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broken shaft of 280 KW SQIM
4

broken shaft of 280 KW SQIM

broken shaft of 280 KW SQIM

(OP)
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.
 

RE: broken shaft of 280 KW SQIM

Misalignment if on a coupled application.

RE: broken shaft of 280 KW SQIM

A general question deserves a general answer (I'm sure others can add more).

Why could a shaft fail?

Stresses can be produced from torsional load, bending load (less likely axial load). Those loads can be static, oscillating, or infrequent transients.

Failure can occur as ductile failure, brittle failure, fatigue failure.

Material can be weakened by stress concentrations from machining (for example shoulder without a radius), from attachments (interference fit hub without a radius), by surface finish and corrosion. By bad heat treat.

We had a pump shaft failure that we blamed on corrosion damage weakening the shaft which reduced fatigue strength.

Often looking at the fracture surface gives clues about the type of failure and types of stresses that caused it. There is quite a bit of textbook guidance about what to look for (I like "Practical Plant Failure Analysis" by Neville Sachs.).  Also Austin Bonnet has some good papers on shaft failure.

If you told us more about the application or the failure appearance we might make a better guess, but still a guess. Probably Mech E general could give better guesses.

From what I gather the  most common scenario is bending load (such as from belt) and failure at a stress concentrator.

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RE: broken shaft of 280 KW SQIM

Quick summary.
Belts too tight.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: broken shaft of 280 KW SQIM

I'll add to the confusion. Excessive vibration. Poor material choice. Some repair of the shaft in that area by welding.

If you want proper answers, then post more details.

Muthu
www.edison.co.in

RE: broken shaft of 280 KW SQIM

2
Hi genman196

A picture or photo of the broken shaft is worth a 1000 words.
Lots more info needed if you want a better answer.
how long as the motor been in service?
What was the torque load?
What was the torque on the shaft?
etc etc.

desertfox

RE: broken shaft of 280 KW SQIM

More confusion: Did it break at the notch of the shaft? Was the break a clean break on the material or rugged one?

RE: broken shaft of 280 KW SQIM

(OP)
Thanks for the valuable info and references you all gave.The motor was belt coupled to a clutch driven flywheel, which allows it to run freewheeling with the flywheel when material extrusion is stopped.The shaft was broken right behind the drive end bearing, inside the motor itself and it was a rugged break.

RE: broken shaft of 280 KW SQIM

Hi genman196

If the break surfaces are approximately at an angle of 45 degrees (see sketch uploaded) then its possible that the failure is due to fatigue.
If the shaft is a ductile material subject to unidirectional repeated torsion, then this type of failure can occur due to fatigue cracks spreading due to tensile stresses perpendicular to the principle planes.
How long as the shaft been in service? Is it only loaded in one direction?
If you can upload a picture or photograph of the failed shaft and anymore detail it might help getting a better answer.
Have a look at the sketch I have uploaded and let us know if that resembles what your broken shaft looks like.
If the shaft as been in service for a while then fatigue is a good possibility.

desertfox

RE: broken shaft of 280 KW SQIM

Any idea of the type of shaft material used? I once had a shaft failure (980kW motor) due to brittle fracture (low energy) where the fracture surface was shiny and flat. In your case, it could be what desertfox described (metal fatigue) due to cyclic loading on the shaft. There are a lot of posters who are very knowledgeable on "fracture mechanics" at the mechanical forums, I guess.

RE: broken shaft of 280 KW SQIM

Hi genman196

Thanks for the picture, it looks like the shaft as failed at 90 degrees to its rotational axis and not as I assumed in my previous post.
The failure picture suggests a shear failure across the section, indicating that the shaft material is weaker in shear than tension (a low carbon steel).
What puzzles me though is the shiny smooth area's which if you look closely seem to have classic beach marking usually associated with fatigue.
Also there appear to be a number of cracks at the outer perimeter which is where I would expect the cracks to start
due to fatigue.
What do the fracture surfaces look like at 90 degrees to the picture you have posted?
I'll try and dig some more information up.

regards

desertfox

RE: broken shaft of 280 KW SQIM

High number of ratchet marks and very small instantaneous zone suggest the primary problem was stress concentration, not high stress.  Radiusing approach on shaft shoulder is the prime suspect. There are limitations of shaft shoulder since it has to coordinate with bearing radius - but there are various ays to overcome this.
 

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RE: broken shaft of 280 KW SQIM

Hi electricpete

It looks to me that the shaft as broken on the straight part of the shaft, level with the outside of the bearing and not at the shoulder side but I could be wrong in which case I would agree with you.
But the failure mode describe at the link in my above post seems to fit the bill, which would suggest poor alignment of shaft to power source.
Perhaps the OP could confirm if this is a new installation or as been stripped for maintainance recently.

desertfox

RE: broken shaft of 280 KW SQIM

Quote (desertfox):

It looks to me that the shaft as broken on the straight part of the shaft, level with the outside of the bearing
I assume the photo is looking at the broken off shaft extension, complete with bearing (no longer part of the motor), based on....

