Bridgeport 1J Step Pulley Motor Bearing Replacement
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  1. #1
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    Default Bridgeport 1J Step Pulley Motor Bearing Replacement

    Howdy Folks,

    A quick search of this forum didn't yield the results I was seeking so forgive me if this has been covered before...

    Back Story: I've had my 1J for 6 years and it's ran like a champ ever since I purchased it. I've done some very general upkeep but decided recently that I would tackle replacing the motor bearings on my Fairbanks Morse QZA 1HP pancake motor. They have been very noisy since I acquired the mill and I decided to do my machine a favor and replace them. HW has the service manual for the QZA series and I followed it to a T and successfully removed and replaced the two bearings (free end 6207 and stator bearing 5605). Everything assembled well and everything moved smoothly without binding or any other cause for concern. My only departure from the repair manual was I used a sealed 6207 bearing in place of an open ball bearing.

    I re-assembled the motor and placed it back on the mill and tried it for the first time this evening. Off the bat, it was noticeably quieter than it was previously. The motor ran for about 2 mins before tripping the overload preset on my VFD. This normally would happen when I was doing some heavy milling and using the power feed but never just with the mill running. Seeing as how I recently had the bearing assembly in the head apart, I removed the drive belt from the step pulley to isolate the source of the added drag. I ran the motor by itself for about 2 mins and tripped the OL again. I decided then to check the running amp setting on the drive as I had never set it outside of what it came from the factory (1HP Teco VFD). It had been set for a running amp value of 3.4 amps when the motor tag called for 4.2 amps. A quick adjustment to the parameter in the VFD and I was off again. I decided to re-engage the spindle and run the whole works to see how things performed. The OL setting on the drive never tripped again, however, the thermal switch on the old motor starter that the VFD is attached to opened up. This also occasionally happened during my tenure as the mill's owner, but only on hot days under sustained running (30 mins with spindle on and loaded).

    Here are my questions/concerns:

    1. I really can't figure out where I may have fouled up the bearing installation to cause excess drag and amp draw. Is there something I did not consider that could cause drag or excessive pre-load? There was no mention of pre-load in the FM repair manual. Do new bearings typically have more drag than ones that are blown out and roaring? Is there a possibility one of the bearings is slightly askew in it's housing?

    2. I ran the mill for a total of 10 minutes and in that time the rotor stator that houses the 5605 bearings got to about 96F. The ambient temperature in my garage was about 34F. Is this normal or should I disassemble and check the bearings? Everything "feels" great and as smooth as glass when I turn the motor over by hand.

    3. Is this all completely normal and has it exposed that I have a faulty thermal switch in the old and likely original Cutler Hammer motor starter that the VFD runs through? Is the old motor starter even needed anymore?

    4. Do I need a new hobby?

    Sorry for the long post. I appreciate any insight you guys may have. Worse case, I bring the motor to a repair shop and let them sort it out. I enjoy tackling these projects myself so I'm willing to torture myself some more.

    Take care,
    Brian

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  3. #2
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    Default

    1. Sealed bearings are good. You worry too much. Once the bearings are running for a little while the additional drag is academic. So I think 10 minutes is
    a good long time. Your overload is acting slow as in the same kind of way that a motor heats up. The current is high to begin with and the overload just heats up
    if it is a thermal acting old type. A modern one might have a time delay built into a semiconductor chip that is connected to a current transformer. Whatever way
    it is a simulation of motor case heating.
    2. Normal.
    3. Not needed. What is the starter thermal overload set to? Measure currents with inductive clamp voltmeter. The reader here assumes that the motor starter
    is connected to the input of the VFD. It would be better to have the input of the VFD connected to a circuit breaker.
    4. Don't mention hobby here. They will boot you out.

    Remove the drive belt. Execute a auto tune command on the VFD.
    Last edited by rons; 01-23-2020 at 06:23 PM.

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    1. I'm an engineer by trade, I overthink everything. It's a curse.
    2. Good to know.
    3. I'll have to check what is on the cover. I'll measure amp draw this evening. I've been looking for an excuse to get a new meter anyway.
    4. It's an expression.

    I will execute the auto tune command this evening and report back with the results.

    Thank you, rons!

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    Default

    I'd think it would be hard to get the bearings in wrong.

    I'd also doubt there is any preload on the motor bearings other than some spring washer. The 1-1/2 hp motor on mine only has a spring washer which really doesn't preload.

    There is more drag on new bearings compared to old ones, unless the old ones are completely fried. The grease is completely gone from old bearings and they typically spin very free, even if they make noise. New bearings have to push all the grease around in the races.

    My guess is something is going on w/ your VFD. The added drag of the new bearings are possibly just enough to push your overloads over the edge.

    It's not normal for your fan cooled motor running 10 minutes to heat to 96F w/ a 35F ambient air temp. My assumption was that your test was run under no load. If the motor was loaded, then yes it could get to 96F.

    I just ran mine for 10 min. in a 60F ambient air temp. From touch, I couldn't tell any difference in temperature of the motor from beginning to end. It was turning the spindle, but no load on the spindle. The motor felt very cool.

    A VFD can overheat a motor, if it's not set right, or defective. Perhaps your autotune will help, but if it doesn't, it still sounds like it's a VFD issue to me. All this is assuming your motor is wired correctly.

    Wanted to mention, you know you aren't supposed to run the VFD output through your cutler/hammer starter, right?

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    Motor tag is the FLA amp rating of the motor - full load amp.

    Running the motor with the belt off, you should not get close to that number for amp draw. I would say 50-60% max.

    You mention VFD - is the motor 3 phase, and the source also? Or are you using the VFD to convert single phase to 3 phase?

    And yes, as was mentioned, the output of the VFD needs to go directly to your mill.


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