Motor Vibration

[JPMacG]

I am on a mission to identify and eliminate the various sources of vibration on my Craftsman/Atlas lathe. I have replaced some pulleys and the countershaft, and stiffened up the cabinet. One source that I have identified is the motor itself. It vibrates when powered. I ran it without the belt so I know it is the motor, not something else. And I noticed that it only vibrates when powered. When I turn the switch off the vibration stops immediately (while the motor is coasting).

It is a 1/2 HP Atlas branded ball bearing induction motor. I think it is the original motor that came with the lathe, circa 1954. I did some reading and apparently torque pulsations are inherent and unavoidable in single phase induction motors.

I'm wondering about a vibration isolation motor mount. Has anyone gone down this path?

 

[ttabbal]

You could make some rubber spacers to absorb vibration and put them between the motor and whatever it mounts to.

I would get a decent 3 phase motor and VFD though. Very smooth and variable speed.

A newer motor will likely run smoother just due to tighter tolerance and better balancing, even single phase.

 

[cathead]

Jon,

After reading this post I am wondering if the bearings could possibly be worn causing an unbalanced condition.
Is there any side play to the bearings? Also could the motor pulley possibly be the problem? It might be worth
your time to take the motor apart and do a bit of inspection.

 

[Dave Paine]

I have several single phase motors in my shop and they do not exhibit vibrations. As cathead stated, worth checking the bearings and the pulley. A set screw on the pulley could easily come loose.

 

[JR49]

Maybe one of you Electrical gurus out there can give some insight as to why this motor would vibrate while supplied with electricity, but stop vibrating when current is switched off even though it is still spinning fast. Thanks, JR49

 

[David S]

Jon did you just get this lathe and notice that the motor vibrates? Or did you own the lathe for some time and the motor used to be fine, but now vibrates. I have a couple of these motors on my machines and none vibrate like you describe.

The fact that it vibrates when powered but doesn't when turned off and coasting down, could possibly indicate bad bearings with excessive play.

David

 

[cathead]

Maybe one of you Electrical gurus out there can give some insight as to why this motor would vibrate while supplied with electricity, but stop vibrating when current is switched off even though it is still spinning fast. Thanks, JR49

Magnetism could be pulling the armature away from center causing unbalance. When the unit is turned off the magnetism is gone as well
as the imbalance. I don't know that this is the case but likely possible.

 

[JPMacG]

Thanks everyone. I just checked a few things... There is no perceptible axial or radial play in the motor shaft. I ran the motor with the pulley off and the vibration level was about the same as with the pulley mounted. It is not like the motor is shaking. It is that I can feel a vibration with my fingertips when I lightly touch the motor housing. I can also feel the same vibration at a reduced level on the lathe bed.

I have owned the lathe for about 25 years. Just recently I got on a kick of fixing it up... replacing some pulleys that wobbled, etc. I now have it improved to the point that the motor vibration is noticeable. I guess I am being obsessive - most hobbyists would probably consider the motor vibration perfectly acceptable.

I checked the motors on my milling machine, drill press and wood lathe and they have a similar vibration. The drill press ( a small Delta) is actually worse. In each case the vibration stops instantly when I turn off the electric to the motor. I really think it is related to the torque varying as the motor turns through the applied magnetic field.

Going to a 3-phase motor would be the best solution. I was just wondering if anyone had experience with vibration dampening motor mounts. I notice that appliance motors (i.e. HVAC blowers) sometimes have soft mounts.

 

[whitmore]

... It vibrates when powered. I ran it without the belt so I know it is the motor, not something else. And I noticed that it only vibrates when powered. When I turn the switch off the vibration stops immediately (while the motor is coasting).

The normal vibration of a single-phase AC motor is the familiar 'refrigerator-hum' at 120 Hz (torque dips twice
per cycle of the 60 Hz AC). It's usually not going to make anything vibrate, though, because the mass of the rotor
acts as a flywheel. If it is a capacitor-run motor, the capacitor can fail, causing excess hum (and eventually
nonstarting). Capacitors are cheap to replace (from generic dealers), but the official repair part might be rare.
You do NOT want a 1954-design identical capacitor. Look up 'PCB capacitor' for too much info.

If the rotor of the motor were to become magnetized, it could cause 60 Hz hum, but that is fairly easy to treat: just run
the motor (maybe with a lightbulb in series, to prevent excess current) into a heavy load, try to nearly stall it. A mechanical
design that damps 120 Hz might not damp 60 Hz vibrations well, and you get 60 Hz torque variations with
an accidentally-magnetized rotor.

If the hum goes away while the motor is coasting, that mainly rules out bearings and bent-shaft and even pulley
imbalance as issues. It doesn't completely rule out mechanical hum, though, because the motor laminations
or windings, if loose, can hum under power, and the fix for that is application of some kind of varnish or impregnation
to fill the cracks.

 

[FOMOGO]

You cold static balance them diy, or if you really want smooth have them dynamically balanced. Mike