The temperature of the water pump shaft is very high; please analyze the reasons

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By June 23, 2020, the inner ring of the bearing on the drive side of this water pump had burned out (the area marked in red); the worn parts were rewelded and machined, and a new bearing was installed. After that, the temperature in that area remained high, so a radiator was added outside the bearing end cover. After one year of operation, on August 13, 2021, it was found that the bearing temperature was as high as 70 degrees, and the shaft temperature at the bearing area of the drive section was as high as 90 degrees; the machine was stopped immediately to investigate the cause. Timely action was taken: the locking nuts outside the bearing housing were tightened (as there were signs of looseness), and the grease in the bearing housing was cleaned and replaced. Five hours after the machine was started up, the bearing temperature was 48 degrees, while the highest temperature at the corresponding shaft area was 73 degrees. Currently, when the pump operates at a low frequency of 45 Hz, the bearing temperature is 50°C, while the temperature of the shaft in that area is 75°C. The higher the rotation speed, the faster the shaft temperature rises. At present, the source of heat on the shaft has not been identified, and the shaft temperature is even higher than that of the bearings; it’s now difficult to determine where the problem lies. The distance between the motor and the pump’s coupling is currently 1 mm, and it is intended to adjust their concentricity. Since the motor is quite large, what are some good methods for adjusting their positions smoothly? I need the experts to help analyze the reason.