plain thrust bearing gurus (large fans) 1

[cruzinonline]

I have a situation with limited and need some advice and information. During the investigation of a shaft failure, it appears the owner installed a thrust collar on the outboard end of a shaft and converted the free end bearing to a thrust bearing. The only evidence available (photos) shows thrust collars on the inboard bearing also (where I expected too see them). It has been my experience that long shafts, especially those exposed to high thermal loads only have one thrust bearing to allow the shaft axial movement due to thermal expansion. I am thinking the cause of the failure may have been aggravated by the high stess caused by the trapped shaft. Rotational fatigue, bending, high vibration from shafrt inbalance (bending). The failure was adjacent to the bearing (highest stress area) on the end that had the thrust collar. Questions to all of you intelligent people, Has anyone heard or seen of a dual thrust setup (approximately 20' apart)? Does anyone agree with my theory? Does anyone have any published information that would (1) show the typical arrangement of bearings and thrust bearings clear enough to demonstrate to people with limited experience (i.e. attorneys) or (2) otherwise support or discount my theory? Your assistance would be appreciated.

 

[JJPellin]

I believe you are correct about the cause of failure. We have four large fans of the type you are describing. All of them are induced draft fans on hydrogen plant reformer heaters. The fan is passing exhaust gas from the heater. Three of these trains have dual drivers. They have a turbine on one end and a motor on the other. There are clutches in between so the fan can be run from either driver. There must be only one thrust bearing. On the opposite end, we had to use a special gear coupling designed for large end float to accomodate the shaft growth. One fan is expected to grow over 0.250" at full operating temperature. Another larger fan is expected to grow over 0.375". If thrust bearings were installed on both ends, the results would be disasterous. The best source of information about proper bearing configuration would come from the fan or bearing manufacturer. Most of these that I have seen use Dodge Sleeve-Oil bearings. Dodge had good literature about the proper set up of their bearings. Do the calculations for shaft growth and diagram it all out. The fan bearings are on separate pedestals and are not connected to the fan box, so don't let them confuse the situation by implying that everything will grow together. These bearings are ofen the same part number for both ends. One of them has a thrust kit installed and the other one does not. Someone probably pulled the spare bearing from the warehouse and put it in with the thrust kit since it was included with the parts. Simple mistake, but very dangerous. A failure like this in one of our hydrogen plants would cost us many millions of dollars in repair cost and lost production. If you can give additinal details on the fan configuration, fan and bearing manufacture, etc, I could comment further.

 

[krb]

I have seen this problem on smaller fans as well. In our case, the mechanics were not aware of the function of the thrust kit and installed it on both bearings. After two failures, we researched the problem and found that the mechanics were installing the thrust kit on both bearings, where one was supposed to be fixed and the other floating. I second JJ's comments about checking with the bearing manufacturer as well as the fan manufacturer about the proper installation of the bearings. You may find it in the maintenance manual for the fan as well.

KRB

 

[JJPellin]

After further thought, I have a few more comments. First, if a 20 foot long shaft went into hard compression because of thermal growth with thrust bearings at both ends, I would expect a thrust bearing failure. Perhaps the thrust bearing did fail, but the growth happened fast enough to buckle the shaft and fracture at the step just inboard of the bearing (highest stress riser). Another possibility is also related to installation. Most of these bearings have spherically seated liners. There is a procedure to unlock the jam bolt on top of the liner to let it align itself within that spherical seat. Then the jam bolt is torqued and locked in place with a jam nut. If they did not follow this procedure, it is possible to put a bending moment into the shaft which could result in fatigue failure. You should analyze the fracture surface on the failed shaft to check for classic signs of fatigue. There is also the chance that both of these theories played a role. When the shaft went into hard compression, it buckled. This would be a rotating bow and would not necessarily result in bending moments that would cause a fatigue crack. But, the bowed shaft would put a large force on the bearing liner which could shift under the jam bolt and move to a misaligned condition.

