Help interpreting vert motor thrust bearing specification 1

[electricpete]

This is a specification requirement applying to a vertical motor which drives a single stage centrifugal pump through rigid coupling. The pump has no thrust bearing of its own. A drawing of the pump is attached.

I’m trying to figure out what this specification means (what are the requirements that the motor really needs to meet to serve this pump)

Paragraph 15. Thrust Bearing Capacity and Rated Life (external load)
- Continuous down-thrust 4100 lbs, 1 year minimum life
- Continuous up-thrust 4100 lbs, 1 year minimum life
- Maximum momentary down-thrus, 5,000 lbs.
- Maximum momentary up-thrust, 5000 lbs.
Minimum allowable continuous down-thrust to prevent damage to guide bearing, 0 lbs.
Momentary periods of up-thrust or lighter down-thrust are permissible.

Going through the 1st 4 bullets, it looks like the motor needs to handle either continuous upthrust (which would be unusual, but not unheard of for balanced impeller designs).

Then the next-to-last doesn’t make sense. If I ignore the previous, I might interpret it as minimum required continuous downthrust requirement, except for the word “allowable” instead of “required”, and the value zero.

Then the last sentence seems out of place in view of previous statements. It almost implies that the pump is designed for continuous downthrust with momentary excursions to lower levels of downthrust or upthrust expected.

Do these requirements make sense to anyone?


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(2B)+(2B)' ?

 

[electricpete]

I left out some words in my commentary:

"Going through the 1st 4 bullets, it looks like the motor needs to handle either continuous upthrust (which would be unusual, but not unheard of for balanced impeller designs)."

should've been:

[*]"Going through the 1st 4 bullets, it looks like the motor needs to handle either continuous upthrust or continuous downthrust(which would be unusual, but not unheard of for balanced impeller designs)."

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Note the quote from the specification is pasted above word for word (nothing left out or altered), including indentation (four bullets, followed by two statements, all under paragraph 15).

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(2B)+(2B)' ?

 

[JJPellin]

The pump is a standard arrangement. It has back wear rings and balance holes to limit the down-thrust when running with high differential pressure. This results in a situation where you can end up with up-thrust if the differential pressure is low (as it would be a very high flow). The specifications you list look like a cut and paste with numbers changed.

As you note, the first four items seem fine. They want bearings that can run in continuous up-thrust or continuous down-thrust up to 4100 lbs with short duration spikes as high as 5000 lbs. The fifth item probably served as a statement of a minimum continuous down-thrust for a different pump. They probably should have eliminated this line rather than changing the value to zero. I agree that with a zero, it seems nonsensical.

The last line would make sense if there was no allowance for continuous up-thrust. But, there is. So, that line should have also been eliminated.

You know much more about motors than I ever hope to know. But, I put all of this together as follows:

Some motor for our deep well pumps have a massive thrust bearing for down-thrust but no bearing at all for up-thrust. Up-thrust is basically impossible and does not need to be accommodated. If the motor were to see any significant up-thrust, it would fail.

Some motors have a 40 degree angular contact thrust bearing for down-thrust but a 15 degree angular contact thrust bearing for up-thrust. This allows for high thrust load in down-thrust but only lower thrust load in up-thrust. This reduces skidding of the normally unloaded up-thrust bearing. But, the motor can still take intermittent up-thrust. If you run this in continuous up-thrust or continuous very light down-thrust, the down-thrust bearing may skid and fail.

Some motors have back-to-back 40 degree angular contact thrust bearings that are sized equally for up-thrust or down-thrust. This gives you the greatest thrust load capability in both directions. But, which-ever bearing is unloaded may have ball skidding which will reduce the bearing life.

The specifications you list suggest that they want the last option. They want to be able to accommodate continuous or intermittent up-thrust or down-thrust. And, they are willing to accept the shortened bearing life that they may experience on the unloaded bearing as a result. Asking for a one year bearing life is not asking for much. Our average motor with ball bearings runs for 10 years between bearing failures.


Johnny Pellin