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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[electricpete]

aolalde/all - your comment as I understood it was that we should be concerned about the permeability of the shaft (this is a 2-pole 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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[Mendit]

Stardelta, At 300F you will of course have clearance over the the shaft diameter, but you will have a great chance of getting it stuck half way in. Always give your self lots of expansion and or clearance, core plates are not always in perfect alignment.
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.

 

[roryrobb]

Pete;
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.

 

[jclough]

Electricpete...

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.