Excessive bearing vibration increases noise
Machinery may never be completely quiet. However, machine builders and design engineers can produce lower-noise machines through a more careful choice of the bearings that are used in their equipment. Bearing rings and balls are not perfectly round, even after grinding and polishing. They still have small imperfections in smoothness.
These machining irregularities manifest as rough or uneven surfaces, introducing a trend for one ring to experience radial movement or oscillation in relation to the other. Consequently, these deviations contribute to bearing vibration and generate undesired noise, emphasising the importance of precision in manufacturing processes to mitigate these imperfections faults and increase bearing life.
The smoothness or quietness of a bearing can be checked by accelerometers which measure bearing vibration at the outer ring, usually with the inner ring rotating at 1800 rpm. To understand how bearing vibration is measured, it is important to understand how vibration works.
In comprehending the methodology behind measuring bearing vibration, it becomes imperative to delve into the principles of vibration itself.
he degree of oscillation within a vibrating object is denoted as displacement. As the outer ring of a bearing undergoes vibration, its outer surface traces a trajectory, ascending to the upper limit, descending to the lower limit, and then returning to the initial point of origin.
The measurement encompassing this journey from the upper to the lower limit is termed peak-to-peak displacement, serving as a crucial metric in evaluating the vibrational characteristics of the bearing.
The wholeoscillation movement from start point through the upper and lower limits and returning to the starting point, is called a cycle. This vibration cycle persists throughout the bearing's rotation, creating a repetitie pattern.
Beyond assessing the mere occurrence of cycles, it is important that we measure the frequency of these vibrations, indicating the number of cycles transpiring within a given perios. This frequency is most commonly expressed as cycles per second (CPS) or Hertz (Hz) — both refer to the same unit of measurement.
It is essential to recognise that the impact of vibration on bearings extends beyond displacement measurements. Vibration in a bearing can accelerate fatigue and shorten bearing life.
Measuring bearing vibration
Vibration velocity in a bearing — a product of displacement and frequency gives us a good indication of the severity of the vibration. When a component within a bearing moves a specific distance (displacement) at a given rate (frequency), it must be moving at a certain speed. The correlation is straightforward: a higher measurement of vibration velocity directly corresponds to an increased level of noise coming from the bearing.
Vibration velocity is measured on a bearing vibration tester, quantifying results in microns per second, or an Anderon meter, measured in Anderons where one Anderon equals 7.5 microns per second.
The readings are categorised into three frequency bands: low (50 to 300 Hz), medium (300 to 1800 Hz) and high (1800 to 10000 Hz).
Although vibration velocity shows the fatigue potential, vibration force can cause deformation to balls and rings and can be very damaging at high frequencies where velocity readings may be quite low. For this reason, we also measure vibration acceleration.
Vibration acceleration — this is an indication of vibratory force (force = mass x acceleration) and since force is damaging at higher frequencies, vibration acceleration is a useful measurement where a bearing will experience vibration frequencies above 2000 Hz. Vibration acceleration is measured in G (9.81 m/s²) but you will often see these measurements converted to decibels (dB).
A low noise/vibration rating is achieved by paying particular attention to the surface finish of the raceways and balls, the roundness of the rings and balls and correct cage design.
Finely filtered low noise greases can also be used. These contain fewer, smaller solid particles which generate noise when they pass between the balls and raceway.
External factors such as surrounding vibration can affect bearing noise. Another problem, particularly with smaller and thin-section bearings, is ring distortion caused by poor shaft or housing roundness. Dirt or dust contamination will also increase noise and vibration levels. Poor fitting practice or incorrect handling is sometimes to blame, causing shock loads which, in turn, create scratches or dents in the raceway.
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For even more information, visit SMB Bearings’ technical information page.