It has been estimated that 60-90 percent of all bearing failures are lubrication-related. Bearing failures most often lead to unplanned downtime, which can impact production as well as affect all related components around the bearing.

Downtime is costly. While the cost varies by incident and by plant, it can add up. Since the most common cause of bearing failure is lubrication-related, it’s clear to see that lubrication is serious business. And for the longest time, that serious business has been conducted in a way that on face value makes perfect sense – but in fact borders on haphazard.

Many technicians, unfortunately, have relied on preventive, time-based lubrication alone. That is, every X number of months the grease gun comes out, and the bearings are lubricated. After all, under-lubrication can be lethal, causing equipment failure, costly repairs and replacements, significant unplanned downtime, and lost profits.

But by relying solely on time-based lubrication, or even a combination of planned maintenance and temperature readings to serve as a proxy for lubrication status, one runs the risk of something just as bad, if not worse: over-lubrication. In fact over-lubrication has been reported both anecdotally and in presentations at various conferences to be the primary cause of premature bearing failure.

Relying on time-based, periodic lubrication assumes bearings need to be greased at defined time periods. Often this evolves into a well-intentioned guessing game at best. Adding more lubrication to a bearing that is already adequately greased is a real risk.

By using ultrasound technology along with standard practices such as removing old grease and replacing it with new, technicians can combine standard time-based maintenance with condition-based, predictive maintenance, gaining in the process both a clearer picture of what’s really going on and better reliability.

How Ultrasound Works

Ultrasonic equipment detects airborne and structure borne ultrasounds that are normally inaudible to the human ear and electronically transposes them into audible signals that a tech can hear through headphones and view on a display panel as decibel (dB) levels. In some instruments, such as the Ultraprobe® 15,000 Touch, the received sound can also be viewed on a spectral analysis screen. With this information a trained technician can interpret the bearing condition in order to determine what, if any, corrective action is needed.

Figure 1 shows a Time Wave Form of a recorded ultrasound of a bearing in the process of being lubricated. The approximately 1 minute sound file shows the bearing before and after lubrication.

Fig 1. Bearing lubrication example 1.

Figure 2 shows a Time Wave Form of a bearing in the process of being lubricated over 13 seconds. Again, a distinct difference before and after lube can be seen.

Fig 2. Bearing lubrication example 2.

Figure 3 shows the bearing dB and amplitude increase as more lube is added and has reached the threshold of over lubrication.

Fig 3. Bearing in the beginning of over lubrication.

The chart in figure 4 shows a trend of decibel readings taken on a drive bearing. The readings are plotted against a baseline dB, a low alarm dB and a more critical high alarm dB level. Ultrasound technology has many advantages:

• It can be used in virtually any environment.

• Learning to use ultrasound technology is relatively easy.

• The technology is relatively inexpensive.

• Modern ultrasonic equipment makes it easy to track trends and store historical data.

• Ultrasonic technology has proven itself to be extremely reliable in predictive maintenance, saving in cost and hours of lost productivity.

Fig 4. Drive bearing lube history.

How to Produce Better Lubrication Practices

Ultrasonic technology helps the lubrication technician take a lot of the guesswork out of lubrication needs. Ultrasound is a localized signal meaning when a sensing probe is applied to a bearing, it will not be affected by crosstalk and allows the technician to hear and monitor the condition of each individual bearing. Ultrasound looks at each individually, much the same way medical ultrasound can detect exactly which artery is clogged or which vein is leaking.

As an example of ultrasound’s efficacy, consider this: a maintenance manager at a large firm reports that, since adopting ultrasound technology and practicing ultrasound-assisted, condition-based monitoring rather than running to put out the fire, his plant has gone from close to 30 rotating equipment failures per year to zero in three years. But how does ultrasound work, exactly, vis-à-vis lubrication?

The first step is establishing both a baseline decibel level and a sound sample. This is ideally done when moving through a route for the first time by first comparing dB levels and sound qualities of similar bearings.

Anomalies will be easily identified. Once established, each bearing can be trended over time for any changes in either amplitude or sound quality.

Generally speaking, when the amplitude of a bearing exceeds 8 dB and there is no difference in the sound quality established at baseline, the bearing needs to be lubricated. To prevent potentially disastrous over-lubrication, the technician will then apply lubrication, a little at a time, until the dB level drops.

Many departments set up their condition-based lubrication programmes by incorporating a two-stage approach. The reliability inspector uses a relatively sophisticated ultrasound instrument to monitor and trend bearings. A report of bearings in need of lubrication is produced.

The lube technician then uses a specialized ultrasound instrument that alerts the technician when to stop adding grease. These instruments can be affixed to a regular grease gun or worn in a holster.

To improve efficiencies, it is good practice for the technician to note when the equipment was last greased and how much grease was used to calculate roughly how much lubrication is used per week. By using ultrasound to lubricate each and every time, the technician produces a historical data that can be used as a guide from previous calculations, helping the department to determine whether the lubrication schedule can be modified, perhaps saving man-hours, and whether the manufacturer’s suggested lubrication amount is accurate (if less is needed, there’s cost-savings potential).

And while most of this discussion has focused on the dangers of under and over-lubrication, ultrasound is just as reliable in picking up other potential bearing failure conditions. The technician, using ultrasound, can hear telltale grinding sounds and other anomalies, which are often accompanied by an amplitude increase.

With regards to lubrication, the advantage of ultrasound is that it is able to isolate bearings and determine their individual needs, thus reducing the possibilities that some bearings are too dry and prevent others from over-lubrication.

Why to Start Using Ultrasound Today

It is always a daunting proposition to make a new investment in technology. Will it pay off? Will my staff actually have an easy time using it? Is it a flash in the pan or a truly reliable modality that will stand the test of time?

While more and more plants are using ultrasound and adopting a predictive and proactive rather than reactive mindset, there are still many that are figuratively using crystal balls and outdated methodologies. The end result is poor reliability, unnecessary man-hours, downtime and lost productivity and profit. While ultrasound can’t cure all reliability ills, it has proven itself – in a plethora of settings – to be a valuable and powerful diagnostic tool that technicians must add to their toolkit.

When it comes to something as important to reliability as lubrication, the question really becomes: Can you afford not to use ultrasound technology? Consider the plant that went from close to 30 bearing failures a year before using ultrasound to having no bearing failures for three years once bringing ultrasound into its maintenance arsenal. It’s no coincidence. Ultrasound works.

For more information on ultrasound technology and to access valuable resources, including a sound library, presentations and articles, visit UE Systems’ website at www.uesystems.eu .

Adrian Messer

Manager of US Operations, UE Systems, Inc.,

adrianm(at)uesystems.com