rotor shaft fracture 2

[chumet1]

vibrating screen motor rotor fractured at bearing journal shoulder (rotor side). set-up of the system installation is shown in one of the pics. motor is mounted on a rigid frame that is mounted to the ground while driving a vibrator through v-belts. my initial assessment is that during vibration, the v-belts exert a radial force on the shaft and due to the motor's rigid mounting it will eventually fail due to the forces. engineers, i am kindly asking for different views on possible causes and solutions to prevent this failure from recurring

 

[desertfox]

Hi

Looks like a fatigue failure due to bending and the area where it failed would have had a high stress concentration.
Read this article below:-

http://www.plant-maintenance.com/articles/rcfa.shtml

The requirements for designing against fatigue are well known, reduce stress concentrations,surface finish,reduce design stresses etc.

 

[rb1957]

it looks to me as though it took a long time to fail ... there looks to be corrosion on the carck face, and a small portion looks to be rapid fracture.

reducing the stress concentration would help, of course; if you're the shaft manufacturer. if you're the equipment operator, maybe look into the rigging, maybe look into replacing the shaft on scheduled maintenance (which'll be cheaper than unscheduled downtime).

Quando Omni Flunkus Moritati

 

[metengr]

The motor shaft failure was caused by high cycle, fatigue crack propagation under bending stress in service. The shiny area signifies rubbing between the fracture surfaces during an early stage of crack propagation.

Since the appearance of the fracture surface was flat with no evidence of final failure, this implies low nominal stress with local stress concentration – so the shaft material is probably adequate in terms of strength. To reduce stress concentration at the shoulder, a larger radius at the shaft transition must be used.

You can also perform periodic inspections of the motor shaft using surface nondestructive testing during shutdowns. The surface nondestructive testing can be done quickly and will detect crack initiation because the cracks initiate in the transition at the OD surface.

 

[JJPellin]

chumet1,

As noted, this looks like classic high cycle fatigue in bending at a stress concentration. I have seen this before on belt driven equipment. The advice of the first two responses is good. I would like to elaborate on their points and add a little bit more.

Reduce the Stress - Move the belts in as close as possible to the face of the motor. Belt tension is the main bending moment and the closer in the sheaves, the less the stress. Make sure that the belt tension is correct and not excessive. Use larger sheaves. The tension in the loaded side of the bent depends on the diameter of the sheaves. The larger the sheaves the less the running tension required to transmit the power. The OEM literature for the best drive components should include a minimum recommended sheave diameter for a NEMA frame motor. This is based on motor bearing loads. But, a load is a load. Larger sheaves will result in lower bending moments while running.

Reduce the Stress Concentration – You may not be able to change this. But, a generous radius at the shaft step would help.

Change the Design – I agree with your assessment of the problem. With the motor mounted on the ground and the driven equipment mounted on rubber mounts, the tension in the belt could be oscillating drastically in operation. You would have much better control of the belt tension if both components were mounted on the same frame. We have large vibratory feeders in our coke crusher that are belt driven. I have never seen a problem with shaft fracture in those assemblies. But, both the motor and the vibrator are mounted on the same frame.


Johnny Pellin

 

[Tmoose]

Is this the OEM design? Can the OEM provide reference installations where this exact configuration is successful?
How many of these machines do you have?
How many hours on this motor?
Was the motor shaft reworked?

I suspect those air mounts potentially invite huge and dangerous variations in belt tension in service, even if set properly initially.
I expect the manufacturer may have a complicated belt tensioning procedure to try to make it work.

The motor manufacturer will have maximum radial loads based on pulley/sheave overhang. I think usually that is for the DE bearing's benefit moreso than the shaft.
I believe some motors for belt drive applications have larger DE bearings, or roller DE bearings or even shafts with upgraded materials.

 

[chumet1]

yes this is the OEM recommended design installation. we have about 7 such installations and i have been told this is not the first time such a failure has happened.
But with the belt tension and pulley over hang, wasnt the shaft supposed to fail after the bearing journal - pulley side not rotor side.
I also googled rosta motor bases that swivel about a pivot to damp shock loads and harmful vibrations. These, i think would mitigate the effects of rigidity on the base

 

[Tmoose]

or a spring loaded belt tensioner.

is there relative motion or change in load from those air mounts ?

 

[rob768]

I think it is very remarkable that this is a one-directional bending fatigue fracture. This one has little to do with belt forces, allthough they will in the end have brought about the fatigue. If it was just bending stress from the belt, it would have shown the characteristics of uniformal bending, i.e. fatigue markings along the entire cicumference (with area of rupture fracture more towards the center).
Here, there is at least a very strong notch effect. Any tool markings? thermal straightening?

I agree with applying a larger radius on the next shaft.

 

[JJPellin]

Rob768, The pattern on the fracture surface would suggest one-directional bending. But, I have seen this in rotating bending with a high stress concentration. Once the crack initiates, the shaft is weaker in one direction and will progress from that point across the shaft as if it was one-directional. The shaft where I experienced this was ductile iron which has a very high notch sensitivity. If this is a standard NEMA frame motor, I would expect a shaft material with much higher toughness. But, I can still believe it is rotating bending.

The other possibility would be if the belt ratio is 1:1, 2:1, etc. and the vibration of the shaker is synchronized to the motor shaft speed. If that was the case, they would be better off to change the belt ratio slightly to anything non-synchronous.

