Metal Fiber Brush (MFB) FAQs

Metal Fiber Brush (MFB) FAQs

Q. What are Metal Fiber Brushes – MFBs?

A. MFBs, which gain their name from the fact that they are constructed of thousands of hair-fine silver alloy fibers running on their tips, offer significant improvement over carbon brushes in both data and current transfer, offering extended brush and slip ring service life, high current capacity, and significant decreases in service acoustic and electrical noise. Our silver alloy fibers offer significantly better service life and higher speeds over gold wire technology at a considerable price advantage.

Q. What are some of the advantages of MFBs?

A. The advantages of MFBs include:

  • Higher current density
    • Current densities of up to 250 Amps/sq-inch are easily obtained.
    • Current densities 1000 Amps/sq-inch achievable under certain conditions.
    • Minimal constriction resistance and heating compared to carbon brushes since each fiber tip carries current.
  • Very low electrical noise across the brush/slip ring interface.
  • Greater service life
    • In EVERY field test using side by side comparisons against carbon brushes, MFBs have produced at least double the carbon brush service life.
    • Non-dimensional wear rates of 3 × 10-11 at sliding speeds of 20 meters/second in an open atmosphere are typical. What does this mean?
    • A 1” long MFB running on a 12” diameter slip ring operating continuously at 1200 RPM will last approximately 24 months.
  • Reduced wear debris
    • The internal space of an MFB is about 5/6 air and 1/6 metal. As a result, the MFB debris from a given volume of brush is 1/6 the volume for the same size carbon brush. This translates to 5/6 less debris than carbon brushes to clean during maintenance.
    • MFB Wear debris is generally larger and heavier than carbon brush wear debris. The density of graphite is about 2.2 g/cm3 while the density of silver is 10.5 g/cm3. MFB wear debris will accumulate in the bottom of a machine where it is easily vacuumed. Filter media is more efficient in collecting MFB debris.
    • MFB wear debris is highly resistive. Wear debris shapes are very irregular. When the particles touch, they only make contact at a very small number of points. There is only a minimal amount of pressure pressing the particles together, so the area of these contact spots is also very small.
    • The path that current must follow through a mass of wear debris is therefore a path through many small, relatively high resistance contact spots, which gives a net high resistance. This theory has been repeatedly proven in field conditions where MFB machines have higher grounds than identical carbon brush machines.
  • Environmental tolerance
    • MFBs do not require a carbon film to operate.
    • MFBs do not require a minimum current density to maintain a film. They operate across a wide spectrum of current densities equally well.
    • Various fiber additives permit satisfactory operation at humidity levels as low as 10% RH.
    • Since there is no carbon film, MFBs operate in open atmosphere and in the presence of atmosphere contaminates that are detrimental to carbon brush films.
  • Acoustic Advantage
    • MFBs operate at about 1/5 the spring force of carbon brushes.
    • MFBs are acoustically quieter than carbon brushes.
  • Friction Advantage
    • MFBs operate at about 1/5 the spring force of carbon brushes.
    • MFBs provide less friction and drag than carbon brushes.

Q. What shapes and sizes do MFB come in?

A. Metal Fiber Brushes come in a variety of sizes and shapes as can be seen in the following photos.

Q. Can Metal Fiber Brushes be used for transmitting data?

A. Yes.

  • Metal Fiber Brushes offer an extremely high data transfer rate across the sliding surface. This has been seen as being in excess of 50 MBaud where the data transfer rate is not limited by the cabling and brush holder capacitance.
  • Brushes sliding on a metal slip ring generate less than 1 millivolt of electrical noise running on either a gold plated or bronze rotor when operating at data rates of 50 MBaud (10 nsec pulse width).

Q. For what types of applications are slip rings suited?

A. MFBs are well-suited for many applications, including:

  • Slip ring power and data applications from:
    • Bi-directional and uni-directional
    • Small and slow: 3” dia. 30 RPM
    • Large and fast: 12” dia. 3600 RPM w/spiral groove
    • Homopolar motor applications
  • Commutation of DC motors ranging from 1 HP to 1,000 HP