Vertical motors with angle-contact ball thrust bearings or spherical roller thrust bearings are designed to operate with a minimum external downthrust load. In some cases this will prevent skidding of the upper bearing. In other cases this prevents the upper bearing springs from loading the lower bearing.

Do you ever perform uncoupled runs of vertical motors such as this? (for post-installation check, for troubleshooting, for balancing etc).

Do you generally take any special precautions related to the concern above (for example perform an adjustment of rotor endplay prior to uncoupled run).

Do you limit the number or duration of uncoupled runs on vertical motors due to this concern?

Have you heard of any bearing problems created by uncoupled run of vertical motor?

I remmeber to have solo run of vertical motors for couple of hours without any issue. In any case there is downward thrust load because of motor rotor weight. In new designs, vertical pumps have their own thrust bearings and they do not depend on motor bearings to take thrust load of the pump.

The concern would apply to some vertical motors and not to others.

Clearly it does not apply to motors with sliding bearings.

My understanding is that a motor repair shop would generally would remove endplay from any motor with rollling element thrust bearings before uncoupled run. But based on plant experience, I think you can get away with it on some motors but not on others.

We have one group of motors with spherical roller thrust bearing with outer ring spring-loaded upwardg. We have run that several times with no major problems. It makes sense that those springs help keep you out of trouble during momentary uptrhrust or during uncoupled run (as long as they don't overload the lower bearing in upthrust).

We have another group of motors that has on top a combination of angle contact bearing and a "gothic-arch" "4-point-contact" "split-inner-ring" bearing. That motor has had bizarre random non-syncronous frequencies (but not bearing fault frequencies) show up heavily during uncoupled run I think at relatively low magnitudes 0.2 - 0.3 ips. I posted that one on the board. We elected to couple it anyway. The vib went away when the machine was coupled and no evidence of any damage caused.

We have another group of motors with two angle contact bearings on the top in a duplex tandem (DT) arrangement. Normal endplay is 0.010 - 0.012. I looked at our ancient records and we have seen one motor that was shut down immediately during uncoupled run (after recording 1.5 ips) when we first tried to run that one uncoupled without adjusting the endplay. The vendor's documents for this series of motors mentions that high vibration may be seen when the motor is run without external downthrust load.

I wonder if it is tied to the bearing configuration. Has anyone done uncoupled run with DT bearing configuration? Other experience or comments?

This message has been edited. Last edited by: electricpete,

I may be about to expose my ignorance but I will confess that I only have experience with back-to-back (DB) arrangements of ACBBs. So ...

The DT arrangement is always going to require more minimum downthrust than the DB arrangement of like bearings. So lacking minimum load during uncoupled runs may be part of the issue but how is it that "normal endplay is 0.010 - 0.012" with the DT arrangement? Is the endplay limited in one direction by the lower bearing? If so, perhaps, something is amiss in the other bearing or in how it or the ACBBs are mounted (i.e. housing bore/depth out of tolerance or not flat/perpendicular to the centerline) in that one case compared to others.

On this motor, the DT pair on top takes the downthrust. A deep groove ball bearing on the bottom takes upthrust. The endplay is adjustable by a locknut screwed to the shaft above the top bearing which pushes down on the bearing inner ring holder to adjust shaft position relative to inner rings. Moving shaft up compared to inner rings decrease endplay. I think you might call this a cross-locating arrangement. We have a drawing which shows the required amount of endplay.

It makes sense to me that any motor with a locating bearing configuration can provide it's own preload and probably woudn't have any problem . That would include the 4-point contact bearing I discussed above and the DB bearing pair in your case. Likewise if you have the cross-locating arrangement I described above with DT on top and deep-groove on bottom and endplay in between, then the only weight on the DT is the motor rotor weight which apparently is not enough.

I am surprised there are not more responses.

I expect there are a lot of people who have run vertical motors uncoupled without problem (most of ours give no problem).

Is there anyone else that has ever had a bad experience (very high vibration) due to the lack of downthrust load when running a vertical motor uncoupled.

Well, I guess I'd like to prove to y'all that there is some reason to think carefully before doing an uncoupled run of a vertical motor, other than the one 1993 experience I previously mentioned where we had very high vibration 1.5 ips.

