Shaft replacement in small motor 9

[electricpete]

We have a 1.5 hp 460v motor driving close-coupled pump. There has been some damage to the shaft which is deemed not repairable

Replacement motor is not available due to the special shaft configuration.

Inspection showed the rotor core appeared tac-welded onto the shaft. I think it is a cast-aluminum rotor configuration - from the outside it just looks like a big steel can.

What has been proposed is to disassemble the rotor (remove shaft from rotor core), reverse-engineer a new shaft, and reassemble the rotor and motor with new shaft.

Do you see any problems with this approach or special considerations? We have a competent machine shop but they don’t work on rotating machines much.

(A preemptive strike – I am not interested in discussing alternate approaches for getting a replacement working motor... just what pitfalls or cautions we should consider for this approach. Thx!).


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[aolalde]

Electricpete; I think you are in the correct track.

Mainly for 2 pole motors, the magnetic flux path goes through the shaft. Make analysis to define the original shaft steel grade and ensure to use the same grade.

Instead of tack welding the rotor laminations to the shaft, make accurate machining to the rotor inside diameter and design the shaft outer diameter to provide interference fit in excess of the motor breakdown torque. Make the rotor-shaft assembly by oven heating the rotor. Do not over tight since the rotor laminations could fracture.

The corners at each diameter step should be rounded to avoid stress concentration.

The shaft journals "fit dimension" should match those specified for the bearing size and type.

 

[stardelta]

Try and bore the damaged shaft out rather than press it to avoid stressing and deforming the rotor. When detailing the shaft dimensions use the outer most edge of the lamination pack as a datum as the pack often distorts around the point of shaft entry. The critical measurements are lamination pack to bearing shoulder at both ends and the distance between both bearing shoulders. Rough out a shaft first then fit into the pack then finish turn ensuring the centers are concentric with the pack. When fitting the new shaft enter it into the pack in the same direction the old one was removed if it was pressed out. Finaly, balance the whole assembly with any keys or couplings used. A machine of this size will probably depend on interference to hold the shaft to the rotor but be prepared to come across a key. We use always EN24T for replacement shafts and have never had a problem relating to loss of efficiency or performance.

 

[electricpete]

You both mention interference fit to transmit the torque. I am not sure how much interference will be required to accomplish that (any thoughts?). I guess we will pay close attention when we disassemble and try to recreate a similar setup to the original.

I can see the reason for boring out the shaft but is that practical? It seems like that would take a very long time.

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[motorman]

Be careful which direction you choose to push the shaft out. That is because when the core laminations are stamped, the die produces a burr. When the core is stacked, the manufacture knows which side of the punching the burr is on and insures that the burrs are all going in the same direction when stacked. Then when the shaft is pressed in place, it goes in the same direction as the burr. Now when pressing the shaft out, if you go against that burr, the shaft could start ratcheting against the bore causing a hang-up. Continue pressing harder then there is the risk that the rotor core could bulge [yield].

 

[electricpete]

I have looked at a lot of copper bar rotors on large motors but I'm embarassed to say I don't have the slightest clue what's going on with the construction of cast aluminum rotors. I know when you look from the outside all you see is shiny smooth appearance with no sign of laminatinos. What is that outer surface? I don't think it can be aluminum (I thought aluminum was poured into bars inside). I don't think it can be core lamination edges because it is perfectly smooth. What is it?

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[electricpete]

I refreshed my memory looking at some pictures and I guess that outer shiny surface is the edges of the laminations. On the small motors I have seen somehow looks different than on larger motors... the edges of the laminations are a lot less distinct from each other.

Sorry for the rambling. I think I answered my own question.

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[stardelta]

Boring out the shaft on a motor that size is a relatively quick and simple operation. After detailing the shaft, grip the pack in the lathe chuck, part off the old shaft as close to the pack as you can. Then run a a drill slightly smaller in diameter than the shaft down the center of the shaft for the length of the pack. The idea being that the now hollow shaft collapses in on itself and loses the interference. You will then find you can place a drift into the hole in the shaft and knock it out without damaging or distorting the pack. Any welds will obviously have to be removed first though. If you have no option but to press the replacement in use 0.002" interference and support the pack as close to the shaft hole as you can. Welds at each end will be of benefit but be careful of heat distortion.

 

[edison123]

stardelta,

I agree it would be ideal to do what you suggested

"When fitting the new shaft enter it into the pack in the same direction the old one was removed if it was pressed out"

However, almost all of the shafts I've replaced have had a shoulder at one end which sits against the rotor bore. In such cases, you have to press in the new shaft in the opposite direction (of removal)only. If you suggest to shift the shoulder to the opposite end in the new shaft, it is not always possible. I haven't any problem with the fit of such oppositely pressed in shafts.

I normally use .001" to .002" interference fits in the new shafts depending on the shaft size.


* Anyone who goes to see a psychiatrist ought to have his head examined *

 

[stardelta]

Hello Edison,
It is true to say that a lot of shafts do have a shoulder, and in addition to my earlier comments must add that if a shaft does have to be pressed out and if the shaft is of differing diameters at each end of the pack, press out from the end with the smaller diameter. Alternativly the shoulder can be machined off if possible as part of the removal process to allow the shaft to be pressed out in the direction of insertion. I think the comments made by all who have contributed to this thread have covered every issue likely to arise relating to the job but you must agree with me in saying no 2 jobs are 100% identical and only a careful examintion of the shaft and rotor will determine the most suitable method in detail.

