How to Judge Bearing Abnormality by Sound
Daily inspection work adopts the Five-Sense Inspection Method to monitor equipment operating conditions and identify and eliminate faults in a timely manner. Generally, the five senses are applied to inspect rolling noise, temperature, vibration, lubrication status and other items of rotating bearings.
The "Five Senses"
They refer to human senses of sight, hearing, smell, touch and taste.
Five-Sense Inspection
It is an inspection method adopted by operators, inspectors and maintenance staff before, during or after equipment operation. With the aid of simple auxiliary tools such as sound listening rods, inspection hammers and thermometers, inspectors check and monitor equipment indicators including pressure, temperature, flow rate, leakage, grease feeding status, abnormal noise, vibration, cracks, wear and looseness on specified positions according to scheduled cycles, relying on visual observation, listening, smelling, touching and tasting.
1. Methods of Five-Sense Inspection
(1) Visual Inspection
Statistics show that 60% of human actions start with visual perception. Visual inspection has a wide range of applications and can be adopted for almost all inspections.
For instance, loose bearing housings can be detected via cracked coating on bearing housings and offset alignment marks between upper and lower bearing shells; changes in oil level and oil color reflect whether the lubrication system operates normally; burnt marks, discoloration, peeling, foreign matters, dust and cracks help discover internal defects of electrical components.
Visual inspection must be carried out carefully. When checking electrical cabinets, inspect not only the panel but also open the cabinet door to observe from all angles. For motor inspection, check the housing, and open the end cover to observe commutation status and spark grade.
(2) Auditory Inspection
Humans are quite sensitive to sound stimuli. The auditory method mainly distinguishes abnormal noise from normal sound.
Mechanical impact, bearing damage, eccentricity, sharp acceleration and deceleration of high-speed rotating machinery will generate multiple vibration sources, loosen various components and produce abnormal impact noise or irregular sound.
The operating sound of relays, contactors, rotating motors and electromagnetic hum of transformers all feature unique normal tones. Familiarity with normal sound makes it easy to detect abnormalities.
In addition, static components such as circuit boards and connectors normally produce no sound. If crackling noise is heard, attention should be paid; this is usually caused by poor contact, loose screws or connectors, or excessive current.
Once abnormal noise is detected, combine it with other sensory judgment or use tools such as sound listening rods to locate the abnormal position.
(3) Olfactory Inspection
Smell plays an important auxiliary role in five-sense inspection. Strange odors indicate definite equipment abnormality.
The olfactory method mainly detects abnormal odor caused by burning. Normally, common electrical equipment has no peculiar smell, so unusual odor can be easily identified. It may be caused by inter-turn short circuit of relay or motor coils, or aging and burnout of insulation. Such failures are highly likely to trigger fire accidents, so abnormal points must be located rapidly, and shutdown treatment shall be implemented when necessary.
(4) Tactile Inspection
Tactile sense is closely associated with vision and hearing. Touch is mainly used to check temperature, vibration and contamination.
Excessively high temperature accelerates insulation deterioration, shortens insulation service life, and easily causes electric shock and burnout accidents. It also notably degrades the performance of electronic circuits.
(5) Taste Inspection
The taste method is rarely used in five-sense inspection. Even in special scenarios requiring rapid identification of acidity or alkalinity, it must be adopted cautiously under the premise of guaranteeing personal safety.
Key Points for Judging Rolling Bearing Abnormality by Sound (Five-Sense Inspection)
Rolling bearings produce distinctive inherent sound depending on their size and operating speed. Distinguishing the causes of various noises greatly facilitates early judgment of bearing damage.
1. Raceway Noise
Raceway noise is a stable and continuous noise excited by rolling elements rolling along raceways during bearing operation. It only draws attention when the sound pressure level or tone is excessively high. In fact, raceway noise generates limited acoustic energy. Under normal conditions, the raceway noise of high-quality 6203 bearing is 25~27 dB. This noise is most typical for single-row deep groove ball bearings subjected to radial load.
Characteristics:
Noise and vibration are random;
Vibration frequency exceeds 1 kHz;
The main noise frequency remains nearly unchanged regardless of rotating speed, while the sound pressure level rises with increasing speed;
The sound pressure level rises sharply when radial internal clearance increases;
Higher bearing housing rigidity leads to lower overall sound pressure level, and the total sound pressure level rises slightly even with speed increase;
Higher lubricant viscosity reduces sound pressure level. For grease lubrication, viscosity and shape/size of soap fibers affect noise magnitude.
