Bearings are essential components in modern machinery. Their main function is to support rotating mechanical parts, reduce the coefficient of friction during movement , and ensure rotational accuracy . Depending on their size and operating speed, rolling bearings produce unique sounds. Understanding the causes of these sounds can greatly help in early detection of bearing damage.
01. Rolling track sound
Raceway noise is a smooth and continuous noise generated when the rolling elements in the raceway of a bearing rotate. It only attracts attention when its sound pressure level or pitch is extremely high. In fact, the sound energy generated by raceway noise is limited. Under normal conditions, the raceway noise of a high-quality 6203 bearing is 25–27 dB . This type of noise is most typical in single-row deep groove ball bearings that bear radial loads.
Raceway noise originates from the inherent vibrations of the inner and outer rings under load. The elastic contact between the inner and outer rings and the rolling elements constitutes a nonlinear vibration system. When lubrication or machining precision is low, inherent vibrations related to these elastic characteristics are excited, which are transmitted into the air and become noise. It is well known that even with the most advanced manufacturing technology, bearing parts will always have slight geometric errors of varying degrees on their working surfaces, causing minute fluctuations between the raceway and rolling elements that excite the inherent vibrations 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.
Features
Noise and vibration are random.
• Vibration frequency above 1kHz ;
Regardless of the rotational speed, the dominant noise frequency remains almost constant, while the sound pressure level increases with the rotational speed.
• As the radial clearance increases, the sound pressure level increases sharply;
The greater the rigidity of the bearing housing, the lower the overall sound pressure level; even if the rotational speed increases, the overall sound pressure level does not increase significantly.
• The higher the viscosity of the lubricant, the lower the sound pressure level. However, for grease lubrication, the viscosity and the shape and size of the soap fibers can both affect the noise level.
02. Sound of falling and rolling
This noise typically occurs at low speeds in large bearings subjected to radial loads. When a bearing operates under radial load, it is divided into a load zone and a non-load zone. If the bearing has a certain radial clearance, the rolling elements in the non-load zone do not contact the inner raceway, but may contact the outer ring due to centrifugal force. Therefore, at low speeds, when the centrifugal force is less than the weight of the rolling elements, the rolling elements will fall and collide with the inner raceway or cage, exciting the bearing's inherent vibration and noise.
Features
It is more likely to occur with grease lubrication and less likely to occur with oil lubrication. It is more likely to occur when using inferior grease.
This often happens in winter;
• This can also easily occur when only radial loads are applied and the radial clearance is large;
• It can also occur within a specific range, and the speed range varies depending on the bearing size;
It could be a continuous sound or a discontinuous sound;
This forced vibration often excites the second and third order bending natural vibrations of the outer ring, thus generating this noise. Preloading can effectively reduce this noise, as can reducing the radial clearance of the bearing after installation. 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.
03. Sharp whistling sound
It originates from 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 sound is sharp and unsettling. This noise is common in large, short cylindrical roller bearings subjected to radial loads. This noise can be prevented by reducing the bearing's radial clearance and using a shallow outer ring raceway structure.
Features
• This is easily caused when the radial clearance of the bearing is large;
It usually occurs in grease lubrication, but is less common in oil lubrication;
• It decreases as the bearing size increases, and often occurs within a certain speed range;
It often occurs in winter;
Its occurrence is irregular and unpredictable, and is related to the amount and performance of the filling agent, as well as the installation and operating conditions.
04. Maintain rack sound
This noise is generated during the rotation of the bearing by the free vibration of the cage and its impact with the rolling elements or raceways. It can occur in all types of bearings, but its sound pressure level is not very high and it is at a low frequency.
Since cage pocket clearance and cage-ring clearance are unavoidable in finished bearings, it is very difficult to completely eliminate cage noise. However, it can be improved by reducing assembly errors and optimizing appropriate clearances and cage movement.
Another type of cage noise is the humming sound caused by the self-excited vibration of the cage due to friction between the cage and the guide surfaces of other bearing components. The stamped cage of deep groove ball bearings is relatively thin, with low bending stiffness in the radial and axial planes and poor overall stability. When the bearing rotates at high speed, it will generate self-excited vibration due to bending deformation, causing a " buzzing sound " .
When a bearing is under radial load and the grease performance is poor, a " clicking " noise will be heard in the early stages of operation. This is mainly due to the rolling elements suddenly accelerating and colliding with the cage after leaving the load area. This impact noise is unavoidable, but it will disappear after running for a period of time.
Features
Both stamped and plastic cages can be produced;
This can occur regardless of whether it is lubricated with thin oil or grease;
• This is most likely to occur when the outer ring is subjected to bending moment;
• This is more likely to occur when the radial clearance is large.
Measures to prevent cage noise
• To ensure stable orbital motion of the cage, ring-guided bearings should be used as much as possible, and sufficient lubrication should be provided to the guide surfaces. The structure of tapered roller bearings under high-speed conditions should be improved by replacing the roller-guided L -type cage with a ring-side flange-guided Z -type cage.
• When a bearing rotates at high speed, the cage vibration amplitude of a bearing with a larger pocket clearance is much greater than that of a bearing with a smaller pocket clearance, so the value of the pocket clearance is particularly important.
Minimize radial clearance as much as possible;
• Maximizing the manufacturing precision of the cage and improving its surface quality helps reduce noise generated by collisions or friction between the rolling elements and the cage;
• Actively adopt advanced cleaning technologies to effectively and thoroughly clean parts and assembled products, thereby improving the cleanliness of bearings.
05. Noise caused by the vibration of rolling elements
When a bearing operates under radial load, only a few rolling elements bear the load. The " spring " support formed by the elastic contact between the rolling elements and the bearing races causes the rolling elements to vibrate periodically as they pass through the radial load line. This causes the bearing center to move vertically up and down or horizontally, generating noise. This type of vibration is called rolling element vibration, and it is more pronounced at low speeds.
The amplitude of this vibration is related to the bearing type, radial load, radial clearance, and number of rolling elements. It is usually small, and only becomes harmful when the amplitude is large. Therefore, reducing radial clearance or applying appropriate preload is often used to reduce it.
