I’m helping a friend this weekend and he said we are going to stone a motor does anyone know how to do this or is my friend pulling my leg? An help would be appreciated..

Jason,
The only stoning I know of is "stoning the commutator" on a DC motor. (using specfic grades of commutator "stones")It helps seat the brushes and "cleans" the comm. It is performed, for the most part, on energized motors so be careful.

Thanks, Jeff

I have heard the terms "stoning" or "grinding" in the context of methods for restoring the surface of slip rings on sync machines or wound rotor induction machines

JasonR

'Stoning' is normally performed on slip ring motors and, more commonly, DC motors.

With DC motors, it is a method to reduce sparking and to help 'refresh' the carbon glazing on the commutator while helping the brushes contour to the commutator itself. Normally, the process involves running the motor at a slower speed and special 'stones' which come in different grades.

The upside is that you can reduce sparking and the associated damage. However, the process does not improve the 'roundness' of the comm nor does it remove flat spots.

Howard

or if your motor commits adultry.

Was that a little Hawthorne humor? No, Hester just had to wear an "A". But maybe the terminology could apply if your motor runs the vent fans at the DEA facility where they burn the confiscated herbal products.

quote:

Originally posted by MotorDoc:
JasonR

'Stoning' is normally performed on slip ring motors and, more commonly, DC motors.

The question becomes: is it worth doing in the grand scheme of things? Would you do it only if the commutator was significantly grooved, or is it something you might do to a decent-looking commutator to 'freshen it up', or is it of dubious value?

Mike:

Only with some groving and if the brushes were sparking. Other times would be if the edges of the comm were feathered. Mostly as a stop-gap to pulling the motor and turning/undercutting the comm.

Another time to do it would be if the operating area was oily and the commutator surface under the brushes became 'thick' and carbon-black. The optimal surface has a buildup of carbon that is a caramel to chocolate color. This surface absorbs some moisture from the atmosphere which acts as a lubricant for the brush contact area.

The caution with stoning a comm, however, is to make sure that the mica in between each bar does not come to close to the surface. Otherwise the brushes may chip against it and sparking will become worse. The mica is undercut to a depth equal it's width, when done correctly, and is sometimes done by hand when the comm is stoned.

It doesn't hurt to check for raised bars and the roundness of the commutator before and after the process. There are several companies that make instruments for commutator profiles and you can also use a dial indicator.

Howard

quote:

Originally posted by MotorDoc:
Only with some groving and if the brushes were sparking.

That's just what I wanted to hear. There is an old directive in our PM system about stoning commutators, and quite frankly it's just not compatible with current safety requirements. I'm just coming up to the time of year (winter) where it would be done, so it's nice to know that I can have a valid reason for _not_ doing it if not truly required.

Thanks Bill,
You just made Monday morn more tolerable.