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

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

 

[Tmoose]

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.

 

[BobM3]

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.

 

[edison123]

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

http://www.edison.co.in

 

[electricpete]

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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[electricpete]

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:

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)

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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[electricpete]

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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[electricpete]

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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[desertfox]

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.

 

[BobM3]

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.

 

[burnt2x]

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)

 

[electricpete]

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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[pmac10]

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.

 

[Tmoose]

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.

 

[genman196]

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.

 

[electricpete]

A quote from Practical Plant Failure Analysis by Neville 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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[desertfox]

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

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

 

[automatic2]

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.

 

[electricpete]

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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[electricpete]

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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