We installed a 330kW 1500 RPM Motor into our plant with a VFD drive unit. For a number of months we had no problems as we had capped the speed at 1300RPM. After a period of time at full speed 1500RPM, we had a failure see report and pictures. We know we have an ambient temperature problem where the motor is operating in extreme temps often above 100F. We have limited the motor speed depending on the ambient temperature. The same failure happened again 3 months later with the same result.
What I am trying to work out is, what caused the inner race of the cylindrical roller bearing to move on the shaft to the point where the rotor dropped onto the windings and shorted it out. Temperature had a factor in it. but what would make the inner race move?

Cool_Fan_Motor_Failure_Report.pdf (199 Kb, 101 downloads) Cool Fan Motor Failure Report

Some thoughts:

The writeup says
"inner race moved on the shaft".

If that is the case, then I would recommend to investigate what repair was done to restor the shaft after the previous failure, and how did they leave the O.D. I expect there should have been a k5 fit between inner ring and shaft.

But it lookis like perhaps instead we have the shaft/inner ring moved compared to the rollers/outer ring/housing? Or was it just moved to take a picture of the inner ring. If the shaft/inner ring moved compared to the rest, then we want to know if there is a locating bearing in this machine (ball bearing, angle contact bearing)? And what about the driven equipment and what type coupling?

Just wondering what type coupling system the motor has. Direct or belt. May have been in your original post and I missed it.

You said this is a direct drive with a coupling right? Is there a snap ring holding the inner race on the shaft? Check the nameplate. It should state what type of bearing to be used on the DE.

An NU type bearing is not the type of bearing to be used in a direct drive application. You need to have side pressure to maintain roller contact with the inner race. Switch to a 6322 bearing.

This message has been edited. Last edited by: THMotorMan,

David,
Like it was mentioned above use of NU type bearing if not adequately loaded will cause roller skidding and bearing overheating. As we don't have all design data the first thing I would do is to re-calculate bearing load and compare it to a minimum load required for this bearing (all info and formulas are available in any SKF catalog). From you problem description it looks like the inner race axial move was the secondary issue and the primary was bearing overheating most possibly due to rollers skidding.

I had a very similar failure on a vacuum pump motor with the same bearings. For years, there was no vibration evidence of any bearing problem, then a sudden and catastrophic lock-up that caused rotor to stator contact and a total loss of the motor, plus prodution, parts..... I'm sure you have heard what I call the $20,000/hour lecture before.

In my case (v-belt drive), there was a total and rapid lock-up of the bearing. Rollers had flat spots and localized bluing where they froze. There was machining on both ends of the cylindrical rollers like they had been pinched. The motor endbell was discolored from high temperatures radiating from the shaft. The end bell had machining on the axial surface of the bearing bore and the endbell itself as did the bearing locknut. The bearing lockwasher was distorted, but inspection showed that no engagement tabs were in a position where they would have held the locknut. It was also missing the tab that engaged it with the shaft.

They theory is that the loose washer allowed the locknut to slowly back off with no real hint of a problem until the point in time that it contacted the end bell. At that point, machining of the endbell began and started to generate heat. The heat expanded the bearing to the point that the axial clearances and then the radial clearances closed up resulting in the lock-up and destruction of the motor. It was agreed that the total time from first contact to failure would be very short and impossible to predict unless you happened to be standing there when it occurred.

This is all theory though, nothing was ever proven. I do have a roller that I kept as a souvenier if you'd like to see it.

quote:

WEG 330kW AC Motor, Serial Number BE53913, Non Drive End (NDE) BRG
6319C3 Drive End (DE) BRG NU322, driven by a VVVF Siemens drive.
1505RPM direct drive.

OK. I see where the drive setup was hidden.

David,

Although vibration data did not penetrate the ALERT level there could be a pattern of looseness in the data. Of course looseness could have also developed rapidly after the last reading. Still it will interesting to see data if possible.

As far as root cause is concerned it is likely the mounting method ( in my understanding it is not interference fit mounting) and/or poor craftsmanship and/or improper static loading condition as other mentioned. The ambient temperature of 100F is not the cause, IMO. In general, even with a direct drive, static loading exerted by the rotor weight should be sufficient in this regard, unless there was a design issue.

David

quote:

( in my understanding it is not interference fit mounting)

It should be an interference fit (of course it might not remain tight if the bearing heats up due to skidding as suggested above).

http://www.easa.com/indus/AR100_0406.pdf
Page 16 of 33 above specifies n6 shaft fit for a designation 22 bearing (I was wrong to say k6 above).

For 110mm bore bearing, n6 means shaft is between 4.3325 and 4.3316

meanwhile bearing bore is between 4.3307 4.3299

The above dimentions create between 0.9 mil interference and 2.6 mil interfrence.
0.9 mil = 4.3316-4.3307
2.6 mil = 4.3325-4.3299
(1 mil = 0.001")

I agree, an interference fit is the right thing to do for motors. I just thought, based on the picture, that extended inner has a set screw or similar method bearing mounting.

