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Contact and Magnetic Sensors for Ultrasonic Bearing Inspection and Acoustic Lubrication

Ever wonder why the readings and/or data collected are inconsistent when using either the magnetic contact sensor or handheld contact sensor during ultrasonic bearing or mechanical inspections? As Jason Tranter, managing director of the Mobius Institute, writes in an article published on entitled, Using a Magnetic Mount Accelerometer for Vibration Data Collection, "Portable sensors are sensitive to how they are mounted on the machine during data collection." Note the emphasis on the word "sensitive."

During the practical exam for a recent training class for Ultrasound Level I, technicians were receiving inconsistent readings when using the magnetic sensor while taking readings on a bearing trainer unit. Upon further inspection, it was determined that the approach the technicians were using was to simply come near the target mounting pad and let the sensor snap onto the pick-up point. In his article, Tranter warns vibration techs that this practice can damage an accelerometer. He recommends approaching the pad from 90 degrees and rolling the magnet onto the surface.

Another area of concern during the practical exam was the decibel response of the three individual bearings of the bearing trainer. When the technician turned the ultrasonic magnetic transducer either clockwise or counterclockwise, or simply nudged the sensor in any direction, a difference of five to 10 decibels could be seen. This difference could mean the difference of changing a bearing today rather than later, scheduling a replacement at the next earliest convenience, having a non-scheduled production stoppage for catastrophic motor or bearing replacement, or losing a weekend spent with the family because of unscheduled maintenance.

The readings in Figure 1 show differences that could be an alert to a potential problem, or simply the result of how technicians touch or contact the target for readings. Even in this particular graph, the readings in the white area of the grid are well within the good range, but if they were in the upper graph in the red, this could be a problem.

A target mounting pad, shown in Figure 2, is commonly referred to as the vibration pad that an accelerometer is placed on for data collecting. It also can be the point where an ultrasound technician comes back to from the previous data collecting reading to keep the readings consistent for comparison purposes.

The ultrasonic contact, or stethoscope probe as some may call it, went through a change in appearance awhile back. This change involved the waveguide, the five- to six-inch rod screwed into the ultrasonic contact sensor that is placed onto the surface of the motor, gearbox, or pump. Earlier versions had a rounded tip; the newer designs seen today are sharply pointed. Why? Mostly because of the movement at the end. A sharp pointed rod or waveguide allows the point to dig into paint and prevent "walking." This walking is friction. Friction equates to higher readings than normal. Yes, some technicians in those days did, and many may still do, position a washer as a reference point for the placement of the waveguide to achieve comparative information or data.

Another problem when using a contact sensor is pressure. How much pressure is too much or too little? An exercise used in Level I and Level II classes is for a team leader, senior technician, or an experienced ultrasound technician to take a reading on a shop grinder. Most maintenance shops have an industrial grinder in the plant's maintenance space, usually one of those two-wheel grinders that is typically older than everyone in the shop or plant.

The exercise goes like this: First, take a marker and mark a spot on the motor for technicians to take their readings. Or, take a washer and adhere it to a preferred pick-up point on the grinder that will allow for a good reading on the bearing of either the left or right grinding wheel. Next, the experienced technician should place the contact probe or point on the mark or in the center of the washer. Then, the technician should apply force on the contact sensor until he or she feels there is a good read of the bearing in question. Lastly, anyone else tasked with using the ultrasound instruments should be encouraged to repeat the steps, with the exception of marking the spot.

Magnetic mount sensors can be a blessing in disguise for those maintenance teams wanting to reduce inconsistent reads or trending data. But, they also could be a problem. Data collection requires repeatable information for good assessment. For sensitivity and diagnosing, the standard handheld sensor (non-magnetic) is typically more responsive, particularly those sensors made by the manufacturer.

Some manufacturers of ultrasound equipment may purchase an ultrasound magnetic sensor from a third-party vendor and include it in their kits. However, those sensors designed and directly manufactured by the manufacturer are typically calibrated to their instruments for peak performance.

Magnetic sensors, such as a two-pole magnetic sensor, are preferred by many since the two-pole magnetic base may mount on flat or rounded surfaces. Standard pick-up points need to be flat and free of dust, dirt, grease and excessive buildup of paint or coatings.

Notice in Figure 4 that the 30 kHz frequency and the 40 sensitivity reads 51 decibels, with the sensor positioned in the opening between the magnetic poles facing up (red dot) on the target mounting pad of a bearing trainer (bearing #3). Figure 5 is the same instrument set at the same frequency and sensitivity, but the sensor is slightly nudged. Notice the difference in decibels.

Although the bearing training unit only shows a nine decibel difference, motors in a plant may vary much more. A nine decibel difference may not seem like much, but imagine preparing for a weekend off and you get a call that overtime has been approved for you to change a critical motor in production that just registered that bearing in the catastrophic range. Had the sensor been positioned for decibel reading and sound quality, there may not have been a difference. That being said, you should never try to manipulate your findings to the lowest possible decibel. However, you should place the magnet on the target and position it for a decibel number and sound quality before storing your readings.

When trending bearings, whenever decibels increasingly go up, no matter which sensor (contact probe or magnetic base sensor), there is a reason. Whether it's general wear or lubrication, it is an indicator of action to be taken. Unlike with vibration, ultrasound depends on the amount of decibels for condition. However, plants with vibration technology on hand should use the ultrasound decibel indication as a time to then use vibration as a final or complementary technology.

With regards to acoustic lubrication, most technicians that are engaged in acoustic lubrication use magnetic sensors. Many are not briefed on the importance of placement for consistent readings. As such, many take readings too far from the bearing's outer race. Others may place the magnet in a position closer to the bearing, resulting in a much greater decibel reading then the previous person who took the reading too far away. As the saying goes, always compare apples to apples and oranges to oranges.

Let's quickly review:

  • Be sure the target mounting pad is free of dirt and grease.
  • Be sure placement is near the bearing for consistent readings.
  • Be sure the pressure placed on the contact probes is consistent among all ultrasound technicians by practicing on a shop grinder or motor.
  • After placing the magnetic sensor on the target, position the magnet by rolling the sensor onto the surface.
  • Keep readings consistent by watching the decibel and listening for sound quality.

To some, these suggestions may be just common sense, however, new employees may be using the equipment for the first time and are not aware that the placement of the sensor can make a big difference. Or, as what happened in a recent class, several seasoned mechanics were not aware of the importance of the positioning and/or pressure needed on either the contact probes or magnetic sensors.

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