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Re: Motor shaft failure analysis
Re: Motor shaft failure analysis -- Paulcor	Post Reply	Top of thread	Forum

Posted by: randykimball ® 09/26/2007, 13:08:28 Author Profile eMail author Edit

If the shaft was under any flex then it will have work hardened at the area of breakage. A crack likely started or connected to the keyway. You are likely correct in that it looks like it was too brittle, because the cyclic work action will have work hardened it until it got too hard to flex without cracking. Upon replacement I would take a serious look at alignment. Have a well qualified millright indicate and align the components. However, realize every shaft has a life.... you haven't indicated how long and how hard a life it had. Is this by some chance a boat shaft on a diesel to screw/prop shaft? If so have the prop balanced and see that the new shaft is straight. See that the stuffing box, engine, and strut are all in alignment. The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them.

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Re: Re: Motor shaft failure analysis
Re: Re: Motor shaft failure analysis -- randykimball	Post Reply	Top of thread	Forum

Posted by: Paulcor ® 09/26/2007, 20:49:49 Author Profile eMail author Edit

Thanks for the response. The shaft was in an electric motor, connected via grid coupling to a gearbox, connected to a paper pulper. It was definitely in flex because the gearbox base is in poor condition. It had been in service 23 months. The grid on the coupling actually broke (and was replaced) a week before the shaft. We didn't replace the entire coupling due to lack of parts. Would the flexing work harden through the shaft or just the outer layers? Do you know whether 4140 is considered ductile or brittle? I am trying to determine if the shaft failed due to the angular misalignment with the gearbox shaft. The fracture face has all the indications of a brittle break (existence of a hinge, river or arrowhead marks, no progression marks). Strangely however, the river marks point to the keyway & hinge. Is this a case of the origin and hinge being at the same location due to some strange crack propagation? Hinges are generally only seen on brittle materials and I thought 4140 was ductile. Maybe it was before it got work hardened. Attached is a photo which I resized to fit the file soze limits. I don't know if it's large enough to see well. Thanks again for your response.

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Re: Re: Re: Motor shaft failure analysis
Re: Re: Re: Motor shaft failure analysis -- Paulcor	Post Reply	Top of thread	Forum

Posted by: Kelly Bramble ® 09/27/2007, 07:34:54 Author Profile eMail author Edit

I'm with Randy, looking at the bottom of the picture it looks like a fatigue crack stsrted then a shear-torsion failure. It allways amazes me how much extra loading can be introduced by improper alignment of load bearing mechanisms.

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Re: Re: Re: Motor shaft failure analysis
Re: Re: Re: Motor shaft failure analysis -- Paulcor	Post Reply	Top of thread	Forum

Posted by: randykimball ® 09/26/2007, 23:20:12 Author Profile eMail author Edit

It broke in typical location, the keyway etc lend a weakened point.... If the coupling is a flex joint it can absorbe very minor mis-alignment. However, this is MY opinion, you should do the best you can afford to get as near to a perfect alignment as possible. Perfection is not possible, therefore we use couplings with various methods of flex. Yes, 4140 is great shaft material. When the coupling was/is replaced this could be an indicator of a problem. It is often wise to inspect the shaft at that time for cracks. Rubbing with a dye that can be seen under a black light is a decent method. After you wipe it off some sticks in the crack structure. I've used the dye that comes "off the shelf" to use as leak detector in airconditioners found at your auto parts dealer. This dye jumps out at you as a bright green glow under a black light when the lights are dimmed. A simple black light flash light will do the job. The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them. Modified by randykimball at Wed, Sep 26, 2007, 23:23:38

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