Shaft replacement in small motor
Shaft replacement in small motor
(OP)
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!).
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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RE: Shaft replacement in small motor
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.
RE: Shaft replacement in small motor
RE: Shaft replacement in small motor
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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RE: Shaft replacement in small motor
RE: Shaft replacement in small motor
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RE: Shaft replacement in small motor
Sorry for the rambling. I think I answered my own question.
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RE: Shaft replacement in small motor
RE: Shaft replacement in small motor
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 *
RE: Shaft replacement in small motor
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.
RE: Shaft replacement in small motor
RE: Shaft replacement in small motor
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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RE: Shaft replacement in small motor
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.
RE: Shaft replacement in small motor
RE: Shaft replacement in small motor
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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RE: Shaft replacement in small motor
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.
RE: Shaft replacement in small motor
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!
RE: Shaft replacement in small motor
Thx.
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RE: Shaft replacement in small motor
RE: Shaft replacement in small motor
Mendit?
Check the interference figures you quote.
RE: Shaft replacement in small motor
The original shaft was 1018 steel and we have available 4140 steel (higher strength) for replacement shaft. I don't know how to verify permeability and I don't understand why it would be important since flux path is toward outside of rotor, not flux. Any comments on whether 4140 would be ok?
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RE: Shaft replacement in small motor
You state the interlaminar insulation will be damaged if we go over 300. This is not true, most laminar insulation systems will withstand 750F without damage.
Remember, when the core is di-cast the temperature of the iron is preheated to 550F then injected with the aluminum which is close on 1500F, there is always some heat transfer, but not enough to damage the core plate.
Pete the use of 4140 is OK however the OEM would not use it as it costs more $ to produce and machine, normal motor shafts are either C1018 for small or C1040 medium size. API 541 and 841, states shafts will be 4140.
RE: Shaft replacement in small motor
4140 is great material but machinability as far as internal stresses goes is sometimes a problem (the stuff moves all over the place). For this size of motor, it would be tough to have to stress relieve the material prior to finish machining the journals and extensions. Be careful with this but it is good stuff when used successfully. Their also correct about the cost. You can buy heat treated and stress relieved materials, but again is it cost effective in this application?
Also, I believe that 300 deg F is hot enough but there is the chance of sticking the shaft due to non-symmetry of the core bore. We not only heat the core stack but we freeze the shaft as well, thus increasing the clearance without endangering the aluminum.
RE: Shaft replacement in small motor
Have you completed your repair yet? If not here are some tips gleaned from many years in the motor manufaturing business.
1. Don't bother with this whole heat shrink process. You are at risk of creating more problems than your avoiding. When pressing a new shaft into a new core its true that an interference fit is the way to go, but if you're doing a repair, a decent epoxy bond is all thats needed on a motor that size. The risk of cracking those cast rotor bars is just too great. Also, concentricity between rotor brg journals and the core od will be poor requiring machining which will mess up the motor performance.
Find a piece of 203EZ and machine a shaft "blank" with journal and shaft extension diameters .010-.020 over final size. You'll want this blank to have a bit of clearance to allow for some epoxy. Five to ten thousandths should do fine.I've used Emerson cummings "stycast" for years with excellent results. Make sure everything is clean and wet both mating surfaces with the epoxy before assembly. make sure that all of the rotor core will end up inside the stator CORE when everything is done. Its ok to shim a bit differently to get the right position but keep the bearing preload within the normal range. After the epoxy has cured chuck on the rotor core and machine or grind as required to meet final size. Viola... a repaired rotor with excellent concentricity that performs just like the one you replaced.