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In trying to understand brushed DC motors, this post has been very helpful but I still have some fundamental questions about the brush mechanism. For instance, what is the purpose of the spring? And since a brush looks mostly like a spring, how did the name "brush" even come about?

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up vote 22 down vote accepted

The purpose of the brushes is to make electrical contact with a rotating conductor (the commutator). Originally, these were bundles of wire that would be dragged across the commutator. At any time, at least a few strands of the wire would be making contact. These bundles, of course, look like "brushes".

Things have improved though, and now we use solid, low-friction, conductive materials for the brushes. It is common to use assorted types of graphite. These brushes must be held against the rotating commutator, and the material eventually wears away and must be replaced. The spring pushes the brush against the commutator, providing good electrical contact as the material slowly wears away.

Here's a representative picture dredged from Google. The dark material is the actual "brush", made of conductive graphite.

brushes

Please note that, unlike the brushes in your link, these don't have a wire connecting to the graphite. This is because the springs themselves are conductive! An additional wire can be used in higher-current applications.

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Well written answer and very helpful representative picture. I totally get it now. – Ben Ogorek Mar 8 at 15:37
    
@BenOgorek Thanks for your kind words. Glad to help! – bitsmack Mar 8 at 16:19
    
Good description the history of why they are called brushes – Tinkerer Mar 8 at 23:21
    
I've always had this mental image in my head of wire brushes pressured against the side of the commutator. Thank you for clearing things up! – Adam Haun Mar 9 at 2:37

The brushes have springs to keep a consistent force between the brushes and the commutator conductors as the brushes wear down. There has to be enough force to make good electrical contact, but not enough to cause excessive wear.

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F = x*k I believe that the force is not consistent! – dotancohen Mar 8 at 9:37
2  
@dotancohen The picture shows the springs at rest. In use they are heavily compressed. If you look at the amount of compression (negative x here) there will be when the conductive material is new vs when it is worn, the amount of compression will vary little between them. Hence the force is roughly constant. – abligh Mar 8 at 10:20

What is the purpose of the spring?

Brushed-DC motors & dynamos generate outward force against the brushes from the motion of the commutator segments. Additionally, the carbon brush you linked is designed as a "sacrificial element" that wears down with use.

The spring functions to keep the brush in contact with the commutator segments in opposition to the outward forces exerted by the segments (both due to surface irregularities [gaps], and imperfect concentricity of the bearings/rings/segments), while also adjusting for the wearing away of the carbon.

How did the name "brush" even come about?

As with many other seeming misnomers, it's a retained reference from a now obsolete implimentation. The first sentence in this paragraph gives the historical context.

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Ah I see. "Early machines used brushes made from strands of copper wire...However, these hard metal brushes tended to scratch and groove the smooth commutator segments, eventually requiring resurfacing of the commutator." – Ben Ogorek Mar 8 at 15:32

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