring powder gently to the shaft diameter area, rotate the rotor evenly, and then lift the rotor out. The coloring requirement is that the contact angle of the thin-walled tile be 135°, with 3–4 contact points per cm2. First, grind the lower bearing shell, and then grind the upper bearing shell. The thickness k of the adjustment shims on both sides of the bearing shells should be equal; it can be calculated using the following formula: k = D – d – Δ, where D is the inner diameter of the bearing shell, d is the outer diameter of the shaft, and Δ is the specified minimum clearance between the bearing shell and the shaft, in millimeters. 2.2 Maintenance of Tilting Bearing Bushings 2.2.1 Removal of Bearing Bushings Remove the bolts and positioning pins on the split surface of the bearing cover, then use jacks to lift the bearing cover gently. Remove the bolts and positioning pins on the bearing housing, and take off the shaft bushings from the radial bearings. Use a special tool to lift the shaft by no more than 0.15 mm, flip the lower bearing shell around the shaft journal to the top, and remove it. Record the position and orientation of each bearing shell segment, loosen and remove the positioning screws on the back of the segments, and then take out each segment one by one. Babbitt slabs must be free from defects such as cracks, chipping, burning, crushing, wear, and roughness. Axial scratches and grooves are not allowed, while the depth of circumferential scratches and grooves should not exceed 0.1 mm. Inspect the tiles for coloring; the babbitt should fit well, with no uneven wear on the surface, and the contact marks should be even along the axial direction. The force-bearing surface on the back of the bearing shell should be smooth; the contact marks with the shell should be uniform along the axial direction and maintain line contact. There should be no signs of scorching or heavy-load damage on the back of the bearing shell. The diameter clearance of the back positioning pin in the pin hole should be no less than 2 mm, and the clearance at the top should be no less than 1.5 mm. The back contact line must pass through the geometric center of the wafer, with symmetrical shapes on both sides of the line. 2.2.2 Inspection of bearing shell The inspection of the bearing shell requires attention to ensure that the mid-surface of the shell fits tightly together, that the positioning pins fit snugly, and that there are no gaps in the mid-surface after the bolts securing it are tightened. The oil seals on both sides of the bearing shell show no wear, and the gaps are within acceptable limits. The upper and lower mating surfaces of the oil seals fit tightly together without pressing against the bearing shell. Check the contact condition inside the bearing housing using red lead powder; at least 80% contact should be present. The left and right sides of the bearing shell are level with the midplane of the bearing housing, and the gaps on both sides are uniform in all directions, not exceeding 0.05 mm. The anti-rotation pin of the bearing shell must not be higher than the midplane of the bearing housing; after tightening the bolts at the midplane, there should be no gaps between the midplane of the bearing shell and the midplane of the bearing housing. 2.2.3 Bearing clearance measurement: The diameter of the lead wire used should be 30%–50% larger than the clearance to be measured. A feeler gauge should be used to ensure that there is no gap in the bearing housing and in the split surface of the bearing seat. Measure the thickness S of the wire at the middle of the two upper tile pieces; then the actual measured gap C is equal to 1.1S. 2.3 Measurement of gear shaft bearing preload: Remove the bolts from the bearing caps of the gear shaft, mark each bearing cap, and place them in their designated positions. Place measurement shims of 0.2–0.3 mm on each side of the gland. Insert a wire with a diameter larger than the thickness of the measurement shims between the upper bearing shell and the gland, then press the shaft bearing cover onto it and tighten the bolts so that the wire is compressed (the wire should not be too long; about 2 cm is sufficient). Finally, loosen the bolts, remove the measuring shims and wire; the difference between the two values obtained from measurement (the thickness of the shim being greater than that of the wire) represents the tension value. If the requirements are not met, the shims can be adjusted or the bearing shells can be lapped. 2.4 Thrust bearing bush maintenance 2.4.1 Inspection of bearing bushes: Visual inspection should reveal that the bearing bush blocks are free from defects such as cracks, wear, or scorching; there should be no radial grooves or scratches on their surface, and the depth of circumferential grooves should not exceed 0.1 mm. For the inspection of red lead coloring, the contact area between the thrust bearing pad and the thrust disc must be no less than 80%. The axial deviation of the thrust disc thickness shall not exceed 0.005 mm; there shall be no radial grooves or scratches on its surface, and the depth of circumferential grooves shall not exceed 0.05 mm. Inspect the front and rear oil seals of the thrust bearing bushing; there should be no wear or scratches, and the split surface should have no misalignment or gaps. The clearance between the oil seal and the shaft should not exceed 0.05–0.1 mm, while the axial clearance of the thrust bearing housing within the bearing housing should not exceed 0.01–0.03 mm. The thrust bearing clearance should be adjusted using shims on the non-working side, with no more than 1 shim being used. The cover on the bearing housing should be installed when measuring the thrust bearing clearance. 2.4.2 Scraping of thrust bearing shells: Remove the shell pieces in sequence, making sure not to mix up the order in which they are removed. The tile is scraped along its axis of rotation (the oil inlet direction) to create a wide edge with a width of 10–12 mm and a depth of about 0.50 mm. Install the lapped bearing shells back in place for a coloring inspection. The upper and lower bearing shells should be considered as a single unit to assess their condition; the affected parts need to be scraped and lapped, and then re-colored and inspected until they meet the required standards. 2.5 Maze seal maintenance: Use a hammer or copper rod with a cross-section smaller than that of the half-ring to strike one side of the half-ring, causing it to rotate and allowing the ring to be removed. After removing the half-ring, clean the ring seat, and insert the new half-ring into it. If the half-rings do not fit well and force should not be applied, scrape off the metal from areas where significant stress is applied. After assembly, the seal should be colored and polished; a layer of colored powder should be applied to the shaft journal that comes into contact with the air seal. The rotor should then be rotated gently once to observe the condition of the air seal. The defective gas seal is scraped and lapped, then repainted and inspected until it meets the requirements. 3 Conclusion In modern production, manufacturing facilities are evolving toward larger size, higher speed, greater automation, and continuous operation. Large-scale units have become key equipment in modern large-scale production systems; any shutdown due to failures can result in the suspension of entire plants, causing enormous losses. In recent years, our factory has placed great emphasis on the quality of maintenance work for the main fans. The availability rate of the main fans in the catalysis workshop has reached 100%. By improving the quality of maintenance of these units, we have ensured the safe and efficient operation of the catalysis plants, resulting in significant economic and social benefits.