Carbon Brushes: Types, Grades and How They Work

Quick Answer

A carbon brush is the sliding electrical contact that carries current between the stationary circuit of an electrical machine and its rotating commutator or slip rings. It works because graphite conducts electricity, lubricates itself, does not weld to copper, and builds a thin film on the commutator that keeps friction and wear low and helps the machine commutate. Brushes are made in five material families: carbon-graphite (hard, for low speeds and loads), natural graphite (soft, for high speeds and slip rings), electrographite (graphitised at very high temperature — the standard for industrial DC machines, exciters and many slip rings), resin-bonded (high resistivity, for machines that commutate with difficulty) and metal-graphite (copper or silver added, for low voltage, high current and grounding). A typical industrial electrographite grade such as EG4 has a nominal current density of 12 A/cm², a maximum peripheral speed of 60 m/s, a contact drop of 0.9–1.9 V per pair, a specific resistance of 6–16 µΩ·m, a friction coefficient of no more than 0.25 and a nominal spring pressure of 20 kPa. Internationally, brush dimensions, terms and test methods are covered by IEC 60136, IEC 60276, IEC 60413 and IEC 60773, and in North America by NEMA CB 1. In Russia and the CIS, GOST 12232-89 sets brush dimensions and two shunt test methods; the properties of each grade are set in the grade's own specification.

A carbon brush is one of the cheapest parts of a DC motor. It is also the one part designed to be worn away in normal service, and the part that decides whether the commutator lasts five years or twenty-five. Pick the right grade and look after it, and a brush machine runs for decades with routine replacement. Pick the wrong one, and the same machine sparks, grooves and needs its commutator machined every year.

This guide explains how carbon brushes work, the main material families and what they are used for, and the standards that define them — as the foundation for the more detailed guides on selection, troubleshooting, ordering and slip-ring duty.

What a Carbon Brush Does

• Conducts current between the stationary connections of the machine and its rotating winding
• Helps commutation in DC and AC commutator machines: as each commutator bar passes under the brush, the coil connected to it is briefly short-circuited and its current reverses. The brush's contact resistance helps control the current circulating in that short-circuited coil, which is why brush grade affects sparking
• Carries continuous current to slip rings in wound-rotor motors, synchronous machines and generators
• Grounds the shaft, diverting shaft currents away from bearings

Why Carbon

No metal sliding contact does all of this as well.

• Graphite lubricates itself. Its layered crystal structure shears easily along the layers, giving low friction against copper and steel.
• It does not weld. Metal-on-metal sliding contacts pick up and weld microscopically; carbon does not bond to copper.
• It tolerates heat and arcing far better than metal contacts.
• It wears instead of the commutator. A brush is designed to be the sacrificial half of the contact.
• It builds a film. In operation, a thin film of copper oxides, graphite and adsorbed water forms on the commutator. The film is what makes the contact stable — lowering friction, protecting the copper and providing the contact resistance that helps commutation. Most brush problems are, at root, film problems. See our guide to commutator and brush troubleshooting.

How Carbon Brushes Are Made

1. Raw materials — petroleum coke, carbon black, natural graphite and synthetic graphite powders, with pitch or resin binders, and copper or silver powder for metal-graphite grades
2. Mixing and forming — the mix is pressed into blocks or near-net-shape brushes
3. Baking — the binder is carbonised at high temperature, forming a solid carbon body
4. Graphitisation (electrographite only) — the baked body is heated in an electric furnace to a very high temperature, typically about 2,200–2,800°C, converting the carbon to a graphitic structure
5. Impregnation and treatment — some grades are impregnated or treated to adjust friction, film formation or performance in unusual atmospheres
6. Machining — blocks are cut and machined to size, with chamfers, bevels and radii
7. Shunt fitting — flexible copper shunts are tamped, riveted or moulded into the brush body
8. Testing — dimensions, specific resistance, hardness, density and strength are checked against the grade specification

The Five Main Grade Families

Indicative family ranges, compiled from brush makers' published data; the datasheet of the actual grade governs. Contact drop is quoted per pair of brushes, one of each polarity.

Carbon-graphite (hard carbon)

Amorphous carbon with some graphite, baked but not graphitised. Hard, somewhat abrasive, with a polishing action on the commutator. Limited in current density and speed. Used on older and smaller machines, low-speed duty and some fractional-horsepower motors.

Natural graphite

Natural flake graphite bonded with resin or pitch. Soft, low friction, quiet, with good film-forming behaviour. Suited to high peripheral speeds and slip rings — including high-speed collector rings — and to machines where a gentle brush is needed. Brushes for turbogenerator collector rings are often natural graphite with a small abrasive content, which keeps the steel ring surface clean.

Electrographite

Carbon bodies graphitised at very high temperature, typically about 2,200–2,800°C. The process removes impurities and creates a strong, uniform graphitic structure with good thermal conductivity and stable contact behaviour. Electrographite grades span a wide range of hardness, resistivity and contact drop, which is why they are the default for industrial DC motors and generators, traction and marine propulsion motors, turbogenerator exciters, synchronous machines and many slip-ring motors.

