Predicting Bearing Failures and Measuring Lubrication Film Thickness in Your Plants Rotating Equipment

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Courtesy of Water Environment Federation (WEF)

ABSTRACT
Typically there are two options of maintenance concerning bearings, one is to lubricate them the other is to change them. Monitoring bearing condition is critical to insure reliability. The equipment in water and wastewater plants is an asset that must be managed and prolonging its life will insure a positive contribution to your plants operating efficiency. By incorporating a condition-monitoring program that involves monitoring both equipment condition and bearing lubrication film thickness, issues such as secondary damage, spills and excessive operating costs are avoided.

The following paper will identify some of the condition-monitoring methods used today. Factors that affect the quality of the film thickness will be discussed followed by real life case studies. The case studies will present issues that are common in all industries including water and wastewater plants followed by solutions.

INTRODUCTION
There are several ways to monitor and measure the severity of bearing problems. Ultrasonic, temperature, vibration analysis, and shock pulse are just a few of the methods used today to detect both bearing lubrication film thickness and bearing condition. Most lubrication practices are often performed on a time basis, which can result in over lubrication causing premature bearing failure.

In many of these monitoring techniques the information regarding the bearing lubrication condition is provided after the problem has developed. The key here is to adopt a balanced combination of technologies and methods to develop a good predictive maintenance program with the goal of maximizing reliability. Since lubrication is the first line of defense for prolonging bearing life it is imperative that the lubrication film thickness between the rolling elements and the raceways be measured and monitored. Lubrication is the vital element in all rolling element bearings.

With all of this said, what about the lubrication film thickness in a rolling element bearing after it is installed but before it ‘s turned on? This is where many bearings start premature failure. As indicated in the chart below and relative to the L10 formula for bearing life, “load” is a commanding factor. The lube pump is turned on and there is no apparent measured vibration alarm. There is no need for oil analysis yet because the machine is new or rebuilt with new and clean lubricant. So, what’s the problem? You may have zero or very little lubrication film thickness between the rolling elements and the raceways of the bearing. If the shaft fit and/or housing fits are too tight or the bearing is slightly cocked or not seated properly your internal clearance within the bearing will be reduced. If there is a slight alignment problem and/or overhung load from over tensioned belts your internal clearance will be reduced. If you are using a super precision bearing i.e. a spindle bearing with very little clearance built in, your film thickness will be reduced. And, the most common installation fault that I have seen in the field is the installation and set up of the tapered bore, double-row spherical roller bearings using a split tapered adapter. The tapered adapter is used as a wedge device to hold the inner race of the bearing onto the shaft. All bearing manufactures have recommended clearances for applying this type of bearing. This type of bearing in a pillow block housing is very common on larger fan applications and is a very popular style for many other applications. If the tapered sleeve (adapter) is drawn up too far by over tightening the spanner nut (a very common practice) the internal clearance of the bearing will be removed thus reducing the lubrication film thickness.

There has always been much discussion and debate regarding the question of how much, how often and what kind of lubricant should be used in rolling element bearings and to many it still remains a mystery. The following chart indicates the many parameters related to the lubrication film thickness and film quality in a rolling element bearing.

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