broken shaft of 280 KW SQIM 4

[genman196]

We had experienced a broken shaft on our 280 KW,SQIM while it was running under load. We dont know what was the real cause,as it was within its nameplate values and the OLR did not trip. Can anyone please give some hints of possible causes of broken shafts for large motors? thanks a lot.

 

[desertfox]

hi genman196

Its a very interesting thread in truth and the responses from members have been very good.
As regards the discussion are you following it okay? do you understand the mechanics of the situation? Let us know if you don't we might be able to explain it better.
Right thanks for the information you have just posted its very useful and I wonder if I can ask you for some more ie:-

Length approx between motor bearing centres?

Width's of both bearings? do you know if there both the same?

Total Belt tension tight and slack side?

WHOA! I am confused now your pulley width is 360mm but your shaft length from drive bearing is only 250mm and the shaft length to where pulley is mounted is 210mm, does mean your pulley extends beyond the end of the motor shaft by 150mm?
or is there a mistake in your dimensions?
If that pulley exceeds the length of your motor shaft then thats where your problem is, your putting a excessive bending moment on that shaft which it wasn't designed for.
Please confirm or deny the above.

regards

desertfox

 

[BobM3]

Genman -

You had both a flywheel and a pulley mounted to the shaft? Is the mass of the pulley and/or flywheel within the ratings for the motor? I ask just so resonance can be excluded as a possibility.

There seems to be some rust on the shaft. Corrosion may be a contributing factor. Any evidence that the bearing inner race may have been slipping on the shaft? You would probably have to remove the bearing to see that.

 

[edison123]

desertfox - Thanks for that shaft stress link. I often get motors with such shaft failures.

As for the pulley extending beyond the shaft, I do get regularly motors with such extensions for rewinds and they seem to work ok with such extension.

And most of the motors have the same dimensioned DE and NDE bearings.

Muthu

http://www.edison.co.in

 

[electricpete]

11 belts and massive pulley hanging way beyond end of the shaft does sound ominous to me, but I don't see a lot of belts. I'll bet the belt/pulley forum can give some guidelines.

I went ahead and did my excercize which was to find the slope and displacement for desertfox' example (attached). The algebra is tedious, but there are graphs of V, M , Slope, Displacement. Thed the results were what Tmoose and desertfox predicted. For desertfox' example problem, the angle at the shaft is 0.0065 minutes. So by the chart there would be no significant reaction moment from the bearing and simple support is a very good approximation. Even with much higher load I think it would still be the case for almost any motor, since the shaft is so beefy. I think the bearing reaction moment for a ball bearing could probably only come into play for a long thin shaft.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

Correction in bold:
"the angle at the shaft is 0.0065 minutes. "
should have been
"the angle at the inboard bearing is 0.0065 minutes. "

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

There was a typo:
Ye(x):=int(Tm(x),x)+CYe;
should have been
Ye(x):=int(Te(x),x)+CYe;

Revised output attached - not a big change. Same conclusion.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[desertfox]

Hi edison123

Your very welcome to the link, its amazing what you come across while searching for info.

Thanks for clarifying about the motor bearings and the shaft extension.
Although I still think having the pulley overhang its shaft by 150mm (assuming that is the case) isn't quite right.

Hi electricpete
I hadn't thought about working out the slope of my beam example to prove whether the bearing moment was significant or not,good thinking,I had missed that point. I checked your calculation with my own calculation and I agree that the angle is very small, my answer was 0.0068' but I used a different method, I can post it if wish.

Hi genman196

In addition to my earlier post requesting information I would like to add the following:-

Bearing outside diameter and if possible can you give the bearing reference.

It might also be useful if you can tell us if there are any differences in the set up between the motor with the shaft failure and its predecessor.

regards

desertfox

 

[genman196]

Hi desertfox

Both DE and NDE bearings are the same, size 6322, described in the SKF Manual as Deep Groove Ball Bearing single row,with width of 50mm and outside diameter of 240mm. The approximate distance between bearing centers is 1000mm, it was a frame size 355 motor.

The pulley width is correct, having quite an overhang of 150mm.Ive talked to some of my older colleagues and they say it is the same set-up as when the machine was installed, with such overhang. The same set-up and installation and adjustment procedures were done during the replacement.

regards

genman196

 

[desertfox]

hi genman196

Well thanks for that information, do you know what the tension is on the belts? ie slack and tensioned side.
If we have the tension in each side of the belts we can work out the torque from that as well as the direct load on the shaft causing the bending and it will enable us to do a rough stress analysis.

desertfox

 

[desertfox]

hi genman196

From the info you have given me so far I have managed to get the bearing information, which allows me to look at the fillet radius dimensions just behind the bearing.
Also the shaft step up at that point is a maximum 149mm in diameter so it goes from 110mm dia to 149mm diameter and from this I should be able to get a stress concentration factor for the fillet radius in the failed area.
I can also work out the bearing reactions now I have the length betwen bearings and do a approx analysis similiar to the example I posted earlier, however without the bending load at the pulley its not a true representation and in addition if I set a stress level and work backwards to get the bending load I get some very large forces, theres definetly some serious loads in that set up.
I need to check with also in the regard to ambient temperature around the motor shaft during operation is it in a very hot atmosphere are normal ambient around 20 degrees C.

regards

desertfox

 

[electricpete]

Also the shaft step up at that point is a maximum 149mm in diameter so it goes from 110mm dia to 149mm diameter

Where did that info (149mm) come from?

I can also work out the bearing reactions now I have the length betwen bearings

I didn't see length between motor bearings. Was it posted?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[desertfox]

hi electricpete

I was kind of hoping genman196 might get those tension figures,still time I suppose.
The diameter 149mm comes from the bearing book, where it gives the maximum diameter for the shoulder against which the bearing rests against. The length between bearings came from gemman in his post yesterday when he kindly furnished the bearing numbers.
Still looking at the stresses I'll post what I can later,hopefully we might get more information, genman196 if you can't get anymore information or you need more time just let us know.

regards

desertfox