Article
Comparison of Single Axis and Triaxial Accelerometers for Route-Based Vibration Analysis

by James C. Robinson and James S. Sparkman, Sr.

ABSTRACT

This paper examines the differences and possible benefits of using a triaxial accelerometer for route-based data acquisition in lieu of a single axis accelerometer. Mounting considerations are reviewed including surface preparation and accelerometer placement. Rare earth flat magnets are recommended to detect higher frequency stress wave activity instead of a two pole style magnet due to its limited frequency response. Frequency response test data is presented and compared between the two types of accelerometers. A Peakvue response test displays the triaxial z-axis response levels compared to a single axis accelerometer. Trended triax and single axis data readings taken over a period of time are listed showing little difference in the two data sets. This article clearly shows how use of a triaxial accelerometer can reduce data collection time.

PREVIEW

“A single axis accelerometer mounted to a two-pole (two feet) magnet is the most common sensor employed in the route-based collection of vibration data for condition monitoring of industrial machinery. The magnet is easily attached to a curved surface (providing two-line contact), so minimal surface preparation is required at the measurement point. Collecting vibration data in this way provides meaningful results for detecting and trending many mechanical faults.

It is desirable to obtain vibration data in the horizontal, vertical, and axial directions. In addition, the capture of higher frequency stress wave activity (in at least one direction) is beneficial for detecting impacts, fatigue, and friction. However, mounting the accelerometer using a two-pole magnet on a curved surface limits its ability to detect higher frequency stress wave activity introduced by friction due to the limited bandwidth (~5 kHz) of the two-pole magnet.

If it is important to reliably detect friction at an early stage, the two-pole magnet should be replaced by a flat rare-earth magnet on either a smooth flat space or a mounting pad. However, three mounting pads might be required at each bearing to take horizontal, vertical, and axial measurements. The inconvenience of providing three mounting pads on each bearing as well as time restraints discourage the acquisition of broadband (up to 15 kHz to 20 kHz) data in the three directions.

In cases in which it is important to reliably detect friction (requiring broadband detection capability), data acquisition is generally restricted to one direction using a single axis accelerometer. An alternative to the single axis accelerometer with a flat magnet is the use of a conventional triax accelerometer, which also requires a special mounting pad or other surface preparation. This method lacks the versatility of a two-pole or flat magnet and hence is not widely used in route-based data acquisition programs.

A new triax accelerometer developed by Emerson Process Management specifically for route-based data acquisition was used in this study. It has the versatility of the single axis accelerometer. Data acquired from the two accelerometers were compared for route-based vibration analysis.

Characteristics of the Emerson Triax Accelerometer
The sensing element for the x-axis is always parallel to the feet (Figure 1); the sensing direction of the y-axis and z-axis are also shown. The two feet are wider than those for the two-pole magnet used with a single axis accelerometer. On a mounting pad the increased area of the feet on the pad (relative to a typical two-pole magnet) has sufficient holding force to provide the desired bandwidth in the z-axis.”

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