Hi.

I need to replace a carbon brush in a vacuum cleaner motor.
The size of the original is 7X11X32
I have carbon brush size:7x11x36
So the new brush is 4mm longer than the original.
Could this addition be a problem?
(i don't want to cut it and deal with reshaping it)
Thanks.

 

Sure, it "could be" a problem,
but how are we to know without seeing the Brush-Holder and Spring arrangement ?

How do You know what the original dimensions of the Brushes were ?

It's fairly easy to shorten Motor-Brushes because they are usually made of reasonably soft material.

Why do You not want to shorten the Brushes if necessary ?
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If it fits into the holder, and the spring is not bottomed when the brushes are pressed against the commutator then it will likely be just fine.

 

If it fits into the holder, and the spring is not bottomed when the brushes are pressed against the commutator then it will likely be just fine.

When you says"the spring is not bottomed",do you mean to check that the spring is not pressed(contracts)to its minimum length?
BTW,I have also 6x11x32 carbon brush(1mm thickness difference),
can it be a better option than the longer one(7x11x36)?
(just for easy comparison,as afore said,the original is 7x11x32)

 

....not pressed(contracts)to its minimum length.....

That's right - there should be some travel left to cater for wobbles in the commutator. Maybe a few mm or more.

Personally I would not use the 6x11 as it will play in the holder and wear. The longer one is best if not too long, but as mentioned above in #2, shortening the brushes is not a big deal if necessary.

 

That's right - there should be some travel left to cater for wobbles in the commutator. Maybe a few mm or more.

Personally I would not use the 6x11 as it will play in the holder and wear. The longer one is best if not too long, but as mentioned above in #2, shortening the brushes is not a big deal if necessary.

The issue with shortening the brush is:that the brush was cut perfect by the factory and on the top surface it has grooves which help the brush to seat on the commutator.So i don't want to ruin that.

 

If break-in of the Brushes takes a little bit longer because of no "grooves",
it won't hurt anything,
and You would probably never notice the slight change in performance as the break-in occurs.
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If break-in of the Brushes takes a little bit longer because of no "grooves",
it won't hurt anything,
and You would probably never notice the slight change in performance as the break-in occurs.
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So,i don't have to reshape(bed)the carbon brush again,right?
I can just cut it straight to the length size,install it and it will bed in the rotor with time?

 

The Brush can be simply cut "square", no shaping is required unless the Motor will be required to
run under maximum-Load from the first instant that the Motor is started.
~90% of the "shaping" of the Brushes to conform perfectly to the Commutator
will probably happen in the first ~20-minutes of low to medium-Load Motor operation.

You still haven't stated how You know how long "new" Brushes "should be".
If the new Brushes were purchased as an exact part-number replacement Brush,
just forget about measuring, and throw them in, and run the Motor as normal.

If You got the Brush Dimensions from some other source, please specify.

If the springs on the Brushes are not binding in any way, with the "un-cut" New-Brushes,
then there is probably no need to shorten them to some specific dimension.

When the new Brushes are installed,
it should be possible to use a very small Screw-Driver to lift the Brushes off of the Commutator,
after which, they should freely snap-back in to contact with the Commutator.

If the Brushes can not be lifted off of the Commutator with the small Screw-Driver,
then they are too long, and they are binding-up,
and must be sanded or ground-down until they can
float freely against the Commutator with Spring-Pressure only.

The Brushes must be free to move,
with only Spring-Pressure applying a light-force against the Commutator.
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.
.

 

The Brush can be simply cut "square", no shaping is required unless the Motor will be required to
run under maximum-Load from the first instant that the Motor is started.
~90% of the "shaping" of the Brushes to conform perfectly to the Commutator
will probably happen in the first ~20-minutes of low to medium-Load Motor operation.

You still haven't stated how You know how long "new" Brushes "should be".
If the new Brushes were purchased as an exact part-number replacement Brush,
just forget about measuring, and throw them in, and run the Motor as normal.

If You got the Brush Dimensions from some other source, please specify.

If the springs on the Brushes are not binding in any way, with the "un-cut" New-Brushes,
then there is probably no need to shorten them to some specific dimension.

When the new Brushes are installed,
it should be possible to use a very small Screw-Driver to lift the Brushes off of the Commutator,
after which, they should freely snap-back in to contact with the Commutator.

If the Brushes can not be lifted off of the Commutator with the small Screw-Driver,
then they are too long, and they are binding-up,
and must be sanded or ground-down until they can
float freely against the Commutator with Spring-Pressure only.

The Brushes must be free to move,
with only Spring-Pressure applying a light-force against the Commutator.
.
.
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I assume that i was too worry about the cutting issue.
Thanks for clarifying and explaining the process how to
check the carbon brush.
Thanks again for your help and also to boostbuck.

 

One thing to keep in mind, not all brushes are created equal, IOW, the carbon-copper ratio depends on the type of use & motor etc, if a vacuum cleaner, these are made to withstand the very high RPM seen on a Universal motor.