Carbon brushes are the small spring-loaded blocks that carry current into the spinning armature of a power tool. They wear down with every minute of running time, and on a tool that earns its keep they are normally the first thing to fail. The good news is that worn brushes are cheap to replace and that the symptoms are unmistakable once you know what to look for.
The most reliable sign is a sudden drop in power that doesn't recover. A drill that used to bore through 4mm steel in a steady run now stalls and labours; a grinder that took the corners off a weld now scrapes and idles. If the tool still runs but feels weak under load, brushes are the first place to look. A second sign is sparking visible through the motor vents — a small amount of blue spark is normal, but bright orange or yellow sparking, or arcs that throw sparks out of the housing, point to brushes that have worn past the safe minimum length. The third sign is a faint smell of burnt electrics during a run that doesn't otherwise feel hot. If your tool starts intermittently — a couple of seconds of running before cutting out — the brush spring may no longer be pushing a worn brush hard enough against the commutator.
The rule of thumb is that a brush needs replacing once it has worn down to roughly 6mm or one-third of its original length, whichever comes first. Replacement is a job most people can do on the bench in fifteen minutes with a screwdriver. Two access caps on the side of the motor housing usually unscrew to release the old brushes and their springs; the new pair drops in the same way, with the curved face seated against the commutator.
Fit matters more than brand. A brush that looks identical to the original can still be a millimetre out on width or thickness, or have a different terminal style at the lead end. The fastest route to the correct fitment is to start from the tool — see our full carbon brushes range, or jump straight to Makita carbon brushes, DeWalt carbon brushes, Bosch carbon brushes or Hitachi and HiKOKI carbon brushes. If you have the original brush to hand, the dimensions stamped on the side are the final word; if you don't, the tool's model number from the rating plate gets you to the same place.
Brushes are the cheapest possible insurance against a dead tool on a Monday morning. Tradespeople who keep a spare pair in the case rarely lose a day to one. For replacement guidance across the rest of the wear parts, see our tool parts hub.