ultrasound for better lubrication.pdf

8/12/2019 Ultrasound for Better Lubrication.pdf

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Benjamin Fried , CTRL Systems Inc.Tags: industrial lubricants

A manufac tu ring fac ilit y had two identical eight-inch bearin gs on amachine. The maintenance technicians knew that one was causing aproblem with the machines operation, as well as compromising thequality of the product. An ultrasonic sensor, the CTRL UL101, wasused to listen to both bearings. One bearing sounded like it wasoperating properly, but the other pr oduced no sound. Suspectingoverlubrication, half the grease was drained from the bearing thatproduced no sound. The ultrasonic sensor was used to monitor thebearing while it was being lubricated until it sounded like a normalbearing. Once properly lubricated, the equipment began operatingperfectly.

Ultrasonic technology (UT) has become widely accepted for the detection of leaks in both pressurized andnonpressurized systems. Most compressor service companies and several manufacturers own some type of ultrasonicsensor for pinpointing leaks. It is easy to cost-justify the purchase of an ultrasonic sensor based upon the high cost of energy loss due to leaks. However, there is another application for ultrasound that consumers, nondestructive testing(NDT) organizations, and even developers and manufacturers of ultrasonic sensors are often not aware of or overlook.UT can be used as a means to detect early wear of components such as bearings and gears due to lack of lubrication or overlubrication.

Ultrasonic Technolog y for LubricationMillions of dollars are spent each year by organizations to monitor critical components of equipment used for production.Preventive maintenance (PM) is widely accepted to help prevent untimely shutdowns and to extend the expected lifespanof bearings and gears. The limitation of a PM program is that it is subjective and largely dependent upon the expertise of the technician. A typical PM may require a technician to visually inspect the bearing for signs of wear or apply one half shot of grease. Although these actions are helpful and even necessary, they are not 100 percent effective and arelimited in their usefulness to improve production.

ound for Better Lubrication http://www.machinerylubrication.com/Articles/Print/67

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Improvements to ultrasonic sensors in the past couple years have enabled organizations to expand their monitoringpractices. Increased sensitivity allows todays sensors to detect smaller signals. This enables the user to detect smaller flaws and detect them even earlier. Small pieces of grit in lubrication, for example, will be detected by the sound of impact against the rollers of a bearing. Better selectivity helps diminish ambient noise or decrease white noise. Analysisand quantification is made easier when the source signal of a bearing being monitored can be distinguished from nearbycomponents. Also, once white noise is diminished, the quality of the signal is improved and is better served for analysis.

As a bearing begins to wear or lubrication becomes less effective, the ultrasonic signature will increase in amplitude andflaws will become apparent.

How Ultrasonic Technology WorksSound is caused by friction, impact, turbulence and electrical discharge. When two objects are rubbed together, theyproduce friction. Not only does friction cause sound, but it also causes heat. Generally speaking, the more friction thereis, the more sound and heat produced. As a component, such as a bearing, heats up, it expands. At some point, it willexpand too much and seize up, causing the equipment to fail and production to cease. Also, when there is a lot of

friction, particles from each component tend to be scraped off. Oil is used to lubricate bearings so that friction isreduced, but as a bearing gets older, it becomes rougher.

Impact is nothing more than a moment of friction. As gears rotate, the cogs interlock. The brief interlocking causesimpact, which produces sound. Turbulence is produced by gas leaks. As gas escapes from one pressure system intoanother, the molecules tumble out. In the case of a vacuum leak, the molecules tumble in. Likewise, electrical dischargeexcites the molecules in the air. Once excited, the turbulent behavior of the molecules produces sound.

When sound is produced, waves of different frequencies will travel from the source in all directions. The distance thesound wave travels and the amplitude depend upon the amount of energy transferred from the source and the frequencyof the wave. Lower frequency sounds travel much farther than high-frequency sounds. Earthquakes produce

low-frequency sounds. An earthquake in California can produce a wave that travels through the earths crust and can bedetected in Japan. Some animals, such as bats, produce high-frequency sounds that do not travel far. These sounds areused to detect small objects, such as insects, from close distances.

