Condition analysis of Rolling Element Bearings

Since one of the analysis techniques involves trending of vibration levels, it is important that the data collection location be marked and the same location be consistently used each time data is collected. Common to most modern portable electronic data collectors is the accelerometer. These are generally constructed with a man-made piezoelectric crystal which generates an output voltage directly proportional to the amount force applied. The accelerometer is usually placed on the bearing cap, or if not accessible, as near as possible to the bearing.

In those instances where it is not possible to safely position the accelerometer by hand, the accelerometer may be permanently stud mounted or glued to the machine and the signal wire terminated in a safe location. Generally the accelerometer will be mounted using a magnet. Both methods are acceptable for general vibration monitoring. In rare instances a stinger (a steel rod threaded onto the end of the accelerometer, do NOT use an aluminum rod) may be attached to the accelerometer to reach a bearing cap located in a tight space, but stingers will alter the signal amplitude and frequency and are not recommended for general usage.

For continuous monitoring of a machine, all of the points of interest use a stud or epoxy mounted accelerometer. The signal wires are then terminated at a common point where they are multiplexed and routed to a permanently mounted data collector. The signals from the data collector are then passed to a computer controller which is programmed to store and process the data. One accelerometer signal can be processed into four presentations; acceleration, velocity, displacement, and enveloped acceleration and these may be processed for different frequency ranges as needed. In other words, the acceleration signal may be presented in one spectrum from 0-30 Hz in velocity to check for balance and alignment. A second spectrum may be generated with a range of 0-1000 Hz to disclose the rotor bar pass frequency. The acceleration signal can also be processed with enveloped acceleration algorisms to check for bearing degradation and finally a high frequency 0-10,000 Hz acceleration signal may be processed to provide a gearbox inspection. In addition other types of sensors can collect operational data such as shaft position, speed, temperature, flow, pressure, etc. Generally any sensor that provides a voltage output can be monitored, the signal can then be collected and stored for evaluation.

Find the original article here on SKF’s website

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