One of mine--the bottom one, is worn much more than the other, and is almost out of reliable contact. I inherited this machine from a friend late Aug 2017. It's 10-12yo, and was parked 7 yrs. I'm wondering how to assess how much work it's done, given the working life is only 5yrs?
working life--used 10%AHr/day or 75%AHr once in a week @ 6kph. Of course I do much more than that.

AHr usually 34,mine 40.

 

Depends on brush composition, of which it is a trade off between copper and carbon, the high copper content is required for high current brushes, the down side is they wear the comm much faster than high carbon content.
Max.