Quote (genman):

The shaft was broken right behind the drive end bearing, inside the motor itself

Quote:

But the failure mode describe at the link in my above post seems to fit the bill, which would suggest poor alignment of shaft to power source.
It could be.  Neville Sachs' book has a large number of pictures/drawings of shaft fracture which helps narrow down the type of stress. However, it also talks about clues of excessive stress.  The large number of ratchet marks is one.  It means multiple almost simultaneous sites of failure intitiation which is characteristic of stress concentration all around the circumference.  It also shows very small instantaneous zone indicates the load on the shaft at time of final failure was low.   The latter in itself is not conclusive, but put them both together along with failure location and known stress concentrator (shoulder) and the diagnosis is strong imo.


 

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RE: broken shaft of 280 KW SQIM

Correction:
"It means multiple almost simultaneous sites of failure intitiation..."
should have been...
"It means multiple almost simultaneous sites of fatigue crack intitiation..."

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RE: broken shaft of 280 KW SQIM

Another correction (last one)
"However, it also talks about clues of excessive stress."
should have been:
"However, it also talks about clues of excessive stress concentration."

The point being what is unusual is not the loading but the the stress concentration.
 

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RE: broken shaft of 280 KW SQIM

Let me backtrack and say that to improve the situation we'd like to address all factors: limit stress concentrations and limit loading.   

There are seemingly-obvious clues I've mentioned that stress concentration is more relevant in this case, but that doesn't have to be the end of the story...

From my limited viewpoint, determining the nature of the stresses that caused the failure is more complicated....I haven't worked through it myself. Will spend some more time when I get a chance.   I didn't mean to downplay anyone else's comments.

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RE: broken shaft of 280 KW SQIM

Hi Genman196

Re-reading your post on the 28th Dec I realised I missed an important point as you stated the motor was belt coupled.
This belt coupling would create a rotating bending stress on the shaft thus creating tensile stresses needed to propagate fatigue cracks in addition to the the torsional shear and shear generated by bending.
Whilst we haven't any load figures to number crunch to verify this, I would put my money on rotational bending fatigue.
If you can provide load details and dimensions of the shaft we might be able to prove it but in addition we would need the shaft material information also.

desertfox

RE: broken shaft of 280 KW SQIM

Hi Genman196

This links quite informative about shaft failure:-

http://books.google.co.uk/books?id=A-nRlT6nUUQC&pg=PA63&lpg=PA63&dq=rotating+bending+failure+
of+rotating+shafts&source=bl&ots=rv47JBAjX8&sig=
zB4kuoisg5DipxN_xnR8DolYLKo&hl=en&ei=7zU6S7jZHJe
8jAeN5rSmDg&sa=X&oi=book_result&ct=result&resnum=7&ved=0CCkQ6AEwBg#v=onepage&q=rotating%20bending%20failure%20of%20rotating%20shafts&f=false

I should of added in my last post that the rotational bending fatigue which I assumed earlier to be purely down to misalignment of the shaft may now not be solely the case.

desertfox

RE: broken shaft of 280 KW SQIM

Any history of motor repairs, especially drive end bearing journal "restoration" via welding or metal spraying?

Even with an unmolested original shaft Waross' evaluation remains a front runner.

Are there other similar installations racking up millions of fully reversed bending stress cycles every week?

I'd be evaluating the belt drive design and installation, including verifying the tensioning method ( twa-a-a-ng) and all the stuff in the Gates' design guide, including sheave diameter, and sheave offset creating greater overhanging loads. One danger is If the designer selected the economical maximum width, smallish diameter sheaves over maximal diametered sheaves. Small diameters force unavoidably greater belt tension for power transmission, and well-intentioned but un-informed installation over-tensioning can really add to that.

I believe Some motor manufacturers spec a minimum drive sheave size to help protect against excessive belt loads, and some spec a maximum radial load and offset directly.   

RE: broken shaft of 280 KW SQIM

Just to verify, you said the break was on the inside of the motor?  Nominal (ignoring stress concentrations) bending stresses would normally be higher on the outboard (between bearing and pulley) side of the bearing.

As suggested earlier, you might want to have some of the members in the "Materials Engineers - Metal and Metallurgy Engineering" take a look at that photo.

RE: broken shaft of 280 KW SQIM

Looks like failure due to excessive shear load. Belt driven loads place one of the worst duties on the shaft. Shear + rotational torsion + cantilever load.

Solution - Increase shaft dia and go for a higher carbon steel shaft material. Or support the belt pulley separately on two bearings.

Muthu
www.edison.co.in

RE: broken shaft of 280 KW SQIM

The failure progressed slowly (beach marks) from the outside in until all that was left was the small football sized area in the middle - called the "instananeous zone".   It is about 5% of the area of the shaft or less.  This 5% was carrying the entire load prior to the failure.   For the guys who say the load is large, would you say a load is large if it can be carried by 5% of the shaft area?  

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RE: broken shaft of 280 KW SQIM

An article by Neville Sachs, whose book has been quoted by myself and others.
http://www.plant-maintenance.com/articles/rcfa.shtml

It is a good overveiw imo.  His book goes into a lot more detail with a lot more examples.

Two passages from this article that I think are directly relevant to the present case:

1st quote relates to size of instantaneous zone:

Quote:

HOW HEAVILY WAS IT LOADED?

