In brief:

• The best overall machinery monitoring program is one that utilizes multiple, integrated monitoring technologies that are well-suited to detect expected failures modes.
• One goal of PdM is to determine how much time is left before a machine will fail, so plans can be made to minimize downtime and damage while still getting the most useful life from the machine.
• An application where ultrasound and vibration work well together is a mechanical inspection.

Reliability-centered maintenance programs are most effective and most profitable when a variety of appropriate technologies and tools are used to complement one another. Vibration analysis and ultrasound are as complementary as two sides of the same coin. Ultrasound is a useful monitoring tool, capable of detecting failing rolling element bearings and over- and under-lubrication conditions. The best overall machinery monitoring program is one that utilizes multiple, integrated monitoring technologies that are well-suited to detect expected failures modes. For low-risk machines, vibration analysis can be performed by a mechanic or operator using a vibration data collector or vibration meter. For machines of higher criticality, a certified vibration analyst should use advanced vibration data collection and analysis hardware and software.

“A lot of people have used vibration as the first indicator,” explains Robert Perez, author of “Is My Machine OK?” “If you go around and take vibration data and your spectra indicate a bearing failure, you can follow up with ultrasonics. Maybe you have early signs of bearing distress, and you follow up with ultrasonics and you hear the bearing is a little noisy. Some people might use that to inject grease in the bearing. Some people use it so they don’t over-grease the bearing. You can optimize your greasing program with ultrasonics. You can use it to determine if you need to repair or replace the bearing. Some people do the same thing with vibration. You see a lot of noise in the bearing, and you add grease to see if it gets quiet. The ultrasonics let you hear what’s going on in the bearing. A lot of vibration programs don‘t have the ultrasonic gun, but it is definitely good for leaks. One person’s critical equipment is not necessarily another’s.”

On a rolling element bearing, it might make sense to use vibration and ultrasound, says Perez. “On a big reciprocating compressor, we use vibration, pressure volume analysis on the cylinder, and then ultrasonics to determine if we have a leaky valve.”

An application where ultrasound and vibration work well together is a mechanical inspection, says Maureen Gribble, director of marketing at UE Systems (www.uesystems.com). “There are a number of ways they support each other,” she explains. “The first is utilizing ultrasound as the primary tool for monitoring bearing condition. The ease, speed, and accuracy of ultrasound inspection has helped many inspection departments reduce their route inspection times by as much as 30%.
Next, while vibration analysis can readily detect faults such as imbalance, cracked shafts, gear faults, and misalignments in high-speed machinery of greater than 60 rpm, detecting faults with vibration analysis becomes increasingly more difficult as the speed of the machinery decreases. This presents an opportunity to utilize ultrasonic analysis as an effective and efficient way to monitor bearings at slower speeds.”

At low speeds, such as 150 rpm, the vibration acceleration signals of a normal bearing and a defective bearing have similar characteristics, continues Gribble. “This makes it hard to note the difference and thus determine that there is a defect,” she says. “With ultrasound, a significant difference can be heard and seen in the magnitude and shape of the signal, making it far more easy to pinpoint and thus correct the defect.”

Figure 1. Utilize ultrasound as the primary tool for monitoring bearing condition.

Ultrasound also can be useful when performing inspections on complex pieces of equipment, such as a gearbox, says Gribble (Figure 1). “While vibration is good for painting the whole picture of what is happening within an asset, ultrasound can really narrow in on a specific area, making localized fault detection easier,” she offers. “When all is said and done, there’s no denying that combining ultrasound and vibration analysis can greatly enhance the diagnostic process.”

Vibration analysis and ultrasound work very well together in a predictive maintenance program, says Jason Tranter, managing director of Mobius Institute (www.mobiusinstitute.com). “However, due to the cost and skill requirements involved with vibration analysis, it can be difficult to test the plant’s rotating machinery frequently enough to provide the earliest warning of a fault condition,” he warns. “One solution is to provide operators with an ultrasound tool to scan machines on a more regular basis. If there is a change in the sound emitted by the machine, the operator can call in the vibration analyst to perform additional measurements so that a detailed assessment of the potential problem can be made. Routine vibration measurement should still be performed to detect any problems not detected by the ultrasound tool, but this strategy reduces the likelihood of fault conditions will be missed.”