Over the past decade or so, it has been interesting to see the evolution of maintenance and reliability. To some, the word maintenance brings a perception of general housekeeping duties such as janitorial tasks or changing light bulbs, but for most, maintenance has become almost synonymous with reliability.
I feel like the reliability field is in a transitional phase as more people are becoming more proactive regarding maintenance rather than being reactive once a failure has happened. Additionally, the condition monitoring tools that are available today are very advanced, and in some cases can give the user almost instantaneous information in order to make a diagnosis on an asset. Mobile devices such as tablets and smartphones have been integrated. For some applications, continuous/remote monitoring is almost a must.
Airborne and structure-borne ultrasound has certainly become a major player in condition monitoring. Once considered just a leak detector, more maintenance and reliability professionals are realizing the benefits associated with using ultrasound for condition monitoring applications, and the I-P-F curve with which we have all become familiar with reflects that trend (Figure 1). We must be concerned not only with the P-F interval, once a failure has been detected, but also with the I-P interval. The I-P interval is where precision maintenance is used to ensure that the equipment is being installed properly using precision balancing and alignment and installing the bearings properly. Precision maintenance is achieved when ultrasound is used to prevent over- and underlubrication of bearings, thus helping to extend the I-P interval.
Figure 1. The I-P-F curve shows ultrasound as being the first technology that detects a failure that is mechanical in nature, such as early stage bearing wear, or subsurface bearing fatigue. Precision lubricating bearings with ultrasound helps to extend the I-P interval.
For example, studies have shown that at least 60% of premature bearing failures can be attributed to lubrication problems, whether the result of overlubrication, underlubrication, the presence of contaminants in the lubricant, or the use of the wrong grease for the wrong application. Ultrasound instruments can be used to prevent over- and underlubrication of bearings. The source of the ultrasonic noise is friction. When a bearing is in need of grease, there is an increase in friction, and consequently an increase in noise or decibel level. When listening to a bearing that is in need of lubrication and watching the decibel level on the display of an ultrasonic instrument, as grease is applied, the inspector will notice a gradual drop in the decibel level, moving eventually back down to a more normal level. When grease enters the bearing housing, there is less friction and less noise, which explains the drop in the decibel level. If the bearing is already overlubricated or is already sufficiently lubricated, as soon as grease is applied, the inspector will notice a gradual increase in the decibel level, letting him or her know that the bearing already had enough grease.
There are two common questions that many first-time users of ultrasound technology for condition monitoring have. The first is, “How do I set baselines?” The second is, “How do I know if what I’m hearing is good or bad?” They are valid questions, given that there is no previous ultrasound data on the equipment to be tested, and the users are simply unfamiliar with using ultrasound for this application.
This article will explore three different techniques for condition monitoring with ultrasound tools. For setting baselines, the comparison method and the historical method will be discussed. The third technique is ultrasound imaging, or the use of recorded ultrasounds to make a better diagnosis as to what’s good and what’s bad. When a recorded ultrasound is played back in spectral analysis software, we can view both the FFT and the time wave form of that sound; this approach helps to paint a picture of what it is that we are hearing. Some ultrasound instruments offer the capability to view the FFT and time wave form view in real time while data is collected – a concept sometimes referred to as ultrasound imaging.
The comparison method
One way to get a quick idea as to what is good and what is bad is by using the comparison approach. With this method, the inspector simply compares the decibel level readings at identical points on identical machines (Figure 2).Using this method, the inspector also begins to train his or her ear as to what rotating equipment sounds like, and it will become obvious that a bearing with a particular fault such as an inner race, or outer race defect, will sound much different than a bearing that is in good condition.
The baseline then can be set based off an average of decibel levels at the compared points. The software may even default to the first reading taken and downloaded; however, the baseline can then be changed in the software as more readings are collected.
Figure 2. The image above shows where readings were taken at 6 identical motors using the comparison method. The decibel level on the 6 motor outboard points were all within three dB of each other except for Motor B. The Motor B outboard point was 12 dB higher than the other 5 motors, thus indicating a need for further inspection or action.