This process is focused on creating carbon brushes, which are composed of a mixture of carbon and a binder, with or without the addition of metal powder, for application in electrical machines.
For more detailed insights, visit Carbon Brush Material.
Carbon brushes are produced economically by blending the raw materials — graphite, coke, metal powder, and a binder — in precise ratios, compressing the mix into its final form within a mold, and binding the components together through a thermal treatment. These steps offer convenience, especially when compression at room temperature is possible, along with embedding the electrical connection lead during the process.
Desirable characteristics for a binder in carbon brushes include:
1. Availability in a powder form with good flow properties.
2. Capability to soften above a certain melting point and possess minimal viscosity thereafter.
3. Effective wetting properties for carbon and metal powders when melted, leading to high mechanical stability and reduced binder content needed.
4. Readiness to cure through cross-linking upon heating in air.
5. Ability to promote the formation of a beneficial patina on the collector ring or slip ring of the machine, enhancing abrasion resistance.
These characteristics should be ideally met, with particular emphasis on the second, third, and fifth conditions.
Traditional binders such as pitch, phenolic resins, and lead often fail to meet all these requirements simultaneously. This shortfall necessitates a higher binder content for sufficient mechanical stability, adversely affecting electrical and thermal conductivity. Consequently, metal-free carbon brushes produced this way are of lower quality, lacking adequate mechanical stability. This necessitates turning to more expensive methods like the production of plate carbons, which involves coking, graphitizing, machining into the desired brush shape, and soldering the lead wire.
The current invention aims to provide a cost-effective method for creating high-quality carbon brushes, including metal-free variants, that meet or exceed the characteristics of conventional brushes made from plate carbon.
SUBJECT MATTER OF THE PRESENT INVENTION:
In the mixing stage, a binder classified as single-bonded aromatic polymers is utilized. These are polymers where aromatic ring groups are connected through a C-C bond or a heteroatom or group linking. This material class is thoroughly detailed in Hochtemperaturbestaendige Kunststoffe [Synthetic Materials Stable at High Temperatures] by E. Behr (Munich 1969) pp. 47ff.
This class of materials meets all the specified requirements: good flow as powders at room temperature, suitable high melting point, excellent carbon wetting due to aromatic content, and easy curing.
Preferred examples include polyarylsulfides with the general formula:
...Ar--S--Ar--S--Ar...
where Ar denotes an aromatic ring system, potentially substituted with alkyl residues. Polyphenylenesulfide, marketed as Ryton by Phillips Petroleum Co., is particularly effective. It has the following formula: ##SPC1##
The binders are combined with carbon in a ratio from 0.5 to 50 parts by weight per 100 parts by weight of carbon, ideally from 3 to 10 parts. The subsequent example elucidates this method.
EXAMPLE 1.
100 parts by weight of natural graphite are thoroughly mixed with 10 parts by weight of finely ground polyphenylenesulfide, and the mixture is compressed in a mold that shapes the brush. A pressure of 4 metric tons per cm2 is applied. The resulting blank is sintered between 300°C and 1000°C, ideally around 350°C, in a protective gas atmosphere such as nitrogen or city gas, composed mainly of hydrogen, methane, and carbon monoxide. After cooling, the carbon brush is ready. Alternatively, 42 parts of natural graphite and 55 parts of powdered copper can be used, resulting in a brush with the specified copper content. The manufacturing conditions remain unchanged.
The subsequent comparison table illustrates the properties of a carbon brush produced via this novel method against a traditional copper-bearing brush and a machined carbon plate brush.
TABLE I
__________________________________________________________________________
Comparison of Properties of a Conventional Copper
Item Containing Carbon Brush, a Metal-Free Carbon
Brush Made According to the Invention and a Metal-
Free Carbon Brush Machined Out of a Plate.
Type Cu-Containing
Nat. Graphite
Natural
Brush with Polypheny-
Graphite
lenesulfide
__________________________________________________________________________
Manufacturing
Pressed in
Pressed in
Cut from
Process Mold Mold Plates
__________________________________________________________________________
spec. elec.
resistance (Ω cm)
100-1000
1800-3000
3000-6000
Hardness (HRc 10/40)
50-70 60-70 60-80
Breakdown Load
(kp) (acc. to PVA
6742 and 10 mm vist.)
15-25 30-40 9-25
Life (hrs.) 1600 2300 2000
Voltage Drop (V)
0.2-0.4 0.2-0.3 1.0-2.0
__________________________________________________________________________
The comparisons demonstrate that carbon brushes produced using polyphenylenesulfide match or surpass copper-bearing brushes in performance and significantly excel in service life. Notably, despite the high specific electrical resistance of the carbon materials, the voltage drop is comparable to that of copper-bearing brushes, indicating excellent patina formation and stability. Compared to the more expensive plate-machined carbon brushes, this method offers similar quality at half the production cost.
This method enables the manufacture of high-quality carbon brushes at very competitive costs, rivaling or surpassing traditional brushes in all key aspects.
The term "heteroatom" refers to atoms other than carbon, typically sulfur, oxygen, nitrogen, phosphorus, and silicon, which act as intermediaries linking carbon atoms in organic compounds.
Brushes with aromatic-polymer binders are sintered at 300°C to 500°C, linking aromatic groups through single bonds over one to four atoms of no more than two of the specified elements.
June 30, 2023
•
Harry
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Superior carbon brushes are essential for all power tools. But how exactly are these integral components created?
In this article, we delve into the primary steps involved in manufacturing carbon brushes.
Need a new carbon brush? Check out our selection here.
Understanding Carbon Brushes and Their Function
Carbon brushes are vital wear components in power tools that transfer electrical current between the stationary and rotating parts of a motor or generator.
Due to carbon's excellent conductivity yet sufficient softness for consistent contact, it is the material of choice. However, they wear down overtime and need replacement when too worn.
Learn more about carbon brushes here.
Step 1: Forming the Carbon Block – Choosing the Right Material
The initial stage involves creating the carbon block from which the brushes will be derived. This block is composed of materials meticulously designed to withstand the specific voltage and wattage requirements of the intended power tools.
While carbon-graphite is commonly used, variations may include copper graphite, silver graphite, natural graphite, or electro graphite.
Step 2: Designing and Cutting the Carbon Brush
Each carbon brush must be tailored to fit a specific tool, necessitating precise dimensions that match the brush holder. This phase involves drafting the designs and cutting the material to the correct size.
Step 3: Smoothing the Brush Body
To ensure consistent contact, the brush body undergoes grinding until smooth.
Step 4: Verifying Brush Body Specifications
Final measurements ensure the brush still adheres to specified tolerances, generally between -0.1mm and 0mm.
Step 5: Embedding the Wire
A conductive wire is added at this stage by drilling a hole into the brush and embedding the wire to ensure the connection to the static component of the tool.
Step 6: Finalizing the Wire Connection
The final step involves welding the terminal to the wire’s end, ensuring proper connection within the power tool system. Some brushes may also include a spring design, which must be manufactured with precise tension, length, diameter, and thickness for optimal pressure against the commutator.
Avoid Using Improper Carbon Sources
Using carbon from old batteries is strongly discouraged due to its hardness, which can cause tool damage. Opt for new, high-quality brushes instead.
Signs of a Subpar Carbon Brush
Characteristics of poorly made brushes include:
- Uneven edges
- Decomposes under heat
- Deposits black residue in the holder
- Emits a burning rubber odor
For top-quality carbon brushes, explore our range online here.
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