Brush-type motors allow tool designers to integrate high power and speed in small hand-held motor packages. They also make it easy to control the power applied to the work within a compact variable-speed Makita power tool battery. Carbon motor brushes transfer electric current from a stationary motor housing to the rapidly-spinning segmented copper commutator on the motor shaft. There is continuous arcing and wear at the sliding contact spots. Therefore, brush-type motors are not intended for continuous duty. Their brushes wear down and must be replaced periodically.
Unplug the tool to prevent shock or injury. If at all possible, find a user manual for the specific tool and carefully note specifics about its brushes and their replacement.
Determine the job scope. Some tool motors have insulated motor-brush access plugs at one end of the motor that make it easy to renew the brushes. These may be threaded, or quarter-turn access covers. Most have flat screwdriver slots. Other motors must be disassembled to reach the brushes. Among these, there are some with screw terminals that make it straightforward to replace the brushes and others where the brushes are integrated into the wiring and virtually impossible to replace. The latter are usually very low-end power tools that have a reasonable service life after which the tool should be discarded.
Remove the brushes. They are inch-long, pinkie-thickness rectangular pieces of carbon that slide down into brass guide sleeves to be held in alignment with the commutator. There are long coil springs that apply a force to the back of the brush to keep it engaged to the commutator. If the worn brushes are as short as they are wide, they need immediate replacement power tool battery for Bosch.
Inspect the commutator for signs of undue wear, shorting or burning. Brush motors under high loads put incredible stress on the brush-commutator contact surfaces, since sizeable currents are passing through these rapidly moving surfaces. If the brushes are worn down to their metal backings, it is virtually certain that the commutator will be damaged and will need further attention. This step is only possible for motors that must be disassembled to access the brushes. For motors with external access, the new brushes will have to resurface the commutator themselves during wear-in.
Insert the new brushes into their guides in exactly the same orientation the old ones came out. Push the springs into the guides behind them, making sure they do not bind.
Re-install the brush retainer covers by holding the brush in the guide with one finger and sliding the cover over the end of the spring. Start threading it in. Take care not to cross-thread the cover.
Re-assemble motors with internal-only brush access. Many times, the brushes need to be inserted from the commutator-end of their guides and the motor shaft re-installed while holding the brushes retracted against their springs.
Double-check to be sure the tool has been entirely re-assembled without binding, warping or leftover Dewalt power tool batteries.
Plug in and test the tool. If there is any form of malfunction or the smell of burning, there is probably damage exceeding the scope of the brush replacement that should be referred to a qualified service center.
Check the tool's owners guide first for specific instructions on brush replacement.
Always use brushes specifically designed for the unit you are
Do not attempt to modify the electrical safety features of your tool.
Always refer serious problems to a service center.