Fatigue failures almost always start on the outside of a shaft at a stress concentration, because the local stress is increased. However, the instantaneous zone (IZ) carries the load in the instant before the part breaks. By looking at the size of the IZ, you can tell the magnitude of the load on the part.
2nd quote relates to the situation where we have small instantaneous zone AND multiple ratchet marks around the outside (indicates stress concentration)

Quote:

Looking at the fracture face, you see a series of ratchet marks. These are the boundaries between adjacent fracture planes, i.e., between each pair of ratchet marks is a fracture origin, and as these individual cracks grow inward they eventually join together on a single plane. The small instantaneous zone indicates the stress at the time when the shaft finally broke was low, but the multiple origins and the ratchet marks show us there was enough stress to cause cracking at many points around the perimeter almost simultaneously.

From this you can conclude that there must have been a significant stress concentration.
(The calculated stress concentration was in the range of 4.0, so the stress in the area of those origins was four times as much as it should have been.)
This last quote applies directly to the photo posted (small IZ, many ratchet marks), agreed?

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RE: broken shaft of 280 KW SQIM

Quote:

Nominal (ignoring stress concentrations) bending stresses would normally be higher on the outboard (between bearing and pulley) side of the bearing
If correct, that is another argument for the role of stress concentration.  However, I didn't follow the logic.  Why would stresses (neglecting concentration) be higher on the pulley side of the bearing than on the motor winding side?  

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RE: broken shaft of 280 KW SQIM

I guess I can see that would be the case if the bearing offers a resisting moment to bending (different than acting like a "simply supported" or "pinned" boundary).

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RE: broken shaft of 280 KW SQIM


Hi Genman196

I think it's fair to say your shaft failed due to rotational bending fatigue, where a number fatigue cracks have grown on the outer perimeter of the shaft as it rotates and slowly precipitated inward until failure occurred.
The smooth beach marks on your photograph show the cracks grew over a period of time and the small eye like feature in the centre of the shaft shows the shaft was lightly loaded at the time of failure in agreement with electricpete's last post.
What isn't clear and there are a number of possibilities as to why the fatigue cracks grew, from what I can see from the posts above are:-

1/
Area of high stress concentration ie: - the fillet radius at the intersection of changing shaft diameters per electricpete's earlier post.

2/
Too much tension on Drive belt to clutch thereby increasing tensile bending load on shaft. (waross,T.Moose)

3/
 Alignment issues between drive and driven shafts.

This list is by no means exhaustive and it might be a single or a combination of these factors above, or indeed some other reason entirely that these fatigue cracks occurred.

BobM also makes a good point about the position of the bending moments which brings me to another request is it possible to provide a sketch or picture of the set up ie: - where the motor shaft is relative to the drive belt and any external supports, in addition can you confirm whether the photo you have posted shows the shaft failure on the straight portion of the shaft or at the intersection of two different shaft diameters.
 

RE: broken shaft of 280 KW SQIM

Pete -

You are correct, the bending moment is not greater on the outboard end of the bearing as I said.  The bending moment is the same on the shaft just inside and just outside the bearing.  

However, the combination of (nominal) bending stresses and (nominal) shear stresses will be higher on the outboard side of the bearing.  It just seems a little unusual for the break to occur on the inboard side of the bearing.

RE: broken shaft of 280 KW SQIM

As posted, the overhanging flywheel plus the belt tension could exert more bending stress on that side of the shaft and will likely initiate cracks at that end.
If indeed the shaft broke just inside the motor (before the DE bearing), the shaft could have been redesigned/repaired/re-surfaced badly causing material fatigue. This failure seems so weird IMO!
Could it be that the shaft was wrongly repaired causing fatigue cracks to grow? (no heat treatment after welding, fillet radius reduced,or fillet not provided near the shaft shoulder)

RE: broken shaft of 280 KW SQIM

Bob - great point.  I was only thinking about bending stresses, not combination of bending and shear stresses which should both be considered.  Imo it is another strong support for the stress concentration hypothesis.

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RE: broken shaft of 280 KW SQIM

no expert on this, but if the motor was reconditioned and a poor fitting bearing used, ie, loose on the shaft, the shaft would rotate faster than the inner race of the bearing thereby wearing the shaft and lessening its cross-sectional area and creating a very odd place for a shaft failure to occur. same could be said for a bearing close to seizure( it doesnt look in best nick in the photo).
after reading some of the well informed responses and learning a hell of a lot, thought id chip in with a simplistic approach.

RE: broken shaft of 280 KW SQIM

I don;t see any replies from Genman for a while.  The picture he posted seems to have the plane of the break within the bearing. It could just as easily be either "side" of the bearing.  The verbal description of "within the motor itself" would seem to indicate it happened nearer the rotor, instead of the drive or outboard end.  A red line indicating the break drawn on this image would make it clearer -  http://www.tamura-engineering.com/images/p009_motor_rotor.gif.

