Replacement Carbon Brushes: Dimensions, Shunts, Terminals and How to Order

Quick Answer

To order a replacement carbon brush that fits and works, a supplier needs seven things: the three dimensions in the standard order t × a × r — tangential (thickness in the direction of rotation), axial (width along the shaft) and radial (length towards the commutator or ring), for example 25 × 32 × 40 mm; the number of shunts and their length, normally measured from the top of the brush to the centre of the terminal hole or slot; the terminal type and hole or slot size; any top and contact-face features — bevels, radius, rubber cushion, contact shoe, split construction; the grade, read from the marking on the brush or from the machine maker's parts list; the machine data — type, rating, speed and commutator or slip-ring diameter; and the quantity per machine. A clear photograph of a used brush next to a ruler, showing the shunts and terminal, answers most of these questions in one go. Never mix grades on one machine: replace the whole set with one grade.

The brush in the photograph is a typical industrial request. A rectangular block, two flexible copper shunts tamped into its top, both joined into a single forked terminal. It arrived as a photo on a sheet of paper — which, with one addition, is exactly how to ask for a quote. The addition is a ruler.

Ordering replacement brushes looks simple and goes wrong surprisingly often. A brush 0.3 mm too thick jams in its holder when it warms up. A shunt 10 mm too short pulls taut before the brush is worn out, and the brush stops touching the commutator with a third of its life left. The wrong grade fitted alongside the old ones turns a healthy machine into a sparking one within days. This guide covers what to measure and what to say so the brushes that arrive are the brushes the machine needs.

The Seven Things a Supplier Needs

1. Dimensions — t × a × r, in millimetres
2. Shunts — number, length, insulation
3. Terminal — type and hole or slot size
4. Top and face features — bevels, radius, cushion, shoe, split construction, wear indicator
5. Grade — from the brush marking or the machine maker's documentation
6. Machine — type, rating, voltage, current, speed, commutator or ring diameter
7. Quantity — brushes per machine, and spares

Dimensions: t × a × r

Brush dimensions are given in a fixed order, used in IEC 60136, in North America's NEMA CB 1 (which calls them thickness, width and length) and in GOST-based specifications:

The contact area is t × a — 8.0 cm² for a 25 × 32 mm brush — and it is the area used to calculate current density. See our guide to carbon brush grade selection and current density.

How to measure

• Measure with a caliper, not a ruler, for t and a. Tolerances on brush thickness and width are tight because the brush must slide freely in its holder without rocking.
• Measure an unworn spare if one exists. A worn brush gives correct t and a, but not r.
• If only worn brushes exist, measure t and a from them, and take r from the holder depth or the machine documentation — or send the holder dimensions and let the supplier work back from them.
• Check the brush holder box as well. A worn holder can let a correctly sized brush rock, and the problem is then blamed on the new brushes.
• State whether dimensions are measured or nominal. "Measured 24.92 × 31.91" tells the supplier the brush is a nominal 25 × 32 with normal tolerance. "Measured 24.3" tells them something is wrong — with the brush or the holder.

Shunts

The shunt (also called the pigtail or lead) carries the current from the brush body to the brush holder or the machine's connection.

Number of shunts. Small brushes have one. Larger brushes and those carrying higher current use two — like the brush in the photograph — so the current is shared and each shunt stays cool. A twin-shunt brush must not be replaced with a single-shunt brush of the same size; the single shunt will run hot and discolour, and in time burn.

Construction. Shunts are stranded flexible copper, plain or tinned, sometimes with insulating sleeving where they could touch earthed parts or each other. Their cross-section is sized for the brush current.

Connection to the brush. The shunt is usually tamped — fixed into a hole in the brush top with conductive powder compacted under pressure — so the joint has low resistance and cannot pull out. Riveted, soldered and moulded-in connections are also used. Tamping is the standard for industrial brushes.

Length. This is the most commonly mis-specified detail. Most manufacturers measure shunt length from the top of the brush to the centre of the terminal hole or slot. State how you measured, and check the length allows for:

• full brush travel down to the minimum wear length, without the shunt pulling taut
• the spring and holder geometry, without the shunt rubbing on the spring or the holder edge

Too short, and the shunt stops the brush feeding before it is worn out. Too long, and the shunt can touch the commutator, the rotor or another brush.

