The majority of vibration sensors used today are single-axis sensors, meaning roughly 95% of what the sensor picks up or detects will be perpendicular to the mounting surface. Certain defects present more prominently in the horizontal plane, while others show up more clearly in the vertical plane. Since sensor placement is critical to obtaining accurate data, it helps tremendously to have an understanding as to what types of defects show up better in which of the three directions that data is taken – vertical, horizontal, and axial.
For example, for horizontally mounted machinery, data that is taken in the horizontal direction will typically show imbalance and/or electrical issues better than in the vertical direction. Conversely, looseness and anti-friction bearing issues will generally show up more clearly when data is taken in the vertical direction (the keyword being “generally” because structural and process-related forces may change the “natural” response.) Angular misalignment will be more evident in the axial direction.
On a bearing is best
Whatever the direction, ideally the sensor should be placed directly on a bearing to get the best possible reading. This is because, in the early stages of failure, rolling element bearings generate high-frequency data. Considering the fact that high-frequency data only travels short distances, the longer the distance from the bearing to the sensor, or more components between the bearing and the sensor means more high-frequency data loss.
Sometimes placing a sensor directly on a bearing just isn’t possible, such as when a fan cover on an electric motor is present. In large motors, the cover can extend as much as 10 inches from the bearing on which you want to place the sensor. In cases like these, all you can do is get as close to the bearing as possible, keeping in mind that the signal generated may be diminished with frequencies at lower levels than expected.
It’s important to remember – the value of vibration analysis is directly dependent upon the accuracy of the data it provides and it begins with correct sensor placement. IVC Technologies provides hands-on training in all of the standard condition monitoring technologies – vibration being our core component. Contact us.