The bending moment diagram would looks something like the lower figure here.
http://www.childs-ceng.demon.co.uk/tutorial/ILEx1.gif
NDE bearing is the left upward arrow. The belt pull is the right upward point arrow.  The DE bearing is the point at X.  Although the >>moment<< is highest right on the bearing centerline, the resulting >>stress<< depends completely on the shaft geometry. Before applying stress concentration factors stress=(Moment)(shaft radius)/I [moment of inertia which varies as the 4th power of the shft diameter].  There will be a somewhat larger shaft diameter to the left of the bearing to form an abutment or shoulder.  Just to the right of the bearing the diameter will be smaller. That just about guarantees the area of highest stress will be outboard of the bearing, and a failure should occur there, unless the material is flawed, or some badly done shaft feature creates a dreaded stress concentration that bumps the localized stress higher than the material's endurance limit.

 

RE: broken shaft of 280 KW SQIM

(OP)
This a pic of the rotor side, the bearing is against a shoulder. The motor that broke was a spare motor we installed when the original motor was due for bearing replacement. When we transferred the pulley to the motor, there was a minute clearance, as we observed that the pulley was too easy to slide in and was confirmed when we ran it without belts. There was a loose banging sound from the pulley side and the motor was immediately stopped. After fitting another pulley, the motor ran loaded for about two months before the failure.im digging as much history about the motor and will post it as soon as possible.I wish i could discuss it as exhaustively but I have very limited if no experience on shaft failure analysis.  
Thanks a lot to all for continuing to take your time to share such valuable information on the failure and i have learned a lot. Happy New Year.  

RE: broken shaft of 280 KW SQIM

A quote from Practical Plant Failure Analysis by Neville Sachs

Quote (Sachs):

INTERPRETING THE INSTANTANEOUS ZONE SHAPE
The IZ is the last piece of the part to fail, and the shape can show us the forces that were actually
acting on the piece immediately before the final fracture. The size of the IZ gives us an idea of the forces at the time of failure and, if there are no progression marks, it is also a good indicator of initiating forces. Furthermore, in Figure 5.12 and Figure 5.13, for the various failure categories. Notes:
1-Plane-bending and reversed-bending failures — the shape of the IZ boundary will generally be convex in the last half of the failure. The presence of sharp changes near the outer surface of the piece is an indication of high stress concentrations.
2 -  Rotational bending — Generally, the higher the total stress, the better centered the IZ.
3- Multiple causes — A pure rotating load will result in a round IZ. The greater the elongation, the greater the proportion of bending load as a cause.
The bolded portion in first paragraph makes a point that the size of instantaneous zone indicates load at the time of final failure but not necessarily at failure initiation – a point also made by desertfox.  The  fact that we have beach marks suggests the magnitude of the cyclic load was changing, so it is hard to argue the load was the same at failure as at initiation.  (the only alternative explanation for beachmarks besides varying magnitude of cyclic load is that the beachmarks indicate individual load cycle striations in the very final stage of failure).  

Item 2 suggests that higher total stress results in better centered IZ.  I think the IZ is fairly centered for the motor posted by genman196.  This particular thumbrule seems to contradict what I have been saying.  The thumbrule doesn't make sense to me since I personally (not a mechanical or materials guy) would expect a centered IZ if uniform stress concentartion around the circumference resulted in simultaneous crack origins at many points around the circumference... regardless of magnitude of the stress.  But I would be interested if anyone can explain the basis/logic for this particular thumbrule.

Item 3 – Suggests we have a round IZ for pure rotating load/stress (I assume that means a load that rotates with respect to shaft, such as stationary belt load).  With the football shaped pattern it suggests presence of a component of simple bending load/stress (not rotating with respect to the shaft).   There is one way I can rationalize in my mind that we have a component of bending load/stress present – the effect of the keyway.... let's say the stationary load is acting toward 6:00 position... when the keyway is at 12:00 and 6:00 we have less stiffness of the shaft and more bending, when the keyway is at 3:00 and 9:00 we have more stiffness of the shaft and less bending.  This aspect tends to create a stress that is simple (rather than rotating) bending in a plane that rotates with the shaft.  It is a small detail that is probably not important as far as a "cause", but it's a satisfying explanation to me to explain/reconcile why we have a component of simple (rather than rotating) bending suggested by the shape of the IZ.   I agree with desertfox and others that the basic nature of the load was rotating bending.

I still think the fact that failure occured at location which was not maximum stress as Bob pointed out is an important consideration pointing toward stress concentration.  But the bottom line, best to keep an open mind.  I also agree with the others, the more information is provided, the better chance of reaching a quality conclusion.   Some possible useful items (some already suggested):
* View of failure from a shallow angle rather than straight on.
* shaft size or bearing p/n
* Motor horsepower rating and speed
* Distance from motor endbell to sheave
* Sheave sizes or at least speed ratio
* number and size of belts
* tensioning procedure and any belt/sheave maintenance history
* shaft material and motor repair history
 

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RE: broken shaft of 280 KW SQIM

Hi Genman196/electricpete

Genman196 thanks for clarifying the view of the failed shaft.
electricpete, now the location of the failure as been confirmed I agree with you about the stress concentration for the following reasons.

