View Full Version : Angle Grinder RPMs Fluctuate - Brushes?

09-19-2012, 07:54 PM
Hello, all,

I have an angle grinder (HFT) whose RPMs go up and down, almost as if someone were messing with a variable speed control...or the AC power was going from 30 to 90 to 60 Hz.

Is this a sign that the brushes need to be replaced? I've never seen anything like it before but now it's also refusing to start when I turn on the switch, which leads me to suspect brushes. The unit is close to 10 yrs old and has a lot of hours on the clock.

I tried to inspect the brushes, but the darn caps that cover the brush holders don't want to come out!

Thanks for any clues.

09-19-2012, 10:11 PM
Usually a sign with Universal motors, the brushes take a beating, especially that age, hopefully they have not gone down to the springs otherwise the com. may be shot!.

09-19-2012, 11:17 PM
Or... if it's a Bosch the electronic module may be bad. I have one that behaves like bad brushes. $70 plus shipping to buy a new module so... it's just spare parts for its brother.

09-19-2012, 11:25 PM
Most likely the brushes but the switches and cords go out too. When they start to stutter, it's an indication the brushes need replaced. I have four of their angle grinders and have to replace one about every other year. Not bad since they all take a beating with heavy use. The thin wire used in the cords will sometimes break where it enters the tool. It usually happens when I don't have a spare cord so I cut the wire and shorten it about 6-8 inches and it's back to work. I've always bought the orange ones. Never once had the gears go out but the plastic handles are a little flimsy. For the price, it's a pretty good tool. I have one that's got to be over five years old with nothing done but change the brushes.

09-20-2012, 08:06 AM
Thanks, guys. I guess after 10 years, considering the price I paid, it doesn't owe me anything anymore.

09-20-2012, 01:14 PM
My dewalt does it but not as drastic. Its noticable though. I already inspected brushes and were fine. I do lube my gears in the head and thought maybe it was to thick of lube csusing it.

09-20-2012, 08:57 PM
There's the possibility that the armature has some end float. When the comm is worn the end float interferes with the seating of the brushes, and this can cause intermittent speed variations. The 'cure' for an older machine can be as simple as cleaning the comm, or it could be as drastic as tossing the machine. In between is any number of things to do with the brushes, broken wiring, and bearings. It would be unusual to be able to replace the bearings easily, so at this point it's better to forget it.