 

[JJPellin]

I have seen on failure that fits within the type that I am describing. One of our fans had a hard radial rub on the larger diameter section of the shaft. I believe that this shaft is 11 inch diameter through the fan box and steps down to 5-7/16 inch through the bearings. A hard rub on the 11" portion bowed the shaft. We had extremely high vibration. The forces from the bowed shaft caused the pillow block bearing on that side to come loose from the base. When I saw it, the entire pillow block housing was rocking back and forth with each shaft rotation. If the hold down bolts had held (or if the housing had been doweled down like it should have been) this could have resulted in a broken shaft. The difference with a shaft buckling under compression would be the tendency to kink at the step from larger diameter down to the smaller, rather than bowing in the larger section.

 

[Tmoose]

I'd look at the instructions that are packed with 'most every bearing that has a fixed/floating provision. Spherical roller pillow blocks come to mind. Those instructions are pretty explicit about ONE axially fixed bearing per shaft.

From the 2004 Dodge bearing catalog, DMR 1203-3, page B16-20. "Provision should be made to permit the free movement of shafting endwise due to temperature changes. One bearnig should serve as theanchor bearing to locate the shaft endwise. ALL OTHER BEARINGS SHOULD PERMIT THE SHAFT TO MOVE FREELY ENDWISE."
After that are some examples when that rule can be broken.
From my experience many airhandlers has frames so weak/soft axially that a floating bearing is not required. Whether a particular frame's softness is an elegant design or a lucky strike makes for interesting debate.

Axial restraint killing one or both thrust bearings makes complete sense. The expansion forces being high enough to bow the shaft requires the bearings/frame being MIGHTY DANGED stout, and that is a stretch for me. More info about actual the shaft "failure" would be necessary.

 

[JJPellin]

In all of our installations that I refer to, the fan bearings are directly mounted to concrete foundations of extreme mass and strength. For this style of blower, the fan bearings are in no way attached or conencted to the fan box. Picture a fan shaft supported on two large cement foundations extending down 6 feet into the groud to resist freeze heave. The fan box is built over the fan wheel and shaft but is not connected in any way to the bearings or the bearing foundations. Then pass gas through the fan box at 675 °F and the shaft will grow by as much as 0.5 inches. In this type of installation, I could expect the bearing mounting to be strong enough to buckle the shaft.

 

[cruzinonline]

You guys have provided a wealth of information and it seems your theories match mine. By the way, the thermal growth is .0006" per inch per 100F, so we knew the growth was going to be very large. The other "funny" thing is the owner of the shaft, "lost" the piece that broke off and the remaining shaft was stored outside with no protection. Needless to say their is not much to analyze and the only thing we have to go on is conjecture. The point about the locking screw was something I missed, thanks for that point. YOur help has been greatly appreciated.

 

[electricpete]

It sounds like everyone assumes rolling bearings.

The terminology "thrust collar" sounds to me like a tilting pad (sliding) thrust bearing.

Can the original poster clarify the bearing configuration?

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[electricpete]

I just reread the subject line: plain thrust bearing which answers my question.

I think now the question is not so straightforward. It could be that thrust bearing on each end of the shaft is acceptable if enough endplay is provided.

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[electricpete]

sorry about my comment "everyone assumed". Not everyone.

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[Mendit]

The dual thrust bearing configuration you asked about on the original post is possible. Some, if not most, Dodge Sleeve Oil bearings have 2 thrust collars. These slide on to the shaft and are secured by 2 set screws, 1 at each side of the bearing called "Fixed". These are for minimum thrust or just axial positioning. Persons not reading install instructions carefully may make the error and put a thrust collar at each end of the assembly "which spells failure" instead of each end of the bearing.

The other type has a single groove cut into the shaft (tightly toleranced) where the thrust collar, split in two equal halves and held together by set screws. This is located in the center of the bearing which is called "Fixed" This type will carry more thrust load due to the positive location in the shaft.

 

[cruzinonline]

The subject shaft had mounted (shrink fit) collars on the drive end and collar on the outboard end. The break was about a 1/4 past the collar. The face of the collar appeared to be "very" clean, i.e. it appeared to have been a running surface. Prior to the failure (within last week) oil analysis failed due to high particulates and the bearing temperature increased. There was no apparant damage to the journal, no scores or other damage noted. I am leaning toward the collar improperly installed and due to thermal expansion of the shaft it was running against the bearing creating the particulates and increased heat.