Fatigue failures usually start at a stress concentration whether it be a mark on the surface of the shaft,a groove, or a fillet radius between two intersecting diameters.
Now we both agree that there was more than one crack initiation point,therefore a fillet radius between two diameters would create a stress raiser uniformly around the shaft.
Now to deal with the stresses, if we consider the shaft
a simply supported beam resting on the two bearings within the motor, it can be shown that the drive bearing takes the greatest reaction from the belt tension(the belt tension acting outside of the two bearing supports). Assuming that the reaction was at the drive bearing centre,then at that point the maximum bending moment would occur and the shearing stress due to bending would be zero,but the shear stress due to torsion would also be maximum.
Move slightly to the right of the bearing, to the area of the fillet radius, the bending stress would be slightly below maximum but the torsional shear stress would still be maximum and some bending shear stress would also be present.
Now if the stress concentration factor is 2 or 3 those nominal stresses in that area are now multiplied by 2 or 3
possibly making it the highest region of stress.
For a fatigue crack to grow it must do so under tensile stresses which are provided via the belt tension and as the shaft rotates,it is subject to cycling tensile stresses which can create multiple cracks.
The IZ in the failure is fairly central and its elongation from circular is due to plane bending,I see the plane bending as being something constant like the mass of the pulley acting in the same direction irrespective of the shaft rotation for example.
I look forward to more information from genman196 as we might home in on a better conclusion, but for now I'll stick with rotational bending fatigue.
Incidentally as the belt tension provides the catalyst for tensile stresses in the shaft I would be checking the motor specification for the allowable over hanging load/ as well as shaft misalignment seeing as the motor was replaced only two months ago.

This site is quite interesting it also shows some shaft fatigue failures and gives some diagrams of critical area's of failure:-

http://vibration.pknu.ac.kr/vibration_pds/motor_diagnosis/Shaft_failure.pdf


This site is from WEG and clearly they state a shaft failure may occur exactly where the OP's shaft as failed.

http://www.ckit.co.za/secure/conveyor/troughed/electric_motors/electric_motors_malfunct.html

Quote:


1.7 - SHAFT FRACTURES

Although bearings traditionally constitute the weaker part, and the shafts are designed with wide safety margins, it is not beyond the realms of possibility that a shaft may fracture by fatigue from bending stress brought about by excessive belt tension.

In most cases, fractures occur right behind the drive end bearing.

As a consequence of alternating bending stress induced by a rotating shaft, fractures travel inwards from the outside of the shaft until the point of rupture is reached when resistance of the remaining shaft cross-section no longer suffices. Avoid additional drilling the shaft (fastening screw holes) as such operations tend to cause stress concentration.

desertfox

RE: broken shaft of 280 KW SQIM

your failure is almost certainly from belt tension. Shafting typically fails at stress risers, even with radiusing.

Check your maintenance records for belt tension, and make sure they are logged going forward. Belt tensions (as well as chain) is difficult to guess at and can easily be set higher than the manufacturers recommendations. It came into focus for use when an 8' fan blade launched out of it's cage.

RE: broken shaft of 280 KW SQIM

Tmoose - here are my thoughts.

An analysis of the inboard bearing modeled as a simple support:

Concentrated point load (reaction force) is applied at the bearing.

The Shear is equal to the integral of distance, so shear takes a step change at the bearing (because the concentrated point load is a singularity like dirac delta... infinite magnitude integrated for infinitessimal distance gives finite result)

Moment is the integral of the shear, so it does not change from one side of the bearing to the other, regardless of area moment of inertia (because shear remains finite, integral over infinitessimal distance from one side of bearing to the other is zero).

So as Bob said, the moment (and therefore bending stress) is the same on both sides of the inboard bearing, but the shear is higher on the pulley side.

Would you agree?

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RE: broken shaft of 280 KW SQIM

One correction
"The Shear is equal to the integral of distance,"
should have been:
"The Shear is equal to the integral of force-per-distance over a distance,"  

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RE: broken shaft of 280 KW SQIM

desertfox - sorry I didn't read your response. I will think about your analysis of the beam problem including torsion stress.

I didn't understand your point about the weight of the pulley creating plane stress.  Are you talking about gravity weight or unbalance?  

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RE: broken shaft of 280 KW SQIM

Hi electricpete

Yes I was just giving a possible example of plane bending ie a gravity load always acting vertically however in this case it might be very small compared with other loads, but read my post I think that the maximum resultant stresses occur at the fillet.
If you ignore the shear stress due to bending for the minute, whatever bending stress you have at the fillet based on the bearing shaft diameter, it is then multiplied by the stress concentration factor, in addition you also have torsional stress at the fillet, which also as to be multiplied by a stress concentration factor.

desertfox

RE: broken shaft of 280 KW SQIM

[quote electricpete]... the moment (and therefore bending stress) is the same on both sides of the inboard bearing, but the shear is higher on the pulley side.[quote]
I was right the moment doens't change from one side to the other.  But I was wrong, the stress does change as Tmoose said - the higher area moment of inertia on larger shaft side (inboard) has lower stress for the same moment.  

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RE: broken shaft of 280 KW SQIM

Hi electricpete

Yes on the side where the larger diameter is the stress will drop, however upto the fillet radius the value of stress is that calculated for the smaller diameter which is then multiplied by the stress concentration factor to give the actual stress in the fillet. The same applies to the torsional stress, the value of stress is that on the smaller diameter right upto the fillet, which is then multiplied by the stress concentration factor.
Once you have passed the fillet radius and get into the larger diameter the stress drops.
Our interest is where the failure occured which is in the fillet radius due to the stress concentration there and its the resultant of the torsional stress and cyclic tensile stress which causes the failure.

desertfox

RE: broken shaft of 280 KW SQIM

I agree - the stress in larger portion of the shaft is not much interest - I was just trying to catch up to understand what Tmoose had said.

