Self-aligning ball bearings
Self-aligning ball bearings are rolling bearings with double-row balls and a spherical outer ring raceway. Their structural features include a spherical outer ring raceway, and a spherical inner ring and rolling elements that can have a certain angular offset relative to the outer ring (usually allowing an angular deviation of 1°-2.5°). These bearings can automatically compensate for coaxiality errors between the shaft and the housing, as well as shaft flexural deformation. They are widely used in textile machinery, papermaking machinery, agricultural machinery, and other applications where shaft misalignment exists, and are an indispensable general-purpose bearing type in the industrial field.
A self-aligning ball bearing consists of an inner ring, outer ring, steel balls, cage, and seals (select models). The design and material selection of each component directly impact bearing performance. The inner ring typically features a double-ribbed design with a deep-groove raceway, creating point contact with the steel balls at a low contact angle (approximately 0°-3°). It primarily carries radial loads, but can also withstand a small amount of axial load (approximately 1/3 of the radial load). The outer ring raceway is spherical, with the center of curvature aligned with the bearing center, which is crucial for self-aligning performance. The outer ring is made of high-carbon chromium bearing steel (GCr15), achieving a hardness of 60-65 HRC after quenching and tempering, and a surface roughness of Ra ≤ 0.08μm, ensuring excellent wear resistance and contact accuracy. The steel balls are made of GCr15SiMn, with a diameter tolerance of G5 and a roundness of ≤ 0.001mm. The surface is super-ground to a roughness of Ra ≤ 0.02μm to reduce rolling friction. The cage is divided into stamped steel cage (20 steel) and solid brass cage (H62). The stamped cage is suitable for low-speed occasions (≤3000r/min), and the brass cage is suitable for high-speed occasions (≤6000r/min). The function of the cage is to evenly separate the steel balls to prevent collision and friction.
Self-aligning ball bearings can be divided into open bearings (unsealable), bearings with double-sided dust covers (2RS), and bearings with double-sided seals (2RZ). Open bearings have good heat dissipation and are suitable for high-temperature applications (≤120°C), but require additional lubrication and sealing devices. 2RS bearings use nitrile rubber seals, which provide excellent sealing performance and prevent lubricant leakage and contaminant intrusion. They are suitable for humid or dusty environments and have an operating temperature range of -40°C to 100°C. 2RZ bearings use steel-framed rubber seals, which have slightly lower sealing performance than 2RS bearings but allow for higher operating temperatures (-40°C to 120°C). According to the size series, they can be divided into 12 series (light and narrow series), 13 series (light and wide series), 22 series (medium and narrow series), and 23 series (medium and wide series). For example, 1205 bearings (inner diameter 25mm, outer diameter 52mm, width 15mm) belong to the light and narrow series and are suitable for light loads; 2310 bearings (inner diameter 50mm, outer diameter 110mm, width 40mm) belong to the medium and wide series and are suitable for medium and heavy loads.
The performance parameters of self-aligning ball bearings primarily include dynamic load rating, static load rating, limiting speed, and radial clearance. These parameters are crucial for bearing selection. The dynamic load rating refers to the maximum radial load a bearing can withstand during its rated life (106 revolutions). The dynamic load rating of 23 series bearings is 50%-80% higher than that of 12 series bearings. For example, the dynamic load rating of a 2310 bearing is 55.8 kN, while that of a 1210 bearing is 22.5 kN. The static load rating refers to the maximum radial load a bearing can withstand when stationary or at low speed, typically 1.2-1.8 times the dynamic load rating. The limiting speed is affected by the cage type and lubrication method. The limiting speed of grease-lubricated bearings with stamped cages is 20%-30% lower than that of bearings with brass cages. For example, the limiting speed of a 1308 bearing (stamped cage) with grease lubrication is 6,000 r/min and 8,000 r/min with oil lubrication. For brass cages, the respective limits are 7,500 r/min and 10,000 r/min. Radial clearance is divided into C2 (small clearance), C0 (normal clearance), C3 (large clearance), and C4 (extra large clearance). The clearance selection needs to be determined according to the operating temperature and installation interference. C3 or C4 clearance should be used in high temperature applications (>80℃) to compensate for thermal expansion; a larger clearance should be used for tight fit installation to avoid heat caused by too small clearance.
The installation and maintenance of self-aligning ball bearings are crucial to their service life. During installation, pay attention to the bearing’s self-aligning properties. The coaxiality error between the shaft and the bearing housing must not exceed the allowable angular deviation of the bearing. Failure to do so will reduce the self-aligning effect and increase bearing wear. When installing the inner ring, use a press to evenly press it with a sleeve, or heat the inner ring (80-100°C) for shrinkage, avoiding direct impact. When installing the outer ring, a transition fit (H7/js6) is typically used between the outer ring and the bearing housing bore. Excessive pressure is not required to ensure that the outer ring can fine-tune with the inner ring’s deflection. Lubrication is selected based on the speed. For low speeds (≤3000 rpm), grease lubrication is used. Use lithium-based grease (NLGI grade 2), filling 1/3-1/2 of the bearing’s internal space. For high speeds, oil lubrication is used. Use an ISO VG32-68 viscosity grade lubricant, supplied via splash lubrication or drip lubrication.
Fault diagnosis and life management of self-aligning ball bearings are key to ensuring reliable operation of equipment. Common faults include wear, rust, cage damage and steel ball peeling. Wear will cause increased vibration and noise, which can be monitored by a vibration detector (vibration speed ≤1.8mm/s); rust is mostly caused by poor lubrication or humid environment, and the lubricant status and sealing performance need to be checked regularly; cage damage is usually caused by improper installation or excessive speed, which manifests as abnormal noise and requires immediate shutdown and replacement. The expected life calculation formula for bearings is L10= (C/P)³×106 revolutions (where C is the rated dynamic load and P is the actual load). Under normal maintenance conditions, the service life of self-aligning ball bearings can reach 10,000-30,000 hours. When the bearing runs to 70% of its expected life, preventive replacement is required to avoid sudden failure and equipment failure. Through correct installation