Selecting appropriate measurement locations when collecting vibration data is important in order to determine the machine condition. Improper mounting locations often lead to gaps or omissions in the analysis, even when using the appropriate sensors, software setup and analysis techniques.
The best locations for collecting vibration data are usually near the bearings; this is because the bearings are in physical contact with the rotating components and provide a solid path to the vibrating components. This reasoning does not apply to non-contact probes and sensors e.g. eddy current probes, which measure displacement at the shaft using magnetic fields.
It may not always be feasible to measure vibration at the bearing locations; this may be the case when there is a safety hazard in doing so, or if the location is not easily accessible. For such cases the measurements should be taken at the closest mechanical path to the bearing; the path should travel along solid metal and avoid gaps and joints that can prevent certain frequencies from being transmitted to the sensor.
When selecting a measurement location, it is important to note if the mounting point resonates or rattles as this will lead to vibration data that does not represent forces generated at the bearing. A solid mounting platform is also important e.g. it may not be possible to mount sensors near the cooling fins of a motor, as shown in Figure 1 below; an improper mounting platform can also lead to data loss.
Figure 1. Cooling fins on a motor make it difficult to mount measurement sensors
Under most circumstances it is useful to measure vibration in 3 orthogonal axes e.g. in the vertical, horizontal and axial directions as shown in Figure 2 below. Measurements should be made at several locations on the machine if possible to obtain a thorough picture of the machine condition.
Figure 2. Recommended measurement directions for collecting vibration data
There are a few important factors to consider when selecting a mounting point for sensors:
- First and foremost, the location should be safe to access.
- The location should be easily accessible, especially when measurements are taken routinely.
- Having a good mechanical path to the bearing in order to gather data at all the frequencies that may be present in the vibration.
Using these rules of thumb can yield the most useful data for vibration analysis.