Raceway noise originates from inherent vibration of rings under load. Elastic contact between rings and rolling elements forms a nonlinear vibration system. Poor lubrication or insufficient machining accuracy excites inherent vibration related to elastic characteristics, which propagates into air as noise.
It is well known that even with state-of-the-art manufacturing technology, tiny geometric errors inevitably exist on working surfaces of bearing parts, causing slight fluctuation between raceways and rolling elements and exciting inherent vibration of the vibration system. Although unavoidable, noise and vibration can be reduced by high-precision machining of component surfaces, proper bearing selection and standardized bearing application.
2. Rolling Element Impact Noise
This noise mostly occurs in large-size bearings operating at low speed under radial load. When a bearing runs under radial load, load zone and non-load zone form inside the bearing. If the bearing has a certain radial internal clearance, rolling elements in the non-load zone do not contact the inner raceway, but may contact the outer ring under centrifugal force.
At low rotating speed, when centrifugal force is smaller than the self-weight of rolling elements, rolling elements drop and collide with inner raceways or cages, exciting inherent vibration and noise of the bearing.
Characteristics:
Likely to occur under grease lubrication, rare under oil lubrication; more frequent with inferior grease;
Frequently encountered in winter;
Easy to generate when only radial load is applied and radial internal clearance is large;
Occurs within a specific speed range, which varies for bearings of different sizes;
Can be continuous or intermittent noise.
The forced vibration often excites the second-order and third-order bending inherent vibration of the outer ring and generates such noise.
Applying preload can effectively reduce this noise by lowering working radial internal clearance after installation. Selecting suitable lubricants also brings improvement. Some foreign manufacturers adopt lightweight rolling elements such as ceramic rollers or hollow rollers to prevent this noise.
3. Squeal
It is intense screaming generated by sliding friction between metal surfaces. Although bearing temperature rise is insignificant, it barely affects the service life of bearings and grease and does not hinder rotation. However, the unpleasant noise causes disturbance. Large short cylindrical roller bearings under radial load often produce such noise.
Characteristics:
Easy to generate when bearing radial internal clearance is large;
Usually occurs with grease lubrication and rarely with oil lubrication;
Weakens as bearing size increases and often appears within a certain speed range;
Frequently occurs in winter;
Random and unpredictable, related to grease filling volume, grease performance, installation and operating conditions.
This noise can be prevented by reducing bearing radial internal clearance and adopting outer rings with shallow raceway structure.
4. Cage Noise
This noise is generated by free vibration of the cage and collision between the cage and rolling elements or rings during bearing rotation. It may occur in all types of bearings with low sound pressure level and low frequency.
Characteristics:
Produced by both stamped cages and plastic cages;
Occurs with both oil lubrication and grease lubrication;
Most likely to generate when the outer ring bears bending moment;
Easy to occur with large radial internal clearance.
Complete elimination of cage noise is extremely difficult because pocket clearance and the clearance between cage and ring are unavoidable for finished bearings. However, improvement can be achieved by minimizing assembly errors and optimizing clearance and cage axial play.
A special type of cage noise is roaring noise triggered by self-excited vibration of the cage due to friction between the cage and guiding surfaces of other bearing components. Stamped cages of deep groove ball bearings are thin, featuring low bending stiffness in radial and axial planes and poor overall stability. High-speed rotation causes bending deformation and self-excited vibration, generating a humming sound.
When a bearing operates under radial load with poor grease performance, clicking noise can be heard at the initial operation stage. This is mainly caused by sudden acceleration of rolling elements after leaving the load zone and collision with the cage. Such collision noise is unavoidable but will disappear after a period of operation.
Measures to prevent cage noise:
Adopt ring-guided design to ensure stable revolution of the cage, and guarantee sufficient lubrication on guiding surfaces. Improve the structure of tapered roller bearings under high-speed conditions, replacing roller-guided L-type cages with ring rib-guided Z-type cages.
During high-speed operation, bearings with large pocket clearance produce far greater cage vibration amplitude than those with small pocket clearance. Therefore, pocket clearance selection is critical. Minimize radial internal clearance as much as possible. Improve cage manufacturing precision and surface quality to reduce noise caused by collision or friction between rolling elements and cages.
Deploy advanced cleaning technology to thoroughly clean spare parts and assembled bearings and improve bearing cleanliness.
When a bearing operates under radial load, only several rolling elements bear internal load. Elastic contact with rings forms a spring support, generating periodic vibration when rolling elements pass the radial load line. Consequently, the rotating shaft center moves vertically or horizontally and triggers noise.
Such vibration is defined as rolling element passing vibration, which is more obvious at low rotating speed. Amplitude is related to bearing type, radial load, radial internal clearance and the number of rolling elements. Amplitude is generally small; hazards only arise when amplitude increases. Therefore, reducing radial internal clearance or applying appropriate preload is commonly adopted for mitigation.
