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Grain Size Variation Causing Early Failure in Brass Motor Brushes

A comparison was made between copper motor brushes with the same product number but from different manufacturers. The motor brushes from one manufacturer were causing excessive wear on the slip ring elements.

Procedure:

  • Photo documentation
  • Hardness and chemical measurements
  • Microstructural analysis: grain size, shape, distribution
  • Photo documentation of microstructure

Conclusion:

It was determined that graphite size and distribution were key in affecting the wear performance of the brushes. The graphite phase was twice as dense in one sample as compared to the other.  There were also significant differences in the size and shape of the graphite particles.  The graphite boundary along the metallic interfaces of the brushes allowed for increased metal-to-metal interface and, consequently, accelerated wear.

 
small graphite
Sample 1: Fine graphite particles
large graphite
Sample 2: Coarse graphite particles

Figure 1 – Chemical Composition

Sample

Cu

Sn

C

Pb

Zn

Fe

Ni

S

P

Si

#1

81.6

9.9

8.0

<0.01

0.14

0.06

<0.01

0.031

0.008

0.16

#2

79.3

10.25

10.2

<0.01

0.14

0.04

<0.01

0.032

<0.001

0.09


 
Figure 2 – Graphite Count

Counts per Inch

Longitudinal

Transverse

Sample #1

814.9

454.4

Sample #2

206.9

221.4