In today's manufacturing environment, cost control is important . . . in fact, it is essential to survival. Profit margins are shrinking; often the difference between profit and loss can be as simple as improving efficiencies. Locating sources of waste and identifying failure conditions can contribute to helping improve the bottom line.
Related to bearing failure, the most influential cause of failure is lubrication related. This includes using the wrong lubricant along with improper lubrication practices such as not enough or too much lubricant. Using improper lubricant can damage a bearing to the point of irreversible failure, causing machine and production shutdown, lost hours and significant downtime. Bearings running with too little lubricant can cause friction requiring more energy to overcome the resistance, which can lead to bearing failure and eventual seizure. Using too much lubricant can also produce heat, break seals and decrease acceptable tolerances, which will lead to bearing failure.
Proper lubrication of bearings is essential as it helps dampen stress distribution. As stated, lack of lubrication will create friction while overlubrication creates grease build up, thickening the area around the bearing, making it difficult to rotate.
In order to eliminate the problems caused by over- or underlubrication as well as several other inspection requirements throughout the plant, many companies around the world have incorporated some form of a condition monitoring program. Condition monitoring is used to check the health or "condition" of operating equipment, as opposed to the other forms of maintenance, such as reactive, in which a failure condition has occurred and maintenance personnel must "react" to the problem, or preventive, where maintenance activities are performed on a set schedule. Any change in monitored fields can alert maintenance personnel of potential failure and allow the repair to be performed on a scheduled, controlled basis.
Traditional lubrication condition monitoring programs include preventive procedures such as time-based lubrication, where lubrication is performed at set, timed intervals with a specified amount of grease applied. However, the issue with time-based lubrication is that if the bearing being lubricated has a sufficient amount of grease already and therefore DOES NOT need lubricant, the inspector is at high risk of overlubrication. Another issue with time-based lubrication is that some bearings may require lubricant to be applied more frequently than assumed through this procedure.
These types of issues do not preclude the need for standard preventive procedures such as removing old, used-up grease and adding new grease. Among the most effective methods available to assist in lubrication inspection is the use of ultrasound technology.
Ultrasound technology
Airborne/structure borne ultrasound instruments receive high frequency emissions produced by operating equipment, electrical emissions, and by leaks. These frequencies typically range from 20 kHz to 100 kHz and are beyond the range of human hearing. The instruments electronically translate ultrasound frequencies through a process called heterodyning, down into the audible range where they are heard through headphones and observed as intensity and/or dB levels on a display panel.
Adding ultrasound monitoring to standard lubrication best practices can prevent potential overlubrication of bearings, which can also lead to fewer bearing failures, extend motor and bearing life, and lead to a decrease in the amount of lubricant used.
And, that affects your bottom line through:
Savings in maintenance costs, lubricant and man-hours
Improved asset availability and reliability
Lubrication: What is too much vs. too little?
While many bearings can fail due to lack of lubrication, overlubrication is considered one of the major causes of bearing failure. Standard preventive time-based procedures may be useful if this practice is followed without any feedback regarding the condition of a bearing. However, it may lead to an overlubricated condition that will eventually cause bearing failure. Many maintenance departments are therefore switching to a combination of preventive and condition-based lubrication.
A condition-based lubrication program requires a combination of trending bearing decibel levels and basic sound analysis. A baseline decibel level is set, along with (if possible) a baseline sound sample, and an inspection schedule is established for periodic testing. When a bearing sound level exceeds 8 dB with no change in the sound quality (usually a smooth, "rushing" sound), the bearing is considered in need of lubrication.
A lubrication technician, while listening to the bearing, will then apply lubricant, a little at a time, until the baseline level is reached. Stopping at that point prevents overlubrication.
Case in point
A maintenance manager for a food manufacturing plant reported at a recent ultrasound conference a savings of $220,000 in their bearing-monitoring program. The company also reported that since initiating ultrasound condition monitoring at another of their facilities more than three years ago, there have been no unannounced failures in the 340 bearings they monitor, and that their motor repair cost was reduced from $2400 to $600 per motor. In fact, all their criteria for rebuilding a motor are based on condition rather than preventive, time-based procedures.
This switch has resulted in a three-fold savings, which translates into $90,000 per year. The benefits of an ultrasonic condition-monitoring program also go beyond bearings. Ultrasound instruments are used to reduce energy costs by locating steam and compressed air leaks and improving equipment availability by locating arcing and tracking problems before flashover in electrical apparatus such as transformers, switchgear, and motor control centers.
How can ultrasound technology provide such accurate results?
During inspection using airborne/structure borne ultrasound, the inspector will sense friction in mechanical instruments. Trending associated amplitude levels and changes in sound quality of a bearing also provide early indication of conditions such as lack of lubrication and overlubrication.
Beyond its ability to detect premature or existing bearing failure or over/underlubrication conditions during a route, the newer digital ultrasonic instruments (such as the Ultraprobe® 10000 and 15000) utilize data management and sound analysis software to predict potential failure conditions.
These digital instruments will enable the plant technician to set baselines, log data, record sound samples for spectral analysis, and analyze sounds. Some of the newer portable digital instruments have onboard spectral analysis and data software to allow for on-the-spot analysis.
Beyond accuracy, what are the advantages of ultrasound related to lubrication analysis?
The basic advantages of ultrasound and ultrasonic instruments are:
1. Provide early warning of impending mechanical failure.
2. Isolate signals to a specific test point.
3. Can determine the quality of a bearing.
4. Can detect exact lubrication needs (i.e. add or stop adding lubricant).
5. Can be used in loud, noisy environments.
6. Can be used on slow-speed bearings.
7. Support and enhance other PdM technologies or can stand on their own in a maintenance program.
So what are you waiting for?
The benefits of ultrasonic condition monitoring are quite substantial. From identification of bearings in need of lubrication to prevention of overlubrication, ultrasound instruments can reduce the amount of grease you need in inventory, improve your asset availability and most importantly, your bottom line.
The initial investment is relatively inexpensive and the return on it could be immense. To help push an ultrasound program along, it is a good idea to attend a training course. There are certification courses available that cover all the major applications and provide all the information necessary to implement a successful program.
Mr. Bandes has been involved with airborne/structure borne ultrasound since 1973. He has lectured and published articles in many highly regarded technical journals. www.uesystems.com
