The copper pigtails on brushes are a very good feature for almost any
application.
They ensure that the current has a low resistance path, instead of relying
on the brush spring and/or the metal guide (usually brass) in the brush
holder.

There is a special compound that's used to bond the pigtail to the brush,
which has been discussed in rec.crafts.metalworking before, but I don't
recall it's name.. but it's not commonly available anyway (and requires a
special process, IIRC).

Brushes with pigtails generally have a metal disk or other type of terminal
that needs to be securely constrained by the brush cap or connected in a
pressure-type terminal. Some of the disk-type terminals have small tabs to
engage slots at the sides of the brush guides, which reduce the risk of the
pigtail twisting tightly as the brush cap is installed.

I've encountered heat damaged brush springs, most often due to improper
brush installation, and have ended up replacing them with other springs for
other motors of similar size, with approximately the same sized brushes.
This has worked well enough, while realizing that the replacements may not
have been optimal as far as extended long term reliability of the motor.

I don't recall ever seeing a method to calculate a proper brush spring, a
far as contact pressure.
There's likely to be a method of selecting the best spring pressure, maybe
based upon the surface area of the brush.

--
Cheers,
WB
..............


"N_Cook" wrote in message
...

It looks as though another feature of low V / high A motors is the brushes
have copper wire tails melded into the graphite , wheras mains ones can
often get away with end of conducting phosphor-bronze spring just resting
against end of brush and no copper braid.