In the past, ultrasound and vibration technologies have been used independently to monitor ball bearings in plant equipment.  However, it is becoming more common to use ultrasonic inspection interfaced with vibration analysis to support predictive maintenance programs for periodic inspection of critical bearings to monitor wear and predict failure.

Research by the National Aeronautics and Space Administration has demonstrated that using ultrasound technology to monitor bearing wear will locate incipient failure before it is detected by traditional heat and vibration methods.  With an ultrasonic instrument, such as UE Systems' Ultraprobe 2000, operators can hear the sound quality of a bearing and monitor amplitude changes on the built-in meter. Using the instrument's demodulated signal in conjunction with vibration analysis, ultrasonic testing zeros in on a worn bearing, allowing users to trend, troubleshoot, and confirm potential bearing problems.  This two-pronged approach enhances vibration monitoring programs.

The ultrasonic instrument expands the ability of the Fast Fourier Transform (FFT) analyzer to deliver a more detailed analysis of regular ultrasonic emissions that can indicate mechanical problems.  For example, low-speed bearings usually are difficult to test or isolate. When the ultrasonic instrument is connected with an FFT analyzer, the demodulated signal is transmitted from the ultrasonic instrument to the vibration analyzer.  This signal is processed so that the ultrasonic components are removed, leaving low-frequency components that the FFT analyzer processes and converts to signals that can be analyzed.  The ultrasonic instrument detects the ultrasonic component of the friction produced by the rotating surfaces of the bearings.

In the case of extremely slow-speed bearings, an operator uses the ultrasonic instrument to listen to the heterodyned sound of the rollers or ball bearings, which are usually large and greased with high-viscosity lubricant.   Generally, no sound is heard because the grease absorbs most of the acoustic energy.  However, a crackling or grinding sound indicates some degree of deformity in the bearing.  This method is especially useful when low-frequency noise masks bearing fault frequency signals from low-frequency accelerometers.

Advanced diagnostic services
Reliability Assessed Maintenance Corp. (RAM), Rancho Cucamonga, CA, provides predictive and proactive maintenance services throughout the United States.  The company works with a variety of commercial and industrial facilities. Among the advanced diagnostic services RAM Corp. offers are ultrasonic analysis, vibration analysis, infrared scanning, motor current signature analysis, and oil analysis.

"Sometimes a company hires us to perform a baseline survey of its equipment," said RAM vice president Gerald Stock. "Other times, customers know they have a problem but can't quite zero in on it.  They've tried to repair it, but the problem keeps returning."

Stock always carries ultrasonic equipment and a vibration FFT analyzer when visiting a customer to test bearings for wear. "I'm a certified vibration specialist, but I use ultrasonics to verify a bad bearing, broken gear tooth, and so forth. It is extremely helpful when I'm dealing with a maintenance manager unfamiliar with vibration spectra indicating a bad bearing; a chart of vibration readings can get a bit confusing," said Stock.  "So I back up my vibration readings with my ultrasonic readings, both by interfacing the Ultraprobe to my data collector (which can be shown to the manager later) and by letting the manager listen to the ultrasonic instrument identify a bad bearing.  It produces a very audible popping sound.  The proof is unmistakable."

The portable ultrasonic instrument is easy to use.   Wearing a headset, Stock touches equipment under inspection with a metal wave guide to promote accurate ultrasonic reception and slowly moves along a predetermined path to take FFT readings at regular intervals.

Frequency tuning enables him to tune into problem sounds while minimizing background interference. The meter mode selection adjusts the meter response from a real-time response to an averaging response, permitting accurate adjustments for mechanical analysis.  Because the instrument is sensitive only to ultrasonic frequencies, it will not respond to low-frequency noises often associated with on-line equipment, making it ideal for use in extremely loud environments.

"For example," said Stock, "I have used the ultrasonic instrument in a metal stamping plant where an extremely critical conveyor bearing often requires immediate analysis.  The environment is very noisy and the stamping of all the presses makes it difficult to determine which machine is the source of the vibration.  Using the ultrasonic instrument with the magnetic coupling makes the problem easy to detect."

The ultrasonic instrument is so sensitive that Stock can hear if a bearing is running dry, if there is too much lubrication, or if there is a small particle of dirt on the ball.  He records the meter readings monthly and uses them to indicate when a bearing should be replaced.  The ultrasonic instrument is extremely light and durable and does not require programming like FFTs, according to Stock.
Such diagnostic tests can save money.  During a routine maintenance check, Stock discovered a bad bearing in an HVAC chiller.   "We took vibration readings to verify the problem and recommended that repairs be made on a 250 hp motor," recalled Stock.  "One bad bearing can bring a chiller to a sudden halt.

"But the greatest advantage of catching a problem early is cost," Stock continued.  "If the motor had failed, it would have had to be pulled out, sent to a rewind shop for extensive repairs, and then reinstalled.  The downtime and inconvenience caused by the breakdown would be significant.  And the difference in cost between a planned repair and a catastrophic repair is probably between three and four hundred percent."