Knowledge of sliding bearing and rolling bearing assembly

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http://p1.pstatp.com/large/dfic-imagehandler/bbc965cd-c4cf-427a-8682-301f88d128ec 1. Assembly of sliding bearings: Sliding bearings are a type of bearing that operates on the principle of sliding friction; they feature smooth operation, low noise levels, and the ability to handle heavy loads as well as significant impact forces. Depending on their structural design, they can be classified into integral types, split types, and pad types. (1) Assembly of integral sliding bearings: Integral sliding bearings, commonly known as bushings, represent the simplest form of sliding bearings. They are assembled using methods such as pressing and hammering; in special cases, hot fitting is employed. Most bushings are made of copper or cast iron. Careful attention should be paid during assembly, and it is possible to use a wooden hammer or a hammer with a wooden block as a pad for striking. When the interference fit tolerance is large, a press is used to insert the bearing. Whether driven in or pressed in, tilting must be prevented; after assembly, the oil grooves and oil holes should be in the required positions. Bearings that are deformed after assembly should have their inner bore repaired; those with smaller dimensions can be shaped using a reamer, while those with larger dimensions should be shaped by scraping. At the same time, care must be taken to keep the clearance between the shaft and its fitting within the specified tolerance range; to prevent the shaft sleeve from rotating during operation, positioning pins or set screws are installed on the contact surface between the shaft sleeve and the housing. Due to the difference in hardness between the box body and the shaft sleeve materials, it is easy for the drill bit to tilt toward the softer material during drilling. Solutions include: first, punching a hole on the side of the harder material using a punch before drilling; second, using a short drill bit to increase the rigidity of the drill bit during drilling. (2) Assembly of split bearings Split bearings, also known as half-bearings, feature a simple structure and are easy to adjust and disassemble. Two bearing shells are fitted onto the shaft, and shims are used at the joint to adjust for an appropriate clearance. ①During the assembly of the bearing shells and the bearing housing, good contact must be maintained between the upper and lower bearing shells and the inner holes of the bearing housing. If this requirement is not met, the inner hole of the bearing housing with thick-walled bearing shells should be used as a reference to grind the back side of the bearing shells; at the same time, the end steps of the bearing shells should be pressed tightly against the ends of the bearing housing. For thin-walled bearings, it is sufficient to raise the mid-surface of the bearing shell by about 0.1 mm above the mid-surface of the bearing housing; there is no need for any grinding or shaping. ②The bearing bush is installed in the bearing housing, and no displacement is allowed, either radially or axially. It is usually stopped and positioned using the steps at both ends of the bearing bush, or by positioning pins. ③For split-type bearing shells, matching grinding is generally carried out using corresponding bearing grinding points; usually, the lower bearing shell is first ground down and then the upper one is ground. To improve efficiency, the bearing shell and cover need not be installed while grinding the lower shell. Once the contact points of the lower bearing shell meet the requirements, the upper bearing shell and cover are pressed in place, and while grinding the upper shell, the contact points of the lower shell are further adjusted. The tightness of the shaft during scraping can be adjusted by changing the thickness of the shims as the number of scraping sessions increases. Once the bearing cover is tightened, the shaft can rotate smoothly without any noticeable clearance; once the contact points meet the requirements, the alignment is complete. ④Measurement of bearing clearance: The size of the bearing clearance can be adjusted by using shims at the mid-surface, or it can be adjusted by directly grinding the bearing shells. The bearing clearance is usually measured using the lead compression method: several pieces of wire with a diameter larger than the bearing clearance are placed on the shaft journal and the split surface, then the nut is tightened to compress the split surface; after that, the nut is unscrewed, the bearing cover is removed, and the compressed wire is carefully taken out. The thickness of each piece of wire is measured using a micrometer, and the bearing clearance can be determined based on the average thickness of the wire. The clearance for ordinary bearings should be 1.5‰–2.5‰ of the shaft diameter in millimeters; a smaller value should be used for larger diameters. For bearings with a shaft diameter of 60 mm, the bearing clearance should be between 0.09–0.15 mm. http://p3.pstatp.com/large/dfic-imagehandler/da381c16-be53-4c03-b75e-55e17b1a192b 2. Assembly of rolling bearings Rolling bearings offer advantages such as low friction, small axial dimensions, easy replacement, and simple maintenance. (1) Technical requirements for assembly: ① The end face of the rolling bearing marked with a code should be facing the visible direction to facilitate identification during replacement. ②The radius of the arc at the journal or housing hole step should be smaller than the radius of the arc at the corresponding position on the bearing. ③After the bearing is installed on the shaft and in the housing hole, there should be no skewing. ④Among the two bearings on the shaft, one must allow the shaft to move as it expands due to heat. ⑤When assembling rolling bearings, it is essential to strictly prevent dirt from entering the bearings. ⑥The assembled bearing should operate smoothly, with low noise, and its operating temperature should generally not exceed 65°C. (2) Assembly method: When assembling a bearing, the most basic requirement is that the axial force applied should act directly on the end face of the bearing rings (when the bearing is mounted on a shaft, the axial force should act directly on the inner ring; when it is mounted in a hole, the force should act directly on the outer ring). Try not to affect the rolling elements. The methods of assembly include hammering, press assembly, hot assembly, cold assembly, etc. ① The hammering method involves using a hammer together with a copper rod and some softer materials to carry out the hammering process. It is important to prevent foreign substances such as copper shavings from falling into the bearing raceways, and one should not use a hammer or punch directly on the inner and outer rings of the bearing, as this could affect the precision of the bearing’s fit or cause damage to the bearing. ②The screw press or hydraulic press assembly method can be used for bearings with large interference tolerances. Before pressing, the shaft and bearings should be leveled and coated with a little lubricant. The pressing speed should not be too fast; once the bearings are in place, the pressure should be removed promptly to prevent damage to the bearings or the shaft. ③The hot mounting method involves placing the bearing in oil and heating it to 80°C–100°C, which causes the inner bore of the bearing to expand; thereafter, the bearing is fitted onto the shaft. This approach helps to prevent damage to both the shaft and the bearing. The hot mounting method is not applicable to bearings with dust covers and seals that are already filled with lubricant inside. (3) The clearance of tapered roller bearings is adjusted after assembly, with the main methods including adjustment using shims, screws, or nuts. (4) When assembling thrust ball bearings, it is necessary to first distinguish between the tight ring and the loose ring; the inner diameter of the tight ring is slightly smaller. After assembly, the tight ring remains relatively stationary with respect to the shaft during operation, as it always rests against the step or the end face of the shaft hole. Otherwise, the bearing will lose its rolling function and experience accelerated wear.