In each of the previous cases mentioned, the human ear cannot hear these sounds. They are out of the range of normalhuman hearing, which means they are below 20 hertz and above 20 kilohertz. Ultrasonic sensors use microphones thatdetect sound at a frequency about 40 kilohertz, which is well above the human range of hearing. The received ultrasoundis then converted or stepped down into an audible range and output to headphones for listening or to a computer for analysis.

By listening to components at the 40 kilohertz range, subtle events can be detected, such as small scratches on the race


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of a bearing or a piece of grit in the lubricant. As a bearing or gear rotates, the scratch or piece of grit produces impactthat is detectable only in the ultrasonic range. Furthermore, the wave produced in the 40 kilohertz range does not travelfar from the source. Therefore, when listening to a bearing in contact mode, sounds from other nearby components donot interfere. This prevents misdiagnosis and provides the ability to quickly pinpoint the real problem.

In one such case, a United States Navy ship participated in a series of onboard tests with ultrasonic technology toevaluate the potential benefit of its implementation in several areas. During the tests, the crew from another ship askedabout the ability of the technology to identify internal pump problems. The electric main lube oil pumps were not putting

out sufficient pressure. The ship was fast approaching a deployment, and the crew could not determine the root cause of the loss of pressure for the pumps with other technologies. The CTRL UL101 ultrasonic inspection system andSoundCTRL were used to test the pumps. Of the four pumps, it was discovered that one of the pumps was bad,producing an abnormal amount of ultrasound, and another was suspect. The pumps were dismantled and opened, andinspection revealed that one pump had damaged components and the other pump was binding.

What to Look For in an Ultrasonic Sensor Ultrasonic sensors used for leak detection and mechanical analysis typically use a piezoelectric microphone with center of frequency of 40 kilohertz to receive the sound wave and convert it into an electronic signal. Prices range quite radicallyfor the different sensors, but there are three ranges of prices to consider. The low range includes sensors that cost fromapproximately $100 to $1,000. The middle range has sensors from $1,500 to $4,000. The upper range can cost as

much as $10,000.

Many low-range sensors are advertised in catalogs and are often strictly used for leak detection. Most use a volumecontrol dial that simply adjusts the volume of the converted audible signal, but does not adjust the sensitivity. Thesesensors are often sold with few attachments or none at all.

Sensors in the high price range are the same as midrange sensors but with add-on software. The software is attachedto the sensor for recording and/or analyzing the signal output. Software used for analyzing ultrasonic signals may haveseveral features such as the ability to record the actual signal, record some type of signal average, calculate theaverage amplitude often using a root mean squared (RMS) method, flag anomalies, and store equipment andmaintenance routes.

Sensors in the middle price range combine features and functionality of both the low- and high-range sensors. Some of these sensors have volume control. Some have sensitivity adjustment. It is important to distinguish between the two.Sensitivity control is necessary for detecting weaker signals from greater distances. It is also necessary to minimize theamount of ambient ultrasound that could interfere with the diagnosis of bearings, gearboxes and valves.

Frequency adjustment is another important feature to consider. The center of frequency of each microphone is 40 kHz. Adjusting away from 40 kHz minimizes the ability of the sensor to distinguish the signal from white noise; therefore it isnot recommended.

Sensors may also include a digital meter or an analog meter. A digital meter will use a relative number, even decibels, toindicate the peak level of ultrasound detected. Analog meters are used to indicate the overall characteristics of thesound and are much more sensitive for the detection of small irregularities. Analog meters are also much more indicativeof the random noise produced by equipment and consume far less power.

