International projectionist (July-Dec 1934)

INTERNATIONAL PROJECTIONIST VOLUME VII NUMBER 4 OCTOBER 1934 BRUSHES: A VITAL LINK IN M. G. MAINTENANCE M. L. Robinson THE purpose of this article is to give users and maintenance men a better understanding of the selection, care, manufacture and application of brushes used in standard motors and generators. There are too many people who regard brushes as plain pieces of carbon, and when new brushes are needed, use anything they can get. This usually results in plenty of trouble and expense later on. The design, manufacture and application of brushes is a very skilled technical procedure requiring extremely high engineering ability. The next time that anyone feels tempted to use a set of brushes out of the "miscellaneous" box this statement should be kept in mind. The manufacture of a popular make of brush requires 36 major manufacturing operations which take 65 days. There are about seven classifications of brushes : . Copper Leaf and Gauze — This class of brush is practically obsolete. Low Grade Carbon Graphite — This brush has a low ampere carrying capacity, is usually abrasive and is limited in use to machines with low peripheral speed commutators where strength and abrasiveness are important. High Grade Carbon Graphite — This type of brush is very popular and has a wide range of uses, has medium friction and is composed of carbon and graphite. Carbon Graphite — This brush is impregnated with lubricating material and is not very popular because it has a number of bad features such as a tendency to stiik in the brush holder and to "gum" up a commutator. Electro Graphitic — A brush of this type is made by a high heat treatment process which increases the carrying capacity and lowers friction. A brush of this type is popular for use with undercut commutators and is usually unabrasive. Selection of Brushes Graphite — Natural or artificial graphite is used in this grade of brush which has a high ampere carrying capacity, medium contact drop, is soft, requires low spring tension. It is used for high amperage, high speed, low or ordinary commercial voltage apparatus and for use on 2200 volt slip rings. Metal Graphite — This is a widely used brush and is made of a copper powder and graphite. Such a brush has a low contact drop and is used on low voltage commutators or slip rings carrying high currents. In determining the type and grade of brush to be used for a machine there are ten factors to take into consideration — namely, capacity, contact drop, friction riding ability, resistance, strength, hardness composition and surface speed. These factors readily explain why it is vitally important to use the grade of brush recommended by the manufacturer of either the machine or the brushes. The specific resistance of a brush is the ohms per inch cube and depends upon the materials used and the process of manufacture. Carrying-capacity of a brush is the current in amperes which one square inch of brush surface on a commutator will carry under normal conditions, without causing the temperature of the brush to rise more than 50 degrees C. When a brush is used on slip rings or for intermittent service on commutators, the rating can be increased from 25 to 40%. Peripheral speed varies with the grade or brush and is given in feet per minute. Hardness of a brush is its ability to withstand pressure and does not bear any relation to abrasiveness. An instrument called the scleroscope is used for determining hardness. This is measured by the height of rebound of a steel ball dropped from a fixed height upon the brush. Contact drop is the voltage drop be [5]