Noise from Bearings
Noise from bearings is a common problem in mechanical systems and is a major source of machine failure. Ultimately, bearings create noise due to friction between the rolling elements and raceways as well as due to misalignment. There are many reasons this occurs and depending on the cause, the remedies also vary. Understanding the source and diagnosis of the noise generated by bearings is critical in solving the problem.
In general, bearing noise can be divided into two categories. Contact noise, which is produced when there is an irregular or direct contact between two bearing surfaces and rolling element noise which is produced when there is no contact between two surfaces and is instead due to wave propagation of the rolling elements.
Contact noise can be further subdivided into various categories including: radial noise, spiked noise, scrapes and scratches, and chirping. Radial noise is primarily caused by a faulty cage and is most intense when the perfect timing of the rolling elements is interrupted. Spiked noise is caused by the interference of the rolling elements and raceways, and typically occurs at high speeds. Scrapes and scratches are the result of small foreign particles which become lodged between surfaces of the bearing and raceway, producing a high pitched grinding noise. Lastly, chirping occurs when the bearing has been disturbed or shocked due to a sudden load change or movement of the machine.
Rolling element noise is typically caused by imperfections on the surface of the race or rolling elements, known as roughness, which propagate waves at a certain frequency through the ball bearings when the bearing is rotating at certain speeds. This noise is usually spread over a wide frequency range and is hard to isolate and diagnose.
For bearings that produce contact noise, the most effective way to fix the problem is to keep the machine in proper alignment, use proper lubrication and ensure that the bearing temperature is low. Alignment ensures that the load from the bearing is evenly distributed and that the raceways are in contact with the rolling elements as much as possible. Oil lubrication reduces the friction between surfaces, reduces heat, and distributes any foreign matter away from the raceway and into the barrel of the bearing. Lastly, keeping the bearing temperature low prevents any foreign matter from melting and adhering onto the raceway surface.
To address rolling element noise, a more complex process is needed. First, the Bearing Domain Analysis method is used to detect the areas of imperfection on the raceways or rolling elements. This process involves scanning the bearing surface with a laser to detect various holes and other surface imperfections, as well as measure any changes in surface heights. The results from the analysis are used to create corrective action through grinding and relapping techniques. This is done by grinding off the surface imperfections to create a smoother surface for the rolling elements to travel on. Ultimately, this results in a reduction of noise and improved performance of the bearing.
In summary, noise from bearings is a common issue that usually results from direct contact between two surfaces or from imperfections on the race or rolling elements. To reduce this noise and improve the performance of the bearing, proper lubrication, alignment, and corrective action techniques must all be employed. By understanding the source and diagnosis of bearing noise, it is possible to properly address and resolve the issue, ensuring that the machine operates more efficiently and effectively.
