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Re: damaging bearings with current

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  • Rick Sparber
    I have heard a lot of compelling arguments on why 1.5V at 1 amp through a bearing can start and sustain an arc that could pit the surface. What if I used 20
    Message 1 of 79 , Jun 4, 2012
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      I have heard a lot of compelling arguments on why 1.5V at 1 amp through a
      bearing can start and sustain an arc that could pit the surface. What if I
      used 20 milli volts at 1 amp?



      Rick



      [Non-text portions of this message have been removed]
    • Rick Sparber
      Richard, I did read the reference and it had a lot of good techniques. The article that detailed my use of a 4 wire connection was hidden in the electronics
      Message 79 of 79 , Jun 26, 2012
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        Richard,

        I did read the reference and it had a lot of good techniques.

        The article that detailed my use of a 4 wire connection was "hidden" in the electronics section because I did not think is was ready for machinists to use. You can see it at

        http://rick.sparber.org/electronics/ueef.pdf

        starting at page 5. I also used a trick not found in the Keithley book that involves measuring the voltage drop across one of the high current probes.

        As previously stated, Lord Kelvin invented the 4 wire connection and I did give me credit.

        Not sure what conclusions I should draw from the Keithley book except that they made a lot of good test equipment.

        Rick


        -----Original Message-----
        From: atlas_craftsman@yahoogroups.com [mailto:atlas_craftsman@yahoogroups.com] On Behalf Of Richard Schaal
        Sent: Tuesday, June 26, 2012 1:56 PM
        To: atlas_craftsman@yahoogroups.com
        Subject: Re: [atlas_craftsman] damaging bearings with current

        Hi Rick,


        This is the article reference that I submitted to the mail group, and the articles you were posting at the time. It's not at all clear that you were using a four-wire connection in the "An Ultra Low Tech, Low Cost Electronic Edge Finder **"


        It was this document that I was referencing when pointing out a need to acknowledge Keithley's contribution as it also provides the OP amp circuitry for a low ohm detector.

        I don't claim to know the derivation of the four wire connection at the time - I just located the article that puts four wire connection and op amps to use, and sent it your way.


        You all can draw your own conclusions.

        - Richard



        ________________________________
        From: Richard Schaal <rschaal_95135@...>
        To: "atlas_craftsman@yahoogroups.com" <atlas_craftsman@yahoogroups.com>
        Sent: Monday, June 4, 2012 8:05 PM
        Subject: Re: [atlas_craftsman] damaging bearings with current


        Hi Rick,

        I went to your website, and looked at two documents that seemed to be related to the current topic, but neither of them had schematics that would get you a real big current through the probes.

        The "A Precision Electronic Cutter Touchdown Detector" document wouldn't do it, and the "An Ultra Low Tech, Low Cost Electronic Edge Finder **" document won't either due to the insulated cutter approach.

        I admit that I've not followed this thread closely, but did you put out a new link to the draft document for this particular approach? Seems to me that perhaps you could use the op amps differently to amplify the reading without drastically increasing the test current even in this low resistance case.

        If I get you correctly at this point, are you saying that the expected resistance measurement at the moment of touchdown of the cutter on the work piece will be flat zero, and that the resistance between the cutter and the work piece, taking the longer path through the tool holder, carriage, ways and spindle will be more than that. -- maybe 0.02 ohms? and this would be in a case where the cutter isn't insulated?

        In any case, perhaps you could make use of one or more of the circuits shown in

        http://www.keithley.com/knowledgecenter/knowledgecenter_pdf/LowLevMsHandbk_1.pdf

        to reduce your test current so you can tell _ZERO_ from some resistance with confidence that you aren't arc welding your bearings.


        Regards,

        Richard




        ________________________________
        From: Rick Sparber <rgsparber@...>
        To: atlas_craftsman@yahoogroups.com
        Sent: Monday, June 4, 2012 7:05 PM
        Subject: RE: [atlas_craftsman] damaging bearings with current



        Bruce,

        The question I have now is will a 20 mV potential open circuit and 1 amp short circuit cause a spark. I have read two white papers that get close to saying "no" but I need more info. I have contacted one author and plan to contact the other.

        See my other email on what I'm trying to do and why bypass the bearings doesn't help.

        Thanks,

        Rick

        -----Original Message-----
        From: atlas_craftsman@yahoogroups.com [mailto:atlas_craftsman@yahoogroups.com] On Behalf Of Bruce .
        Sent: Monday, June 04, 2012 4:25 AM
        To: atlas_craftsman@yahoogroups.com
        Subject: Re: [atlas_craftsman] damaging bearings with current

        1) Yes, 1.5V can give a spark. Try it with a dry cell and a wire.

        2) Conceivably, a 1.5V spark could cause damage. A 12V spark certainly can. It's called welding. Or EDM.

        3) Sparks will only happen if power is interrupted AND conditions are right for sparking. But it seems a good bet they will occur. If electrical contact is continuous, there will be no sparking. But how do you guarantee that through bearings?

        4) The damage a spark will do is current-related. The very low resistance may (depending upon total resistance) correspond to high current. If your circuit will tolerate it, you might use a resistor (maybe 100k ohms) to limit current.

        5) The damage sparks will do will not be as dramatic as the train Jon mentions. Rather, I'd expect bearings to become pitted OR speckled with "weld splatter". In the long run, this could still cause serious damage.

        6) Motors transmit electric power to the rotor all the time using carbon brushes and copper rings. In this case, the copper is not essential, but you want to make contact at a non-essential part or an added sacrificial part, and a pressed-on copper ring might be just the
        ticket. You can get the carbon rod from the inside of a standard
        (dead) zinc-acid battery.




        [Non-text portions of this message have been removed]



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