Carbon Brush - help

[barcaloco]

Hello i'm trying to repair a drill and just wondered if anyone knew roughly what length a carbon brush (originally 25mm long) should be replaced??

The tool gets a small jolt when the trigger is pressed and it's like the motor does a half rotation and stops, the brushes are currently 10-11mm long and there's no obvious signs of damage or burning.
I've done a continuity test on the power/switch and it's ok from the cable through the switch so it's getting power.

any advice would be appreciated.

 

[MarkieSparkie]

The manufacturers recommended end limit is usually 1/3rd of the new brush length, but it's often wise to replace them before they reach the limit. Check the manufacturers instructions for precise wear limits. Some manufacturers, Metabo for example, build their power tools with automatic brush monitors that illuminate an LED when the brushes require replacing.

 

[barcaloco]

a 1/3 of original length i'll remember that, i know what you mean about the light i have a makita that lights up but this is an older hilti, i don't think it has any kind of LED on it, it's that old.

i'll try replacing the brushes then and see if it works, thanks for the help

 

[Des 56]

When you say it does half a turn then stops
Does it then continue to turn on the next operation without having to rotate the armature a little or does the process always repeat your description

 

[barcaloco]

When you press and hold the trigger you can hear the motor rotate a little then there's nothing at all, then when the trigger is released then pressed again it does the same, rotates a little, and stops.

 

[barcaloco]

When you press and hold the trigger you can hear the motor rotate a little then there's nothing at all, then when the trigger is released then pressed again it does the same, rotates a little, and stops.

sorry that was inaccurate, i blew the drill out n cleaned connections etc and tried again, the motor starts off fast for a second then stops and its buzzing, then when the trigger is released and pressed again it starts fast again for a second then turns VERY slowly and continues to turn slowly with the buzzing noise.

it must be the brushes......

 

[barcaloco]

While i was cleaning the tool i noticed there's a small board and it looks like there's 2 small 5v capacitors in there, could these be the real problem? if they have blown will it stop the motor from running?

 

[MarkieSparkie]

Hi mate, Change the brushes first. You have probably worn the existing set beyond their wear limit and now there is insufficient spring tension to maintain a low resistance contact with the commutator, causing the behavior you are witnessing. The two small capacitors you mention are most likely radio frequency interference (RFI) suppressors, they are designed with a special construction to fail open circuit for safety. If this happens the result is increased sparking at the commutator, generating RFI likely to effect local radio and TV reception. Generally RFI capacitors are hardy beasts that have self-healing properties and rarely fail.