Cylindrical roller bearings inner race is normally shrink fitted. We use oil bath or induction heater to heat the inner race and slip it on to the shaft against shaft shoulder. The inner race is locked at one end by the shaft shoulder and by lock nut at the other end. The lock nut thread is such that it tightens against the direction of rotation of the shaft. If the threads are not confirming to this rule and we are relying on washer to lock the nut, wahser may get loosened during running and the lock nut may also get loosend afterwards. The inner race will now not be locked and may move axially and get heated up increasing its ID which will result in its rotation with respect to shaft. That is one assumption , may be the case with your bearing.
Secondly, as a bad practice we mount even big bearing by hydraulic press which results in removal of shaft material in microns. Several such mounting , dismounting in the long run make the inner race loose on the shaft which may have relative motion with respect to the shaft thereby causing overheating, increase of ID and its axial movement on the shaft.
Kindly consider the above two postulations.
PS:Only he who maintains the equipment can decide best as to what went wrong with his equipment.
Regards.
Irshad Akhtar

Electricpete.
Reason we believe the inner race moved ont he shaft is, Inner race was approx 35mm away from the shoulder on the shaft. The rollers d seized onto the inner race by approx 5mm. The None Drive End bearing was a 6319C3

Ralph.
Coupling is a Cone Ring coupling with bolts and rubbers, the coupling gap was approx 4mm and laser aligned when installed.

DavidG.
I will post the vibration Sectrums Monday which were taken 2 days before the failure to see if anyone would of made the call.

quote:

Inner race was approx 35mm away from the shoulder on the shaft

I think I can see that. I have labeled that dimension in attachment slide 1.

quote:

The rollers d seized onto the inner race by approx 5mm

Are you saying there was only 5mm of axial overlap left between the rollers/outer-ring and the inner ring? (labeled as attached?) That would indicate that the outer ring has moved in the housing even further than the inner ring moved on the shaft. Or is there some other meaning to the 5 mm? And if the rollers/outer ring didn't move to disengage from the inner ring, I'm still unclear about how this "allowed the stator to drop onto the windings"
==========================
This bearing I don't think has any lockwasher and not butted against any shaft shoulder.

Attached slide 1, I have labeled the components (tell me if I have labelled wrong). Coupling to the right, motor winding to the left. I assume the bearing is installed onto the shaft from the coupling end. That would normally mean shaft shoulder on the winding end of the bearing (hidden from our view), and locknut on the coupling end of the bearing. It doesn't seem likely that you have that configuration here, because it looks like your bearing slid toward the winding which would be right past where that shaft shoulder should have been. And if there is a locknut on the coupling end of the bearing, I certainly don't see any threads for it. Also no snap ring groove in sight.

So it looks like there is no locknut associated with this bearing and no shaft shoulder to push the bearing inner ring against when you install it. The shaft shoulder on the coupling end has no obvious funciton other than perhaps a point of reference for installing the bearing. Am I correct, or have I misunderstood?

I have seen a lot of applications with no locknut, but never seen a bearing without a shaft shoulder on the winding side to position the bearing inner ring against during installation. Of course I work mostly with deep groove bearings... maybe roller bearings are different.

Is it possible there is another shoulder back further towards the winding, and the bearing was never seated against it to begin with? I would ask the shop for a picture of the shaft after bearing is removed.
==========================
Now some rambling thoughts about thermal expansion and sliding of inner ring.

Assuming the bearing and shaft both had similar temperature coefficients around that of carbon steel ~ 6.5 E-6 inch / (inch – F)

For the 4.3" bearing OD to grow thermally by 1 mil (the minimum interference) would require 35F temperature difference. To grow 2 mil would require 70F temperature difference. To grow 3 mil would require 105 temperature difference.

So it is certainly conceivable the bearing could lose interference from abnormal bearing heating.

And clearance at the ring which is rotating with respect to the load is much more severe than clearance which is stationary with respect to the load. Attached slides 2 and 3 is an interesting example from SKF. For a 0.5 mil clearance at 3600 rpm, they predict 1.4 miles relative movement in 24 hours! (yes, miles, not mils). In reality, I think surface roughness, ovality, etc would keep things in place at 0.5 mil. But once relative movement starts at the seat of the rotating inner race, it can degrade rapidly.

Also worth mentioning, there is not a lot of torque on the inner ring (only bearing friction torque) and ideally no axial thrust on a roller bearing.
============
Motorman, Alex, and David are definitely right that roller bearing is not typically used outside of belted application. Do you know the rotor weight? (if so we can compare to thumbrules for cylindrical roller bearing minimum loading to prevent skidding).

This message has been edited. Last edited by: electricpete,

Labeled.ppt (952 Kb, 29 downloads)

quote:

Ralph.
Coupling is a Cone Ring coupling with bolts and rubbers, the coupling gap was approx 4mm and laser aligned when installed.

Thanks David.

My concern was not really the coupling but whether or not it was coupled or belt driven with an NU or N type bearing. Personally I think this is not the best DE bearing to use direct coupled.

Another thought. Was the slippage of the inner race a result or the cause of the failure?

