DC Motor started surging or thrusting, the shaft would slide back and forth almost a quarter inch

[alexander123097]

This application is on a roll grinder. The motor is turning a grinding wheel using v belts from motor to grinding wheel spindle. 40hp DC mill duty motor mounted horizontal on a carriage, the carriage traverses back and forth. The motor is driven by a digital drive using an analog tach feedback. The motor has sleeve bears (babbit), do not the spec. on end play (axial movement of the shaft). I work in an electrical maintenance & control department at a steel mill, the grinder mentioned is a roll grinder
First called on problem because it had threw belts, told motor had not been installed that long ago. Do not know exactly why the replacement was needed or how long it was in operation. Belts were replaced and monitored operation and motor was surging or thrusting, the shaft would slide back and forth almost a quarter inch. Operations and machine maintenance said there has always been end play but nothing like this. The tach assembly had a love-joy coupling and it was not mounted correctly. Set the drive up to run from tach feedback to voltage feedback and tuned to compensate, operation looked ok & operated for a few days. A new tach arrived with proper machined shaft and a different coupling, it is a collapsible type. Did not stay close to problem but Area wanted these parts installed and switched back to tach control, it had been operating that way for years without issues. Later, about 4 days, the motor started surging or thrusting, the shaft would slide back and forth almost a quarter inch. There was darkness in the bearing oiler so the decision was made to change motor because of the bearing, although it did not feel hot. A new motor, repaired at the same machine shop that has done all these motors, was already onsite so decision was made to replace. This motor had been ran for hours offsite by the motor supplier with no load only putting voltage on the buss and letting it running giving it a pass fail test, it passed. This motor was installed but unable to put in operation. It looked ok when first installed but after running an hour it started surging or thrusting, the shaft would slide back and forth almost a quarter inch. Removed the belts (removing load) & connected an analyzer up and monitor. Motor would act normal at low speeds but when turned up to high RPM it would thrust. Also it would thrust when the carriage that it sets on moves at either RPMs. Adjusted the field as high as possible (probably could not operate at this setting and problem remained). Did not see motor current on analyzer surge, it looked steady. Had an alignment group check the motor level, it was good. Had a GE Drives person come in but he could not find issue with drive. He is not an application grinders person only works on drives on many different types of systems. We all were and are left with no more ideas. This motor never was able to be put in service. The motor supplier states and it did run ok at his shop using a DC supply. Another motor was coming back from machine shop so the decision was made to install and align getting all people together and start it up and monitor operation. Did that and the first roll it ground all looked ok. Vested people left, it ran maybe an hour total, then it started thrusting slightly( not like the motor that was removed). But is that normal or is this how the problem started?
The grinder was shut down until tomorrow so we all have a chance to think it over and get a person coming in that works on grinders. Not sure it he will have further input. Do not want to compromise the motor by continuing to run it with questions. I believe he will say replace drive but do not know. I am looking for a spare drive we may have that we can temp install just to run motor if asked. At this point will follow his thoughts because we are at a loss at our site. Our department clovesr so much area and systems do not know exact details of all equipment. How much end play on a 40hp motor that is belt driven not coupled to a gearbox, no idea.
This site has been a great resource for me. Anyone have any thoughts on this?

 

[dArsonval]

Alexander;

When you state “mill duty motor”, it conjures the typical split case style with a tapered shaft and threaded
end for a locking nut. These motor types could be series, shunt, or compound wound, and normally have ball
bearings. Nameplate data (if it’s attached to the motor) would clarify exactly what kind of motor you’re referring to.
I’m hazarding a polite guess your installation is not actually a mill duty type, and perhaps the term is loosely used
here for illustration.

Your post is somewhat difficult to follow due to the many events you’re writing about.
How many motor installations and removal have actually occurred?

This sentence, “…told motor had not been installed that long ago. Do not know exactly why the replacement
was needed or how long it was in operation…” seems to refer to the original motor, and your first replacement.

Further into your post you’re saying, “…A new motor, repaired at the same machine shop that has done all
these motors, was already onsite so decision was made to replace.”

And further into your post you’re stating, “…Another motor was coming back from machine shop so the
decision was made to install and align getting all people together and start it up and monitor operation…”.

Are you up to 4 motors? The original, the first replacement, second replacement, and now third?

An additional conflicting portion of your post refers to a GE drives person whom could not find any issue
with drive, yet the motor was never able to be put in service alluding the drive was not compatible for motor.

With all the variables you’re discussing, and not knowing anything about each individual motor mentioned
makes replying to your post difficult.

