Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation
There is a science and an art to greasing. Yes, there is an "art" to greasing and here's why.
Which part of greasing is science and which part of greasing is art? Calculating the oil film thickness required to successfully support a ball bearing in its raceway at a given speed, temperature, load, and geometry, is a science. Knowing how often to apply the grease and in what amount is the art. The word "art" is the politically correct term that replaces the word "guesswork." For it is in the application of the scientifically formulated grease that many companies stumble. The old adage "some is good, so more must be better" does not hold a fond place in grease lubrication theory.
However thorough grease manufacturers are in recommending grease type, quantity, and application frequency, and regardless of whether the recommendation is based upon test stand data or empirical field data, it all boils down to the person holding the grease gun. The application of grease is subjective and is totally dependent upon the knowledge of the person doing the greasing. Attempts are underway to diminish the Achilles heal of greasing by means of education, automation, and diagnostics. On one hand there are lubrication classes to go to and on the other hand there are automated grease dispensers to buy and diagnostic sensors to use. All three are attempts at eliminating the guesswork out of greasing.
What then is so unpredictable and frustrating about this function? Greasing a piece of equipment is not as simple as adding oil. Adding oil is so simple that a child in the sixth grade who can read and understand a label can perform this function without complications. Every single piece of rotating equipment that is oil lubricated has a level marker with clear instructions "fill with oil to this level." It can't get any simpler. It either case, whether it be oil or grease, both functions are attempts at delivering the minimum required oil needed to develop an adequate film barrier between the rolling element and the raceway without generating an excessive amount of heat. When it comes to accomplishing this task oil is an easier method to use than grease. At best, greasing a piece of equipment is a blind attempt at developing that minimum oil film barrier. A predominant failure mode for a greased bearing is over greasing. The analogous failure mode for oil would be in over filling the equipment well above the "fill line." In the case of over filling a bearing with oil, the rolling elements churn through the oil and generate a significant amount of heat. It's the same for over greasing. The rolling elements churning through the grease generate a great deal of heat that in turn causes an excessive amount of oil to come out of the grease. The process feeds upon itself and the situation only gets worse with more and more oil coming out of suspension.
If the cavity that surrounds the bearing is large enough in size the excess oil from the grease should splash outwards away from the bearing and leave only enough oil to create an adequate film barrier. That assumption is totally correct if and only if there is sufficient cavity space, and, that space isn't plum full of grease. Why would the cavity be completely full of grease? The cavity could be completely full of grease if 1) the application information for quantity and frequency is excessive, 2) instructions are given to fill the bearing full of grease until it is expelled from the drain plug, 3) the cavity size is too small in the first place, 4) the cavity space is full of thickener and not grease, and most important 5) an untrained person on an off-shift finds that the bearing is running hotter than normal and pumps the housing full of grease (happens more times than I care to think about). In all of these cases the outcome is severe. The bearing overheats and the unit is taken off-line for inspection and/or repair. In either case, the unit is not making money for the company. With a grease, as the oil comes out of suspension and lubricates the rolling elements the oil will eventually degrade, decompose, oxidize and disappear. The thickener, on the other hand, will remain indefinitely. It doesn't decompose and disappear; it accumulates. The thickener is the substance in the grease that holds the oil in suspension. The accumulation of the thickener within the bearing cavity can be a problem, especially in high temperature grease applications. It is very important to match the temperature performance of the grease with the expected operating temperature environment of the bearing. The more often a bearing runs hot, the greater the likelihood that more oil than normal will come out of suspension and more thickener will accumulate.
