Carbon Brushes for DC Motors and Generators

Terry Taylor, Senior Consultant, IDCON INC

Carbon brush grades are usually classified according to the manufacturing processes and the types of carbons, graphites, and other ingredients used. The 4 main carbon brush grade families are listed below:

Carbon-graphites
Electrographitics
Graphites
Metal-graphites

1. Carbon-Graphite Brushes

Carbon-graphite brushes made their entrance early on in brush industry history. These are generally limited to lower current densities (45 amps/sq.in.) and are used in older, slower speed motors with a maximum surface speed of 4000 ft./min. These are high friction brushes that make them very unattractive for present day use on commutators.

2. Electrographitic Brushes

Electrographitic Brushes are baked at temperatures exceeding 2400C. These brushes are fairly porous which allows them to be treated with various organic resins. These treatments generally increase strength and improve the brush’s lubricating ability. This results in increased brush life especially in high operating temperature and low humidity environments.

These brushes are generally capable of continuous operation at 80 amps/sq.in. and at surface speeds of 7000 ft./min. These brushes are commonly found in many industrial, transportation and mining industries.

3. Graphite Brushes

Graphite Brushes are composed of natural or artificial graphite bonded with resin or pitch to form a soft brush material. These brushes are characterized by their controlled filming and excellent riding qualities on both commutators and slip rings at brush current densities of less than 45 amp/sq.in. to 60 amps/sq.in. These are not capable of sustained operation at higher current densities.

The fast filming properties of graphite brushes is very beneficial in protecting commutators in contaminated atmospheres such as those seen in paper mills.

4. Metal Graphite Brushes

Metal Graphite Brushes are generally made from natural graphite and finely divided metal powders. Copper is the most common metallic constituent but silver, tin, lead and other metals are sometimes used. These brushes are often ideal for a wide variety of applications because of their low resistivity.

These brushes are often the solution to particular commutation problems that can’t be solved with more conventional brushes.

Carbon brushes are very complex and finding a solution for a particular commutation problem, it takes patience, trial and error, and the assistance of your carbon brush provider.

Interested in learning about DC Motor Brush Holders?

Use P-F Intervals to Map, Avert Failures

The potential-to-functional failure interval (P-F interval) is one of the most important concepts when it comes to performing Reliability-Centered Maintenance (RCM). Remarkably, the P-F interval is also one of the most misunderstood RCM concepts. The failure mode analysis becomes even more complicated when you are dealing with several P-F intervals for one failure mode. This paper will help clarify the P-F interval and the decision-making process when dealing with multiple P-F intervals.

The RCM Trap

This column is likely to create a lot of reactions from the academia of reliability and maintenance management, and all comments are welcome. Reliability Centered Maintenance (RCM) has its place, but many times plants jump into training programs and attempt to implement RCM long before they are ready for it. This application makes RCM more of a trap, than a helpful tool to implement.

Dealing with a Reduced Maintenance Staff

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Reliability Centered Maintenance is a step-by-step instructional tool for how to analyze a system's failure modes and define how to prevent or find those failures early. RCM become a very detailed study of things we already know, you will often hear the justification "it's to make sure we don't miss anything." If you are considering implementing a reliability centered maintenance program, you. should be aware of the problems you may run into.

Design for Maintainability

Maintainability can be defined as the ease in time and resources of retaining equipment in or restoring it to a specified operational condition. It directly affects the bottom line because it can impact operations, downtime, maintenance costs, and safety. Maintainability is an important aspect of any system's lifecycle, but process plant engineers typically give it little direct consideration. This is primarily the result of a short-term view of capital project costs that fails to consider lifecycle costs and downstream activities.

Communications: The OEM Partnership

When maintenance is allowed to interact with the plant and production Original Equipment Manufacturer (OEM), a prosperous relationship for both parties will often ensue. This OEM partnership results in first-hand information for maintenance to set up its maintenance approach, and valuable operational and design feedback for the OEM. There are seven main circumstances in which maintenance will interact or communicate with an OEM and form an OEM partnership.

Carbon Brushes for DC Motors and Generators