Here are two references:
#1 - Westinghouse Instruction Leaflet 3030-D1 - "Instructions for Large AC Motors - LifeLine D Vertical Induction Motor; Frames 5000, 5800, 6800" page 11 reads as follows:
"Note that if motor is run without downthrust, excessive vibration may occur."


#2 - US Motors http://www.usmotors.com/Service/faq10.htm
"I. [FOR] Spherical Roller Thrust Bearings and Angular Contact Bearings (With Springs)...
"Motors built with spherical roller thrust or angular contact bearings with springs require a minimum external thrust load, sufficient to compress upper die springs and unload lower guide bearing from axial spring thrust. Refer to the motor's spring thrust plate for required minimum thrust. NOTE: Do not run motor without load for more than fifteen minutes, as lower bearing damage may occur and improper seating of thrust bearing may cause vibration."

Note there are two concerns identified in the US motors note (the last sentence):

1 - damage of thrust bearing (top) due to lack of load when required. This whole section applies to motors with springs. I believe that springs are employed only in cross-locating arrangements similar to what I have described where the bearing cannot preload itself. So I don't think motors in springs are the only ones this applies to... it also applies to my motor with DT pair (and no springs) and upthrust taken by lower deep groove.

2 - For the bearing with springs at top thrust bearing, when you remove the external load, the springs may lift the rotor and apply upthrust to the lower (deep groove) radial bearing which can damage that bearing if run for an extended period.

MY RECOMMENDATION based on the above: if it's the first time you have run the vertical motor uncoupled and you don't know the bearing configuration is, then monitor the vib carefully during initial start (in case it's vibrating like crazy) and limit duration of the run to 15 minutes (per US motors). Although if you have sliding (vs rolling) thrust bearing or back-to-back angle contact (DB) bearings, these concerns do not apply.

A side note - when discussing our motors with an OEM's yesterday, I discovered we have another unusual configuration. We have one set of vertical motors with back-to-back angle contact (thrust) bearing at the bottom and floating deep groove (guide) bearing at the top. I have no concerns with this arrangement for uncoupled run since it has back to back, but this is the first I ever heard of thrust bearing on the bottom of a vertical motor.

electripete,
The motors with spherical roller bearings have little radial stability with no load. These are "high thrust load" motors. They require a min. thrust load to operate (may reqire 1400 lbs down thrust depending on bearing size). In a motor shop,they are adjusted to .002 to .003 clearence to run for a short time. The angular contact bearings bearings in tandem are adjusted about the same. Back to back can run longer with no problems. The sphericals use the springs to limit the down thrust load on the top bearing. These have a min-max thrust load. Going below the min, the rolling elements slide. Above the max, the bearing and oil limits are exceeded and break downs occur.

Knowing the bearing configuration is critical.

This post will refer to the motor with DT bearing pair on top taking downthrust and deep groove on bottom taking upthrust.

We got some instructions from the OEM to remove all the endplay prior to the uncoupled run.

Now I'm wondering about the difference between removing all endplay and leaving 0.002-0.003 mils. I'm thinking that leaving 0.002-0.003 may be easier to accomplish without fear of over-preloading and may reduce the vib, but it would be more effective to completely remove the enplay if practical in the configuration we're looking at.

The room where these motors are located happens to have a crane in the overhead.

Our plan was to use a crane with a load cell to lift the rotor to the upper limit of travel by applying just a little more load than the known weight of the motor rotor (monitoring the load cell reading). Then, we'll tighten the lock nut (we don't drop the rotor a few mils before we tighten). This is of course prep for the uncoupled run and we'll restore the endplay before coupling up the motor.

Maybe that's extra work and crane/load cell are not always handy. But I don't think it would create any problems, do you? One thing to note is that in this configuration, thermal expansion of the shaft can only increase the endplay above that which was set at room temperature.

This message has been edited. Last edited by: electricpete,

electripete,
Why are you wanting to run these motors with no load? Is there a problem or are you just courious? Couriousity is ok, but can get expensive$$$ One customer had a habit of running all repaired motors in their work shop before installing them. They had no experence with the high thrust motors and ruined two sets of bearings. They thought the motor shop was building them wrong. Cost them $$ Bearings cost $1200 each.
I have used rigs that mimmick the load of the pump. Talk about extra work!

