Vertical Thrust Bearing in hallow shaft pump question

[Mysterrose]

I am new to vertical hallow shaft pump motors and have a newbie question here. Why are the thrust bearings mounted on the non drive end of the vertical hallow shaft pump motor ? Wouldn't you want the thrust bearing on the DE so it can support the weight of the motor? Is this done for assembly or maintenance purposes? Does it have to do with the fact it's a hallow shaft?

Realize it may be a dumb question, but would really like to understand why this is done. Please help

 

[electricpete]

Some thoughts: for hollow shaft motor, the pump shaft comes up inside the hollow motor shaft to some form of coupling at the top. With thrust bearing at top, the thrust does not have to be carried through the length of the motor hollow shaft.... But if thrust bearing were at bottom, the pump downthrust would cause compression loading along the whole length of the hollow shaft. If things are not perfectly centered, compression loading can create a bow, which tends to increase vibration. Also bearings at top tend to be a little more accessible, although not necessarily easy to get to.



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(2B)+(2B)' ?

 

[1gibson]

Coupling at the top of motor near thrust bearing is used to adjust lift setting of the vertical pump, so it needs to be accessible.

You can't compress a vertical pump shaft, especially hollow shaft, it will bow out and damage bearings. Same reason you don't want to operate a vertical pump in upthrust.

DE thrust bearing supporting the weight of the motor... You completely lost me there, sketch a free body diagram and see if you still have the question.

 

[SteveWag]

The location of the thrust bearing on the top of the motor, and the non-reverse ratchet, if supplied, allows the bearing to be in compression, allows the shaft stretch to be easily compensated for and completely isolates the hung weight and hydraulic down thrust from the motor bearings. The weight and down thrust is many times an order of magnitude or greater than the rotor weight of the motor. With the thrust bearing at the top, the weight is passed through the frame of the motor to the support. The only reason for the hollow shaft is to allow the top mounted bearing. Also, if you ever design a pump support/floor, make sure to take in to account the hydraulic down thrust in addition to the pump/motor weight.

Steve

 

[Mysterrose]

1gibson: Meant DE thrust bearing supporting the weight of the rotor. Typed a letter wrong there (typed m, meant r). When I sketch my free body diagrams with the thrust bearing in either the DE or the NDE, the bearing is in compression by the weight of the Rotor.

electricpete: I can see that. Coupling below the thrust bearing puts hallow shaft in tension so don't have issues with buckling. Coupling above thrust bearing puts shaft in compression so you would have issues with buckling.

SteveWag: Downthrust force might be beyond my comprehension level at the moment.

Thanks for the help guys!!! I'm starting to make heads or tails of this.

 

[SteveWag]

The hollow shaft motor will run just fine with the top-mounted thrust bearing removed. The rotor is supported by bearings that only support the rotor of the motor.

The hydraulic down thrust is a function of the mounting depth, diameter of the impellor(s), and the discharge pressure. All of the weight of the water AND the discharge pressure spread over the impellers, is counteracted by or supported by the vertical line shaft. This is why line shaft stretch is calculated at shutoff head.

Steve

 

[electricpete]

The hollow shaft motor will run just fine with the top-mounted thrust bearing removed. The rotor is supported by bearings that only support the rotor of the motor.

I believe it depends on the pump. Some pumps have their own thrust bearing, others do not.


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