Does the transverse channel of a triaxial accelerometer provide measurements equivalent to those of a single-axis accelerometer oriented in the same direction? This is an important question for maintenance teams who rely on sensors to detect subtle vibration changes in machinery to stay ahead of breakdowns.
The first factor to consider is that different locations on a bearing housing will provide different structural transmission paths for vibration generated from the bearing. Each different transmission path might have different frequency response characteristics as well. Consider the following diagram:
The horizontal measurement at the triaxial sensor (triax) depends on the input force at the bearing (F) and the frequency response of the structural transmission path between the bearing and the location (1) of the triaxial sensor. The measurement at the single-axis sensor depends on the same input force (F) but is transmitted through a different structural path to the location (2) of the single-axis sensor.
For sufficiently low frequencies comparable with typical machinery rotation rates or sufficiently tight, solid bearing housings, the frequency response of the two paths will be extremely close. When considering high frequency vibration (greater than, say, 1 kHz) the response might be different depending on the mass, stiffness, and geometry of the machine casing. The two locations might measure different amplitudes.
High-frequency vibration produced by faults such as deteriorating rolling element bearings or high-speed gear meshes is transmitted acoustically through the structure in stress waves, as opposed to overall translation (in phase) of the entire machine. Subsequent interpretation of these components in terms of the directionality of the underlying forces can be extremely complicated. Such vibration tends to be omni-directional as the stress waves are scattered at structural interfaces.
When measuring machinery vibration to map diagnostic trends, the difference in frequency response is usually not relevant. Both measurements are repeatable, so analysis should detect any increasing trends.
If the bearing housing dimensions are large relative to the overall size of the machine, then the rotational components of motion might also have an effect on the difference between the two measurements. For example, if the machine is rocking on its foundation, then the horizontal component of motion is actually higher at the triaxial location (1) than the single-axis location (2). However, both measurements provide a repeatable indication of the force generated at the bearing.
Although both locations provide horizontal measurements from the same bearing housing, the triaxial sensor really measures motion tangential to the axis of rotation, while the single-axis measures radial motion. In the case of a soft foot or bearing housing rotation, the motion would not be detected by the horizontal measurement at location (2), but would be pronounced horizontally at the triax location (1). Many mechanical forces generated in typical machinery result in such tangential motions of the casing. The triax is a better tool to measure tangential vibration than the single-axis sensor.
In a predictive maintenance program where many machines are monitored on a regular basis, triaxial data collection offers a significant cost and labor-saving benefit, especially when data can be collected simultaneously from all three channels. Only one sensor attachment and measurement cycle is needed to monitor each bearing. Only one surface preparation at each bearing is necessary. Collecting three-axis data from each location with no additional labor gives analysts a much more complete dataset to diagnose machinery faults than if a smaller set of selected single-axis measurements were made.
While high-frequency response might be different between a shear measurement made with a triaxial accelerometer and a radial measurement in the same direction as a single-axis accelerometer, the benefits to using a triax include cost savings, convenience, and improved tangential sensitivity.
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