Jet belt sander- motor gets way too hot, motor doesn't get hot... why?

pontiac428

pontiac428

John Newman
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This is my Jet 2" belt sander/grinder. There are many like it, but this one is mine. My dad passed it down to me while he was in the tool and equipment business in the 1980s. He kept the 3/4 horse version, and I got the 1/2 hp (which means it has enough power half the time). Anyway, this guy has been a real workhorse for me.

I was running it the other day, and it was dropping RPMs until startup engaged, then it would run up to speed and start dropping RPM again. The start circuit would engage and disengage with every run up, tick-tock, tick-tock. It wasn't doing that last time I used it. I put my hand on the motor, and blew up with bad English when I found it to be around 200 degrees Fahrenheit.

The first thing I suspected were the bearings, but when I fired it up again later the problem had vanished. There wasn't any kind of sound coming from or drag fighting the motor shaft, so maybe it's not bearings. It didn't get hot again, and ran normally, so maybe it's gone... but I know no mechanical problem stays gone for long.

Can anyone give me some tips for how to troubleshoot an intermittent like this? It's easier to troubleshoot a fault as it is occurring. Next time it heats up, I'll find out where the hot spots are with an infrared thermometer, but if it doesn't come back, what are some likely culprits? This has been a good machine for a long time, I'd like to keep it that way.

Thanks!

IMAG1364.jpg
 
The only thing on the electrical side I can think of is to check the run capacitor (if it has one), including dirty/corroded or loose contacts.
 
There are no visible capacitors on the body. The base casting has a small cavity for the switch and wiring. I might have to separate it from the base, invert it, and see what is underneath.
 
There may be a cap in the base- the overheating is a bad omen though- a winding could be shorting internally
(which basically means it's shot)
Let's hope it's the former :)
 
The base is hollow and contains mysteries!
Do your self a favor and dont run until further inspection.
The start circuit is kicking in when the motor speed slows to the point this allows this mechanical device to kick in.
Check for a lost, broken spring (s) on the contact plate. Under one of the motor (bells). Under the base is your caps and wire terminations.
I think you will find both start and run caps and your run cap is probably bad.
They are cheap so buy brand new ones as they decay over time whether used or unused.
 
Okay, looks like I have to clear off some bench space to check the caps and the centrifugal start mechanism. I can manage that, and I took a couple extra days to make a long long weekend out of the holiday just to spend in the shop. I appreciate all the help in finding a starting point, I don't get into AC motors unless I have to. This is one I'd like to keep going as long as possible. I'll report back...
 
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