Identifying bearings by sound

Thread Content

Identifying bearings by sound – Noise of rolling bearings. Depending on their size and operating speed, rolling bearings have their own distinct sounds. Being able to identify the sources of the following noises is very helpful for predicting bearing damage in advance. 1. Raceway noise: Raceway noise is a steady and continuous sound generated as the rolling elements roll within the raceways while the bearing is rotating; it only attracts attention when its sound pressure level or pitch becomes very high. In fact, the acoustic energy generated by the raceway noise is limited; under normal conditions, the raceway noise of high-quality 6203 bearings ranges from 25 to 27 dB. This type of noise is most typical in single-row deep groove ball bearings that are designed to withstand radial loads, and it has the following characteristics: a. The noise and vibration are random in nature ； b. Vibration frequency above 1kHz ； c. Regardless of how the speed changes, the main frequency of the noise remains almost constant, while the sound pressure level increases as the speed rises ； d. As the radial clearance increases, the sound pressure level rises sharply ； e. As the stiffness of the bearing housing increases, the overall sound pressure level decreases; even when the rotation speed rises, the increase in the overall sound pressure level is minimal ； f. The higher the viscosity of the lubricant, the lower the sound pressure level; however, in the case of grease lubrication, both its viscosity and the shape and size of the soap fibers can affect the noise level. The source of the raceway noise is the natural vibration of the ring under load. The elastic contact between the rings and the rolling elements constitutes a nonlinear vibration system. When the lubrication or machining precision is low, natural vibrations associated with this elastic property are induced, which transform into noise when transmitted into the air. As is well known, even when bearing components are manufactured using the most advanced modern manufacturing techniques, their working surfaces always possess minor geometric errors to varying degrees, which cause slight fluctuations between the raceways and the rolling elements and thereby trigger the natural vibrations of the vibration system. Although it is inevitable, it is possible to machine the working surface of the parts with high precision, select bearings appropriately, and use them precisely to reduce noise and vibration. 7. Rolling sound of falling objects: This type of noise generally occurs in large bearings that operate at low speeds and are subjected to radial loads. When the bearing operates under radial loads, there are load-bearing areas and non-load-bearing areas within the bearing. If the bearing has a certain amount of radial play, the rolling elements in the non-load-bearing areas do not come into contact with the inner race; however, due to centrifugal force, they may come into contact with the outer ring. Therefore, at low speeds, when the centrifugal force is less than the weight of the rolling elements, these elements fall and collide with the inner race or the cage, thereby inducing the bearing’s natural vibrations and noise. This phenomenon is characterized by the following: a. It occurs more easily under grease lubrication and less easily under oil lubrication. It occurs more easily when inferior grease is used. b. It often occurs in winter. c. It also occurs easily when only radial loads are applied and the radial clearance is large. d. They are also generated within a certain range, and bearings of different sizes have different speed ranges as well. e. It can be a continuous sound or a discontinuous sound. f. This forced vibration often excites the second and third order bending natural vibrations of the outer ring, thereby generating this noise. This noise can be effectively reduced by using a preload method, which reduces the radial play of the bearings after installation; choosing good lubricants can also help to alleviate this issue. Some foreign companies employ technical solutions such as lightweight rolling elements like ceramic rollers or hollow rollers to prevent the occurrence of such noise. 3. High-pitched screeching sound: This is a rather intense screeching noise resulting from sliding friction between metals. Although the bearing temperature does not rise much at this time, it has little impact on the bearing’s lifespan or that of the lubricant, nor does it affect rotation. However, this unpleasant sound is disturbing. Such noise is common in large short cylindrical roller bearings that are subjected to radial loads. Its characteristics are as follows: a. It tends to occur when the bearing has a large radial clearance. b. It usually occurs in grease lubrication, while it is less common in oil lubrication. 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 it is related to the amount of filler used, the performance, as well as the installation and operating conditions. This type of noise can be prevented by reducing the radial play of the bearings and using a shallow outer ring raceway design. 4. Retainer noise: This noise is generated by the free vibration of the retainer during the rotation of the bearing, as well as by its collisions with the rolling elements or rings. It can occur in various types of bearings, but its sound pressure level is not very high and it is at a low frequency. Its features are: a. Both stamped retainers and plastic retainers can be produced. b. It occurs regardless of whether it is lubricated with thin oil or grease. c. It occurs most easily when the outer ring is subjected to bending moments. d. It is prone to occur when the radial clearance is large. Since gaps in the retainer holes as well as gaps between the retainer and the rings are inevitable in finished bearings, it is very difficult to completely eliminate retainer noise; however, this can be improved by reducing assembly errors, selecting appropriate gaps, and controlling the amount of movement of the retainer. Another type of abnormal noise generated by the cage is the humming sound caused by the self-excited vibration of the cage due to friction between the cage and the guiding surfaces of other bearing components. The stamping retainer of deep groove ball bearings is thin, resulting in low bending stiffness in both the radial and axial directions; this leads to poor overall stability. When the bearing rotates at high speeds, bending deformation causes self-excited vibrations, giving rise to a \"buzzing\" sound. When a bearing is under radial load and the grease performance is poor, a \"clicking\" noise can be heard at the beginning of operation. This noise is primarily caused by the rolling elements accelerating suddenly after leaving the area under load, resulting in collisions with the cage; such collision noises are inevitable but will disappear after the bearing has been running for some time. Measures to prevent cage noise are as follows: a. To stabilize the orbital motion of the cage, it is advisable to use ring guidance as much as possible and to ensure adequate lubrication of the guiding surfaces. The design of tapered roller bearings for high-speed operation should be improved, by replacing the L-shaped cages that use roller guidance with Z-shaped cages that utilize ring flanges for guidance. When the bearing is rotating at high speeds, the vibration amplitude of the retainer in bearings with a larger clearance between the pockets is much greater than that in bearings with a smaller such clearance; therefore, it is particularly important to choose an appropriate value for this clearance. 9 Be careful to minimize the radial clearance as much as possible. d. Striving to improve the manufacturing precision of the retainer price and enhancing the surface quality of the retainer helps to reduce the noise generated by collisions or friction between the rolling elements and the retainer. e. Actively adopt advanced cleaning technologies to effectively and thoroughly clean the spare parts as well as the products after assembly, thereby improving the cleanliness of the bearings. 5. Vibration of the rolling elements: When a bearing operates under radial loads, only a few rolling elements inside it bear these loads. The elastic contact with the rings creates a \"spring\"-like support that causes the rolling elements to vibrate periodically along the line of action of the radial loads; as a result, the center of the rotating shaft moves up and down or horizontally, which in turn generates noise. This type of vibration is known as vibration caused by the rolling elements, and it is more apparent, especially during low-speed operation. Its amplitude is related to the bearing type, radial load, radial clearance, and the number of rolling elements. Typically, this amplitude is small; it only becomes harmful when the amplitude is large. To reduce this, radial clearance is often reduced or an appropriate preload is applied.