Where to place the vibration sensor depends on what data you wish to see. Certain defects show up better in the horizontal direction while other defects show up better in the vertical direction. So which location should I choose to place my vibration sensor? Most sensors in use today are single-axis sensors, so generally 95% of what they pick up or detect is in line with the sensor. Therefore, since placement of the sensor is crucial, some thought should be given as to what data shows up best in which of the three directions—vertical, horizontal, and axial. Data taken in the vertical direction will typically show looseness better than in the horizontal direction (at least on a horizontally mounted piece of equipment); however the horizontal will show unbalance better than the vertical. Axial vibration will show angular misalignment better than a radial reading will.
Also, it should be considered that in the real world there are times that the sensor cannot be placed directly on a bearing, such as with the non-drive end bearing on an electric motor due to the fan cover; on large motors this cover can extend 10 in. or more from the bearing, and considering that rolling element bearing generate high frequency data in early stages of failure and high frequency data only travels short distances, the data can and will be diminished the further from the bearing that you take your reading. In these cases you simply get as close as possible to the bearing knowing the generated signal may be diminished; in such case you should pay special attention to the frequencies present as they may be at lower levels than expected. If you are dealing with vertical equipment it’s typically stiffer in-line with the discharge than perpendicular to the discharge and that will affect your data as well.
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Condition Monitoring, Vibration Analysis by Gary James CRL