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AC Motor shaft sheared
5

AC Motor shaft sheared

RE: AC Motor shaft sheared

That docx crap doesn't cut it for about 80% of us. Any chance you can post a jpg in here?

You can use the above for direct posts.

Keith Cress
kcress - http://www.flaminsystems.com

RE: AC Motor shaft sheared

(OP)
OK, may have over done it, but I'll catch on


RE: AC Motor shaft sheared

Tossing out a guess, it appears to be an [overhung load with rotational bending] fracture... based on the photos posted of the shaft break.

What is the motor's application? Is it a belt drive arrangement by chance?

And thank you for posting pictures!

John

RE: AC Motor shaft sheared

OK thanks, that worked, as did the close-ups. Thanks!


Yikes. I see what you mean. Not clear on what we're seeing though. Is the bottom picture the shaft having left the rotor and heading into a machine (pump)? What kind of load is the shaft driving would help with the possible answers.

Typical shaft breaks are like this. Something stresses the shaft and then the stress fracture moves across the shortest path until the shaft fails. I could see the HAZ from welding the fan on being the precipitator.

Keith Cress
kcress - http://www.flaminsystems.com

RE: AC Motor shaft sheared

Odd that the shaft sheared not on the outboard of the bearing but on the inboard at the rotor core end. What was that welding for?

Muthu
www.edison.co.in

RE: AC Motor shaft sheared

(OP)
The shaft sheared right going into the rotor. The other picture is of where the shaft goes out of the housing, and that flat end is what was attached to the rotor. It's a hydraulic elevator pump motor. As for the welding marks, have no idea. Was purchased as a new motor. Almost looks like it was "welded" to the rotor. That is almost too flat a fracture.

Thanks everyone

RE: AC Motor shaft sheared

Could you have had any power interruptions of a second or so?
Re-energizing out of phase with the residual EMF may cause torsional stress on the shaft.
That, combined with the local stress of the welding may have been a cause.

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

RE: AC Motor shaft sheared

Bill; Don't motor shafts that fail typically shear off cleanly like this one?

Keith Cress
kcress - http://www.flaminsystems.com

RE: AC Motor shaft sheared

This is a shear failure rather than a torsional one. Such shear failures happen typically at bearing ends, not at rotor core end and most of them are due to tangential loads like V belt pulleys, gears etc. or due to stress risers at the inadequately radiused (?) shaft steps. OP should post the pics of the shaft on the outboard of the bearing.

Muthu
www.edison.co.in

RE: AC Motor shaft sheared

This shaft break is interesting to me because somewhere in my archives of electric motor failures,
I have photographs of nearly the identical fracture as posted by the OP.

I still remember the color of the motor as green, it had aluminum end brackets with no bearing retainers etc.

Not to beat up on Reuland.... but to see this identical failure serves cause that...
perhaps this motor construction was unfit for the service it was engineered to provide.

Note the knurled shaft to provide a "fit" with the rotor laminations and the welds
to assist in holding it fast to the rotor. It's not a solid press fit.
It's not a motor that would endure a lot of mechanical stresses.

None of the above pinpoints the exact cause of the failure. Just highlighting that this particular
name brand of motor or its design has had this type of failure attributed to its history in certain applications.

John

RE: AC Motor shaft sheared

(OP)
OK, made a mistake. Motor was from a traction elevator. Couple more pictures to offer different view
First picture shows where the shaft is attached to the flange that couples the motor to the brake.

RE: AC Motor shaft sheared

I'm wondering if the shaft failed or even just cracked a long time ago, and someone decided that rather than replace it, they would weld it onto the rotor, as in "We can't have the elevator be down for weeks waiting for a new motor from Rueland, so weld it for now and we will order a replacement and put it in when it gets here...".

Then some bean counter said "Is it working now? Yes? Then no need to spend money on a new one..."

Side note: now that we know there is a mechanical brake, it's likely that there is something wrong with the brake system. Elevator motors must have very heavy duty brakes capable of stopping a fully loaded elevator in free fall. If the brake engages while the motor is still energized, you can definitely break the shaft.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: AC Motor shaft sheared

Agree it was an inherent crack which snapped the shaft when it was braked. If it is a 'new motor', OP should claim the warranty.

Muthu
www.edison.co.in

RE: AC Motor shaft sheared

Hi

Looks like a fatigue failure to me, I can see river lines going radiating from the large dark area which have may of been a flaw in the shaft.
I'll do some digging and post later.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: AC Motor shaft sheared

I'm a little confused, the first and 2nd pictures are both halves of the same break, right?
The 2nd pic shows a keyway.
I don't see the same keyway in the 1st pic. Is it hiding there at around the 6:00 position? Is the shaft keyed to the core? Why would the keyway extend beyond the core?

I guess there was welding between shaft and core endplate at 12:00 and 6:00 positions to prevent core migrating axially on the shaft? Seems to have deteriorated, any idea why? (maybe during the failure).

The plane of the shaft failure seems to be about 1/4" into the core.

