Root cause analysis of ball bearings
Root cause analysis of ball bearings
(OP)
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?
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?





RE: Root cause analysis of ball bearings
If you increase no. of balls load distribution b/n balls will reduce naturally.
But i would suggest you to use case carburize bearing rather than Through harden,
This may be one of the reason for dent on ball.Numerous failures means not only because of mountings but I also suggest you recheck your bearing selection calculation.
9108K This is the NSK equivalent TImken bearing
RE: Root cause analysis of ball bearings
Also, please be advised that case hardening is not likely to solve the problem. It "might" reduce the damage but if you are impacting the bearings hard enough to brinell the raceway of a through hardened bearing case hardened material is not a magic bullet despite decades of marketing by Timken.
The best fix would be a process improvement either to use appropriate sleeves to spread the load out over the entire bearing or better yet invest in an induction heater (if it fits your process) or other thermal process to eliminate the impacts all together.
Attached is some general information on correct mounting procedures.
Additional information here:
http://www.koyousa.com/brochures/pdfs/koyotraining...
RE: Root cause analysis of ball bearings
RE: Root cause analysis of ball bearings
This bearing is from a paper sheeter and it has been seleced from the OEM. When it fails, the sheeter goes offline and then our fitters oull the roll , dismount the bearing and fit new ones in.
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
The reason why i suspect an incorrect mounting method is because of the following:
during mounting, two bearings need to be fitted (one fixed and one floating).. the fitters fir the floating end first using appropriate fitting tool. they then proceed to fit the fixed end bearing again using appropriate fitting tools.Howeve, when they are fitting the fixed end brg, there is nothing supporting the float end brg. My suspicios is that whils fitting the fixed end brg, the load is being transferred to the float end and since it has nothig supporting the inner and outer ring together, this may be causing brinelling.
RE: Root cause analysis of ball bearings
I wondered what condition constitutes a "failure" for you.
Noisy operation?
Smoking destruction?
Excessive clearance develops?
Something else altogether?
Dan T
RE: Root cause analysis of ball bearings
the rolls stop turning when the bearing fails. I have not been on the shop floor so I am not 100% sure what constituted the failure. I was called after and asked to come up with a reliability improvement technique.
RE: Root cause analysis of ball bearings
I don't agree the explanation given by you regarding the load transfer due to mounting of bearings.
Usually float bearing will not take much of the load as that of fixed bearing.
one of the reason may be mounting procedure,please check the mounting of float bearings check that the float bearing is mounted properly to allow to float on the shaft.
Also please check the fitting practices.
Also please check is there any excessive vibrations in the system.
If you have photographs of damaged bearings please do share,it will helps lot to trace mode of failure.
Pradeep
RE: Root cause analysis of ball bearings
Vibration analysis can often detect developing problems. One of those $500 vibration "pens" is a lot better than nothing.
RE: Root cause analysis of ball bearings
Mike Halloran
Pembroke Pines, FL, USA
RE: Root cause analysis of ball bearings
This bearing is from a paper sheeter and it has been seleced from the OEM. When it fails, the sheeter goes offline and then our fitters oull the roll , dismount the bearing and fit new ones in.
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
The reason why i suspect an incorrect mounting method is because of the following:
during mounting, two bearings need to be fitted (one fixed and one floating).. the fitters fir the floating end first using appropriate fitting tool. they then proceed to fit the fixed end bearing again using appropriate fitting tools.Howeve, when they are fitting the fixed end brg, there is nothing supporting the float end brg. My suspicios is that whils fitting the fixed end brg, the load is being transferred to the float end and since it has nothig supporting the inner and outer ring together, this may be causing brinelling."
If this was how the bearings were being mounted most likely you would not have a problem.
Look for something else.
RE: Root cause analysis of ball bearings
We have agreed, as "MikeHalloran" that we are making a jig with two end plates and two bearing sleeve. This will be used in conjunction with a hydraulic plunger to press fit the bearings in place.
RE: Root cause analysis of ball bearings
If you are referring to axial support during installation, and installation is by pressing or driving the bearing onto the roll journal, I'd say "supporting" either outer ring is not a good idea. The force to install the bearing should be applied only to the inner race. The force to resist motion of the roll should only be applied to the roll.
Or, are you referring to the situation shown in Figure 6, on page 13 of the SKF Guide available here?
http://www.ien.com/uploadedfiles/ien/Blog/SKFBeari...
The shaft shoulders and abutments and fillet must be VERY nicely made to avoid tilting the bearing even when the cylindrical diameter is an interference fit.
RE: Root cause analysis of ball bearings
nice info
thanks
Mfgenggear
RE: Root cause analysis of ball bearings
Does the bearing sit idle for any time, with or without proper lubrication.
RE: Root cause analysis of ball bearings
the bearings do not sit idle. it is a 6008 VV.
the only analytical assessment I did is to undertake a performance assessment of the bearing during operation: check whether equivalent load rating, equivalent static load rating, critical speed are all within requirements and they were.
RE: Root cause analysis of ball bearings
RE: Root cause analysis of ball bearings
A few questions>
Is the damage all around the race?
Is there any color on balls or race?
Can you take another picture with without the cage and with a more oblique view.
RE: Root cause analysis of ball bearings
i would say its showing signs of dents at ball pitch followed by flaking.
no, damage is not all around the race
no colour on balls.
sorry, other pictures did not turn out good.
RE: Root cause analysis of ball bearings
The balls are shiny and the damage is very local and on exact ball pitch.
That kind of damage can, in my world, only occur as a result of inappropriate mounting method.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Root cause analysis of ball bearings
"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.
RE: Root cause analysis of ball bearings
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
RE: Root cause analysis of ball bearings
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.
RE: Root cause analysis of ball bearings
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.
RE: Root cause analysis of ball bearings
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.