My eldest son suddenly showed some interest in using some of my machine tools as he picked up a project from work that involved drilling and tapping 4 6-32 holes at 90 degrees to each other into the circumference of a spindle. In order to do this we needed to make a jig to facilitate the process. This was the first time we really ran my Bridgeport mill for a lengthy period of time. It worked flawlessly the first night, but when we went to use it the second night it would not turn on and would blow the overload reset on the VFD I have controlling it.
At first I figured it was a bearing, though I was scratching my head because it made no abnormal noises the night before and I would have expected to hear something before the bearings totally seized up. After taking the motor off we found we could turn the shaft by gripping the pulley but it required significant effort. I received a lot of great counsel from the folks on the Bridgeport Mill Yahoo group. I checked the resistance on the 3 leads going into the motor with a multimeter - realizing fully that this is only a partial test but not having a megger it was the best I could do. Everything appeared okay to my novice eyes.
I then pulled the top housing off and as soon as I separated it slightly from the main stator housing it spun freely on its bearing - definitely not ceased. Similarly when separating the bottom housing from the main stator housing it too spun on its bearing quite freely. I was a bit confused as I had figured it was a seized bearing.
On this particular motor the top housing contains a greased bearing (external zirk for greasing it) and it has a plate behind the bearing towards the rotor. This plate is screwed on with 3 screws from the top of the motor. If you don't take out the screws then the rotor will come out with the top housing. The bottom housing does not need to come off to service the top bearing and the rotor. Not the less I took it all apart before I fully realized that :-)
The problem actually appears to be gunk having perhaps fallen into the gap between the rotor and the stator as seen in the picture below. The rotor has heavy dirty grease on its side. The inside of the stator also had similar deposits.
I did my best to clean the rotor and the top bearing and plate as well as the top and bottom housings with mineral spirits, careful to not soak the rotor in the mineral spirits, not sure if it would matter but thought it best to not take a chance. For the stator I used a cloth soaked in Denatured Alcohol and wiped the stator surfaces and any loose crap on the winding insulation leading to the stator. Figured denatured alcohol would evaporate much faster then mineral spirits leaving less likelihood of something shorting out there - no idea if that is true, just my thought process (I'm sure someone will laugh at me).
I cleaned all the grease out of the top reservoir, even replaced the zirk. Did my best to repack the bearing with grease and then reassembled the top housing making sure all the shims were in place first (see pictures of housings). Then filled the top reservoir with grease until it came out the plugged hole used to push out the old grease when doing a "grease change" once a year (according to the motor plate). I bolted the bottom housing to the stator housing then carefully lowered the rotor and top housing back into the stator after adding the shims to the bottom housing. Tightened up all the bolts (ughh, broke one, so back apart and drill out the broken bolt, then reassembly. I also replace the zirk and the 3 screws holding the reservoir bottom plate.
I remounted onto the mill then rechecked the resistance to ground and to each lead with the multimeter again. All seemed well, so I hooked it back up the VFD and cringingly hit the start button. The motor started right up without any hesitation and spun up and down through the whole range of the variable speed on the VFD.
It took me about 4 hrs in total - including cautious disassembly, cleaning which took forever, a run to the hardware store for bolts and screws, fixing the grease gun (what a mess) and final assembly and testing.
I'm not saying this is a fix for all motors, actually I was completely surprised by the actual cause. But looking at those end housings in the before picture shows there was a ton of gunk on them - about an 1/8" thick in places so it doesn't surprise me something fell in there. I can't say how long my cleaning will last - if the grease reservoir is allowing grease to escape down the shaft eventually more might find its way into that same spot and we'll be back at square one. The cost of the repair was $5.91 for the bolts, screws and some washers - only because I bought type 8 bolts (why I don't know).
Obviously if you don't know what you are doing, be extremely cautious when messing with electrical components such as this.
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