- Most bearing failures are lubrication-related.
- Relying on time-based, periodic lubrication assumes bearings need to be greased at defined time periods.
- 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.
Keeping a handle on lubrication is easy, right? All one needs to do is make sure the right lubricant is used in the right amount and at the right time. Not so fast; if only it were that simple.
Most 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 its face makes perfect sense but in fact borders on haphazardness.
Many technicians, unfortunately, have relied on preventive, time-based lubrication alone, that is, every certain 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 can cause premature bearing failure.
Figure 1. By using ultrasound technology technicians can gain a clearer picture of what’s really going on and improve reliability.
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 (Figure 1).
Ultrasound and lubrication
Ultrasonic equipment detects airborne and structure-borne ultrasounds normally inaudible to the human ear and electronically transposes them into audible signals that a technician can hear through headphones and view on a display panel as decibel (dB) levels. In some instruments, 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.
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 dB 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’s 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 programs 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 (Figure 2).