About Servo Motors

Did you check the manual, or ask the supplier of the motor????

You have more than one of this brand of motor I see, they should be willing to help you.

AC Servo Motor Driver Operating Manual

actually this machine is 12 years old and the machine manufacture company was already closed,

It shouldn't be a problem for a competent welder and machinist working together to rebuild the motor shaft.

Is it a PM servo? If yes it gets a bit more complicated. And anyway shafts don't just brake like that. That motor has suffered one too many end of road crashing stopping shocks, and old habits die hard. S.M.

You ran the machine so hard that you broke the motor shaft!

There certainly are many highly competent welders out there capable of welding anything and then there are others. I personally don't know of any encoder that must be welded to the motor though. All of the encoders I've encountered always had some keyed or set screw coupler to allow disassembly. A very few motors had integral encoders that could not removed even with a torch. My only concern for the encoder is if this encoder has an ancient multi-pole magnetic encoder. The individual magnets were added to the spinning shaft and a coil picked up the field as it went by. Don't heat these magnets too much.

As for encoder alignment, I'm much more worried about the alignment of the whole machine when the motor shaft broke. Motors shafts rarely break, more often the motor just stalls. Peak motor currents are tested by deliberately stalling the motor shaft. Other shafts may break if one has gear reduction.

Unless I'm very mistaken the encoder will reset at a "home or null" point in the process. So the encoder could be at any angle to what is the normal, the program will compensate for any offset. The encoder only counts

I'm basing this on previous experience with CNC machines, which would return to the "home or null position" once every X number of operations.

I don't care how accurate the encoder is, it has to be "nulled" every so often.

By that virtue it could be 180° out for the first run, but the first operation before running the machine is to null the home points. Therefore everything should be correct….. in theory…… possibly…… I hate machines!!!!! They fight back.

If the shaft broke it was overloaded since motor shafts are designed to fatigue and in general do not break if loading is respected. The section where it broke indicates a combination of torsion and bending over the motor manufacturer limits.

If you weld it, the welded section will be less resistant than the original material and the risk of a rapid new breakage is high. Great attention has to be given to the filet radius between the diameter in front and behind the section where it broke. It could be even of interest to modify the design in order to reduce the stress level and adapt it to the material property as welded. If you make a sketch with dimensions and send a picture of the broken section it could be possible to make some suggestions.

I would consider the change of the motor as the best solution if you do not want to have new head aches.

dear sir,

this is PM rotor and i didnt opened the shaft at its non driving end, because i am thinking that if i remove the encoder from shaft can we get the proper null position of the rotor? and also please tell me what type of encoder is this in pic 3 ? its not seems like optical encoder

That photo was worth 1000 words. Your servo is probably caput since it looks shaft broke at bearing's inner side, and I bet there was enough movement to give much more damage than you see now. Servos have real small gap and their rotors don't like much touching stators. S.M.

Thank you for the pictures. I would ask if possible for 2 views of the broken sections taken in almost axial direction (with light from the side to see the surface area) in order to analyze the type of fracture. Could you also measure the diameter of the shaft end in p1 behind the failure and give the shaft diameter which is in the bearing? It will help to make a correct expertise of what was the reason for the accident. You would have to look at the rotor if traces are present or not. If not you may change the shaft if yes there is risk of local overheating due to friction and this leads to loss of magnetization so that the motor torque can be degraded.

1- the motor manufacturer does exist in Germany and is a big company

2- the sensor is either a resolver (standard) or a sin-cos (special demand).

3- verify please if the shaft has d= 28 mm and the key is straight and 8 mm wide

I think the motor you have is an older series and you could have problems with spare parts. If you want to repair you will have to do it by yourself.

With this kind of sensor you have an automatic zero since this depends on the position of the rotating coil with respect to the stationary coils.

The problem is that according to the picture it appears that the rotor was cast on the pre-machined shaft and the final dimensions were obtained after in order to have a good coaxiality between bearing surfaces and rotor cylindrical surface, as mentioned, gaps have to be small for a high magnetic field. However I think a maintenance is possible by making a new shaft end. During welding the rest of the rotor especially the PM must be cooled. The same good coaxiality has to be obtained this is COMPULSORY.

I am asking myself which would be the best repair solution and the more I think the less I am tempted by welding.

It would be of interest to check which motor of the newer series is interchangeable with yours this being the BEST solution.

What's the cost to fix compared to new? In my opinion, the force that caused failure would be more likely to reoccur after shaft repair... And added cost for aditional downtime.

Ensure you refit in the same position relative to the rotor (as when taken off) so that the encoder is in the same place relative to the poles. Otherwise, you will need to retune and/or reposition later when refitted. The relative pole position is essential to the servo controller (drive) for motor control and may also be used in the position controller as mentioned in another post.

The fatigue type break seems to be at the diameter reduction before the drive end assembly bearing, maybe you should check the radius or have a complete new shaft fitted maybe improving this area?

...what are the precautions should be taken when removing encoder?

Don't break it and remember how it goes back together.

dear mr. Wal sir

what i want to know,after we remove the encoder, if we connect at the same position which we will mark during removal of encoder then servo motor will run?

i mean there is some relatonship between rotor /stator current and encoder signal,otherwise motor will trip on over current fault or feedback fault.

what i am thinking is right or wrong?

Read about"resolvers" instead of keeping on "encoders". According to the catalogue of Baumüller (which yoy may find as I did) they have NO ENCODERs but only RESOVERS. You may find explanations about function by "google-ing".

If the encoder is not damaged and it is reassembled and connected exactly to its original configuration then it will work.

No magic here. Just an application of correct work practices, discipline, care, skill, observation and know how.

How were you planning on repairing the schnapped scharft? That's a little more critical then removing and reinstating an encoder.

Will your repair method clash with the encoder mounting?

What is the difference between encoder and resolver ?

Functionally a resolver is an analog only position detecting device while most encoders produce an incremental digital output. A resolver identifies its position magnetically while an encoder identifies its position optically. There are encoders that produce an analog sine and cosine output just like many resolvers, but these are very high precision devices capable of micron and even sub-micron movements. There are some absolute encoders that immediately report there position in some fashion but most are incremental encoders (even the analog ones) that have a unique index position that must be first detected to reset a counter to permit absolute measurements. Many orders of magnitude of increments are possible with an encoder for one index signal. In contrast a resolver has at most a few dozen poles where that can produce ambiguous positions.

My apologies, that's not really well phrased. But I hope you get the idea.

Thanks but the question was not for me but for the OP who continued to use the wrong term although I gave the information that the angular feed-back device is not an encoder.

In fact my question was provocative to bring him to have a look at the differences since technique is a precise stuff at least when it comes to definitions. It is good that ou gave the explanation but I doubt that it will be read. I bet the next post makes again use of "encoder"!

You have not to apologize my formulation was neither clear enough.