Any chance of a photo?? ■

Dear prosenthal,

Have you done a crack analysis?

A crack analysis can tell you the nature and the cause of the failure.

It is probably a material fatigue problem.

Suppose the cause of the failure is bending stresses, then you can investigate

how bending stresses are generated and possibly estimate the magnitude of this stresses by calculation,

based on the operational loads.

The load can be found by measuring the drive power.

Also investigate whether the cracks are initiated by f.i. chemical corrosion, causing loss of material and thereby creating higher stresses.

And this is only a minor example of the possibilities of a thorough research effort.

success

Teus ■

Are the screws stainless steel or carbon steel?

Are you adding chlorine to the process before the centrifuge? (I have seen even low levels of chlorination do bad things to different metals very quickly)

It is hard to see from the pictures but it looks like the failure is on the centre pipe and you say it is 3" from the auger flange, I am assuming this is the drive flange (or drive disc). Can you clarify this?

Are you getting buildup inside the trough? I have seen screws with material hardened and caked inside the trough so that it was keeping the centre shaft deflected and putting a bending load on the centre pipe.

Put an amprobe with a "max" value function on the drive for one shift to see what the maximum current draw is. If it is below the nameplate Full Load Current you are not "overloading" as the manufacturer claims. ■

I think this is a definite case for getting an independent metallurgical examination of the failure.

If it's a fatigue failure, which seems possible, then "excessive stress" is not necessary just an oscillating tensile stress is sufficient.

The corrosion 3" from the weld is odd, seems rather a long way away for a heat affected zone problem?

Try this forum for some specialist advice

http://www.eng-tips.com/threadminder.cfm?pid=338 ■

Is the plant conveying only sludge? Now, rags may be the cause of these failures. Rags and shafted conveyors do not mix well. Shaftless Conveyors are usually always used when rags are a concern. Those lines you are seeing on the shafts are more likely a result of the manufacturing process when they weld those pipes they attach the electrode to the pipe and weld as it rotates. The arcing will cause those type of superficial defects. Did you have those cracks X-rayed? You will conclusively find that they are not cracks at all. ■

There is something decidedly wrong when failures show such as this on screws that have never been placed into service. The fact that the cracks on each side do not meet would suggect that the shaft has been subjected to repetitive bending, rather than rotation, but the location away from the undressed weld with attended stress concentrations is not compatable with an overhung cantilever loading. A combination of fatigue and stress corrosion can cause rapid failure, but cracks on a new item and in a similar place to others in service is really weird. I confess to being baffled. A drawing of the whole screw and arrangement would be useful to better assess the duty. ■