Vertical Motor ? 2

[THooper]

Please share your experience on what can cause an up thrust on a vertical small deep well pump motor. Our facility rebuilt a 75hp 2-pole vertical motor only, a couple months ago and it was pulled for high vibration on the top end (thrust bearing end) After close inspection the lower guide bearing shows signs inside the outer race, a wear grove as if the bearing was operating in an up thrust position for an extended length of time. End play adjustments are at specifications of .008

Note: The lower guide bearing also shows signs of heat and lubrication was starting to breakdown. The top end looks fine?

Thanks in advance!
motorhead

 

[vanstoja]

Some possibilities for upthrust loading in a rebuilt vertical pump motor are:
1. The stator iron and rotor iron are not properly aligned giving a magnetic upthrust force proportional to the offset distance between stator and rotor magnetic centers and to some power function of airgap electromagnetic flux density.
2. There is a differential pressure across the rotor core due to a change in pressure distribution of the motor cooling fluid. Did you change cooling fan blade numbers or diameters or introduce screens or other blockages in the coolant flowpath?
3. The pump impeller discharge centerline has been moved upward, reducing impeller hub to casing clearance which increases backside leakage flow velocity, reduces static pressure and increases axial hydraulic upthrust. If impeller displacement prevents backside hub leakage flow and increases front shroudside leakage flow and pressure level then front shroud will have higher uptrust force.
4. Post motor rebuild operating flow/head conditions have changed. Axial hydraulic downthrust due to impeller eye pressure imbalance varies with the square of running speed (it's a function of head) so lower running speed will increase hydraulic upthrust over the entire operating flow range. If head generation in hub to shroud streamtubes varies with flowrate then thrust reversal may occur before head runout at high flowrates and you'll have axial hydraulic upthrust. Same thing can happen at very low flows if impeller flow stalls, becomes unstable giving a drooping head characteristic toward shutoff.
5. All the other things I haven't thought of.

 

[jbartos]

Suggestion: Contact the motor-pump set manufacturer tech support.

 

[jbartos]

Suggestion: Visit

http://www.geocities.com/worrapol_tpe/mvn.html

http://www.china-tianyi-pump.com/SerialInfo.asp?serialid=38

etc. for more info
Also, please provide more info about the type of pump and motor you have.
The motor power supply quality could be checked for harmonics and common mode currents.

 

[THooper]

The customer has found a problem with the pump impeller discharge centerline was moved, reducing impeller to hub casing clearance just as Vanstoja has described in #3 comments. The application is up and running fine now.
Thanks for the good tips.

Kind Regards
motorhead

 

[jbartos]

Question to the question in the first posting: What about "The top end looks fine?"

 

[THooper]

Jbartos, what I was relaying by the top end looks fine is after a complete evaluation of the motor, all the micrometry machine fits are in tolerance, thrust bearing showed no signs of wear or heat, oil analysis checked out good, All electrical test are good, The only problem detected was the lower guide bearing.

The motor was completely recondition again and put back in to service. It's nice to work with an honest customer, once communicated of the signs of up thrust on the lower guide bearing the customer inspected the pump and found the problem with the impeller and our facility did not have to EAT this repair.

 

[jbartos]

Question: Why had to be the motor completely reconditioned, when the lower guide bearing was affected only, and the top end looked fine?

 

[THooper]

When inspecting and checking all machine fits in the evaluation process the bearings have to be removed. Once removed we will not use them again and take a chance, because most bearings will encounter internal damage in the removal process. The windings did not need to be dipped and baked again, but it was requested since the motor was apart and it doesn't hurt to get all the varnish build up you can on an old winding. (customer request)

Why take any chances, it would be more cost effective to replace both bearings then take a chance and have down time again because maybe the top bearing has some internal damage from operating in an up thrust positon also, there is no signs of this, but why take the chance.

 

[electricpete]

We never re-use bearings either. Too much chance of damage during removal and remounting. Big investment in labor will have to be made whether you replace them or not, and only a small investment in material for new bearings (especially on these small machines).... so why not.

Sounds like the varnish was overkill but the customer is always right (right?)

 

[jbartos]

Question: When you did such a good repair, have you installed better quality bearings?

 

[THooper]

We use skf exclusively, In our shops opinion the best quality bearing money can buy, They cost a little more but have proven to be very reliable.... It's not uncommon to achieve 8-10 years service life if properly maintenance is adhered to of the application in service.

 

[THooper]

electricpete is correct, the customer is not only right, but the customer is always KING, The customer REPRESENTATIVE that is, he/she is the one sticking their neck out making recommendations where to send their motor repairs and counting on a quality repair from whomever facility they are putting their trust in.