Many plants perform route-based, machinery condition monitoring with portable vibration analyzers. The most common vibration sensor used is a single-axis accelerometer mounted to a 2-pole magnet, which can easily attach to a variety of surfaces including curved ones. Vibration data collected from these sensors provide meaningful results for the detection and trending of many mechanical faults.
However, mounting these accelerometers via a 2-pole magnet on a curve surface limits the ability of the sensor to detect the higher frequency stress wave activity introduced by metal impacting, machinery friction, and fatigue cracking. This is because the 2-pole magnet mounting method limits the sensor from reaching its optimal high frequency response range of 10-20kHz.
To detect these stress waves, the two-pole magnet needs to be replaced with a flat rare-earth magnet and a smooth, flat place, such as a mounting pad, on the machinery’s surface. Three mounting pads could be required if horizontal, vertical and axial measurement are to be acquired for each bearing—something not very convenient. This difficulty discourages the acquisition of the optimal broadband vibration data, 10-20kHz.
An alternative to the single-axis sensor and three flat, rare-earth magnets is to use a conventional triaxial accelerometer. The downside is that it requires a special mounting pad or additional machinery surface preparation.
To address these issues, a triaxial accelerometer was developed by Emerson’s Machinery Health Management team specifically for route-based vibration data acquisition. When the triaxial accelerometer is placed on a curved surface, the performance of the sensor in the z direction is approximately equivalent to the single-axis accelerometer attached to a 2-pole magnet placed on the curved surface.
The design needed to have the versatility of a 2-pole magnet, to have the magnet be integral to the sensor assembly, to sense in the z-direction flat within 3 dB up to frequencies of 10 kHz when mounted on a flat surface or mounting pad, and to sense in the x-y horizontal plane flat within 3dB up to 3.5 kHz.
When the triaxial accelerometer is placed on a flat surface (such as a mounting pad), the performance of the sensor in the z direction is approximately equivalent to the single-axis accelerometer with a flat rare earth magnet attached to a flat surface (mounting pad).
The sensing element for the x-direction is always parallel to the feet. The z-direction is always parallel with the axis of the sensor and the y-sensing direction is perpendicular to the feet (as shown in the picture). When placed on the keyed mounting pad, the increased area (compared with a 2-pole magnet with a narrower foot arrangement) between feet and keyed mounting pad provides sufficient holding force to collect the desired high frequency response in the z-direction.
The triax sensor provides an equivalent sensor in the z-sensing direction (best mounted on the machine in the horizontal direction to get closest to the load zone) to the commonly used single-axis accelerometer when on a curved surface. Additionally, the x-y sensors can be used where x generally is axial and y is vertical (assuming z is horizontal). If the triax sensor is placed on a smooth flat surface, such as the keyed mounting pad, the frequency response of the z sensor is adequate for detecting high frequency stress wave activity accompanying impacting, friction, and fatiguing.
Based on feedback from end users, the Machinery Health Management team notes that the use of the triax sensor in the route mode of data collection in typical plant conditions has reduced data collection time on an average of 30%-50% versus route collection using a single axis accelerometer.