What is the purpose of the evenly spaced axial grooves along the shaft? I'm not familiar with it. I get the wild idea that may have been done because they had difficulty assembling and this was easier than remachining the diameters to reduce interference / increase clearance.

Based on the odd location of failure my wild first guess fwiw is there is a manufacturing related stress riser of some kind. I didn't read the other responses closely, I apoligize if I ignored better answers (I don't think I'd know the right answer if I saw it in this case).



=====================================
(2B)+(2B)' ?

RE: AC Motor shaft sheared

Pete; Now that you mentioned it, take a close look at the 6:00 o-clock position.
I think that I see a keyway with a key still inserted. Not sure.
I suspect that whatever the cause, the welding stress may have contributed to the location.

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

RE: AC Motor shaft sheared

How is the motor controlled?
This could be a fracture caused by "reclose effect" due to the supply being interrupted for a very short period of time and reclosed partially out of phase.
Start/delta starter, soft starter with one or more SCRs misfiring, frequent "Auto-Off_Manual switching while it is running, rapid switching from forward rotation to reverse operation.
The flux in the rotor can take more than a second to significantly reduce, so all open transition switching can result in very high torque transients.
The two weld points would probably create stress points and weak spots to transient torques.

Mark Empson
Advanced Motor Control Ltd

RE: AC Motor shaft sheared

My thoughts fwiw

The description above figure 6 indicates fatigue failure from rotating bending stress. That is a pretty common failure mode, and yes it sort of resembles that (I would always defer to a materials guy and someone that looks very closely at the failure).

Location of such failure would typically occur at the location along the shaft where bending stress is maximum. IF we draw a shear and bending moment drawing based on radial load applied on the shaft extension, under a BIG SIMPLIFYING ASSUMPTION that the shaft cross section is uniform, then the max bending moment does not occur anywhere close to this location.

That suggests that the interaction between the core and shaft has some role, one of three ways I think of: 1 - a source of stress from interference; 2 - as a stress concentrator at the end of the core (*); or 3 – as an effective change in shaft diameter which invalidates the above simplifying assumption and thereby changes the location of the maximum bending stress. I’d have to try to work out a shear and bending moment diagram to see if #3 makes sense. It’s a little more complicated without that simplifying assumption, I’m not sure what that diagram would look like at the moment.

* the stress concentrator could also be those welds.

Still waiting to hear about the keyway. Certainly if the keyway stops abruptly with a squared-off fashion (rather than rounded or sledrunner) exactly at the plane of the failure, that would be a significant fact.

What is the effect of those axial grooves in the shaft? (knurling as John called it). At a minimum, they reduce the effective diameter of the shaft slightly which reduces fatigue strength slightly. But it is well known that poor surface conditions of the shaft (typically from corrosion) can reduce the fatigue strength (I guess it forms initiator locations for cracks). Could those grooves act like an extreme case of poor shaft surface condition and greatly reduce the shaft fatigue strength? John - how common is knurling of the shaft under the rotor core....and does it typically extend beyond the core as it does here? (if the purpose was to prevent relative movement, then the shaft knurling could terminate before the end of the core... a location which may see locally higher bending stress.


=====================================
(2B)+(2B)' ?

RE: AC Motor shaft sheared

Just thinking out loud (broadening the possibilities, I’m not in a position to narrow them), I wonder if excessive interference between core and shaft might play a role.
I looked up the effect of interefence fit on shaft fatigue strength:

Quote (Machine Elements Life and Design, CRC Press)


“A special emphasis should be placed on the decrease in fatigue strength caused by press fits. This kind of stress raiser is not connected with any change in the shaft’s shape, but it may reduce the fatigue strength even to 25–30% of that of the plain shaft. This effect is caused both by additional stresses where the hub and shaft surfaces make contact (see also Chapter 2, Section 2.1) and by fretting damage to the shaft surface layer. The formation of fretting is associated with the relative motion of the contacting surfaces under the influence of bending moments and torque. A mechanism in which this motion appears has been described in Chapter 2.”

They are talking in particular about interference between coupling hub and shaft, but the same principle should apply to a postulated interference between shaft and core.
They don’t say where the failure would occur in such case, but I’m pretty sure it’s at the end of the hub (core in this case), for two reasons:
1 - in the center of the core, the added stress is pure compression, there is no shear component from the compression. I think there is a shear component at the ends.
2 – The discussion following the above paragraph indicates some means to mitigate the problem, and it involves changing the profile of shaft or hub at the ends.

Also the fretting they mention would be most severe at the ends.

None of this considers the grooved/knurled shaft. John suggested these are not normally used with press fit / interference fit.


=====================================
(2B)+(2B)' ?

RE: AC Motor shaft sheared

I think it's your 2 AND 3 at the same time Pete.

2) The stress riser due to the HAZ (Heat Affect Zone) of those welds.
AND
3) The effective change in shaft diameter where it leave the rotor... in fact almost exactly where it leaves the rotor.