As an example of a high temperature application in one particular plant, fired kiln trunnion bearings have large bearing cavities. Some of these bearings have been in service for nearly ten years, running continuously. The secret to their longevity is in pulling the bearing covers every six months and removing all of the thickener that has accumulated. The lesson of pulling the covers was learned the hard way when one of the bearings failed. Inspection of the bearing showed fresh grease engulfing an inordinate amount of spent thickener. The new lubrication mechanic swore that he greased the bearing according to tradition. When asked when was the last time he pulled the caps and removed the thickener the mechanic had a blank stare on his face. He was greasing the bearing according to tradition. He stopped greasing when he felt resistance. He just didn't realize that the resistance in this case was the ever-increasing accumulation of spent thickener that he was supposed to periodically remove. As the amount of spent thickener increased, the amount of available oil that could be delivered to the bearing decreased. The bearing failed due to a lack of lubrication. Periodically removing the spent thickener was a small secret that had never been passed on by the retiring lubrication mechanic. The procedure to this day is still handed down, like Native American history, by word of mouth.
When it comes to knowing how to grease a bearing the ultimate slap in the face is to install a sealed bearing in place of a greased bearing. It's somebody's way of saying there is a lack of faith in the rigor of greasing correctly. Sealed bearings take the guesswork out of figuring out the amount and frequency of greasing. They are a dream come true. You pay a price though. What happens when the oil in the grease is expended? The bearing will fail, that's what will happen unless some sort of predictive diagnostics catches it in time (e.g. vibration diagnostics, infrared thermal diagnostics). Still, the service life of a sealed bearing is several orders greater than a manually greased bearing, but it doesn't have to be that way.
The art of greasing is knowing exactly how the grease enters the bearing, where it will be flung, what it will cling to, where it will accumulate, the path it takes through the bearing, and whether there is a drain plug, and if so, its location. All of these conditions will affect how much oil is dispensed out of the grease and how much oil will accumulate in the rolling element's pathway. All of these conditions are totally invisible to the lubrication mechanic. That is why greasing is a blind attempt at delivering a lubricant. Having knowledge of all of this is still not enough. It is through inspection that any recommendation is validated or refined. Through inspection the lubrication mechanic can see all of these things mentioned and on top of that he can ascertain where the spent thickener will accumulate and whether the location of that accumulation will interfere with grease delivery, oil delivery, or purging the system.
Figure 1 shows a cross section of a split bearing. In this particular style the grease enters the raceway through a hole marked "A". It is located where the two bearing outer races join together. Some of the grease accumulates in the region marked "B" between every roller. The rest of the grease is forced outwards from "B" toward the bearing cavity on either side, and is marked "C." Figure 2 shows the bearing cavity regions marked "C" without the bearing in it. Although there appears to be ample room in the cavity for grease figure 3 shows how very little space there is once the bearing insert is put inside the bearing cavity region. It is easy to see that it would not take very much excess grease to contact the rotating element that in turn would cause churning of the grease and subsequent heat buildup within the grease and bearing element. Other bearing designs have large cavities that could easily accommodate more grease and still other designs force the grease into the roller pathway from the side instead of through the raceway as in this design. This design tends to spray the excess grease onto the cavity wall. Packing the cavities with grease in this bearing is a sure path to failure. The subsequent periodic greasing has no place to go.
Over several years eight bearings were removed because they were running hot and were thought to be on the verge of failure when in fact they were over greased A bearing cavity with too much grease was recovered and is shown in figure 4. There was so much grease that the rotating labyrinth seals were in contact with the grease. The pen in figure 4 shows the excess grease buildup. Another view of the grease buildup is shown in the figure to the left. Note the well defined shape of the grease in the figure as illustrated by the white line. The bearing cage and retaining ring element were in contact with the grease and pushed it aside until there was no place to go and the grease took on the mirror image of the rotating assembly. Some of the grease in the figure was removed in the middle for illustration of how much grease was in the cavity. When there is this much grease in the cavity the contacting surfaces will generate a lot of frictional heat that will draw out more oil than required. The excess oil finds its way into the rolling elements which in turn churns the oil and generates more heat. It is an insidious spiraling operation that feeds upon itself like a wild fire.
So, don't ever give the lubrication mechanic a hard time because he wants to pull the cap off a bearing and take a look at the grease. It just might be that he knows ...
"the art of greasing."
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