1. Rolling track sound
Raceway noise is a smooth and continuous noise generated by the rolling elements in the raceway when the bearing rotates. It only attracts attention when its sound pressure level or pitch is extremely high. In fact, the acoustic energy generated by raceway noise is limited. For example, under normal conditions, the raceway noise of a high-quality 6203 bearing is 25–27 dB. This noise is most typical in single-row deep groove ball bearings that bear radial loads, and it has the following characteristics: a. Noise and vibration are random; b. Vibration frequency is above 1 kHz; c. Regardless of the rotational speed, the dominant noise frequency remains almost constant, while the sound pressure level increases with increasing speed; d. When the radial clearance increases, the sound pressure level increases sharply; e. The higher the rigidity of the bearing housing, the lower the overall sound pressure level; even with increased speed, the overall sound pressure level does not increase significantly; f. The higher the viscosity of the lubricant, the lower the sound pressure level, but for grease lubrication, its viscosity and the shape and size of the soap fibers can affect the noise level.
Raceway noise originates from the inherent vibration of the raceways under load. The elastic contact between the raceways and rolling elements constitutes a nonlinear vibration system. When lubrication or machining precision is insufficient, inherent vibrations related to this elastic characteristic are excited, which are transmitted into the air and become noise. It is well known that even with the most advanced manufacturing techniques, bearing parts will always have slight geometric errors of varying degrees on their working surfaces, causing minute fluctuations between the raceways and rolling elements that excite the inherent vibration of the system. Although this is unavoidable, noise and vibration can be reduced by using high-precision machining of the working surfaces of the parts, proper selection of bearings, and precise use of bearings.
2. The sound of a falling object rolling
This noise typically occurs in large bearings subjected to radial loads at low speeds. When a bearing operates under radial load, the rolling elements in the unloaded area do not contact the inner raceway if the bearing has a certain radial clearance. However, due to centrifugal force, they may contact the outer ring. Therefore, at low speeds, when the centrifugal force is less than the weight of the rolling elements, the rolling elements fall and collide with the inner raceway or cage, exciting the bearing's inherent vibration and noise. This noise has the following characteristics: a. It is easily generated with grease lubrication but less so with oil lubrication. It is more likely to occur when using inferior grease. b. It often occurs in winter. c. It is also easily generated when only radial loads are applied and the radial clearance is large. d. It can also occur within a specific range, and the speed range varies for bearings of different sizes. e. It can be continuous or intermittent. f. This forced vibration often excites the second and third order bending natural vibrations of the outer ring, thus producing this noise. This noise can be effectively reduced by using preloading methods, reducing the radial clearance of the bearing after installation, and using a good lubricant can also improve the situation. Some foreign companies use lightweight rolling elements, such as ceramic rollers or hollow rollers, to prevent this noise from being generated.
3. Sharp whistling sound
This is a rather loud screeching sound generated by sliding friction between metals. Although the bearing temperature rise is not high at this time, and it has little impact on the bearing life and grease life, nor does it affect rotation, the unpleasant sound is disturbing. This noise is particularly common in large, short cylindrical roller bearings subjected to radial loads. Its characteristics are: a. It is easily generated when the bearing radial clearance is large. b. It usually occurs in grease-lubricated bearings, and is less common in oil-lubricated bearings. c. It decreases as the bearing size increases, and often occurs within a certain speed range. d. It often occurs in winter. e. Its occurrence is irregular and unpredictable, and is related to the amount and performance of the grease, as well as installation and operating conditions. This noise can be prevented by reducing the bearing radial clearance and using a shallow outer ring raceway structure.
