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.
Proper lubrication of bearings and other rotating pieces of equipment is essential, but it is not the only maintenance technique. Listening at 40kHz offers the technician the unique opportunity to diagnose budding failures in mechanical equipment before they occur. Software that collects data samples and trends them over time can pinpoint significant changes and flag test points for further service. Using ultrasound CBM, a maintenance department can develop a turn-key system of data sampling and team management communication that can greatly aid in the lubrication, repair, and replacement process of critical pieces of equipment.