1. A bearing failure investigation identified several possible reasons for failure including excessive load due to higher stock size, temperature loss causing lower operating temperatures, and higher loads on the bearing side where the pass was running. 2. Calculations showed loads were higher when temperature was lower, and the non-drive side bearing experienced much higher loads. Estimated bearing life was much shorter under lower temperature conditions. 3. Other potential factors included electrical fluting from welding in the housing, vibration from issues with the bearing housing and stand, and problems with the bearing assembly process and fitting between bearings and rolls.

1. Bearing Failure RCA
Possible Reason:-
1) Excessive loaddue tohigherstockSize.
2) Temperature loss –130Sq billethave higherrollingtime,there ischancestodropin
Temperature.
3) ChockBearingfail mostof the time whichside passisrunning( Drive Side/NonDriveSide)
4) Electrical flutinginbearingdue weldingonchock.
5) Fatigue failure due tovibrationinstand.
6) Assemblyproblem.
Explanation –
Bearing 24034 characteristics
Dynamic Load – 963000N – 98 Ton
Static load – 1460000N – 148 Ton
Fatigue Start from the load – 137000N -14 Ton
Top Speed– If Lubricant is Grease – 1500RPM
Stock Size
C1- 15.5mm
C2- Ø 21mm
C3- 12.5mm
C4 – Ø16.5 mm
1. Excessive loaddue to higherstock
RollingMill Loadcalculatedbasedonstandardformula.
H0- Initial Heightof the stock
Hf- Final Heightof the stock
R- Radiusof the Roll
L- Contact lengthof the Roll
L= Sqrt{R(H0-HF)}

2. S1- Flowstressat the entrance (140Mpa)
S2- FlowStressat the exit(200Mpa)
S0- (S1+S2)/2
μ-co efficientof friction.(Consider0.5)
b- Widthof the pass
Ha = (H0+Hf)/2
Q=μ x Lp/Ha
S0= (S1+S2)/2
P- Load
P= S0x {(Lpx b/Q) X (exp(Q)-1)}
So,the rollingloadbyusingthisformulaissoon
C1- 291302N -29 Ton
C2 – 204773N -20 Ton
C3- 184275N – 18 Ton
C4 – 144662N – 14 Ton
By the calculation itseemsthatloadisnot too muchundernormal operatingcondition.The 24034
dynamicloadisaround 98 tonundercontinue load.
2. Temperature loss
It is observedthatin130x130 billettail endistempisconsiderablylow .Sometimesdelayinbitingon
roughingroll androllinglengthsignificantlyincreasedue to130 sq billettempcause thatthe tail end
temperature isdropped.Here we consideringthe tempattail endgoesbelow the recrystalizationtemp
i.e.720*C to 750*C. thenloadbycalculation asfollowings.
C1- 621160 N- 63 Ton
C2 – 445610 N- 45 Ton

3. C3 – 368047 N – 37 Ton
C4- 335295 N – 34 Ton
Note- Flowstress we consider362 Mpa for calculatingthe rollingload.
3. Chock Bearing fail most of the time which side pass is running( Drive Side/Non Drive Side)
Let we considerside collargapbetween roll andpassis100 mm inall roll.Inour mill barrel lengthis
490mm. If we are usinglastpasses Non drive side thenloadineach bearingis as followings.
Stand Combined Load High Temp Low Temp
High Temp Low Temp Drive Side Non Drive Drive Side Non Drive
C1 29 Ton 63 Ton 5.92 Ton 23.08 Ton 12.86 Ton 50.14 Ton
C2 20 Ton 45 Ton 4.08 Ton 15.92 Ton 9.18 Ton 35.82 Ton
C3 18 Ton 37 Ton 3.67 Ton 14.33 Ton 7.55 Ton 29.45 Ton
C4 14 Ton 34 Ton 2.86 Ton 11.14 Ton 6.94 Ton 27.06 Ton
By this table it is alsovery clear that there is always be higherload inbearing which side pass is
running.
Bearing Life Calculation
Calculatingthe bearingLife bythe formula
L10= (C/P)t
x (B/N) x a

4. Where,
L10 = Life inHours
C = Radial dynamicloadratingof bearinginNewton
P = Actual DynamicLoad workingonbearinginNewton
t = Dependsonbearingtype
For Rollerbearingt=10/3
Ball bearingt= 3
B = ISOfactor 106
/60
N= Rotational speed
a = Life adjustmentfactor(1whennottakingenvironmentconditionLikecontamination.)
Stand Roll RPM MinimumLife inHighTemp(InHours) MinimumLife inlow Temp(InHours)
C1 183 11366 856
C2 276 25990 1741
C3 326 31248 2831
C4 475 49646 2576
Note:- RPM sample takenwhile 16mmisrunningfromHMI. RPMwill change accordingto roll dia.,Mill
speed,andstocksize.
It is very clear that temperature is significantlyaffectingbearinglife. Some minor effectonlife due to
roll RPM.
4. Electrical fluting
In our mill housingconditionisnotsogood,side linearisnotfixedproperlyandpackingatside linerare
weldingtime totime whenever roll change ormill under maintenance. The housingiscastedandthe
linerismade of MS plate that’swhyweldingisbrokingagainandagain.Itis alsoobservedthatspindle
couplingare weldedsometimesdue tohole mismatchorboltnotavailable.
In all weldingearthingisnotgivenclosertothe weldingplace thenelectriccurrentpassthroughall body
of standeventhroughthe bearing.Thiselectrical currentgeneratingelectricflutinginside bearingcause
seriousdamage.

5. 5. Fatigue failure due to vibrationin stand.
As we discussedbefore the standconditionisnotsogoodlinearisnotproper.There isgap in between
chock and andhousing.All standsare three highstandsupperscrew thatpressingthe topchock is also
verylong.The gear and motorlevel islow andthe chocksheightishighconsiderably.
Thisall factor generatingvibrationinstand. Causesinsidebearingrollercontinue hammeringitscage. In
that case chances of cage failure inbearing.
6. Assemblyproblem.
Duringinvestigationwe foundthatrightnow we are not usinganytype seal (Retainerseal,V seal,O-ring
etc.) In chockassemblyand insteadof taperinside bore we are usingstraightbore bearing.Tomake
makingstraightbore we doingweldingonneckthengrindingthatisalsonotacceptable.
It isalso foundthatroll neckhardnessislessandit isworn outveryfrequently.Itwillaffectthe fitting
betweenroll andbearing.