Ultrasound for Condition Monitoring and Acoustic Lubrication for Condition-Based Maintenance


With all the hype about acoustic lubrication instruments, you would think these instruments, once turned on, would do the job for you. Far from it! Knowledge is power, but, as Albert Einstein said, Information is not knowledge. The only source of knowledge is experience. You need experience to gain wisdom.”

When implementing an acoustic lubrication program, “know” the people you are placing into this position. They must have electric motor knowledge and experience. Acoustic lubrication is more than just putting on a set of headphones and pumping grease into the bearing. It’s more than just the sound of the bearing, in fact, it’s only about the decibels.

Figure 1: Using an ultrasound instrument to trend decibel readings for trending data (Photo courtesy of The Ultrasound Institute)

Ultrasound as a Condition Monitoring Tool

During acoustic lubrication, there is an inflection point, which is the point where the decibel goes upward or increases. Not catching the inflection point, could be the difference between over lubrication and under lubrication.

Let’s use a visit to a paper mill as an example. The person performing the lubrication said the large motor required 14 ounces or one tube of grease. When asked if he thought this was too much, he replied: “No, the person who just retired had always put that amount in.”

However, when asked to listen with an ultrasound instrument to the bearings he had just lubricated, he insisted the bearing must be bad after hearing a popping noise. Most of what he was hearing was the result of pumping 14 ounces of grease into the motor. Using too much grease can end up in the motor housing, blow the grease seal, push grease into the motor, causing it to overheat by insulating the windings, and possibly cause premature failure. Not to mention the internal pressure on the bearing itself.

Afterward, a visual inspection of the motor was done for grease exiting the motor casing. No grease exiting the motor was found, so he continued to think he was doing everything right.

It is not uncommon for a maintenance team to give the acoustic lubrication program to the least experienced worker. Not a mechanic, not a technician, but someone with little mechanical experience. This is a formula for disaster.

Ultrasound is an excellent condition monitoring (CM) instrument. CM is a type of predictive maintenance that involves using sensors to measure the status of an asset over time during operation. With CM, maintenance is only performed when the data shows that.

Affordability, ease of use and one-point data point retrieval versus multiple points, such as vertical, horizontal and axial, are some of ultrasound’s benefits. If you prefer less downtime, less man-hours lubricating motors, early detection of potential failures, less motor maintenance and less grease purchased, then implement ultrasound testing as another CM tool.

On the P-F curve, ultrasound is first to detect a defect, doing so before vibration and before infrared. Granted, because it is so early in detection, it can be very subjective. But, being too early in detection depends on the criticality of the asset being monitored.

Figure 2: P-F curve shows ultrasound as “Early Signal 1”

Having vibration analysis as your primary CM tool requires having enough vibration technicians and vibration instruments to cover all your assets. Implementing ultrasound requires setting a baseline, like vibration data, and trending 30, 60 and 90 days.

Acoustic Lubrication in Condition-Based Maintenance

Condition-based maintenance (CBM) is described as maintenance “when need arises.” This maintenance is performed after one or more indicators (e.g., vibration, temperature, or decibels) show that equipment performance is deteriorating or going to fail.

When trending bearings with ultrasound, once the reading is eight to 10 decibels above the previous baseline, then CBM, or you could say conditioned-based lubrication, is in order. In this case, acoustic lubrication would be the CBM.

To use an ultrasound instrument to view the decibel at the time of lubrication, you must be able to identify a beginning point. One option is the half-stroke method. As you lubricate using the half-stroke method, when the inflection point is noted, stop and give it 10 to 15 seconds to return to the decibel reading prior to the inflection point. (Note: In freezing or subzero weather, you may need to wait 20 to 30 seconds for the decibel to come back to the point prior to the increase.)

If the reading does not return to where it was prior to the inflection point, then stop lubricating. The half-stroke used confirms the bearing is not in need of any more lubrication. The procedure is complete. However, had the decibel dropped back to the starting point or less, another half-stroke would be warranted. Repeat this action until you reach the inflection point again.

Figure 3: Using a grease gun with handheld acoustic lubrication instrument with decibel displayed and sensor (Photo courtesy of SDT Solutions)


Why Condition Based Lubrication?

