Root cause analysis of ball bearings 2

[Nivrah]

Hi all,
I have an NSK deep groove ball bearing designation num 6008.This has 12 ball elements. Can anyone point out another bearing manufacturer equivalent that has the same number of balls? or more!

Secondly: We have had numerous failure of this 6008 NSK and I suspect that it may be mounting problem from our fitters imparting shock loads to it since there are dents at ball pitch of the stationary inner raceway.
I am looking at different scenarios and one of the question I have :
if I utilise an equivalent bearing with more ball elements, this may help reduce the load distribution on the ball elements, right?

 

[Tmoose]

"it is a rotating outer ring with a stationary outer ring load. So the inner ring is a tight fit and the outer ring is a loose fit"
If the load is mostly in one direction, like gravity on a horizontal roll, or web tension on process roll, the outer race should be the "tight" fit, and the inner can/should be "loose" . Like the front wheel bearings on many RWD drive cars of the 60s and 70s.

Do you have a picture of the outer races and the roll bearing bore?
If the fit is incorrect I'd expect outer race to creep and wear out the housing and itself, but not cause the bearings to fail as shown.

Is that corrosion visible on the race off to the left?

Do you have pictures of the other side of the inner race? If the load is one-directional relative to a non-rotating inner ring then ~ 180 degrees of the ring will be unloaded in service. I'd expect minimal damage on that side other than debris sliding by. Brinneling from pure axial load during installation should be visible there too.

 

[mfgenggear]

Nivrah

all of the above suggestions are all very good

My suggestion is this .
get a digital camera, get your best mechanics to assemble the sub assembly with bearings.
record the actual process, ask the mechanics politely if you may obtain pictures for visual aids.
I believe he or she would be glad to help.

inspect the shaft dimensions prior to assembly.
document the proper procedure for assembly
require Inspection to validate assembly.

My opinion those bearing have being knocked out of round.
so they will self destruct. from your picture it does not appear to be an easy
bearings to assemble. so misalignment is possible.
I having difficult understanding which are the pressed members. I am bad.
but it is commen if the bearing does not insert correctly it will have to be tapped
to straighten it out. thus may be causing damage.

part of the above process is to discuss with the mechanics is as follows
#1) do they have the correct tools.
#2) is the current assembly procedure correct.
#3) what are other options available to promote easier assembly.
#4) if all the above is correct, then the problem is else ware, bad bearings, damaged during shipment, or during stocking.
Incorrect bearings, Boxes tossed around with extra strong shipping employees.
#5) inspect the bearing for any visual damage, do fast simple run out verifications
#6) then start looking at design issues. bad design, or maybe a better design.
discuss this a s group effort with manufacturing, manufacturing engineering. & any senior design engineers.
but be very careful of any changes, and it's ramifications.

Mfgenggear

 

[Nivrah]

@Tmoose:
You are right in the light of the rotating outer being a tight fit and a loose fit for the stationary inner. Prior to the failure, the mechanics did it the other way around,i.e, they made the housing bore loose and the stationary inner tight...very tight indeed(m6 for the shaft).

The exterior surface of the outer ring showed heat marks...most likely for the fact that it has been spinning due to the loose fit. This heat generation has caused lubrication degradation. There is no corrosion and the seals have been taken off during disassembly for myself to examine. As for the other side of the inner race, there is no damage.

To mfgenggear:
I have since revised the fits and put in a procedure for the fitters/mechanics to use. The fits corresponding to this application have been revised to a tight fit for the rotating outer(M6) and a loose fit for the stationary inner (g6).
The issue with the mechanics is that most of them are above 40 years old so they do not like being told what to do. To manage this culture problem, I scooped out a formal procedure and applied it to mill wide mechanics with the approval of senior engineers.

 

[Tmoose]

With two seal mounting rings shown on the inner race I think the original design is a double sealed (or perhaps shielded) bearing. Either way, the bearing is not regreasable by conventional means. INjecting grease will at best do nothing, and and worst can jam the shields/seals into the cage restricting free motion and creating debris. IF the seal mounts to the inner race the sliding surface of the seal is the faster moving outer (higher surface feet/minute) race. This adds to heat generation (at low speed no problem) and simply shortens seal life (may be a big problem.

A bearing with improperly loose fit creeps, not spins, and wears out the seat ( shaft or housing) and itself. The heat generation is not necessarily much at all. I'd look harder at the lube degradation (possibly from contamination) being what caused the race discoloration and the early spalling of the load zone of the inner race.
Stripping one seal or both seals and modifying slingers, flingers and end covers would permit some level of re-greasing.
As pointed out in other recent threads re-greasing can go a long way toward mitigating bearing contamination problems.

Creating a floating bearing with loose fitted inner rings will require some thought.

 

[BearingDave]

I agree with most of the suggestions above.

Stationary inner ring, rotating outer ring load = loose fit on shaft and tight fit on housing. The machining tolerances you have selected for the shaft (g6) and housing (M6) appear correct based on the documentation I have (although mine shows M7 to be precise).

The bearing being sealed means that most likely this was not an issue with lubrication. We would have seen abrasive wear on most of the raceway if that was the case. In your pictures, the spalling is localised which does indeed seem to point to brinelling caused by a shock load which occured either during operation or (most likely) during mounting.

If I were you, just to be sure, I would also verify geometrical tolerances on both the shaft and housing.