Motor Shaft Failure

Not an expert on this failure, but I have seen a few. This does not look like a fatigue failure to me, it looks like over stress.

If the system became jammed suddenly, then the shaft would be put under a huge and sudden stress and would fail. That failure would result in one end of the shaft free spinning and the other end stopped. The result will look like melted metal on the ends of the shaft. (Seen that a few times) Fatigue usually starts with a crack that grows to the point of failure.

Laby

focus on the fracture is too soft when I blow up in photoshop to examine closer. What clues are telling you fatigue?

What is the source of the red color smudged on the fracture?

milo

I agree with the others it doesn't look like a Fatigue failure. A fatigued shaft would have zig zag cracks running from the outside toward the centre or on softer metal a twisting and narrowing at the fracture point. It may be overload failure or most probably a granular fault in the shaft during manufacture which has developed into a crack and finally failed. Check the rest of the machine for and ensure there is no unnecessary loading such as stiff bearings which might cause the shaft to heat up taking it outside design limits

Just a few questions -

1. What is the speed of the motor? hp?
2. Does it rotate in one direction only? Or does it reverse? If so, how often?
3. How much overhung weight is on the shaft?
4. Did the shaft break right at the start of the keyway notch?
5. How long was this motor in service?
6. Is it interlocked to trip out if the gearing gets jammed?
7. What is the possibility of wear damage occurring to the shaft? For example the gear coming loose and scarring the shaft, or multiple replacements of the gear causing damage, or ......?
8. How are your initial shaft alignment procedures?
9. Was the misalignment that you mention present before or did it occur as a result the failure?
   
10. Does the motor suffer from a soft foot? Or possibility loose anchors?

May be it is due to failure of brake which has not released the motorshaft for rotation. Faracture indicates it is due to bad material for shaft as inner core seems to have been soft.

Suresh Sharma

Or perhaps the exterior case of the shaft was hardened while leaving a softer (tougher) core.

Unless the shaft was through hardened, it is quite likely that the interior is softer than the exterior, regardless of heat treatment. If shaft was cold drawn the od will be harder than midradius and center; if it was hardened but not through hardened, the rate of heat leaving the OD is much greater than that leaving the ID, thus harder on OD than mid radius and center.

So the "seems to have been soft" appearance is not necessarily an abnormality-

It is in fact a normal expectancy for the larger diameter involved as long as it wasn't through hardened. Without knowing the design and heat treatment of the shaft, and with out sharp focus to allow us to see gradations and beach marks, we have absolutely no justification for saying "bad material" or Material failure" when the only obvious signs from the photo are of torsional overload as others have mentioned.

Better photo will let us see the beach marks and pinpoint orign (s) (Most likely plural), and thus nail down the exact failure mode.

I continue to hope for a sharper photo of the fracture surface so we can do this.

In absence of definitive photo, and indisputable evidence, a wise man reserves comments about causality, and defaming material. Knowing that there were in fact out of control mechanical issues in the equipment tells us that the material was likely facing nondesign forces; The fracture failure mode will help us understand what those forces were.

Ultimately, the poster needs to get his eqiuipment back in design order. The entire system.

The shaft material is not an issue based on facts available at this time.

milo

Hi Milo,

I appreciate your long comment,but please note that it was my assumption as I have said "may be".I agree with you without clear picture we can not arrive at right conclusion. How about brake failure,it could have been one of the causes.

Normally here in India motor shafts are mfd from En9 steel, which carbon steel and is mostly fully hardened.It looks that shaft was case hardened as core has remained soft.Again it is my guess not a conclusion.

Suresh Sharma.

Hi suresh. no harm, no foul.

The statement "Faracture indicates it is due to bad material for shaft as inner core seems to have been soft." seemed to stand alone. thus my clarification.

milo

Please check the following:

1-If the "racks" are positioned correctly and not out of alignment. Most racks are replacable and thus subject to minor infractions that under load become major issues.

2-Verify that motor brake is properly installed and clearances are within tolerance. Also, with a drive gear on both ends of the motor, I am assuming you are incorporating two brake units. If so, are they being energized or de-energized equally? (see comment #3).

3-If everything checks out mechanically, then look at the PLC for motor brake operation issues. From the description of previous brake problems, I would look at the PLC parameters, as signal voltage may be an issue here.

My background is in the plastics industry with blowmolding and injection molding; you would not believe the incredible amounts of stress that can be compounded by a rack system.

Good Luck, post back if you need to........

Could be a combination of both . First an alignment issue fatigues the shaft , then an overload of stress because of a mechanical problem . The overload may have been within the limits of the shaft had it not been fatigued by the alignment . Without NDT testing prior to failure it may be tough to tell . When it is repaired , install strain guages on the shaft and it will tell you where your stresses are originating .

Good Luck