The "Five Senses"
They refer to human senses of sight, hearing, smell, touch and taste.
Five-Sense Inspection
It is an inspection method adopted by operators, inspectors and maintenance staff before, during or after equipment operation. With the aid of simple auxiliary tools such as sound listening rods, inspection hammers and thermometers, inspectors check and monitor equipment indicators including pressure, temperature, flow rate, leakage, grease feeding status, abnormal noise, vibration, cracks, wear and looseness on specified positions according to scheduled cycles, relying on visual observation, listening, smelling, touching and tasting.
1. Methods of Five-Sense Inspection
(1) Visual Inspection
Statistics show that 60% of human actions start with visual perception. Visual inspection has a wide range of applications and can be adopted for almost all inspections.
For instance, loose bearing housings can be detected via cracked coating on bearing housings and offset alignment marks between upper and lower bearing shells; changes in oil level and oil color reflect whether the lubrication system operates normally; burnt marks, discoloration, peeling, foreign matters, dust and cracks help discover internal defects of electrical components.
Visual inspection must be carried out carefully. When checking electrical cabinets, inspect not only the panel but also open the cabinet door to observe from all angles. For motor inspection, check the housing, and open the end cover to observe commutation status and spark grade.
(2) Auditory Inspection
Humans are quite sensitive to sound stimuli. The auditory method mainly distinguishes abnormal noise from normal sound.
Mechanical impact, bearing damage, eccentricity, sharp acceleration and deceleration of high-speed rotating machinery will generate multiple vibration sources, loosen various components and produce abnormal impact noise or irregular sound.
The operating sound of relays, contactors, rotating motors and electromagnetic hum of transformers all feature unique normal tones. Familiarity with normal sound makes it easy to detect abnormalities.
In addition, static components such as circuit boards and connectors normally produce no sound. If crackling noise is heard, attention should be paid; this is usually caused by poor contact, loose screws or connectors, or excessive current.
Once abnormal noise is detected, combine it with other sensory judgment or use tools such as sound listening rods to locate the abnormal position.
(3) Olfactory Inspection
Smell plays an important auxiliary role in five-sense inspection. Strange odors indicate definite equipment abnormality.
The olfactory method mainly detects abnormal odor caused by burning. Normally, common electrical equipment has no peculiar smell, so unusual odor can be easily identified. It may be caused by inter-turn short circuit of relay or motor coils, or aging and burnout of insulation. Such failures are highly likely to trigger fire accidents, so abnormal points must be located rapidly, and shutdown treatment shall be implemented when necessary.
(4) Tactile Inspection
Tactile sense is closely associated with vision and hearing. Touch is mainly used to check temperature, vibration and contamination.
Excessively high temperature accelerates insulation deterioration, shortens insulation service life, and easily causes electric shock and burnout accidents. It also notably degrades the performance of electronic circuits.
(5) Taste Inspection
The taste method is rarely used in five-sense inspection. Even in special scenarios requiring rapid identification of acidity or alkalinity, it must be adopted cautiously under the premise of guaranteeing personal safety.
Key Points for Judging Rolling Bearing Abnormality by Sound (Five-Sense Inspection)
Rolling bearings produce distinctive inherent sound depending on their size and operating speed. Distinguishing the causes of various noises greatly facilitates early judgment of bearing damage.
1. Raceway Noise
Raceway noise is a stable and continuous noise excited by rolling elements rolling along raceways during bearing operation. It only draws attention when the sound pressure level or tone is excessively high. In fact, raceway noise generates limited acoustic energy. Under normal conditions, the raceway noise of high-quality 6203 bearing is 25~27 dB. This noise is most typical for single-row deep groove ball bearings subjected to radial load.
Characteristics:
Noise and vibration are random;
Vibration frequency exceeds 1 kHz;
The main noise frequency remains nearly unchanged regardless of rotating speed, while the sound pressure level rises with increasing speed;
The sound pressure level rises sharply when radial internal clearance increases;
Higher bearing housing rigidity leads to lower overall sound pressure level, and the total sound pressure level rises slightly even with speed increase;
Higher lubricant viscosity reduces sound pressure level. For grease lubrication, viscosity and shape/size of soap fibers affect noise magnitude.
Raceway noise originates from inherent vibration of rings under load. Elastic contact between rings and rolling elements forms a nonlinear vibration system. Poor lubrication or insufficient machining accuracy excites inherent vibration related to elastic characteristics, which propagates into air as noise.
