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  • question about brushes

    This may be a stupid question...

    Can someone tell me why there are brushes in tools? What is their purpose and how can you tell if they need to be replaced?

  • #2
    Re: question about brushes

    Basically the brushes are what carry current to the motor coil (rotor windings). The most common DC electrical motors are made up of a field assembly which are the magnets sorrounding the windings. The copper winding rotates on a spindle which has a comutator towards the end. The comutator is connected directly to the ends of the coil and is set up alternatingly. This is what the brushes come into contact with. As the spindle spins each brush briefly shorts out two comutator segments generating torque.
    Last edited by Velosapien; 06-08-2007, 03:49 PM.

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    • #3
      Re: question about brushes

      Oh, you can usually tell if they are worn out once there is very little material left between the spring load system that pushes them in and the comutator. Some motors have auto stop brushes which have a means of stopping the brush from going further in once its sufficiently worn out.

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      • #4
        Re: question about brushes

        Velosapien did a great job of explaining them I'll just add a few more things. Brushes are a wear item since they rub against the commutator. The commutator wears too...sometimes it needs to be refinished or replaced.

        Here is a link to another explanation and picture.



        http://hyperphysics.phy-astr.gsu.edu...ic/comtat.html



        http://hyperphysics.phy-astr.gsu.edu...motdct.html#c1

        Another explanation...might be easier to understand.



        http://www.howstuffworks.com/motor1.htm

        Some motors have replaceable brushes. Some, like many high end tools, even have ports to get to the brushes more easily. In some, you have to take apart the motor casing to get to the brushes. In others, the brushes are installed in the motor casing and the motor casing is crimped shut. These types aren't designed to be serviced. If the brushes wear out you need to replace the entire motor. Having said that, the nonserviceable motors are generally less expensive than the ones with that are serviceable.

        Ridgid tools use motors with non-serviceable brushes.

        All this could go away if tool manufacturers would move to brushless motors. In brushless motors the windings are on the inside of the case and do not move, therefore no brushes are necessary. They cost a bit more for the switching controller, but are considerably more efficient (longer battery life and more power) and the only wear items are the motor bearings. Panasonic makes an impact driver that uses a brushless motor and, if I recall, Fein makes a drill with a brushless motor....other than that....they are pretty scarce.
        Last edited by Disaster; 06-10-2007, 08:18 AM.

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        • #5
          Re: question about brushes

          If the brushes are for a motor that's gets lots of use it's good to have a spare set of them. When the ones in the motor are worn to 1/2 the length of the new ones then it's wise to replace them. Also if there's less than 1/8" of carbon left, replace the brushes fast.

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          • #6
            Re: question about brushes

            To add my two cents here, it's also a good idea to blow out the air vents around the brushes of your motor every once in a while with a compressor air hose. Dust tends to accumulate in there, espcially tools like circular saws that get exposed to a lot of sawdust. It just keeps the motor clean and any dust buildup from causing problems.

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            • #7
              Re: question about brushes

              Skeeter has a good idea about keeping the air vent holes clean and also blowing out the brush holders when you check the brushes. I'll add that it's wise not to blast compressed air into a motor through the vent holes. You'll do far less harm if you put the crevice tool on the hose of a good vacuum cleaner and suck the dirt out. If you have the motor apart then compresed air works well. What you don't want to do is blast any dirt (especially like sand) into the motor. You can ruin a bearing or cut the insulation off the winding wires.

              It's a good idea to take your power tools to a good service shop once a year (more like 2-3 months if used all the time) for basic service and checking over. The grease in a gear case needs to be removed and replaced. As to just how often that depends on the tool and how much you use it.

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              • #8
                Re: question about brushes

                But unfortunately dust and small particles get sucked into the motor when you're using it anyway. It's almost unavoidable because the fan pulls air through the motor to keep it cool. Along with that air comes whatever debris is flying off the blade end of the tool. On tools like circular saws, and angle grinders, there can be quite a bit of stuff flying off.

                I know that DeWalt, at least in their cordless tooll manuals anyway, recommend holding the trigger on (motor running full blast) while blowing dry air at the vents. I wouldn't use 120PSI with the nozzle right up against the vents, but I can't see a regular air nozzle at a moderate pressure like 40PSI and held back from the tool a reasonable distance causing any problems.

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                • #9
                  Re: question about brushes

                  Definately a good idea to blow out dust from the brushess every once it a while. I had a Dewalt drill that smoked out badly from the brush area until I blew all the dust out from in there.

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