Some other features of ultrasound equipment to consider include the design of the sensor for fitting in hard-to-reachareas and the material used to make the sensor. If used in harsh environments, a plastic sensor or one that is poorlymanufactured may not last long. Battery usage is also a consideration. Rechargeable batteries can be expensive and donot last much more than a shift or two. A nine-volt battery should be able to operate the ultrasonic sensor for severalshifts, at least. Additionally, some manufacturers offer devices that have been manufactured according to intrinsicallysafe standards. These sensors should come with a certificate of conformance, verifying that they may be safely used inhazardous environments.


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Warning signs of problems relating to lubrication can be subtle but critical. Lubricant is used to decrease friction of components. If a component is underlubricated, the level of friction and sound will increase. If the component isoverlubricated, there will be little-to-no sound produced at all.

Such subtle changes will be detected only with a sensitive sensor with a large signal-to-noise ratio (SNR). Everyelectrical component produces some small amount of noise when working. The sensor should be able to distinguish thiswhite noise from the input signal. If there is too much white noise, analysis is not possible. Additionally, too much whitenoise can dampen the input signal, making it undetectable. It is important to make sure the test system is adequate for

the applications before it is purchased.

How to Incorporate UT into an Effective PdM ProgramTo obtain the full benefits of UT, it is important to select the right sensor for the lubrication program. It is even moreimportant to spend the right time and energy implementing the ultrasonic technology correctly. Most UT companies willwork with the customer to develop an effective turnkey program.

The first step in developing a program is to become familiar with the characteristics of ultrasound, the ultrasonic sensor and the software. Most companies offer some sort of product and technology training. It is best to have on-site trainingso that the individuals who will be using the equipment can gain some immediate insight and the trainer can learn moreabout the maintenance departments goals and challenges.

The second step is to identify the critical components or areas where ultrasound will be used. There are thousands of places to use ultrasound, and it can be overwhelming to do everything at once. However, it is usually just a fewcomponents that cost an organization the most money. If an oil analysis program is already in place, those componentsthat have been frequently identified as having problems should be the first ones to be included in the ultrasound program.Ultrasound can be used to identify the causes of the problems, such as poor installation or misalignment.

Third, it is important to develop an inspection schedule that meets the organizations needs but does not put too muchburden on the maintenance staff. It is vital to monitor the identified components on a regular basis, such as twice per week. Once baselines are established, large changes in the ultrasonic signal will indicate changes in the operation,lubrication and condition of the component.

For example, a normal bearing typically produces a soft, whirring sound and a relatively even wave form when viewedwith analysis software (Figure 1). (To hear the audio of the bearings depicted in Figures 1 through 3, visithttp://media.noria.com/poa/200411/audio.htm .)

Figure 1. Normal Running Bearing

If a bearing is damaged, intermittent pops or grating may be heard, depending on the rotation of the bearing and thedegree of damage. Analysis system software will reflect a heightened crackle count, and abnormal spikes in thewaveform will be visible (Figure 2).


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Figure 2. Damaged Bearing

If a bearing is underlubricated, there will be an increased sound of friction. The sound intensity of the bearing willincrease, and it will emit an intense scraping sound. The root mean squared (RMS) value of the signal will also increase,and the height of the waveform will visibly increase (Figure 3).

Figure 3. Underlubr icated Bearing

Finally, the technicians recordings and notes should be reviewed. Ultrasound is instantaneous; with the right sensor, anexperienced technician will detect lubrication problems right away. The latest changes in ultrasonic sensors will further enhance the users ability. The most effective program will also consist of comparing record history for criticalcomponents obtained from several different technologies and programs including oil analysis, ultrasound and preventivemaintenance.

Once properly implemented, ultrasonic technology will decrease the costs of maintenance and production, increasesafety standards, and increase quality control. In other words, your organization will run a lot smoother.

Editors NoteBenjamin Fried is employed by CTRL Systems in the product development and support division. For more information

about ultrasonic technology and turnkey implementation, contact CTRL Systems at 877-287-5797. Additional resourcessuch as test techniques, frequently asked questions and multimedia clips that demonstrate the characteristics of variouscomponent conditions are available at www.ctrlsys.com .


ultrasound for better lubrication.pdf

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