Back to the subject of the different types of bending. In the context of rotating shaft discussion there are two types of bending: plane bending and rotating bending.  In the way that Sachs uses these terms they are applied only  in the reference frame of the rotor.

So rotating bending for the rotor would occur when the plane of bending rotates with respect to the rotor. The most common scenario would be if the direction of force were constant for a stationary observer (such as belt load or gravity).  That would lead to the round IZ.

Plane bending occurs when there is a component of bending that remains in a plane that rotates with the rotor.  (It looks like plane bending in the rotor reference frame).  It is tough for me to imagine how this can occur other than   the type of scenario I described...constant direction force in the stationary reference frame (like belt) combined with assymetry in rotor stiffness (like keyway).
1 - When keyway aligns with belt the shaft tilts toward the belt.  
2 - Rotate 90 degrees and shaft tilts away from the belt.
3 -  Rotate another 90 degrees and shaft again tilts toward the belt.
Comparing 1 and 3, the shaft tilted the same way in our stationary reference frame, but it tilted opposite direction in the rotor reference frame.  There is a component of shaft bending back and forth in a plane that always contains the keyway and the point 180 from it. That is the plane bending (to my understanding)
 

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RE: broken shaft of 280 KW SQIM

Also there is a simple symmetry argument that supports the logic of a round IZ for pure rotating bending... if we start with the ASSUMPTION that the cracks started uniformly around the shaft... then in the presence of symmetric shaft and a load like a belt, there is no reason than anyone one of the cracks will reach a certain radius before the other... by symmetry they all reach a given radius at the same time (circular pattern at any time including final fracture and IZ).

To get anything different (under that starting assumption uniform cracks all the way around), we need some type of assymetry. It's hard to create an assymetry in the applied force if we have something like belt load or gravity, but we can create an assymetry in the shaft (keyway) that leads to assymetric stresses and assymetric crack growth... proceeds at different rates for different portions of the rotor and one portion reaches a given radius  before the others and the pattern is not a circle but oval or football.

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RE: broken shaft of 280 KW SQIM

Hi electricpete

Thinking further on this plane bending I can't see on the shaft in question how you could have a plane bending component (which makes my earlier statement about mass of the components incorrect),any variation in shaft stiffness as it rotated would only vary the cyclic stress at that point.
Looking again a the photo of the shaft the IZ is only very slightly elongated which suggests mainly rotating bending fatigue as previously agreed.
Reading Sachs reference again page 66,fig5.7 and 5.8 gives an example of a shaft that initially had fractures due to rotational bending, but after after the fracture got halfway through the shaft, the final failure came from plane bending, the plane bending however appears to come from the blow received from the cutting roll which occured after every revolution.
So in that case the operating forces changed, in regard to the OP's shaft we are not aware of any changes to the operating force which might give rise to plane bending.
I agree with you that the force would have to rotate with the shaft constantly to have any plane bending in this case.

desertfox

RE: broken shaft of 280 KW SQIM

hi genman196

Happy new year to you.

I am not sure how familiar you are with bending moment and shear diagrams etc.
Anyway I just sketched out some dimensions and put numbers on them to give a feel for what were talking about.
I haven't included any torsional or bending shear stress in the calcs, but concentrated on demonstrating how the bending stress changes, due to the changes in shaft diameter
and the fillet radius area. It can be seen from my example that the bending stress in the fillet radius is 2.8 times the nominal bending stress in the 2" diameter ie 235.56lb/in^2 increasing to 659.568 lb/in^2 before dropping down to 29.44lb/in^2 in the 4" diameter.
Whilst my example is just a static case if you now imagine the shaft rotating, the bending stress in the fillet radius alternates from tensile stress to compressive stress every 180 degrees of rotation and it is this alternating stress which causes the fatigue. As you have probably read already
this stress does not have to be the yield or tensile stress of the shaft material but can be a value significantly lower.
Also if in my example we assume the 40lb load is correct for this particular set up, but due to an error during maintenance this 40lb is now set at say 80lb we automatically double the stress in the 2" diameter shaft portion, which is then multiplied up by the stress concentration factor of 2.8 in the region of the fillet radius, which could possibly cause a failure.
In your particular case it could be belt over tension or misalignment of the belt that as increased the stresses over the previous motor setup.
Anyway we will wait for you to post some more info as stated in your last post.

desertfox

RE: broken shaft of 280 KW SQIM

You neglected shear and included stress concentration factor, and your conclusion was stress was highest on the winding side of the bearing.

Bob included shear stress and excluded stress concentration and concluded the maximum combined (shear and bending) stress occurs on the pulley side of the bearing.

Two different statements, both true.

To find which is more relevant or closer to the true answer, we need to look at magnitudes as you have begun to do.  A quick calculation of shear stress:  area is pi inch^2, shear force is 40/pi  lbf/inch^2 ~ 13 psi... not too important.  