Our plant routinely does uncoupled runs on motors:

#1 - When troubleshooting a vibration problem on a coupled machine. If we uncouple and the vibration goes away we generally think it's not the motor... must be misalignment or the pump.

#2 - When installing a motor received from the shop. That way if there was some problem introduced during shop work, transport, or installation, we find it earlier and can react sooner (don't have to wait until after we couple up). We also check direction of rotation (wouldn't want to find out about wrong direction after coupled up), bearing tempertuares etc. Likewise if uncoupled run is good and a vib problem emerges only after we couple up, we have a better view of the possible causes (for example misalignment would be one of the prime suspects).

#3 - Also we have occasionally balanced vertical motors in place.

Last weekend we had a fairly unusual situation. The pump was damaged since excessive force was used during removal of a spacer coupling which didn't have enough clearance (until you cleared the boss fits) and had hubs galled onto the shaft. Over the weekend the plant was well on their way to doing an uncoupled run of my DT-bearing motor to make sure the motor wasn't also damaged during that coupling removal (without any plans for endplay adjustment). Luckily they called me up with an unrelated question and I put on the brakes since I had heard about the 1993 problems during uncoupled run of a sister motor. But some folks at the plant were pretty surprised and skeptical and a little mad when I told them they couldnt' run that motor uncoupled. To prevent that from happening again, they have now asked me to develop a procedure for doing uncoupled runs of our vertical motors when necessary. I spent some time this week trying to learn more about this subject.

I think these type of uncoupled runs are fairly common practice in plants (Do a lot of you guys do uncoupled runs in the plants?). Also I think plant personnel are generally unaware of the unique concerns for uncoupled runs on certain vertical motors.

This message has been edited. Last edited by: electricpete,

quote:

Originally posted by lake man:
I have used rigs that mimmick the load of the pump. Talk about extra work!

Can you briefly describe what kind of rig might be used to provide downthrust on a vertical motor? Thx.

Basicly, a bearing housing with angular contact bearings (DB) mounted to a center hub. Above the hub is a plate contacting the outer race and springs above the plate. The hub was threaded to except a headshaft(be mindful of thread direction). The top of the housing was machined for the flange of the motor. You mount the housing to the motor, thread in a headshaft of the proper length,and apply a load just like adjusting a pump. Turn the line shaft nut to deflect the springs. We made several tops to fit the housing so it would fit a couple of motors.The bearing housing uses oil for lub. This makes more since in a shop than the field. We had several for the common motors that we did. Somewhere I have a picture of the set-up, not sure where now. I built them in the 70's. A good machinist should be able to figure it out.

Pete:
Sorry for not keeping up with this thread but just another thought... With the DB arrangement, the load must go through each of the rows of rolling elements. With the DT arrangement, the load is shared by the two bearings. Of course, the amount that each individual bearing supports is theoretically indeterminate. Taking that a step farther in the realm of possibilities ... Do you suppose that the 1993 problem was the result of a bad pairing of bearings such that one bearing supported the majority of the load and the other bearing elements skidded in their races which created the objectionable vibration? I believe that, many times (always?), the faces of paired bearings are ground to avoid unequal load sharing - perhaps that pair was not ground correctly or there were other dimensional issues between the two bearings.

Maybe you're way beyond me on this (and maybe I should frequent this board more) but I suppose that a bearing manufacturer might suggest this as a possibility (especially if it is someone else's make of bearings).

--H

H –

Thanks for your comments.

That's a good point about the need for matched pairs or universal-ground bearings in duplex arrangements. I haven't given that any thought. I don't really have any information about the pedigree of the DT bearing pair that was in there (and remain in there).

In the case of our 1993 problem, I am fairly certain the cause of the problem was the lack of downthrust as you mentioned in your earlier post. The reasons why I am fairly certain:
1 – the vibration did not occur when coupled (loaded), only when uncoupled (unloaded).
2 – As mentioned here http://www.usmotors.com/Service/faq10.htm running certain motors without external load can cause excessive vibration
3 – We talked to the OEM for this motor and he has given us instructions that the endplay has to be removed before running the motor uncoupled (and then restored before putting the motor in service). We have incorporated this into our site procedures for this motor.

This message has been edited. Last edited by: electricpete,