Terminals

Give the terminal type, the slot or hole diameter, and — if the terminal is angled — the angle and orientation relative to the brush faces. The brush in the photograph uses a single forked terminal joining both shunts.

Top and Contact-Face Features

Many brushes are not plain blocks. Features to check and describe:

• Top bevel or top radius — shaped to suit the spring finger or pressure plate. The bevel angle and direction matter.
• Top rubber cushion — a pad bonded to the top that absorbs vibration and shock, common on traction and high-vibration machines.
• Contact shoe or metal top plate — spreads the spring load or gives the spring a wear-resistant seat.
• Contact face — flat, radiused to the commutator or ring diameter, or bevelled for brushes set at an angle to the radial direction (reaction or trailing holders). Ordering brushes with the face pre-radiused shortens bedding-in.
• Chamfers — on the edges, to prevent chipping.
• Split or sandwich construction — two or more pieces side by side, sometimes bonded, which improve contact and commutation on difficult machines. A split brush must not be replaced with a solid one.
• Grooves and slots — for spring fingers or guides.
• Wear indicators — marks, or signalling contacts that switch when the brush reaches its wear limit.

When in doubt, photograph the brush from all six sides.

Identifying the Grade

The grade — the brush material — decides whether the machine commutates cleanly and how fast the brush and commutator wear. Sources for it, in order of reliability:

1. The machine maker's spare parts list for that machine
2. The marking on the brush — grade codes are usually printed or engraved on a side face, though they wear and smudge
3. The previous order or the plant's stores records
4. A sample sent for identification, where no record exists

If the original grade is unknown or no longer available, the replacement should be selected for the machine — current density, speed, duty and environment — rather than guessed by appearance. Two black brushes can be completely different materials.

Kasko Mak branded EG4 brushes are 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 0.9–1.9 V per pair of brushes, specific resistance of 6–16 µΩ·m, a friction coefficient of 0.25 maximum and a nominal spring pressure of 20 kPa, with dimensions and test methods to GOST 12232-89. See our guide to carbon brush types and grades.

Grade Rules That Prevent Trouble

• One grade per machine. Different grades have different contact drops and friction. Mixed on one commutator, they share current unevenly, film the commutator unevenly and spark.
• Replace a full set at once where practical, and at least all brushes on one machine with the same grade. Where brushes must be changed a few at a time, as on turbogenerator collectors changed under load, brush makers advise replacing no more than about a fifth of the set at once.
• Changing grade is an engineering decision, not a stores decision: it needs a reason — wear, sparking, a changed duty — and a check against the machine's operating conditions.
• New brushes need bedding in so the face matches the commutator or ring curvature before full load. See our guide to commutator and brush troubleshooting.

How Many to Order

• Brushes per machine = brush arms × brushes per arm (for a commutator), or rings × brushes per ring (for slip rings)
• Running spares — at least one full set per machine in stores for critical machines, because brush wear is predictable but failures and sparking incidents are not
• Planned replacement — track wear in millimetres per 1,000 operating hours and order before the brushes reach their wear limit (the machine maker's wear mark or minimum length), not when they do

A change in wear rate is more useful than the wear rate itself. Brushes that suddenly wear twice as fast are telling you about the machine: load, film, contamination or a mechanical fault.

Sending a Photo for a Quote

A good enquiry photograph:

• Brush on a plain light background, with a ruler or caliper in the frame
• Shunts laid out straight, so their length and the terminal are visible
• Side faces shown, so the grade marking can be read
• Top and contact face shown separately, if they have bevels, pads or a radius
• The brush holder, if the brushes are worn or the holder is suspected

Add the machine nameplate photo and the number of brushes on the machine, and most enquiries can be quoted without further questions.

Common Ordering Mistakes

1. Dimensions in the wrong order. "32 × 25 × 40" and "25 × 32 × 40" are different brushes, and one of them will not fit the holder.

   Prevention: Always give dimensions as t × a × r, and label them if there is any doubt.

2. Radial length taken from a worn brush. The new brush is ordered short.

   Prevention: Measure r from an unworn spare or from the holder, or ask the supplier to confirm it from the machine data.

3. Shunt length measured to the end of the terminal instead of the centre of the hole or slot. The new shunt is too short.

   Prevention: State the measurement convention, or send a photo with a ruler along the shunt.