Recognizing the relatively small impact of shear stress, it is reasonable to suspect as you said that the highest combined stress will occur at the winding side of the bearing at the shoulder, even if well radiused.

I vote you a lps.

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RE: broken shaft of 280 KW SQIM

Hi electricpete

Well I would say the highest stress,even combined will be on the winding side.
Firstly which is why I asked the OP which side the failure was on, if and only if there was a stress concentration factor on the outer side of the drive bearing, which had a bigger impact than at the fillet radius would it have failed on that side.
Had that been the case though, the chances are that we would have had only one or possibly two fracture sites and clearly we have several or more.
As you rightly pointed out, the shear stress outside the drive bearing due to bending is 13 psi falling to half that value on the winding side, however the only stress concentration factor exists on the winding side so it can only magnify stresses on that side, so your shear stress due to bending as well as the bending stress would get magnified.
More significantly than that consider the torsional shear stress due to the 6" dia pulley and the 40lb force on the 2" shaft, it works out to 76.4lbs/in^2 far more significant than the shear stress due to bending.
Now this torsional shear stress is also subject to a stress concentration factor at the fillet radius which according to my graphs is about 2.
Therefore before we get the resultant combined stress we have to magnify the individual stresses at the fillet radius.
I will look at this later I hadn't said this earlier as I was trying to keep it simple while still making the point.

regards

desertfox

RE: broken shaft of 280 KW SQIM

hi electricpete

Thanks for the lps


desertfox

RE: broken shaft of 280 KW SQIM

e-pete posted "Plane bending occurs when there is a component of bending that remains in a plane that rotates with the rotor. "  

I believe an example of that would be "force" unbalance, and could only cause bending stress in the shaft between the bearings. Since the magnitude of that force would likely be less than 20% of the rotor weight (lest the vibration analysts storm the castle with pitch forks and torches), the effect would be superimposed on the constant vertical gravity load ( assuming the popular horizontal shaft configuration ).  So although the unbalance force is ~ constant value and ~ constant direction relative to the shaft, the vertical load would vary from 80% rotor weight > 120% rotor weight once per rev. It seems to me if that was a significant load the ODE of motor shafts would be failing all the time. I base this on ODE bearing typically being smaller than De bearings.  If the bearing IDs are the same then my assumption is flawed.

Here is a link to A Baldor premium efficiency motor spec.
http://www.baldor.com/pdf/TEFCSpec.pdf
"All shafts shall be precision machined from high-strength carbon steel suitable for belt and pulley drives (except as limited by 3600 RPM motors)."

Section 3.4.18 seems to be based on belt drive loading being the extreme case that effectively defines the motor shaft dimensions and material.  The effect of unavoidable stress concentrations etc vs allowable endurance ( fatigue resisisting) stress for the material would all be part of the shaft design.  I'd bet a dollar that the shaft design uses endurance limits for "infinite" theoretical life, so stress concentrations are harmless, UNLESS design loads are exceeded.
 

RE: broken shaft of 280 KW SQIM

Hi T.moose

According to my Ball & Roller Bearings Theory,Design and Application (Wiley Publishers) the Drive End and Non Drive end bearings for motrs are normally the same size for economical reasons, associated with the high precision machining of the housings.
The plane bending failure came from the Sachs links above which discusses how the beach marks are observed on the fracture surface as compared with the skewed beach marks with a rotational bending failure.
The plane bending failure shows beach marks progressing straight down till it encounters the IZ zone and involves one way loading similiar to a leaf spring or beam under a static load.
I don't think in this case plane bending is involved with the failure but if it were involved the load would of had to rotate with the shaft to produce the beach marks as described by Sachs and I agree with electricpete on that.
I also agree with you, that the motor shaft would have been designed for a life span and its likely that either the belt tension or misalignment of the belt as resulted in the failure, however we cannot be 100% sure.

desertfox

RE: broken shaft of 280 KW SQIM

Tmoose - unbalance load would cause the shaft to deflect and then stay that way (not continue to flex) since the bow rotates unchanged with the shaft.   Imo, a rotating assymetry such as a keyway (combined with load such as belt) provides a good credible mechanism for the plane bending.  In this case the bow is sensitive to shaft position, but reverses direction twice per revolution creating what I think Sachs calls plane bending.

desertfox
It is common practice to treat a ball bearing as a simple support.  However it can provide some moment resistance such as described here (click on download)
http://www.nsk.com/services/basicknowledge/technicalreport/05distribution.html
On page 5.8.2 is a discussion of ball bearing moment response as a function of angle.   I haven't studied it much.  I expect it should be a function of not only angle but load because I have read in another reference that a ball bearing accomodates misalignment through it's clearances... there is a free angle of misalignment where no moment resistance is offerered, under no radial load.  You can't bend it that far without moment when radial load is present because the radial load also wants to use up the clearance ina certain way.   I am a little bit confused about the way they have treated radial loading on this page.  In the figure 4 there is a plot of moment load vs angle for 6208 40mm bore, just a little smaller than desertfox' example 2" shaft (about 50mm).   In theory with some work, the bending moment diagram might be integrated to give an angle to see what kind of moment is present.... if it is small then we know the simply supported approximation is good here.  If it is large, then I think some iterative solution is necessary to adjust the diagram to account for the moment, which in turn depends on the diagram (slope).  In any case I'm still confused why radial load is not explicitly accounted in Figure 4.

