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Small and medium-sized asynchronous motors generally use rolling bearings, as they are easy to install and simple to maintain. However, improper selection can also lead to serious accidents. (1) Rolling bearing failures: Bearing failures are mainly manifested as overheating and abnormal noises. 1. Overheating: During operation, if the bearing temperature of an electric motor exceeds 95°C, it indicates that the bearing has experienced an overheating problem. The main reasons for this include improper bearing installation – where the upper part of the end cover is not properly fitted, resulting in misalignment of the bearing holes at both ends – or damaged bearings, which can cause the balls (or rollers) to make abnormal contact with the inner and outer rings, thereby increasing frictional losses and leading to overheating. Another reason could be too loose or too tight a fit between the inner ring of the bearing and the shaft journal, or between the outer ring of the bearing and the bearing hole in the end cover. Furthermore, eccentricity of the inner circle of the bearing cover or improper assembly can also increase the frictional losses of the bearing; thirdly, poor quality of the lubricating grease or an excessive amount of it used can cause problems as well. 2. Abnormal noises: A properly functioning bearing should produce a uniform sound level during operation. Common abnormal noises and their causes are as follows: ① Obvious rolling and vibrating sounds indicate that the bearing clearance is too large; ② A muffled sound with a deep tone is caused by impurities in the lubricant; ③ Irregular knocking sounds suggest that some of the balls have cracked or fallen out; ④ A whistling-like screeching sound accompanied by rolling noises indicates severe lack of oil in the bearing. When a rolling bearing fails, corresponding measures can usually be taken based on the cause to resolve the issue. If the bearing has an excessive clearance or is already damaged, it generally cannot be repaired and must be replaced with a new, identical, qualified unit. The general bearing model is printed on the end face of the bearing. Tables 10-11 and 10-12 list the bearing models used in J2, JO2, and Y series motors for reference. Table 10-11: Rolling bearing models for J2 and JO2 series motors – Frame numbers, synchronous speed of 3000 r/min, synchronous speed of 1500 r/min or less, shaft-end side, non-shaft-end side. 1 2 3 4 5 6 7 8 9 204 205 206 208 309 310 311 314 317 204 205 206 208 309 310 311 314 317 204 305 306 308 309 2310 2311 2314 2317 204 305 306 308 309 310 311 314 317. Table 10-12: Rolling bearing models and allowable vibration values for Y series motors – Center height (mm), number of poles: Y(IP23), Y(IP44); drive end, non-drive end. Model, vibration value (m/s); Model, vibration value (m/s)