Resin-bonded

Graphite bonded with a synthetic resin that is not carbonised. High specific resistance and high contact drop, which strongly damp commutation sparking. Used on machines that commutate with difficulty — AC commutator motors and universal motors in tools and appliances, and some DC machines with poor commutating conditions.

Metal-graphite: copper-graphite and silver-graphite

Graphite with copper or silver powder, in proportions from modest to very high metal content. Very low specific resistance and contact drop, high permissible current density, but lower speed capability. Used where voltage is low and current is high — low-voltage DC machines, automotive and battery-powered motors, some slip-ring duties — and, as silver-graphite, for signal slip rings and shaft grounding brushes where very low and stable contact resistance matters. Their wear dust is conductive, so metal-graphite grades are not used on high-voltage or totally enclosed machines.

Electrographite in Practice: The EG4 Grade

Kasko Mak branded EG4 brushes are an electrographite grade produced for DC machines, slip-ring motors, generators and turbogenerator exciters. The grade combines low contact resistance with a low friction coefficient for stable commutation and low commutator and slip-ring wear in continuous industrial duty.

Typical applications: turbogenerator exciters and excitation slip rings on units below 200 MW, rolling mill drive motors, marine propulsion motors, DC generators and converters, single-armature converters, low-power DC motors and general-purpose machines, synchronous motors and slip-ring induction motors.

How to check whether EG4 — or any grade — suits a particular machine is covered in our guide to carbon brush grade selection.

Brush Construction

A brush is more than its material:

• Shunts — single or twin flexible copper leads, tamped, riveted or moulded into the brush top
• Terminals — forked, ring, flat blade or bare shunt end
• Top features — bevels and radii for the spring finger, rubber cushions against vibration, contact shoes
• Contact face — flat, radiused to the commutator diameter, or bevelled for angled holders
• Split and sandwich brushes — two or more pieces side by side for better contact and commutation on difficult machines
• Wear indicators — marks or signalling contacts

Our guide to replacement carbon brushes covers how to specify each of these.

Where Each Family Is Used

Slip-ring and exciter duties are covered in our guide to slip ring and exciter brushes.

Standards and Grade Designations

IEC

• IEC 60136:2024 — dimensions, marking and testing of carbon brushes, and dimensions of brush holders, including the clearance between brush and holder box
• IEC 60276:2018 — definitions and nomenclature for carbon brushes, brush holders, commutators and slip rings
• IEC 60413:2026 — test procedures for determining the physical properties of brush materials, such as density, resistivity, flexural strength, hardness and ash content; the 2026 second edition replaced the 1972 original
• IEC 60773:2021 — test methods and apparatus for measuring the operational characteristics of brushes, including the black-band test for DC machines

NEMA

• NEMA CB 1 — Brushes for Electrical Machines: definitions, dimensions and tolerances, and tests of physical properties and shunt connections, used in North America

Dimensions are given as t × a × r: tangential (thickness), axial (width) and radial (length).

GOST

In Russia and many CIS markets — where a large installed base of DC drives, rolling mills, turbogenerators and exciters was built to Soviet standards — brush dimensions follow GOST 12232-89, the interstate equivalent of IEC 136-86. Its scope is narrower than often assumed: it sets brush dimensions and tolerances, and two test methods, one for the contact resistance between brush and shunt and one for the force needed to pull the shunt out. It does not define grades or their performance. EG4's dimensions and test methods follow it.

The rest sits elsewhere. Russian national standards cover general technical requirements (GOST R 52157-2003, which groups brushes into graphite, carbon-graphite, electrographite and metal-graphite) and brush types and sizes (GOST R 53617-2009). Terms, including the tangential, axial and radial dimensions, are defined in GOST 21888-82. The properties of each grade, such as current density, speed limit, contact drop and friction, are set in the grade's own technical specification.

Soviet and Russian grade names carry a prefix that indicates the material family:

The prefixes are a convention rather than a standard, and not entirely consistent: Г20 to Г23, for example, are carbon-graphite grades despite the Г. Within the ЭГ family, EG4 is a low-resistivity grade (6–16 µΩ·m), close to the graphite grade Г3. Traction grades such as ЭГ61А, ЭГ74 and ЭГ84 run at roughly 30–75 µΩ·m with a higher contact drop, which helps commutation on heavily loaded traction motors.

Manufacturer grade codes

Brush manufacturers designate grades with their own codes. Cross-reference tables between manufacturers exist, but they are approximate — two "equivalent" grades can differ in contact drop, friction and film behaviour. For a critical machine, check a replacement grade against the operating conditions rather than relying on a table alone.

Common Mistakes

1. Choosing by appearance or price. Two black brushes can be completely different materials.

   Prevention: Identify the grade from records or the brush marking, or select it for the machine's operating conditions.

2. Mixing grades on one machine. Uneven current sharing, uneven film and sparking.

   Prevention: Replace complete sets with one grade.