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RE: broken shaft of 280 KW SQIM

I should have mentioned the reason I mention all this: to the extent the bearing offers a moment reaction to the shaft slope, it tends to reduce the bendings stress on winding side compared to pulley side.  

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RE: broken shaft of 280 KW SQIM

Moment is moment.  To get stress or deflection/slope the shaft geometry and material E must be entered. It would take a heap of shaft bending or slope just to use up the clearance in a single ball bearing. I believe the bearing would run hot, and full of HFBD type vibration, and maybe even noisy the instant it was asked to run while handling any real moment.

RE: broken shaft of 280 KW SQIM

Hi Desertfox,

FAG used to give that Wiley book away for free off their website around 1998. That's where I got mine. Nice survey type text.

EPete and others undoubtabley have way more current motor experience than I do, and Genman could report what size bearings his motor has, but I think it is not uncommon for motors in the ~ under 200 HP range to have different DE and ODE bearings. I believe some are even set up so a roller bearing can be directly substituted for the standard ball bearing when serious radial loads (belt drive) are anticipated.

I could not find an online motor catalog with the level of detail necessary to correct my misconception.  

RE: broken shaft of 280 KW SQIM

(OP)
Thanks desertfox for helping me out about the discussion and thanks all for sharing the information, I try to learn as much as i can from this thread and its a whole lot!
Sorry for the delay in posting more details,
shaft diameter where pulley is mounted : 100mm
shaft diameter where bearing is mounted : 110mm
shaft lenght extending from bearing : 250mm
shaft lenght where pulley is mounted : 210mm
pulley diameter : 470mm
pulley width : 360mm
flywheel diameter :1200mm
motor rpm : 980 rpm
Pulley is coupled by 11 belts to the flywheel.I have no details about the shaft material.
Also sent a pic which shows keyway position relative to the break.im compiling some pics of the installation which could be of help and will post again soon. thanks.

RE: broken shaft of 280 KW SQIM

Tmoose - I suspect you are right since I checked several references and they all treated a ball bearing as a simple support.  I will try  solving it lataer tonight to satisfy my curiosity
y' = Integral {M(x)}dx + C1
y = Integral {y'(x)}dx + C2
M(x) as solved already
C1 and C2 solved  satisfy y=0 at both bearings.
convert y' to moment using the chart and compare to moment diagram

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RE: broken shaft of 280 KW SQIM

Hi electricpete
Thanks for the interesting link.
Right, have looked at the link its quite heavy reading, in fact some of it goes right over my head, I would agree that the bearing may take off some of the bending stress at the winding side of shaft, but I doubt it would take it all off, if it did then the shaft from the drive bearing outwards could be treated as a cantilever beam and the shear force and bending moment would stop at the drive bearing. In addition whatever stress is in the shaft at the winding side, it is magnified by the stress concentration factor, this also should include the rotor mass which has been omitted from my simple calculation.
Now in truth the majority of motor failures are due to the drive end bearing failing for one reason or another,I saw a pi chart for motor failures and I think 51% was drive bearing failure and about 2% shaft/coupling failures.
The design methods we use however are a simplification and truthfully the shaft is supported by bearings which are neither simple supports nor "built in" so its not easy to be 100% sure exactly where the maximum stress is, we can only approximate and I think the simple supported model is the best as you have indicated.
Just going back to the rigidity of the bearing I would also go along with T.moose's statement regarding excessive bending of the shaft to take up all the bearing clearance without knowing about it fairly soon, because just looking at figure2 on page13, the inner to outer race angular misalignment (dependant on load) can be about 1/6 to 1/2 a degree which when taken to the end of the motor shaft would be quite a deflection from the horizontal. Also looking at fig4 on page 14, for a small angular misalignment in the range above,it would generate a large moment in the bearing which I feel wouldn't go unseen.

this link posted earlier shows critical areas for shaft failure and its anywhere from the drive bearing to the shaft end.

http://vibration.pknu.ac.kr/vibration_pds/motor_diagnosis/Shaft_failure.pdf

Hi T.moose I am jealous you got the book for nothing!
I had to pay for mine at a discarded library book sale, but it was only a £1 and at least its hardback. I looked for information on motor bearing arrangements too, but drew a blank like you, I'll carry on looking though.

Regards

desertfox

RE: broken shaft of 280 KW SQIM

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

RE: broken shaft of 280 KW SQIM

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.   

RE: broken shaft of 280 KW SQIM

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
www.edison.co.in

RE: broken shaft of 280 KW SQIM

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.

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RE: broken shaft of 280 KW SQIM

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. "

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RE: broken shaft of 280 KW SQIM

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

RE: broken shaft of 280 KW SQIM

(OP)
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
 

RE: broken shaft of 280 KW SQIM

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

RE: broken shaft of 280 KW SQIM

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

RE: broken shaft of 280 KW SQIM

Quote:

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?

Quote:

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?

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RE: broken shaft of 280 KW SQIM

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

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