It is well known that even with state-of-the-art manufacturing technology, tiny geometric errors inevitably exist on working surfaces of bearing parts, causing slight fluctuation between raceways and rolling elements and exciting inherent vibration of the vibration system. Although unavoidable, noise and vibration can be reduced by high-precision machining of component surfaces, proper bearing selection and standardized bearing application.
2. Rolling Element Impact Noise
This noise mostly occurs in large-size bearings operating at low speed under radial load. When a bearing runs under radial load, load zone and non-load zone form inside the bearing. If the bearing has a certain radial internal clearance, rolling elements in the non-load zone do not contact the inner raceway, but may contact the outer ring under centrifugal force.
At low rotating speed, when centrifugal force is smaller than the self-weight of rolling elements, rolling elements drop and collide with inner raceways or cages, exciting inherent vibration and noise of the bearing.
Characteristics:
Likely to occur under grease lubrication, rare under oil lubrication; more frequent with inferior grease;
Frequently encountered in winter;
Easy to generate when only radial load is applied and radial internal clearance is large;
Occurs within a specific speed range, which varies for bearings of different sizes;
Can be continuous or intermittent noise.
The forced vibration often excites the second-order and third-order bending inherent vibration of the outer ring and generates such noise.
Applying preload can effectively reduce this noise by lowering working radial internal clearance after installation. Selecting suitable lubricants also brings improvement. Some foreign manufacturers adopt lightweight rolling elements such as ceramic rollers or hollow rollers to prevent this noise.
3. Squeal
It is intense screaming generated by sliding friction between metal surfaces. Although bearing temperature rise is insignificant, it barely affects the service life of bearings and grease and does not hinder rotation. However, the unpleasant noise causes disturbance. Large short cylindrical roller bearings under radial load often produce such noise.
Characteristics:
Easy to generate when bearing radial internal clearance is large;
Usually occurs with grease lubrication and rarely with oil lubrication;
Weakens as bearing size increases and often appears within a certain speed range;
Frequently occurs in winter;
Random and unpredictable, related to grease filling volume, grease performance, installation and operating conditions.
This noise can be prevented by reducing bearing radial internal clearance and adopting outer rings with shallow raceway structure.
4. Cage Noise
This noise is generated by free vibration of the cage and collision between the cage and rolling elements or rings during bearing rotation. It may occur in all types of bearings with low sound pressure level and low frequency.
Characteristics:
Produced by both stamped cages and plastic cages;
Occurs with both oil lubrication and grease lubrication;
Most likely to generate when the outer ring bears bending moment;
Easy to occur with large radial internal clearance.
Complete elimination of cage noise is extremely difficult because pocket clearance and the clearance between cage and ring are unavoidable for finished bearings. However, improvement can be achieved by minimizing assembly errors and optimizing clearance and cage axial play.
A special type of cage noise is roaring noise triggered by self-excited vibration of the cage due to friction between the cage and guiding surfaces of other bearing components. Stamped cages of deep groove ball bearings are thin, featuring low bending stiffness in radial and axial planes and poor overall stability. High-speed rotation causes bending deformation and self-excited vibration, generating a humming sound.
When a bearing operates under radial load with poor grease performance, clicking noise can be heard at the initial operation stage. This is mainly caused by sudden acceleration of rolling elements after leaving the load zone and collision with the cage. Such collision noise is unavoidable but will disappear after a period of operation.
Measures to prevent cage noise:
Adopt ring-guided design to ensure stable revolution of the cage, and guarantee sufficient lubrication on guiding surfaces. Improve the structure of tapered roller bearings under high-speed conditions, replacing roller-guided L-type cages with ring rib-guided Z-type cages.
During high-speed operation, bearings with large pocket clearance produce far greater cage vibration amplitude than those with small pocket clearance. Therefore, pocket clearance selection is critical. Minimize radial internal clearance as much as possible. Improve cage manufacturing precision and surface quality to reduce noise caused by collision or friction between rolling elements and cages.
Deploy advanced cleaning technology to thoroughly clean spare parts and assembled bearings and improve bearing cleanliness.
When a bearing operates under radial load, only several rolling elements bear internal load. Elastic contact with rings forms a spring support, generating periodic vibration when rolling elements pass the radial load line. Consequently, the rotating shaft center moves vertically or horizontally and triggers noise.
Such vibration is defined as rolling element passing vibration, which is more obvious at low rotating speed. Amplitude is related to bearing type, radial load, radial internal clearance and the number of rolling elements. Amplitude is generally small; hazards only arise when amplitude increases. Therefore, reducing radial internal clearance or applying appropriate preload is commonly adopted for mitigation.
