Total end play on electric motors driving pumps 1

[RotEquipEngr]

I would like some input from others involved with rotating equipment in either a refinery or power generation plant.

What would some of you consider excessive end play in an electric motor that is driving a centrifugal pump?

We have had several pumps which had gotten into a pattern of changing the mechanical seals every couple of days. We changed the pump, rebuilt the pump, changed the seal, ect. It was found that the electric motor had 0.025 of end play. After contacting the motor manufacturer and a motor shop, they both indicated that they would not consider 0.025 on the electric motor excessive. However, after we changed out the electric motor with 0.025 end play, the mechanical seal problems stopped. The shim pack type couplings which we are using are good per data sheet for about 0.031. The total axial thrust in pumps using angluar contact bearings is usually set at 0.002 -0.004. I would like some general input and guidance on when some of you think an electric motor needs to be replaced or rebuilt; is 0.015 too much? 0.020 too much?

Thank You.

 

[electricpete]

We have shim pack couplings are used with sleeve bearing machines that have 0.5" endplay on the motor (10" endplay on the pump Kingsbury thrust bearing). We don't see any problems except that at low flow the motor shaft visibly moves axially at a few hz. We imagine that small movement of the pump within it's clearance is exciting the resonance of the motor mass on the coupling spring.

My reaction (right or wrong) about your situation is as follows: The only way I can imagine that reducing the motor endplay makes your seal last long is if your motor endplay was limiting axial movement... that shouldn't be the case if you have much smaller endplay at the pump. Are you sure the pump endplay is set correctly?


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[JJPellin]

Electricpete is correct. The motor end play should not affect seal life on a pump with a flexible coupling and motor bearings. I would be concerned about motor end play on a direct drive pump where the impeller mounts on the end of the motor shaft. I would also be concerned about the end play on a motor that is rigidly coupling to a vertical in-line pump. For any installation with a flexible coupling, it is important to set the distance between shaft ends correctly so that the pump and motor are not thrusting against one another excessively.

However, with all that said, the end play you mention of 0.025" would be excessive for most bearing configurations.

Our motor repair specifications require the following:

“Shaft Endplay (D) - shall be between 0.001” and 0.003” for back-to-back thrust bearing applications and approximately 10 times the internal radial clearance of the bearing (posted in bearing manufacturers application books) for single radial bearing applications. This figure will vary from about 0.006” to 0.026” depending on the bearing size.”

We have had to work closely with our motor repair shops to get them to adhere to this specification.


Johnny Pellin

 

[RotEquipEngr]

Total pump axial movement should not exceed 0.004 when using two angular contact thrust bearings. This figure is baed on the pump operating at 100 degrees F or less. If we are using a double row bearing for a thrust bearing, we may allow .001 to .007, depending on what the manufacturer recommends. I do understand that motors using the old babbited sleeves will have more end play. The majority of our electric motors are running ball bearings with C3 bearing fits. On a C3 fit, I have always used an allowable end play of 0.003 - 0.005, at ambient temperature. Of course, if the application is a high heat application, more will be allowed for thermal growth; this I am ok with since we will also allow for thermal growth in the pump. At ambient temp. using a C3 fit, what would be the maximum axial movement you would allow?

 

[JJPellin]

As I indicated, it depends on the bearing size and type. For a deep groove radial bearing size 309 with no thrust bearing, the internal radial clearance for a C3 fit would be 0.0008" to 0.0013". We would allow axial endplay equal to 10 times the internal radial clearance. So, for this same bearing we would allow 0.008" to 0.013". If you try to hold a radial bearing to 0.005" end play, you may have a very hard time. Even if everthing is perfect, you can have more end play than that just from the normal internal clearance within the bearing.

Johnny Pellin

 

[bingopin]

I've seen many older motors up to about 50hp with as much as .075 end play. These motors were driving typical horizontal centrifigal pumps (thrust bearings and flexible couplings). These motors had a spring washer behind the fan end motor bearing. Still some of them running today. No problems to speak of.

 

[RotEquipEngr]

JJPellin:

10 times the internal radial clearance works for me. After using this on a couple of bearings I know we have running in our electric motors, I am comfortable with the range of axial movement this yields. I would like to know if this is a figure you developed there where you work, or if this is an industry standard used by electric motor manufacturers. This is the first time I have heard this rule of thumb for determining acceptable axial movement in electric motors.

Thank you for the valuable info.

 

[JJPellin]

This is an internal pump repair specification that we developed. It was developed by our electrical group, so I don't know what they used as a basis. I would expect that they would have reviewed motor repair specifications from our motor repair shops and motor manufacturers and come up with one that is agreeable to all parties. But I don't know that for certain. We purchase all motors from Siemens. They may have a specification that we adapted.

Johnny Pellin