Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation
Yes, you can make a round peg fit into a square hole, that is ... if you try. No matter how idiot proof a design, there is always a way to get around the logic of it all and thwart common sense. This article is about the miraculous operation of a thrust bearing operating with an unstable oil film thickness, and if that bearing could speak, oh what a yarn it would weave.
Simply put, people that had no business putting this bearing together, put it together. You simply had to see it to believe it. I don't think even Riply himself could believe this one, but it's true. Read on ...
An induced draft fan with pillow block babbitt bearings lost its oil supply several times over a period of a couple of months. First, the outboard fan bearing failed, then the inboard thrust bearing failed (shown above). This fan had been a bad actor for quite some time irrespective of the oil problem. It seemed that the inboard thrust bearing vibration was high. Even after the inboard fan bearing was replaced, the vibration was still high. The bearing halves shown above are the top (above left) and bottom (above right) inserts that go inside the inboard thrust bearing pillow block housing. The same thrust collar is shown in each picture for illustrative purposes.
The failed inboard thrust bearing was inspected. The inspection notes and commentary follow:
Top Half View Close Up
The bearing half in this picture has the thrust plates installed correctly for illustrative purposes. The oil delivery ports that are milled into the thrust plates are oriented such that they line up with the oil piping holes drilled into the bearing insert. This allows the oil to be delivered directly to the shaft where it is then evenly distributed via the oil relief grooves located along the bearing split line. To see more detail of the front and back sides of the thrust plates click on the thrust collar or any oil delivery port. The thrust collar fits snugly inside a groove on the shaft where it is locked onto the shaft by means of two set screws.
Bottom Half View Close Up
Again, the thrust plates in this picture are installed correctly for illustrative purposes; they are solid on the bottom in order to establish the oil film. A rabbit groove is cut into the thrust plates on only one side (see close ups in the picture above). On that side an anti-rotation tab fits into the groove (see picture left). The primary function of the anti-rotation tab is to prevent the thrust plates from rotating with the thrust collar. If they rotated, then an oil film would not develop properly. On another note, the thrust plates cannot be installed incorrectly as long as a mechanic orients the groove to the tab. Fool proof, right? .... Wrong ... In this particular case, the mechanic installed the thrust plates correctly on the right side. However, for reasons only known to the mechanic and God (and I don't even think God understands either), the mechanic couldn't get the thrust plates to fit on the left side. They were installed incorrectly because the mechanic couldn't get the anti-rotation tab to fit where there was no groove in the thrust plates. So what did he do? He didn't try turning the thrust plates around, he just left the anti-rotation tab out! Come off it! No way! ... Yes, he left the tab out! Look at the pictures to the lower left and lower right. These pictures show the bear insert halves without the thrust plates installed. You can see the oil port holes where the oil enters the top of the bearing. Look REAL CLOSE. See anything unusual? See the discoloration on the insert? The thrust plates left a footprint. The picture on the left shows a correctly installed set of thrust plates, while the picture on the right shows an incorrectly installed set of thrust plates. If you haven't already looked at the close ups of the thrust plates for the "top half" picture above, look at them now because you'll understand the shape of the footprint better.
The thrust plate on the left has the milled groove through which the oil can be delivered to the bearing. The thrust plate on the right has the oil relief grooves. Notice how the oil relief groove footprints don't line up with the oil port. This side of the bearing didn't get any oil. What's worse is that any oil it did receive on the bottom half wound up being relieved through the milled grooves that were in the wrong location. The oil film on this side was partially collapsed. It's unknown whether the incorrectly installed thrust plates rotated periodically. These pictures would tend to indicate that they did not rotate very often, if at all. What's even more miraculous is the fact that the improperly installed thrust plates were on the "loaded" side. They were loaded up in the axial direction while the fan was running. The correctly installed thrust plates were on the "unloaded" side. The fact that this bearing survived for an entire year assembled incorrectly is a tribute to the bearing designers.
Top Half Bearing Insert - Left Side
Top Half Bearing Insert- Right Side
|A tall tale? Fabricated up? A figment of my imagination?
When I saw this I feared the worst. I figured that if somebody who didn't know anything about this kind of thrust bearing put the new bearing together, that person would "play it safe" and install the new one in exactly the same manner as the way in which the old failed bearing came apart. What did Forest Gump say in the movie "Forest Gump." In the movie he aptly put it "Stupid is as stupid does, sir." Was it possible? It sure was, as I found out when tearing it apart. The findings are in the "Vibration - Case Study No. 83: Incorrectly Installed Pillow Block Thrust Plate" article. It's on the vibration of this particular fan. We had a weird vibration waveform for an entire year and did not know what was causing it. Now we did.
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