Carbon brush

Sliding contacts produce an electrical connection to the moving parts. Examples are the carbon brushes and commutators of motors, pantographs ( contact strip / catenary ), slip ring systems or rotary switches and potentiometers.

Sliding or sliding contacts are made as sliding bearings always consists of two materials of different hardness. At high powers and voltages usually the pairing copper / graphite is used, while precious metals are used in very small power and signal transmissions. Rotary switches for signaling purposes to use materials such as brass, copper or silver, palladium and gold; these does not wear, but the good contact and low contact resistance in the foreground.

The macroscopic and microscopic operations on a sliding contact with and without current flow are fundamentally different. The electro-migration plays a role, in particular with direct current, and metal / metal combinations.

History

1833 William Ritchie built a machine that were used in the Copper springs as Stromübertragungselemet. 1884 appeared a patent which was the talk of brushes. This form of sliding contact was also the godfather of still in use today throughout the world name of this component - carbon brush.

However, it soon realized that copper than Gleitkontaktwerkstoff a bad life. Through strong sparking a high wear both the brushes and the commutator is triggered.

In 1885, Professor George Forbes published a patent, it is proposed in the carbon current transmission in sliding contacts.

But it took until the beginning of the 20th century, to this idea has been implemented commercially. At the latest with the development of electric railways ended the period of the copper brushes. The economic development of key industries such as steel or mining, or from global companies such as Siemens, AEG, BBC would not have been possible without electric drives with carbon brushes as a sliding contact.

Carbon brushes are mainly used in electric motors (so-called universal motors, including washing machines, vacuum cleaners, electric hand-held devices ), but also to portal cranes, ferris wheels, carousels etc.. When pantographs of electric railways they are called abrasive strip for potentiometers are they called grinders. Small dc electric motors in CD players, cassette tape recorders or similar. on the other hand often have precious metal brushes.

Advantages and disadvantages of graphite

Graphite has over metallic friction materials the following advantages:

  • With the temperature increasing electrical conductivity
  • Low friction due to the layered arrangement of the crystal planes of the graphite
  • No melting, so no tendency to weld with the counter material
  • Low density
  • Environmentally friendly
  • Good chemical resistance

A disadvantage is the semiconducting property of graphite, this causes even at low currents a certain voltage drop, however, is not significantly increases at greater power. This prevents the use of graphite sliding contacts at very small voltages or for measurement purposes. For potentiometers for measuring angles, in acceleration or pressure sensors as well as for small DC motors with low operating voltage is therefore used precious metal sliding contacts.

Carbon brush motors form a so-called patina on the copper collector or grinding partner. This dark brown layer is not a defect and no pollution, but the color is characteristic of a good condition of a collector or slip ring. If it is, however, partly black, this indicates excessive sparking or partially contrasting brushes.

Graphite sliding contacts: Production and requirements

The production of carbon brushes or graphite Gleitkontaktstücken begins with the selection and preparation of raw materials: in addition to natural graphite, artificial graphite, cokes, carbon blacks and pitches are used. After mixing the components and shaping processes in a fire is carried out at up to 3000 ° C, which plays an important role in adjusting the properties of the final products. Treatments such as impregnation with resins or the addition of metal components allow for further modification. Abrasive machining steps provide the components then the final contour. See also carbon brush.

Temperature, flow, humidity, counter material, contact pressure and environmental conditions are the main parameters which have influence on the running behavior of electrical sliding contacts.

Areas of electrical sliding contacts

The main areas of application are:

  • Stationary DC drives from a few watts to several megawatts
  • Railway traction motors and long distance transport,
  • Slip-ring motors and generators, such as power plants or wind turbines,
  • Low-voltage motors, including actuators on machinery and motor vehicles ( eg fan, windshield wipers)
  • Small and micro motors in household appliances and audio and video recording and playback devices
  • Pantographs for electric rail vehicles
  • Sliding contacts for moving electricity consumers ( gantry cranes, carousels, etc.)
  • Potentiometers, sliders, angle sensors, acceleration sensors, incremental encoders
Stockert Radio Telescope Plain bearing Electromigration Noble metal Pitch (resin)
268552
de