You may be thinking: Why not simply perform a preventive maintenance (PM) action to lubricate a motor with two to three shots of grease? The answer is simple: The motor does not require any grease. It’s not unusual for maintenance to have a scheduled PM requiring two to three full strokes of grease into the bearing. It’s also not unusual for maintenance to put four to five pumps of grease instead of the recommended two to three pumps.

Many organizations have had great success after implementing acoustic lubrication. Your maintenance team may have experienced some hiccups implementing acoustic lubrication. But, don’t get discouraged. One of the leading reasons why this practice gets dropped or is underutilized is the fear of over greasing.

Make this practice a slow and deliberate one. If using a manual grease gun, make sure you have positive pressure throughout the half-stroke of the grease gun.


Figure 4: Performing acoustic lubrication using a manual grease gun and an ultrasound instrument (Photo courtesy of The Ultrasound Institute)


There may be instances where motors require more than 10 half-strokes of the grease gun. It is important to watch the decibel of the ultrasound receiver. Watch for the inflection point or db rise in the sound level. When there is a rise or inflection of the db reading, stop and allow 15 to 30 seconds for the decibel to return to where it was prior to the inflection. If the db reading returns to its position prior to inflection, apply another half-stroke of the grease gun. Look again for the inflection point to return to where it was prior to the last increase. Once the decibel rises and does not return in 15 to 30 seconds, then you are done. Record the readings and move to the next bearing.

However, if you are performing this method of acoustic lubrication and you have completed 10 or more half-strokes and the decibel difference is not noted, then take a moment and look over the motor for signs of grease being pushed out. Look around the bearing, motor casing, grease seals, lightning holes, etc. If the motor still requires grease, then continue the acoustic lubrication practice.

Delivery of the grease is important. But, whether it’s a Zerk fitting or a button and hook setup, what matters is consistent decibel readings for trending.

Button and hook hardware is often preferable due to the strong lock and hold of the button that is threaded into the motor casing. A Zerk female adapter tends to move around when mated to the male Zerk fitting, sometimes causing inconsistent db readings. Repeatability of the decibel is important for trending purposes. You may want to discuss with upper management the removal and replacement of Zerk fittings with a button and hook setup.

Figure 5: The button and hook setup for greasing motors, with (left) button prior to being placed over the button and (right) hook after being pulled atop button (Photo courtesy of The Ultrasound Institute)


Again, implementing ultrasound as a condition monitoring practice may indicate an early defect, failure, or a potential failure early on and not just a need for lubrication. Today, many maintenance departments are using vibration instruments to trend motor bearings. However, a great deal of them only have enough instruments to service a small portion of the facility’s motor bearings.

Ultrasound used in a condition monitoring program can serve to offer a viable solution in two ways. First, it provides a means to detect early signs of defects, failure, or simply when lubrication is needed. Second, it determines if there is a problem beyond the need for lubrication. For example, let’s say the bearing has a rise of eight to 10 or more decibels since your last report and you perform condition-based lubrication, such as acoustic lubrication. If the decibels don’t go down, does that mean the bearing doesn’t need any grease? Your next step, depending upon the criticality of this motor, is to put this bearing on watch. Or, possibly call for a vibration analysis of the asset.

Figure 6: Using a grease gun with a mounted acoustic lubrication instrument with decibel displayed and sensor and Zerk fitting (Photo courtesy of UE Systems, Inc.)


This, too, would be condition monitoring. Whether you are using ultrasound or vibration to trend motor bearings, it is condition monitoring. In fact, ultrasound can be a stand-alone tool for a large majority of your mechanical, electrical and energy maintenance and inspections.

Jim Hall

Jim Hall, CRL, is the Executive Director of The Ultrasound Institute (TUI). Jim has been in the ultrasonic market for over 25 years and has trained many Fortune 500 companies in the use of airborne ultrasound, including the electrical power and generation, pulp and paper, automotive and aviation industries. Jim has been a contributing writer for Uptime® Magazine’s (ultrasound segment) since the magazine’s inception.
www.theultrasoundinstitute.com

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