At the SKF website, there is an equation and on-line calculator for minimum load for various types of bearings.

http://www.skf.com/portal/skf/home/products?maincatalog...ng=en&newlink=1_4_12

quote:

SKF Interactive Engineering Catlogue
In order to provide satisfactory operation, single row cylindrical roller bearings, like all ball and roller bearings, must always be subjected to a given minimum load, particularly if they are to operate at high speeds or are subjected to high accelerations or rapid changes in the direction of load. Under such conditions, the inertia forces of the rollers and cage, and the friction in the lubricant, can have a detrimental influence on the rolling conditions in the bearing arrangement and may cause damaging sliding movements to occur between the rollers and raceways.
The requisite minimum load to be applied to single row cylindrical roller bearings can be estimated using

Frm = Kr (6 + 4*n / nr) * (dm/100)^2

where
Frm = minimum radial load, kN
kr = minimum load factor (see product table) [0.15]
n = rotational speed, r/min [1500]
nr = reference speed, r/min (see product table) [3000]
dm = mean diameter of bearing [175]
= 0,5 (d + D), mm

When starting up at low temperatures or when the lubricant is highly viscous, even greater minimum loads may be required. The weight of the components supported by the bearing, together with external forces, generally exceeds the requisite minimum load. If this is not the case, the single row cylindrical roller bearing must be subjected to an additional radial load.

If you open the link and click on the blue equation (Frm = ....), an on-line calculator is opened that does the calculation for you. I entered the part number (NU-322) and a speed of 1500rpm, and the calculator told me the minimum load was 3,680 Newtons (see attached), which is the force produced by a weight of 368kg. Assuming half the weight of the rotor is seen at each bearing, that load would be created by a rotor weight of 368*2 = 736kg ~ 1600 lbm.

If your rotor weighs less than 1600 lbm, you are not meeting this requirement. (assuming the loading comes from weight of the rotor... neglecting loading from misalignment etc). You might want to double-check my unit conversions.

SKF_322_MinLoad.ppt (382 Kb, 20 downloads)

Electricpete;
The inner race moved towards the coupling, not the outer race into the windings, see attached .ppt pages 1 & 2

DavidG;
Page 3 is the velocity and Spike Energy spectrums from this bearing taken 3 days before the failure.

Ralph;
I'm thinking the slippage of the inner race was the result and not the cause. from all the testing, vibration, temperature, visual inspections and thermal inspections. Temperature caused the inner race to expand enough so that it could move along the shaft untill the cylindrical rollers could not support the rotor and this caused it to hit the windings.

But am still not sure what caused the temperature problems.
The latest motor in has Thermocouples on both bearings 6319 & 6322 and set up into our alarm system, when they reach a certain temperature we reduce the speed of the motor and the line speed, these usually happen on hot ambient days during summer.
We have to wait 20+ weeks for a specially designed motor which can operate in these high ambient temperatures.

MCL4_Upleg_Blower1.ppt (784 Kb, 17 downloads)

Let me restate, an NU bearing should not be used in a direct drive application. Switch to a 6322.

What ambient temperatures are you talking about?
I live in AZ, and we service plenty of motors that run fully loaded in full sun (and other very hot spots) with no problems. The proper bearing for the application makes all the difference, as does the grease.

What were your axial measurements? An NU bearing has far more play than others. Possibly the rollers were moving in the axial direction, generating friction.

THMotorMan;

We have changed the Drive End Bearing to a 6322 and so far we have not had any failures.

Ambient temperatures around the motor have reached 66 degree's C (150F), majority of this is caused due to the fact that this motor sits ontop of steel flooring approx 30m (100foot) above a zinaneal pot.
We have tried to protect the motor as much as possible from radiant temps, plugging holes, deflecting the heat away from the motor.
See thermal image for hottest location on the motor, its not always the Drive End bearing

Thermal_Images_Upleg.pdf (354 Kb, 20 downloads)

Dear All;

FAG - Recommends in Motor Applications as following;
1) Expansion of inner ring after mounting because of interference ( n6 in shaft for this application)
2)expansion of the inner ring due to Thermal influence in operation as a temp differnce of 5 - 10'C will be there between the inner and outer ring resulting in furthur reduction of operating clearance.
Hence it is always recommended to use bearings with C3 clearance in application where the shaft dia exceeds 80mm. also it is recommended to measure the unmounted and mounted radial clearance ( Radial clearance to be measured in the bottom most roller by lifitng the brg as in NU Roller is guided in Outer ring)to facilate required operating clearance

quote:

Bearing Failure Caused by Heat and ??????

Interesting thread...first time I've read it. The answer to your original question is "inadequate clearance". I've always said that a bearing fails for one of two basic reasons: 1) too little clearance with too little lubrication. 2) adequate clearance, but mechanical failure (bearing loses geometry).

I always say that the proper bearing, properly installed, properly lubricated, and properly loaded.... will run almost forever. It really is that simple. If it fails, one of the 4 "propers" above was violated. When a bearing fails, just ask the four questions above, and you will get to the source of the problem.