 

[stardelta]

Incidently I have a formula somewhere that determines the amount of interference required when replacing a shaft, I will try to find it and post it but I am not sure if it will apply to a laminated pack.

 

[electricpete]

I have seen equations for calculating required interference on couplings based on area and torque.

But you're right that laminations could complicate the issue.

For one thing, the interference formula is based on relatively smooth surfaces I think.

For another thing, isn't the roughness on the lamination pack several mils? I think that would make it challenging to determine what the actual intereference is.

One more issue on flywheels there is exansion of the bore from centrifugal effect which makes the interference while rotating become much less than while stationary. Is it safe to assume that effect is negligible for motors due to smaller diameter?

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[Tmoose]

The shaft replacement and brand new manufacturing process is less care and precision, and more methods to allow correction after the traumatic high stress rotor installation.

A shop with lots of motor experience earned with pain, burns and scars will have the best chance of success.

Probably need to Leave the bearing journals and other important shaft features rough machined before installing the rotor. The shrink/press fit will almost certainly distort things, including the rotor OD. Some would shrink a sleeve and clamp the rotor from the ends to keep the lams stable and the cast bars safe. The best partial motors put the laminations on a sleeve set up for hydraulic relaxation and rotor removal

I'd Start researching the required power to determine the max airgap you can accept, and be prepared to find it is 0.005 to 0.010 less than that. The re-installed rotor OD will most likely require some refinishing to achieve good concentricy.

 

[sparky1976]

What we've done when we change the shaft from rotor is same as most of the guys in this forum, the only difference is when w insert the new shaft to rotor, we freeze the shaft.

 

[electricpete]

Thanks guys.

Tmoose - I am really confused by your last paragraph. Maybe you can explain the whole paragraph to me again in little pieces.

To my thinking the average airgap should be the same after reassembly as when we found it. And all we have to worry about is concentricity... I don't understand why there would be a big problem there.


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[Tmoose]

"The re-installed rotor OD will most likely require some refinishing to achieve good concentricy."
The concentricity I was referring to is OD-to-bearing journals, not rotor OD-to-stator ID.

After the old rotor is shrunk or pressed on the new shaft, the OD will almost certainly have offensive runout and unseemly bumps and steps. I believe you will find that is because during new manufacture the rotor OD was oversized, knowing the installation would tweak the shaft. And one of the processes on the new rotor was turning the OD down the right size, simultaneously correcting runout and removing all high spots.

If the new shaft journals were left semi-finished until after rotor installation, then the journals can be finished to improve the average OD-to-bearing journal concentricity. But the rotor OD will still
have horrible and unseemly bumps and steps.
Naturally and properly the temptation will be overwhelming to "turn" the rotor OD (machine in a lathe - be sure to use a sharp tool to avoid smearing the laminations). In order to get the rotor OD to "clean up" it will have to be turned undersized. And then the resulting airgap will be "excessive."

Normally at that point discussions will erupt about how much power or efficiency will be lost due to the excessive airgap. But, since this is an emergency repair, you have to use it as-is, so maybe those discussions will be mercifully short.

 

[stardelta]

If the operation is carried out correctly there should be no need to skim the rotor pack and increase the air-gap. Most are seeming to assume that the now replaced shaft is now true and that the axis of which forms the datum. This is not the case and it cannot be assumed to be so. After the new shaft has been fitted all further machining operations must be carried out using the two further most edges of the rotor pack as the datum. This will keep the rotor and stator packs concentric and the airgap constant when the machine is assembled. I mentioned earlier the need to finish turn after the new shaft had been fitted and this should be done relative to the rotor pack at each end. You cannot make a new shaft to finished dimensions, fit it to the rotor pack and expect it to run true. There is no guarantee that the centerline of the rotor pack bore will follow the required centerline of the new shaft, more than likely it will be off center and skewed. Particularly with larger machines. BTW my idea of true is 0.003" max

1 rough out new shaft
2 fit into rotor pack
3 support in lathe with 4 jaw chuck with a rotating center in tailstock.
4 clock rotor and get it running true over the full length of the laminated pack.
5 machine shaft extensions to required dimensions and finish turn journals and shaft realative to rotor pack
6 cut any required keyways
7 dynamic balance to compensate for any eccentricity between shaft and rotor.

There is no other way of doing it with any accuracy.
Period!

 

[electricpete]

When reassembling the rotor onto new shaft, is it OK to heat to approx 300F?

Thx.

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[Mendit]

Pete, It is OK to heat the rotor to 600F (normal factory preheat temperature) try to use an oven with indirect heating or electric, set the rotor vertical and support on a plate with a hole large enough to allow the shaft to pass through. Your fit of iron to shaft should be in the region of 0.0002 - 0.0005" tight per inch of shaft core diameter, it tend to be bigger for the 1st inch at each end. (make sure you measure toward the center of the iron) If it has a key make sure the key is well bevelled at the end (bullet shape) and not to tight in the laminations. If the is tight it will almost certainly bind and cause it to seize. When the core is hot (allow good soak time) drop the shaft in by hand and use a shoulder to locate the iron accurately. Balance after the rotor has cooled.

 

[stardelta]

300 Degs should be an absolute maximum, no way would I go higher, you will damage the interlamination insulation and have a drastic drop in efficiency. 300 degs will be enough to overcome 0.002 to 0.003" so there is no need to go higher.
Mendit?
Check the interference figures you quote.