AC motor: bended shafts and broken bearing 2

[Ledombre]

We have following situation:

AC motor, 160kW, framesize 315, B5 flange-flange mounted to a gearbox.
Coupling on mototshaft with a key (DIN standard) coupled to gearbox by a spline to the gearbox (rigid coupling in my opinion)

On 1 motor we have suffered a DE bearing problem. bearing is damaged, after removing of the bearing, it seems the inner ring has rotated on the shaft. Shaft diameter measured 80,01mm. This is OK. DE bearing is non located, therefore it can only be a radial force which has damaged the bearing.
Shaft run out and flange tolorances of the motor are checked and are OK acc IEC/DIN standards.
I expect a misalignment is the problem, from gearbox side we have no info yet.

On 2 motors we have a found bended shafts (bearings not damaged yet) shaft run out: 0,18mm and 0,10mm (acc. standards limit is 0,06mm)
After dissasembly we have found the shafts are bended like a banana. (keyway = 0 degrees: at DE +0,18, at middle of rotor -0,26, at NDE +0,14mm)
Both shaft have the same shape: at DE keyway maximum deviation, at rotor package at opposite (180 degrees) maximum deviation, at NDE maximum deviation in same direction as DE key.

Looking to the broken bearing it seems we may observe a radial force due to misalignment. Is it possible the shafts are bended due to the same axial force due to misalignment? Or should the bearing be damaged earlier.
A second cause can be a force in the shaft material. (isn't it strange both shafts are bended at the same position, at keyway?)
Shaftmaterial is 42CrMo4 (not heat treated)

Had anyone experience with bended motorshafts?

 

[JJPellin]

We have seen bent motor shafts and failures of drive keys and couplings. The ones I recall fall into two categories.

One category is pumps drawing from storage tanks where the piping was not designed properly for flexibility. The pipe strain would cause the pump to move resulting in extreme misalignment across the drive coupling and catastrophic coupling failure. These have progressed to the point where the motor shaft was bent at a 90 degree angle and the coupling was thrown out.

The other category was from motors driving high speed compressors. We had a number of coupling failures, sheared drive keys, broken coupling shim packs or bolts and bent motor shafts. We eventually determined that the compressors were being hit with liquid causing extreme high torque to the point of failing the coupling.

You did not mention what this motor/gearbox is driving.


Johnny Pellin

 

[1gibson]

Is the inertia of the motor rotor sufficient to cause this damage if the driven equipment locked up or came to an abrupt stop? Maybe even a not-so-abrupt stop; what does a typical "spin down" test look like? Turn the machine off and watch the shaft, maybe take a video on your cell phone for reference. Does it go from 10 rpm to stationary? 50 rpm to stationary?

 

[chicopee]

I know this is not the right arena but it offends me after reading statements such as: AC motor: "bended" shafts and broken bearing;...we have a found "bended" shafts (bearings not damaged yet...." Some of you guys are really killing the English language and I just wish that you were more meticulous on your writing.

 

[Artisi]

Chicopee: Have you ever considered that English might be the OP's 2nd or even 3rd language, so how about cutting a bit of slack in the interim of establishing some facts. Also, I would think that meticulous in or with is better than "on your writing".

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Ledombre]

I am sorry for my bad english. I am from europe.

The motor is driving a winch. An abrubt stop can be an emergency stop (2 brakes: 1 at NDE of the motor and 1 at the drum of the winch.
2 motors are connected to 1 winch. Motors are driving in master slave configuration. (what happens for example if 1 brake at NDE motorshaft is 500ms faster as the other one?

 

[Artisi]

Ledombre: Nothing wrong with your English, no need to apologise.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[desertfox]

Hi
Can you provide a sketch or photograph of the set up please.
Also how long have these motors and winches been in service or are they new?
Relating to the point you mention about the brakes, if one brake was applied before the other I think that would only apply torsion to the shafts and not bending as you describe, if there is a over hanging load on the motor driving end, then that would cause a bending mode in the shaft.
Have the winches been overloaded?

 

[1gibson]

If there is slop in the key/keyways, I can imagine the coupling trying to "roll over" the key. This might egg the coupling bore (suggest that be checked) and would generate a force perpendicular to the shaft, at the key. The more emergency brake cycles it sees, the sloppier it gets, the larger the force.

Imagine a key sitting in keyways of a shaft and coupling, assembled. What happens when a large force is applied to rotating the key? One corner of the key push down into the shaft keyway, and 180* apart, the other corner of the key pushes up against the coupling. The reason the key begins to rotate is a large shock load and a sloppy fit. If the coupling is rigidly located, that key movement is going to shove the shaft around.

If you switch to a dual keyway design for this coupling, it may reduce or eliminate the shaft bending. Then you get to find the next weakest link.

 

[EnergyMix]

Ledombre and others: As an aside, just to let you know, Eng-Tips now has a translation forum (see my signature for the forum number). It won't answer technical posts, but if you feel that Google translate isn't really giving you the best translation, you can post there in your native language and someone will answer (hopefully) with how it should be phrased.

Any multi-lingual folks are encouraged to add it to their thread minders and if, they see a green-star, to check it out and see if their translation services are needed.

Thanks!

Want to know the do's and don'ts of Eng-Tips? Read FAQ731-376.
English not your native language? Looking for some help in getting your question across to others or understanding their answers? Go to forum1529.

 

[racookpe1978]

You say the motor was driving a winch. It is a different way of looking at the problem, but could the net weight, the total inertial load plus a mass load, could have been forced onto the shaft of the motor?

That is, could the shaft have simply bent because it was forced to carry too large a weight at too far a distance?