Vibration generation at high speeds
When bearings operate at high speeds, high over-rolling frequencies are generated in the bearing and a high-pitched noise can be expected from the application. What is perceived as “bearing noise” is the audible effect of the vibration generated by the bearing and transmitted through the surrounding structure. The surrounding structure also contributes to the attenuation or amplification of the noise characteristics of the application. In addressing noise issues in high-speed bearing applications, it is useful to consider the following additional aspects.
Excitation due to a varying number of loaded rolling elementsWhen a radial load is applied to a bearing, the number of rolling elements carrying the load varies slightly during operation, this means alternating between 2–3–2–3. This generates a displacement in the direction of the load. The resulting vibration cannot be avoided, but can be reduced by applying an axial preload to load all the rolling elements. This, however, is not possible for cylindrical roller, needle roller and CARB toroidal roller bearings and is not recommended for double-row bearings..
Accuracy of associated componentsIn cases where there is a tight fit between the bearing ring and the housing or shaft, the bearing ring may take the shape of the adjacent component. If form deviations are present, these may cause vibrations during operation. Therefore, it is important to machine the shaft and housing seats to the required tolerances (→ Tolerances for total radial run-out).
Presence of local raceway damage or indentations caused by solid contaminants also reduce the accuracy of the raceway microgeometry and increase vibrations in the bearing. High cleanliness of the lubricant and protection from solid contaminants can help to reduce bearing noise issues in an application.
Influence of the bearing on the vibration behaviour of the applicationIn many applications, bearing stiffness is of the same order as the stiffness of the surrounding structure. This opens the possibility of reducing vibrations in an application by either replacing the bearing or adjusting the preload or clearance in the bearing arrangement. There are three ways to reduce vibration:
- Remove the critical excitation vibration from the application.
- Dampen the critical excitation vibration between excitant component and resonant components.
- Change the stiffness of the structure to change the critical frequency.