Keith Cress
kcress - http://www.flaminsystems.com

RE: AC Motor shaft sheared




Gold line start and crack propagation direction.
Red circle is the last bit holding that actually got twisted off.

Note on the terminating end of the gold crack where the shaft was punched in as the shaft bent on separation. The twist-off would happen at the 'last end' of the crack failure.

My theory anyway.

Keith Cress
kcress - http://www.flaminsystems.com

RE: AC Motor shaft sheared

Thanks Keith. I didn't see that view before - now I can see there is the key sticking up at 3:00 in that photo. The top of the key is rounded and the key is welded to the core and the shaft. Please disregard all my questions about the key and keyway.

IF the propogation is in the direction you say (*) THEN it started in the vicinity of that keyway / weld.
(* however, I think I can see beach marks running parallel to your arrow and I thought the failure propogates perpendicular to the beach marks ),

=====================================
(2B)+(2B)' ?

RE: AC Motor shaft sheared

Nice picture Keith. I have suspected that the shaft was keyed to the rotor core as well as the press fit.
Suspicions are stronger now.
Marke and I have both suggested that the shaft was weakened by the welding and then suffered a torque event. Either by a re-closer event or by Jeff's suggestion of inappropriate breaking.

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

RE: AC Motor shaft sheared

Hi Keith

Excellent job it's the photo mark up.

Electricpete , I too struggled with the Bach marks and its possible that there was more than one crack on the outer edges of the shaft albeit that the keyway would have been the weak point especially having been welded.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: AC Motor shaft sheared

One more data point, the inside corners of the key way do not appear radiused.

I have no reason to doubt what Keith says. But if I were investigating at our plant, I'd give it to our degreed metalurgist who dabbles in failure analysis part time. I've done that before and he comes up with some remarkable subtle observations which I would never have noticed or understood, based on both the macro appearance and the under-the-microscope appearance. We're lucky to have him.

=====================================
(2B)+(2B)' ?

RE: AC Motor shaft sheared

fwiw, my best swag based on attempting to read beachmarks (which are perpendicular to direction of crack propagation) is that the cracks initiated on the inside corners of the keyway, rather than the weld (although view from the other side of the key hidden in this photo might be different). Again, I'm no expert and I would trust a knowledgeable person with closeup inspection far more.



=====================================
(2B)+(2B)' ?

RE: AC Motor shaft sheared

Noting the requalification of the motor being from a "traction elevator"... is the output shaft
driving a pulley, or is it directly coupled? (I may have missed that answer if it was given.)

On the knurling:
Knurling is an economical way of creating an obstructing fit between the rotor laminations and shaft.
This type of fit relies upon raising the shaft surface metal to compensate for the shaft diameter being undersized for the hole it's mating with.
Hence, the additional necessary application of weld(s) in this instance... to keep the rotor in its proper location.
Typically this type of assembly is found in much smaller horsepower arrangements.

Knurled shaft fits on larger motors are usually confined to cooling fan fits, etc.

As Marke suggests, the weld points could have contributed to creating stress points,
"but" typically weld points like this are found on many motors in industry without any adverse effect.

Another observation related to the motor's design:
It's not very often the rotor/shaft portion of an electric motor is SMALLER in diameter than its working output shafts.
(They are out there. A soaking furnace motor comes to mind to name one example.)
In looking at the re-attached photo from earlier in the thread, note how the rotor is
mating with a shaft that is smaller in diameter than what is passing through the light duty
6200 series bearing heading toward the brake.
Consider how much more robust the rotor portion would be if the shaft remained the same diameter or slightly larger than the brake output shaft end.

The shaft failure appears to be "text book" rotational bending.
At this point, it's conceivable that an identical motor could be reinstalled for service, and
within a period of time have the same failure.

If a new shaft was made to replace the broken one, it would serve greatly to make it a "keyed"
press fit that would bottom against a shaft shoulder and not rely upon a couple of spot welds.

With this being a somewhat OEM motor failure, I'd suggest more input from the manufacturer
of the equipment, and the maker of the motor to learn if there is something amiss here.

John


RE: AC Motor shaft sheared

Quote:

note how the rotor is
mating with a shaft that is smaller in diameter than what is passing through the light duty
6200 series bearing heading toward the [load]
Good point. (I knew something looked odd about that picture). That is opposite of the vast majority of motors (and other rotating equipment) where the shaft gets steadily larger as you move to the center. In addition to crafting a weak spot, it changes the bending moment profile along the length of the shaft (for a given assumed radial load on shaft extension)
=====================================
(2B)+(2B)' ?

RE: AC Motor shaft sheared

Hi

Just looking at the photo electricpete posted on the 7th oct, it appears to me that the green lines Pete marked out actually appear to come from the silvery coloured area ( where we believe final fracture took place), I know this goes against the common thinking but I wonder if the crack started opposite the keyway and made its way across, I can see a crack in the outer diameter just above the silvery coloured area in peters photos.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

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