3. Using a high-contact-drop grade where it is not needed. Unnecessary electrical loss and heat.

   Prevention: Match contact drop to the machine's commutation difficulty.

4. Using a metal-graphite grade at high speed, or in a high-voltage or totally enclosed machine. Excessive wear and heating at speed; conductive dust that tracks across insulation.

   Prevention: Check peripheral speed against the grade's maximum, and keep metal-graphite grades out of high-voltage and totally enclosed machines.

5. Blaming the brush for a machine fault. Runout, high mica, weak springs and worn holders cause sparking with any grade.

   Prevention: Inspect the commutator, holders and springs before changing grade.

6. Ignoring the environment. Dry air, silicone, oil and abrasive dust destroy the film whatever the grade.

   Prevention: Review the atmosphere inside the machine as part of grade selection.

7. No spares for critical machines. A sparking incident stops the machine while brushes are made.

   Prevention: Hold at least one full set per critical machine.

Supply from Kasko Makine

Kasko Mak branded EG4 carbon brushes

• EG4 electrographite grade for DC machines, slip-ring motors, generators and turbogenerator exciters
• Produced in 25 × 32 × 40 mm and in other sizes to order, to customer drawings and from reference samples
• Single or twin flexible copper shunts, tamped and secured in the brush body
• Forked, ring, flat-blade or bare-end terminals
• Top rubber cushion, contact shoe, and radial or bevelled contact face options
• Homogeneous electrographitised body, machined to size and chamfered
• Dimensions and test methods to GOST 12232-89
• Dimensional, resistivity and hardness checks before dispatch; certificate of conformity with each order

Engineering support

Send the brush dimensions, shunt and terminal details and a photo, with the machine type, current, speed and commutator or ring diameter. We will confirm whether EG4 suits the duty and quote.

Send your brush details and machine data — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading: Carbon Brush Series

• How to Select a Carbon Brush Grade: Current Density, Speed and Contact Drop
• Commutator and Carbon Brush Problems: Diagnosis Guide
• Replacement Carbon Brushes: Dimensions, Shunts and Terminals
• Slip Ring and Exciter Brushes

Frequently Asked Questions

Q: What is a carbon brush?
A: A carbon brush is a sliding electrical contact, usually a machined block of carbon or graphite with a flexible copper shunt, that carries current between the stationary circuit of an electrical machine and its rotating commutator or slip rings. In DC and AC commutator machines it also helps commutation, and dedicated brushes are used to ground motor shafts and protect bearings from shaft currents.

Q: What are the main types of carbon brushes?
A: The five main families are carbon-graphite, which is hard and suited to low speeds and loads; natural graphite, which is soft and suited to high speeds and slip rings; electrographite, graphitised at very high temperature and the standard for industrial DC machines and exciters; resin-bonded, with high resistivity for machines that commutate with difficulty; and metal-graphite, with copper or silver for low voltage, high current and grounding.

Q: What is an electrographite carbon brush?
A: An electrographite brush is made from carbon that has been baked and then heated in an electric furnace to a very high temperature, converting it to a graphitic structure. The result is a strong, uniform, thermally conductive material with stable contact behaviour, used for industrial DC motors and generators, traction and marine motors, turbogenerator exciters, synchronous machines and many slip-ring motors.

Q: When are copper-graphite brushes used?
A: Copper-graphite brushes have very low specific resistance and contact drop and can carry high current density, but at lower peripheral speeds than electrographite. They are used where voltage is low and current is high, such as low-voltage DC machines, automotive and battery-powered motors and some slip-ring duties. Silver-graphite brushes are used for signal rings and shaft grounding.

Q: Why do carbon brushes wear?
A: Carbon brushes are designed as the sacrificial half of the sliding contact, so they wear instead of the commutator or slip ring. Normal wear depends on grade, current density, speed, spring pressure and the condition of the commutator film. Abnormally fast wear usually points to light load, very dry air, silicone or oil contamination, abrasive dust, excessive spring pressure or a mechanical fault.

Q: What standards apply to carbon brushes?
A: IEC 60136 covers dimensions, marking and testing of brushes and brush-holder dimensions, IEC 60276 definitions and nomenclature, IEC 60413 test procedures for the physical properties of brush materials and IEC 60773 test methods for operational characteristics. North America uses NEMA CB 1. In Russia and the CIS, GOST 12232-89 sets dimensions and shunt test methods, while grade properties are set in each grade's own specification. Dimensions are given as tangential × axial × radial.

Q: What are the properties of the EG4 carbon brush grade?
A: Kasko Mak branded EG4 is an electrographite grade with a nominal current density of 12 A/cm², a maximum peripheral speed of 60 m/s, a contact voltage drop of 0.9 to 1.9 V per pair, a specific resistance of 6 to 16 µΩ·m, a friction coefficient of no more than 0.25 and a nominal spring pressure of 20 kPa, with dimensions and test methods to GOST 12232-89.