Johnny Pellin

 

[Walterke]

I remember a typical example for this kind of fractures from my school days.
In old days, train wheels were made with the wheels fixed to the axis, so the axis+the wheels would rotate around a fixed bearing.
Because of the weight of the train, the wheel axis was bent so, with every rotation, the axis would get bent in every different direction.
Eventually a crack would appear and every rotation of the wheels, the crack was subject to a sinusoidal cycle of tension and pressure, prying the crack open further and further with every turn. Because of the pressure every cycle, you'd see a perfectly smooth surface, followed by a brittle crack when the axis finally failed.

Seems to be what happened here, but the pictures are too blurry to see.

NX 7.5.5.4 with Teamcenter 8 on win7 64
Intel Xeon @3.2GHz
8GB RAM
Nvidia Quadro 2000

 

[rob768]

John

I agree it could still be related to a bending from the belt, but the absence of an any evidence of any hint of unidirectional bending at least strongly suggest a large notch influence during initiation and probably a large portion of the early life (your high stress concentration). such influence will govern the early life, but after that the uniformity of the stress will take over. You will not get a symmetric fracture, but I expect you not will get the area of rupture one the outside.
It is well possible that such defect is the cause, and not the stress levels per se.

I fully agree with you on the belt. If there is a 2:1 ratio, and the drive being elastically mounted as it is, it may well be shaking at resonance, or un perhaps an irregularity on the belt?
In case of resonance: change the rubber of the mounts should do the trick

 

[ipajewski]

I don't have a whole bunch of experience analyzing fatigue failures like this, but I work with vibrating screens like this everyday. I'll point out a few things:

1) The motor is in the ideal location. A screen like that vibrates with a 1/4" to 5/8" total stroke along a straight line (800-100 vpm probably). The motor is set up such that the drive line is perpendicular to the motion of the screen, so the screen's motion doesn't cause much strech on the belts during normal operation.

2) Using a "Rosta" style motor mount is always a good idea. An elastic motor mount will significantly reduce the amplitude of the screen's motion when it coasts down through spring resonance. I'll bet good money that your shaft see initial cracks form during a coast down cycle. When a rigid motor mount is used, you will typically see the drive belts loosen and violenty snap tight repeatedly while the screen's motion is erratic (incidently, snapping belts will also make the motion even worse).

3) I'll also ask the obvious question. Can you move the drive sheave closer to the bearing on the motor?

 

[MikeHalloran]

You have room to move the motor "back" toward the NDE a couple of feet, and couple it to a sturdy jackshaft that can carry the belt sheave. The bearing adjacent the sheave, and the shaft supporting the sheave, can then be almost arbitrarily large.
The sheave could be supported between two bearings, if you don't mind cranking the outboard bearing support around a little so you can get the belts over it without unbolting anything. ... or just make the shaft big, and cantilever the sheave.

First, check the specs of the original motor, and whatever motor is there now, to see if they're even rated to carry a radial load at all. It's fairly common for the 'default' bearings on an electric motor to be rated just to carry the weight of the rotor.









Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

Is the break on the rotor side of the bearing seat?

 

[electricpete]

But with the belt tension and pulley over hang, wasnt the shaft supposed to fail after the bearing journal - pulley side not rotor side

The failure presumably occurs from bending stress, which depend on the moment.

So we can analyse using a shear a bending moment diagrem.

If it's a ball bearing, then as Tmoose taught me once (I think it was him), it acts like a simple support.
Simple support exerts no reaction moment, so there is no step change in moment at the support (you'd have to move a distance away to see a change in moment). So the moment immediately on one side is practically the same as the moment immediately on the other side. Without a big change in moment from one side to the other, the location of highest stress concentration would be most relevant in determining location of failure.

If cylindrical roller bearing, I'm not sure how to model. Seems like it would be more capable of exerting a reaction moment than a ball bearing.


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

 

[Sparweb]

A diagram of the shear and bending-moment diagrams, which I believe are applicable to this case:

The motor bearings are at RA and RB, being the "rear" bearing at A and the "shaft" bearing at B, and the pulley load is at C.
I'm surprised to see the fracture on (what appears to be) the LEFT side of RB.
That may be why many responders are considering the possibility of an initial flaw on the shaft, even while many others are looking at the service loads.


STF

 

[chumet1]

thank you so much for the informative contributions you made. i did the following modifications on the system:
-pushed the sheave 20mm inside, now have an overhang of around 20mm which is just enough to avoid sheave interfering with the motor frame
-made sure the machinist employed larger radius on shaft dimensional transitions
-i am currently working on changing the motor base from rigid to a flexible one

one thing i didnt understand though was the effect of sheave radius ratios

 

[RajuKanumuri]

1) Cause: high cycle fatigue.
2) Solution:
a) Immediate Action: Move the belt close to the motor
i) Move the pulley close to the motor.
ii) Try to use SPC type belt and reduce the number of belts 3 to 2.
b) Short term: Contact motor manufactures
i) Ask them for a Crain duty (Heavy duty) rating motor.
ii) Ask them to use the VA type bearing in the motor (bearing used in vibrating screens).
c) Long term : if the above solutions not working, you have to use Jack shaft arrangement, please see the sketch).
(Isolate the motor from the “v” belt loads with jack shaft: suspend the pulley between the Barings on a jock shaft and connect the jack shaft with motor with a pin and bush coupling)

 

[RajuKanumuri]

i am sorry, i need to add one more action before we jump to the long term solution.
b2) move the motor on to the screen itself with a flexible loop motor power cable (to absorb the vibrations)