General
Every bearing, good or bad, new or old, produces ultrasonic friction as the internal rolling elements turn against the inner and outer raceway. A good bearing will produce less acoustic energy than a bearing with typical wear flaws such as pitting, spalling, flattening of the balls, and scarring of the raceway. Likewise, the friction absorbing properties of grease means a well-lubricated bearing produces less friction than a bearing that lacks lubrication. As the lubricant's viscosity deteriorates, and/or the bearing's material composition stresses and fatigues, the friction and the corresponding ultrasonic emissions increase. Monitor these high frequency acoustic vibrations with an ultrasonic detection system to determine proper lubrication intervals and predict when the bearing is entering its FIRST stages of wear.
Acoustic Vibration Monitoring vs Vibration Analysis
Acoustic vibration should not be confused with low frequency vibration measurements. Low frequency vibration measurements (velocity or displacement) indicate a bearing in an advanced state of wear and provide information about root cause of premature failure (misalignment, imbalance, etc.). Normally there is only a small window of opportunity to schedule downtime prior since the bearing is already advanced to a failure state. High frequency acoustic vibration monitoring controls the evolution of the bearing, indicates necessary lubrication intervals, and triggers alarms before the bearing enters failure state.
The earliest indication of change means corrective action can be taken sooner to increase the machine's longevity. If one goal of predictive maintenance is to extend the lifespan of production machines then ultrasonic inspection must play an integral role. No other condition monitoring technology can give this kind of advanced warning.
The Technique
Monitoring bearing condition requires the SDT 170 M or 170 MD operating in contact mode. The type of sensor used depends on the application but normally the Contact Probe with Mechanical frequency selected. Other sensor options are the Magnetic Sensor or the permanent mounted Threaded Sensor.
Ultrasonic acoustic energy is measured by the SDT 170 unit and displayed on the generous 3 cm x 6 cm LCD screen giving the user a quantitative value for trending. At the same time the ultrasonic signal is converted to an audible signal that is output to the headset. Technicians can actually hear ultrasound while totally eliminating the normal parasitic sounds associated with conventional audible frequency methods (screwdriver to cheekbone or doctor's stethoscope). The combination of both quantitative and qualitative information equals a most powerful diagnostic tool.
At the outset, the goal is to establish a baseline or normal operating range for each bearing to be checked. Throughout the life of a bearing its ultrasonic level should remain relatively constant (+/- 4 or 5 dBmV). As the condition of the bearing evolves, increases in acoustic energy due to either lubrication breakdown or structural breakdown are monitored. Increases correspond with an elevated reading (dBmV) on the SDT 170. Trending acoustic energy with the Data Logger software allows the user to accurately predict when lubrication should be applied to a bearing (see application note on predicting lubrication intervals), and when the bearing itself is entering early failure stages. Remember, trending acoustic vibration warns us of the earliest signs of change in a bearing. Diligent use of the information gathered will result in better lubrication practices and extended bearing life. The ultimate result is a much larger window through which to schedule repairs and change outs.To optimise the use of the SDT 170 unit, it is essential to listen to the bearings. Listening to the noise of a bearing provides significant information on the measured values. A regular noise associated to stable values indicates a normal functioning. A strident high pitched noise indicates a lack of lubrication, an overload or a rotation speed not adapted to the features of the bearing. A crackling noise associated to unstable or high dB values indicates faults, wear or loose metal particles in the lubricant. Use the headphones in conjunction with the digital readout to perform a total diagnostic predictive maintenance.
Comparative vs Correlative Technique
Each bearing "emits" its own sound or ultrasound and its value is expressed in dBmV by the unit SDT170. This emission is directly influenced by the following factors:
In other words, there is no absolute chart for comparing measurements of a particular bearing. Two identical bearings evolving in different environments may produce different acoustical emissions. When starting a condition monitoring program it may be useful to compare similar machines to each other. But respect to the above mentioned variables must be given to have reliable results. The goal is to establish baseline acoustic vibration levels for each bearing individually and correlate the current ultrasonic readings against historical data.
In general, the critical operating threshold is fixed at a value of 15 dBmV above the baseline. An increase of +8 - +10 dBmV is reason to suspect a dramatic change in the lubrication condition. Listen to the ultrasound of the bearing while applying new lubricant. When the grease reaches the bearing's internal components both the perceived sound in the headset and the measured digital value to should return to baseline. If the acoustic vibration levels remain high after lubrication is applied it is reasonable to assume the bearing has entered early failure state (check that the grease was able to reach the bearing).
Use of the contact probe
In most circumstances, the contact probe is the best sensor to monitor a bearing. The contact probe is a standard accessory in the SDT 170M and MD versions and an option in the SDT 170S version.
To ensure repeatability of the measurements taken with the contact probe, it is imperative to follow these rules:
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