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Ultrasound for Predictive Maintenance (PdM)

Predictive maintenance goes a step beyond PM by monitoring the condition of critical components in order to predict failures that could result in unnecessary shutdowns. In the past, these programs have often been expensive to implement, difficult to learn, and cumbersome to execute. Trends are shifting, however, and many industrial organizations are making a move toward a more predictive system of maintenance. This is partially because a lack of manpower and time to address maintenance issues has made PdM is more crucial than ever before, but also because the available technologies and methods of implementation are more cost-effective and easy to learn than they have been in the recent past.

While PdM has traditionally been performed through outstanding technologies such as vibration analysis, infrared, or oil analysis, one of the simplest and most cost-effective methods of PdM has proven to be ultrasound acoustic listening. While most people are only aware of ultrasound transmission technologies such as medical ultrasound, ultrasound acoustic listening is a less commonly known method for leak detection, non-pressurized vessel testing, electrical failure detection, and the condition-based monitoring of critical components. A single high-quality ultrasonic inspection device can be used for all of these applications. These devices are lightweight, ruggedized, and non-destructive. When paired with an intuitive software application for data collection, ultrasonic hardware can be used to develop an effective turnkey PdM program.

Why Ultrasound?

The top end of the human auditory scale is around 20 kHz. High quality ultrasound detection equipment listens at a frequency of 40kHz and translates that higher frequency sound into the human auditory range through a noise-canceling headset. At 40kHz, ultrasound is produced by friction, impact, turbulence, or electrical discharge. This gives the technician the benefit of hearing compressed gas and vacuum leaks, budding mechanical failures, steam trap and valve failures, and electrical failure, even if the industrial environment is incredibly loud.

As it relates to maintenance for mechanical equipment, friction and impact are the key sources of ultrasound that this paper will consider, as they are the by-products of mechanical equipment. For instance, a roller bearing will produce friction as the shaft and balls roll around the bearing’s center. This friction, in turn, causes sound, but it also causes heat. Issues such as imbalance, improper installation, or debris in the bearing can cause an increase of friction. An increase in friction will generally result in an increase of sound and heat, as well. As the component heats up, it will expand. Eventually, the bearing will expand too much, seizing up and causing the equipment to fail. Another by-product of increased friction is spalling: as the components expand, particles from each component are scraped off and cause further damage. Bearings are usually lubricated to reduce this friction, but lubrication cannot entirely stop the aging process that causes bearings to become rougher with time.

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