S.C.I.M. shrink ring / retaining ring

[electricpete]

This is a 13,200kv 8,000 hp motor.

The rotor has steel rings called shrink rings or retainign rings. From external view, they are outbooard of each copper endrings, but I assume the copper endrings also extend underneath these rings.

I know these rings provide a heat sink for the copper end rings. I also know they have a purpose to stabilize the position of the copper endring... but I'm not sure what forces they are countering (centrifugal force... thermal expansion of endring?)
Question 1- can you add to or clarify the function of these retaining rings.

We measured runout (TIR) of one of these retaining rings to be 0.057". The adjacent copper end ring and the other-end copper end ring and retaining ring all had TIR approx 0.010" or lower. Typical readings for these motors are 0.005" are less.

Question 2: What would cause high TIR? Does it need to be corrected (by machining)... or is rotor balancing enough? Is this ring typically steel that would be magnetic, or is it typically non-magnetic stainless steel? (yes I will try a magnet).

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

Also the nominal airgap in this machine is 0.125". This condition was found during routine refurbishment. There is no evidence of any rub.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[macmckim]

Is the motor 2 pole?

 

[electricpete]

This is a 4-pole.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

Slides 1 and 2 of attached file are overview of shrink ring.

Slide 3 shows possible "chuck" marks on the shrink ring... which could indicate the rotor was squeezed at some point in the past and created the runout (*)

Slide 4 shows new item of concern – I'll call it a "divot". (*) To me it looks like a casting defect although the shop is sure it's not a cast part. At a minimum we will do some NDE to look for cracks that might have started from this area... and smooth it out to prevent crack formation. The shop will do some evaluation whether the resulting reduction in area and/or stress concentration suggests the material will approach any failure limits.... including during motor start (to decide whether we live with this or replace it).

QUESTION 3 – What do you think might have caused this "divot"

* Both the divot and chuck marks are not in the same track where the runout was measured... runout varies smoothly... the bad readings were not the result of hitting a bad spot.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[ScottyUK]

I rather suspect that the end ring is forging and not a casting, and that the mark is a leftover from the forging process. I'm a little surprised it wasn't either machined out or rejected as scrap unless the stress in the ring is relatively low. I've not seen any similar marks on generator rotor end-rings although they are both bigger in diameter and operating at 2-pole speed (3000 rpm over here) so much more highly stressed.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[electricpete]

One thing to note - portions of this ring are heated and shrunk on over the copper endring... which creates a source of stress even without centrifugal force.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[burnt2x]

Answer on Q3:
I'm no expert but divot looks like it is caused by sand- inclusion during casting!
Assuming it is a forged one, forged metals are supposed to be of "low-porosity" and continuous all along the material unless the billet was heated above specs during prepping causing it to cling to the dies.
The usual procedure in attaching things like this is through force-fitting, i.e. aluminum ring is heated while the other mating material is kept cold and when both are mated, they stick very tightly as if they were welded.

 

[electricpete]

Thanks guys. I'm also with Scotty, this application doesn't seem like one where casting would typically be used.
I'm alsoe with with burnt2x, the only place I have seen this type of thing in my limited experience is in cast materials. It may be possible with forging as alluded by Scotty but I haven't seen it.

The shop will be doing some more research on several questions related to repair of this, I'll let you know what we come up with.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[edison123]

pete

I have done such rotor end rings with steps for fitting the retaining rings but they were all 3000 RPM. The RR is usually cast steel. The RR's used to lock down the end rings to withstand centrifugal forces. They are not heat sinks.

Muthu

http://www.edison.co.in

 

[electricpete]

Thanks. The shop tells me these rings are nonmagnetic stainless steel. Can stainless steel be cast?

Regarding the function. The shrink fit ring is in intimate contact to the endring so does affect the thermal performance although I agree that's not the primary function.

I now think the primary function is to minimize the radial expansion of endring which tends to put stresses at the ring/bar joints (see Nailen page 3-10 for discussion of end ring expansion). It is more common on 2-pole machines because the starting transient and stresses are more severe there. It is probably used on this machine because it was designed for use with huge flywheel that makes it a very severe start. For sync machines the retaining ring has a different function probably mostly countering centrifugal force.

=====================================
(2B)+(2B)' ?

 

[desertfox]

Hi electricpete

If its cast stainless steel its probably sand cast.
looking at the pictures you posted the piece of of the retaining ring looks like it was cast but can't be 100%.
Have a look at this book link its very informative, it talks about why the rings may be none magnetic and the stresses caused be heating etc.

http://books.google.co.uk/books?id=nxfiqMblF34C&pg=PA205&lpg=PA205&dq=purpose+of

+electric+motor+retaining+rings&source=bl&ots=5R336xwtny&sig
=dc1SlJxcbJmGpL_dW1XnRHRg03Q&hl=en&ei=9whfTI3ODJ720wTdqf3
TBw&sa=X&oi=book_result&ct=result&resnum=9&ved=0CDYQ6AEwCDgU#v=onepage&q&f=false

desertfox