4. Twin-shunt brush replaced with a single-shunt brush. The single shunt overheats.

   Prevention: Match the number and cross-section of shunts to the original.

5. Split brush replaced with a solid brush. Commutation deteriorates.

   Prevention: Check for split or sandwich construction and specify it.

6. Grade guessed from appearance. The replacement has a different contact drop and the machine sparks.

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

7. New grade mixed with old brushes on the same machine. Uneven current sharing and film.

   Prevention: Replace all brushes on a machine with one grade.

8. No bedding-in. New flat-faced brushes run on a fraction of their face for the first hours, overheat and spark.

   Prevention: Order brushes with a radiused face where possible, and bed them in before full load.

9. 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
• Nominal current density 12 A/cm²; maximum peripheral speed 60 m/s; contact voltage drop 0.9–1.9 V per pair; specific resistance 6–16 µΩ·m; friction coefficient 0.25 maximum; nominal spring pressure 20 kPa
• 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
• Terminals: forked, ring, flat blade, or bare shunt end
• Options: top rubber cushion, contact shoe, radial or bevelled contact face
• Homogeneous electrographitised body, machined to size and chamfered
• Dimensions and test methods to GOST 12232-89

Quality and documentation

Dimensional inspection, resistivity checks and hardness checks before dispatch, with a certificate of conformity supplied with each order.

Engineering support

Send a photo of the brush with a ruler, the three dimensions, the shunt and terminal details, the machine nameplate and the quantity. If the original grade is unknown, include the machine's current, speed and commutator or ring diameter, and we will check EG4 against the operating conditions before quoting.

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

Continue Reading: Carbon Brush Series

• Carbon Brushes: Types, Grades and How They Work
• How to Select a Carbon Brush Grade: Current Density, Speed and Contact Drop
• Commutator and Carbon Brush Problems: Diagnosis Guide
• Slip Ring and Exciter Brushes

Frequently Asked Questions

Q: How are carbon brush dimensions measured?
A: Dimensions are given as t × a × r: t is the tangential thickness in the direction of rotation, a is the axial width along the shaft, and r is the radial length towards the commutator or slip ring, the dimension that wears. A 25 × 32 × 40 mm brush is 25 mm thick, 32 mm wide and 40 mm long. Measure t and a with a caliper, and r from an unworn brush or the holder.

Q: How is carbon brush shunt length measured?
A: Most manufacturers measure shunt length from the top of the brush to the centre of the terminal hole or slot. Because some measure to the end of the terminal, state the convention when ordering or send a photograph with a ruler along the shunt. The length must let the brush travel to its wear limit without the shunt pulling taut, and must not let the shunt touch rotating parts.

Q: Why do some carbon brushes have two shunts?
A: Larger brushes and those carrying higher current use two flexible copper shunts so the current is shared and each shunt stays cool, and so the connection has some redundancy. Replacing a twin-shunt brush with a single-shunt brush of the same size overloads the single shunt, which then overheats, discolours and can eventually burn through.

Q: Can different carbon brush grades be mixed on one machine?
A: No. Different grades have different contact voltage drops and friction, so brushes of mixed grades share current unevenly, form an uneven film on the commutator and cause sparking and accelerated wear. All brushes on a machine should be the same grade, and a change of grade should be made for the whole set at once, after checking it against the machine's operating conditions.

Q: What information is needed to order replacement carbon brushes?
A: Give the three dimensions in t × a × r order, the number and length of shunts, the terminal type and hole or slot size, any top or face features such as bevels, a rubber cushion or split construction, the grade, the machine type, rating, speed and commutator or ring diameter, and the quantity. A photograph of the brush beside a ruler, plus the machine nameplate, answers most of this.

Q: How do I identify the grade of an old carbon brush?
A: The most reliable source is the machine maker's spare parts list, followed by the grade code printed or engraved on the brush side face, and the plant's previous orders. If no record exists, a sample can be sent for identification, or the replacement can be selected for the machine's current density, speed, duty and environment. Two brushes that look identical can be completely different materials.

Q: How many spare carbon brushes should be kept in stock?
A: For each critical machine, hold at least one complete set — every brush on the machine, in the same grade — because a sparking incident or damaged brush can stop the machine while new brushes are made. Plan routine replacement from the recorded wear rate in millimetres per 1,000 operating hours, and reorder before brushes reach their wear limit rather than when they do.