# DC Carbon Brush PM - Usable Length Measurement



## MDShunk (Jan 7, 2007)

When a manufacturer only specifies the minimum length of a carbon brush for a DC motor and does not have a pictorial or text explanation of how the measurement is taken---- Is there a standard as to whether you measure the brush length at the outermost points of the concave area (its longest dimension) or at the shallowest point of the concaved area (its shortest dimension)?


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## telsa (May 22, 2015)

Brushes are pushed ahead by some spring mechanism. 

This determines their contact force... which is critical.

So since all of the springs are factory engineered... the force has to be calculated from the point of contact. What's really being measured is how far the force springs have expanded// lost 'push.'

That has to be from the minimal length where the wear pattern scallops out the arc.

The edges are almost irrelevant to the passage of current. 

Once a certain point is crossed, the driving spring is too weak and contact with the commutator is so lousy that you start to get way too much arcing. 

For monster DC commutators ( trains ) there are special truing blocks that re-polish the brushes back to factory specs. Good grief are they huge. ( This final polishing is after a re-grinding on yet another custom machine.) This truing is performed at the same time that the commutators are lathed back to perfection, and with a trick tool cut back// trimmed so that each segment does not befoul the next. The whole process is quite an art, and expensive... hence the virtually universal shift to AC drive motors with solid state triggers.


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## MDShunk (Jan 7, 2007)

telsa said:


> That has to be from the minimal length where the wear pattern scallops out the arc.


That's what I would reasonably think, but I've been unsuccessful in finding any real documentation stating such.


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## 460 Delta (May 9, 2018)

If I’m looking at this correctly, it seems that you measure from the outside. This is from a book published in 1977 by Union Carbide.


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## frenchelectrican (Mar 15, 2007)

For the brushes I generally go by narrowest part of concave on brushes and check the spring tension too to make sure it still in factory specs.

Some DC motors will have X numbers of hours of brush life so when it get close to that numbers you have to be ready to change brushes and clean and repolish the commuter bars up again. 

Just be aware with frequent reversing rotation the brush life can shorten up some compared to monorotation ( only one rotation )


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## Bird dog (Oct 27, 2015)

FWIW...

https://www.google.com/search?q=dc+...nance&ie=utf-8&oe=utf-8&client=firefox-b-1-ab


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## John Valdes (May 17, 2007)

I think one manufacture had a line on the brush to indicate when to change. I think it was Helwig?

You cannot see how deep the brush leads/pig tails are inserted. So that would be my concern allowing the twisted copper leads to make contact with the comm.
This all that matters IMO. To not let the pig tail ever come into contact with the commutator.
This will ruin a comm in little time. And this cannot be fixed with a stone.
The comm has to be removed, turned down and undercut to remove the scars.
This requires a full tear down.
And once it broken down, you might as well do a PM on it.  Only a good motor shop should be trusted to work on DC motors.



You can bust open an old brush (same brush manufacturer) and measure how deep it goes. Then use that measurement to mark new brushed before installing.
I have never done this and it only came to mind typing this post.


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## MDShunk (Jan 7, 2007)

John Valdes said:


> ....This will ruin a comm in little time. And this cannot be fixed with a stone.
> The comm has to be removed, turned down and undercut to remove the scars....


Check out this new, modern twist, on comm turning. We have some DC motors with encoder feedback that are used for precise positioning. When the comms are turned down and re-undercut at the motor shop, the comm is effectively smaller in diameter than it was before. It faults out the positioning controllers during some movements (particularly long moves with additive error) because X number of pulses no longer equals X number of degrees-minutes-seconds of movement. The controller thnks the motor is slipping or something (I honestly doesn't know what it thinks). The only effective repair option that I've found when comm scoring is observed in these cases is a new armature. I wish I could just change the "program", but that's not possible/practical with some embedded robot controllers. A new armature isn't the worst option in the world anyhow.


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