Conveyor gearbox output shaft failure

[Cagou]

Could you help with the failure analysis of this shaft?
The shaft is the gearbox output shaft toward a low speed coupling. The gearbox is on a conveyor secondary drive assembly.

 

[desertfox]

Hi Cagou

Can you give us some information about the failure?


regards

desertfox

 

[Cagou]

Oooops, here is the picture...

 

[desertfox]

Hi Cagou
How long as the shaft been in service? and does it see a constant torque load or variable.
The failure to me looks like a fatigue failure and the river lines point toward the centre of the shaft, toward a little redish blue mark on the photo which suggests the area were the crack or cracks propagated from.
Thats about as much as I can say without more information such as material of shaft, loading, temperature etc.
I would further add that if the crack as I believe started from the centre of the shaft it would appear to be a material flaw that was there from day one perhaps from a incomplete heat treatment for example.

Regards

desertfox

 

[digger200]

I can see starts of fatique cracks at 8 o'clock & 3 o'clock ? Are there stress raisers here? Torsional failure? How do the calcs (including the K factors for the fillet) compare.

 

[diskullman]

A simple answer would be overload. Did anything else happen to the conveyor? Something jammed?

Russell Giuliano

 

[metengr]

Cagou;
The most practical advice I could offer as a practicing metallurgist, and not offending anyone, is to obtain the services of a competent metallurgical lab and have a failure analysis performed. I have been burned just relying on pictures to offer a root cause.

 

[TVP]

Cagou,

metengr offered the best course of action, however, I am going to jump in and offer my opinion nontheless! There are at least four different fatigue crack origins on the outside diameter of that shaft, with two occurring in the locations that digger200 identified, and another two occurring at the 6 o'clock and 9 o'clock. There may be one at the 5 o'clock position too. The shaft definitely failed by overload after the fatigue cracks grew to ~ 60% of the diameter of the shaft. It is difficult to tell whether or not bending or torsion is the most significant contributor to the initiation and growth of these cracks. You definitely need to have a thorough metallurgical analysis performed of the fractured shaft.

 

[TomFin]

Observing the radial cracks I also notice a subtle texture difference which appears to create a circular fine grit pattern just a little off-center (quadrant II for geometry gurus). This is typical in variable loaded shafts subject to torsional loads. The presence of cracks stemming from the edge of the rim would indicate the absence of notch in the shaft that would act as a stress riser.
So Cagou, was there a stress riser?



Failure is a prerequisite of successful design

 

[CoryPad]

It is definitely a combination of bending and rotating fatigue. In ASM Handbook Volume 11 Failure Analysis and Prevention, Failure of Shafts section, Fatigue Failures article, are several schematic images of shaft fracture surfaces with different failure modes. Yours most closely corresponds with Rotating-Bending Fatigue, Medium Stress Concentration.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Cagou]

Thank you for your elements of answer, I agree with CoryPad for the combination of bending and rotating fatigue. The conveyor has 2 primary drives and one secondary drive (the one that failed). The 3 drives are 500kW, design of the conveyor is 1400tph, demand power in normal operation is 1121kW. Reducers have an exact ratio of 24.916:1. The drives are torque arm type and shaft mounted. The weight of the total assembly (swing base, motor, HS coupling, hydraulic brakes, HS coupling,bearings, reducer, LS coupling) is 11.3 metric tonnes. Speed of rotation of output shaft is 59.4tr/min. My understanding (but I have to discuss this with the VSD people) is that the secondary drive is driven as a slave and doesn't see the same torque as the primary drives, the drives are variable speed to provide controlled starting and stopping. My concern is mostly the assembly type shaft mounted in this application.
The conveyor has been in service intermittently to unload shipments. In total I would say it has been running under load 2 weeks in total.

 

[Newmanite]

It appears at first look the be a "cyclic" load failure.In which direction does the shaft turn and in relation to the large raises what direction are they in relation to shaft rotation?
Some other checks I would be doing are things like the braking system,I am assuming that you have dual hydraulic pressures and also dual times for decelleration and full braking force?
And cuurent applied to the electric motor for drive(I seriously doubt you have a problem here but hey whilst maintenance are "doing nothing")

 

[Cagou]

I checked the current draw at the time of the failure, and no peak was registered, After releasing the brakes manually we turned the gearbox without any problem. You're right for the braking system, there were some alarm reported, but I thing the intensity would have peaked.
I have to check the load applied on the shaft if the gearbox assembly was resting on it.

 

[GregLocock]

I have seen a failure a bit like that where a shaft drove a (huge) V belt. I'm a bit baffled by the initiation and the short life.

Given the large size of the shaft, and the probable expense, you really need to take it to some grownups to look at.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[JJPellin]

I recently went through a similar exercise when we broke the input shaft on a lobe blower. Before we were able to complete the RCFA, the shaft broke again. The look of the fracture surface is very similar. We found that ours was high cycle fatigue in bending. The root cause ended up being a machining error by the manufacturer. We asked for a print of the shaft. It was supposed to have a 0.030" radius. Instead, they had plunged the tool in too far and created a relief cut that was not properly radiused. Our material was cast ductile iron which is very notch sensitive. The above replies are correct that you need to send both sides of the fracture to a lab for metallurgical analysis. But, you also need to verify with the manufacturer that the material of the shaft and the geometry in the area of the fracture are correct. Especially if this is a cast material, a very slight change in step geometry can change a shaft that runs for 20 years into a shaft that a fails in a few weeks. Even if the shaft is manufactured to the original design, perhaps the design is bad. An FEA would show how any stress concentration may be affecting the fatigue life at this location.

Johnny Pellin

 

[PersonalProfile]

There are a large number of possible issues. Considering the expensive, I have assumed are involved here, would recommend getting someone to complete an investigation.
You mentioned LS coupling, is it a parallel or taper fit shaft (from the photo, I assumed it was a hollow mounted reducer). If your LS coupling has excessive run out and your torque arm is rigid, your drive (via output shaft) will see full LS torque of the reducer - which it probably wont be designed for. The failure does not appear to have necking at the radii as I would have expected associated with this method of overload (though I am not a materials expert).
Being a secondary drive, the 2:1 drive ration means if the primary drive "sheds" 10 % of its share of the total load, the secondary drive see "20%" (not just when running, what about when starting). As you have VSD’s would probably have reasonable control.
There are plenty of other issues.
Regards,
Lyle

 

[rob768]

This has all charateristics of a rotating bending fatigue crack. Multiple initiation along periphery, relatively small rupture area, so no excessive load. Poor machining or sharp fillet?

 

[polleke]

Hello Cagou,

I work at EMO bulkterminal in Rotterdam and we have had a few broken drive shafts in the same looking matter as yours.
The cause was not one thing but a combination of some things.
We changed from mechanical drum brakes to hydraulic disc brakes. In case of emergency stops the brakingforce of the discs is higher than the drumbrake was. Another thing were our couplings. We drive with Voith turbo couplings (hydraulic) and when starting under full load these couplings sometimes blow their safety plug and lose their oil. When filled again, the filling was sometimes too much.
The coupling became too stiff and caused too much torque.
The fixation of the reaction arm of the driveset in some cases was not flexible enough, it allowed too less movement of the gearbox and the drumshaft
Like Lylebrown00 and Greglocock say: Have someone who knows his business make a full investigation an report

Succes,
Polleke