Pillow Block Bearing Fan Replacement

[MedicineEng]

Hi All:
A couple of weeks ago, there was an interesting discussion here at the workshop on 2 opposite sides and there was no clear winner, so I've decided to put that to this forum to hear your opinion:

In a fan shaft with 2 pillow block bearings installed, when you replace one of the bearings because it is faulty, do you also replace the other, even if doesn't show signs of immediate problem?


Thanks for your opinions.

 

[JJPellin]

No.

Johnny Pellin

 

[dvd]

It depends on cost of downtime versus cost of bearing. Unless it was ridiculously easy to change at a later date, I would probably want to swap it out. You are right there and have the drive disassembled. May want to look at hours on the bearing since last replacment/repair/rebuild, but, someone will raise hell if only one bearing is replaced and then the other bearing fails when the system is back online.

 

[TugboatEng]

A top tier building management company assumes it costs $55 to replace a lightbulb between labor and parts. The labor is the same to replace both bulbs and they have both been exposed to the same conditions that caused the first to fail. You need analyze you're own costs.

Recently, I had a few failures on line shaft bearings (6 inch diameter). These don't have a regular history of failure so I opted to only replace the components that had failed. But, if you have bearings that tend to wear out together OR require substantial labor to get to, it's best to replace all together.

 

[arunmrao]

Depends. Last week I had a bearing failure on Over Head Crane (15ton). One bearing was damaged, while the other was good. But I decided to replace both the bearings with seals.

The cost of bearing is insignificant in this case compared to downtime and risk in bringing the gearbox down.

 

[Stick.]

This is highly dependent on the finer details of the situation, but in general I would start with the assumption of replacing both and then use any complicating factors (large increase in cost/time to replace the second one, low consequences if a second failure occurs, etc) to talk myself out of that position.

 

[Tmoose]

In addition to the basic cost vs downtime etc questions, I would want to fully understand how the bearing is "faulty" and repair history.

The kinds of things on on my mind are -
- Over hung vs center mounted fan
- Cylindrical bore bearings and even adapter mount bearing loosening and creeping and wearing themselves and the shaft. These are installation issue.
- Belt drive "Drive end bearing" loosening as above, or not lasting full expected life before developing internal spalls etc. I'd be thinking about belt tension and etc.
- Direct coupled drive end bearing being under-loaded causing the rolling elements to skid and thus smearing and spalling etc.
_ A hot overhung fan whose Non Drive End bearing is "faulty." C3 clearance, presence of heat flinger, or other details that are required but missing will not be overcome by an in kind bearing replacement.

 

[drawoh]

MedicineEng,

I admit am not a maintenance guy. Are you replacing a faulty bearing or an old one. Ball bearings fatigue to destruction. Journal bearings wear out. Just how expensive are pillow blocks anyway?

--
JHG

 

[electricpete]

I don't think we would automatically replace the other non-pillow block bearing except in very unusual situation where the downtime / unavailability of the machine was at a premium (for machines where operations/production dept doesn't want to remove machine from service or make it unavailable for standby start by tagging it out for maintenance).

Another factor in your decision is your condition monitoring. Typically rolling element bearing degradation can be seen in vibration monitoring long before any degradation that might threaten reliability. So if you have a good vib monitoring program, that argues not to replace the other bearing unless you see some signs of degradation on that bearing too. The warning time might be lower in high speed high-loaded bearings but for most of our stuff we can watch bearing defects develop for a long time before we feel a need to write a work order.

=====================================
(2B)+(2B)' ?

 

[MedicineEng]

Thank you all for your feedback.
I am also on the field of replacing both pillow blocks since the equipment is offline either way, technician and tools are already in place, so the marginal cost of replacing the not-yet faulty bearing is much lower than setting up everything again to replace the other pillow block bearing once it fails.
We currently don't have a vibration monitoring program in place as most of our machinery is fairly small and distributed, so maintaining a vibration monitoring program doesn't make a lot of economical sense.

@Drawoh: our pillow blocks bearings cost anywhere between 500-1500USD each, depending on the size and type.

 

[drawoh]

MedicineEng,

What does your downtime cost?[ ]

--
JHG

 

[dvd]

Besides that, if you have one fixed and one expansion bearing, it is easier to ensure that they are properly installed when it happens as a pair. Expansion/expansion or fixed/fixed bearings would create problems.