AC motor: bended shafts and broken bearing
AC motor: bended shafts and broken bearing
(OP)
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?
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?





RE: AC motor: bended shafts and broken bearing
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
RE: AC motor: bended shafts and broken bearing
RE: AC motor: bended shafts and broken bearing
RE: AC motor: bended shafts and broken bearing
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.)
RE: AC motor: bended shafts and broken bearing
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?
RE: AC motor: bended shafts and broken bearing
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.)
RE: AC motor: bended shafts and broken bearing
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?
RE: AC motor: bended shafts and broken bearing
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.
RE: AC motor: bended shafts and broken bearing
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: Eng-Tips.com Forum Policies.
English not your native language? Looking for some help in getting your question across to others or understanding their answers? Go to forum1529: Translation Assistance for Engineers.
RE: AC motor: bended shafts and broken bearing
That is, could the shaft have simply bent because it was forced to carry too large a weight at too far a distance?