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ECS
Brushes: Selection Guidelines  PDF
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Physical Properties
Resistivity
Resistance is the opposition offered by a material to the flow
of electric current.
Resistivity usually has less influence on brush performance
than contact drop, coefficient of friction or thermal conductivity.
Hardness
Hardness of brush material is
measured by the Shore Scleroscope method, observing the height of rebound
of a special weight when released from a standard distance above the
test specimen. Values
are expressed from 0 to 140.
Quite often, hardness is erroneously blamed
for excessive commutator wear, when in fact there is little relationship
between the two.
Transverse Strength
This is the breaking
strength of a material when tested as a simple beam under load. It
is measured by supporting a specimen on two knife-edges, then applying
stress at the top center until the specimen breaks.
Apparent Density
Density of brush material
is determined by dividing a specimen’s
weight in grams by its volume in cubic centimeter.
Contact Properties
Surfaces that have rubbed together for some time
appear to be in intimate contact with each other. In reality,
the net area of true contact is very small, usually about 10-4mm2.
The very small contact is made up of a number of points that can sustain
the load pressure without further distortion or crushing.
Contact Drop
Contact Drop is the voltage
drop between the brush and commutator or slip ring. It is influenced
by many factors including brush grade, current density, spring pressure,
friction, vibration and ambient conditions.
Contact drop of a brush grade
under commutating condition can differ appreciably from that of the same
grade on a slip ring.
Since repeatability and accuracy of contact drop
depend upon many variables, the following total value groupings are used:
| Very High |
Greater than 3.6 volts |
| High |
2.4 to 3.6 volts |
| Medium |
1.4 to 2.3 volts |
| Low |
0.8 to 1.3 volts |
| Very Low |
Less than 0.8 volts |
Contact drop
figures are totals of both polarities
Coefficient of Friction
Brush friction
is the tangential force at the interface of the brush and the pressure
perpendicular to the sliding surface. The
values used are:
| High |
Greater than 0.20 |
| Medium |
0.15 to 0.20 |
| Low |
0.10 to 0.14 |
| Very Low |
Less than 0.10 |
The values listed in the tables are typical of good practice on average
machines.
Normal Maximum Peripheral Speed
Allowable
maximum peripheral speed depends upon the characteristics of the brush
material, spring pressure, current density, collector condition, vibration,
and atmospheric conditions. Therefore,
the maximum speed values listed in the tables are only approximations.
Spring Pressure
The spring force needed for satisfactory brush operation will
vary with each application.
Insufficient force results in poor contact,
sparking, overheating and rapid wear due to electrical erosion. Excessive
force causes high friction and increased mechanical wear.
In the spring
pressure curve AB indicates a rapid increase in rate of wear due to burning
caused by lack of contact between brush and collector.
CD shows wear increasing
from abrasive frictional forces. BC indicates
a reduction in electrical wear nearly balanced by an increase in mechanical
wear.
 Click
diagram for a larger image.
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Guidelines
> Brushes Grade Descriptions
> Brushes Tables of
Properties
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