John

 

[alexander123097]

1. We call these heavy frame mill duty motors, your correct about the windings. I cannot give nameplate today I am not at work.
2. The motor current mounted is the third but the first was removed because of a broken shaft. I do not know the history of that event at this time.
3. this refers to the motor that replaced the org. taht had a broken shaft.
4.The motor stated new was a repaired motor and is the motor that we could not get to operate and was replaced by this most recent motor.
5.yes on four. but really do not have that many spares. I believe the supplier has three total.
6.What I mean by not going into service is the thrusting does not allow proper grinding. The wheel needs to turn constant or it will transfer the speed pattern into the roll it is grinding.
I realize this is a difficult path to follow. I tryed explaining best I could, probably should have done better. We are pointing at the machine shop repair because we do not know what else because the operation of the drive looks correct. But not sure where the root problem is.

 

[dArsonval]

Thank you for the additional clarification.

I’m still hesitant to acknowledge we’re discussing a Mill Duty Motor as produced by Westinghouse
in its early days, or General Electric for example as shown here in product literature:

http://www.gepowerconversion.com/sites/gepc/files/product/GEA-17259 Mill Duty MD800.pdf

A belt drive Direct Current motor “thrusting”, (meaning unstable current readings, speed, etc.) tends to point
toward the power supply driving it, AND its direct load. Some drives are just not compatible with certain D.C.
motors and the fact that different “motor swaps’ have occurred during the troubleshooting process makes this
a valid issue to consider. Not because there’s a problem with either. More so, because of a the motor / drive
compatibility issue.

The original failure of the very first motor draws a Red Flag as well.
Mill duty motors are extremely robust. A possible improper repair may have contributed to its shaft failure,
or the driven load mechanical aspects were never fully investigated.

My multiple hunch is this:

A random spare motor was found to be adequate to replace the original failure. It happened to be a sleeve
bearing motor which did not matter at the time because of production requirements.
It ran for as long as possible until its miss-application reared its ugly head.

The original motor used in the application needs to be identified, and its comparable replacement
put in place. It’s a best guess on a late Sunday afternoon.

I'm hanging the Service Center you're working with [on the Hook] on this ; )

Let’s hope you figure this mess out on Monday morning.
Or at least by Monday afternoon!

Always enjoying this forum.

John

 

[alexander123097]

Thanks for the reply. If time tomorrow I will post the nameplate and Monday's actions. It always helps others when there is follow ups on problems posted, as well member's replying.

 

[alexander123097]

The motor installed nameplate: 40hp, 151amps, 240v, shunt, duty 24hrs., field amps 3.51-.870, resistance 25c 54.4, service factor 1.15, frame S-504.A, comm end sleeve, sytle 40K2058

 

[alexander123097]

Anyone have a spec sheet on a Westinghouse motor nameplate: 40hp, 151amps, 240v, shunt, duty 24hrs., field amps 3.51-.870, resistance 25c 54.4, service factor 1.15, frame S-504.A, comm end sleeve, sytle 40K2058? Interested in knowing the spec for end play (axial movement of the shaft).

Update: started grinding with this motor installed and current on analyzer looked good however tach signal has noise. This operation looks better than previous motor. However it has only ran about 1 hour.

 

[dArsonval]

Alexander;

The 1/4” endplay is not excessive provided it’s possible to push on the armature shaft at each end
(while the motor is running) to note that it is indeed “floating” freely and not thrusting against either
one of the bearings.

Because every split case mill duty motor I ever had the opportunity to work on were always ball bearing
style units like as shown in the URL below, I was reluctant to believe you were working on a unit with sleeve bearings.

http://www.pbase.com/visual_first/image/155421174

Wellllll, in scrolling through volume 3 of a 1942 American Technical Society book, I found a photo of
a General Electric mill duty split case motor with oil lubricated sleeve bearings. And while these volumes
would be considered laymen’s literature in some respects, it does mention end thrust as much as 3/8”.

As for finding a spec? One could embark on a research project looking for that kind of info.
There’s all kinds of charts related to sleeve bearing clearance, but next to nothing on thrust dimensions.
Someone here in the forum may have more to contribute on this though, ‘Ya never know.

One other note related to the black oil mentioned earlier. I’ve seen where a repair tech is over zealous
with Permatex in sealing up a bearing housing resulting in dark oil. But this would normally be caught/revealed
during in-shop tests and be corrected before allowing the motor out the door to the customer.

Enjoying the forum,

John