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1948-49 Theatre Catalog, 7th Edition, Page 485 (472)

1948-49 Theatre Catalog, 7th Edition
1948-49 Theatre Catalog
1948-49 Theatre Catalog, 7th Edition, Page 485
Page 485

1948-49 Theatre Catalog, 7th Edition, Page 485

Factors Affecting the Operation of Brushes

Small in Size and Inexpensive, too Little Attention is Given Their Maintenance and Effect 011 Equipment

Most theatres are located in districts where direct current power supply is not available. In such cases, the operation of DC. projection lamps requires the installation of some type of rectifying or converting device to change the alternating current supplied by the power lines to direct current for use at the arc. In many theatres a motor-generator set or some type of synchronous converter is used for this purpose and carbon brushes are required on the DC. side of all machines of this type.

Reasonable attention to factors affecting the operation of brushes will be well repaid by the improved results and greater reliability of operation thus obtained.

As is true of all rotating equipment, careful attention should be given to the thorough lubrication of bearings. At the same time, care should be exercised to prevent oil getting on the commutator of the machine where it has a tendency to impair the mica insulation of the seg ments as well as to interfere with the operation of the brushes.

Accurate alignment of the edges of the brushes with the commutator seg ments and equal spacing of brushes around the periphery of the commutator are likewise essential to good commutation and cool operation of the unit. A convenient way to check brush spacing is to wrap a strip of paper tightly around the commutator and, with a pencil, mark the point at which the corresponding edge of each brush bears on the strip. Measuring the distance between these marks will then Show whether or not the brush spacing is equal. If found unequal, this condition should be corrected, the method of correction depending on the construction of the machine. '

It. is obviously important to keep the brush holders and their supporting studs firmly secured. Loose brush holders permit vibration and displacement of the brushes from the correct position.

When new brushes are installed they should be sanded in to fit the commutator accurately. A good way to do this is to cut strips of sandpaper slightly wider than the brush. Raise a brush and place the strip of sandpaper under it with the rough side against the carbon. Let the brush down on the sandpaper and pull the latter through in the same direction as the commutator rotates, repeating this operation until all of the brushes have been sanded to full contact from edge to edge. It is not advisable to pull the strip of sandpaper back and forth under the brush as there is some play of the brush in its holder and this method of sanding tends to form a double face which prevents full contact from edge to edge.

The continuing search by the Editors of THEATRE CATALOG for factual data relating to all manner of equipment and supplies used by theatres, and its eventual recording for the reference of the theatre man, turned up TBRUSHESME which is Chapter 13 of the new 4th Edition of the National Projector Carbon Handbook. Despite their -minute size and price, this important theatre item merits attention.

The tension on all brushes should be as nearly the same as the adjustment provided on the brush holders will permit. The actual value of the tension should conform to the recommendation of the manufacturer of the brush grade in use. In most cases, a pressure of two pounds per square inch of brush contact face will give good results.

Maintenance of a good commutator surface merits careful attention on the part of the operator. Undercut commutators should be carefully inspected'for the presence of side mica and where found it should be removed. Burned bar edges, streaks and minor defects in commutator surface can usually be removed by polishing with a commutator stone or with fine sandpaper secured to a block which has been formed to the radius of the commutator. Serious flat spots or deep grooves, however, require turning or grinding of the commutator. This should be done by a shop properly equipped to handle work of this character.

Until the commutator of a new machine becomes thoroughly seasoned, it is possible that the segments may loosen and high or low bars become apparent. The clamp or "V" ring of the commutator should be examined from time to time and, when evidence of looseness is discovered, firmly tightened. This should be done when the machine is hot. Any evidence of failure of the mica insulation of the commutator segments should also receive immediate attention.

All electrical connections on the generating equipment should be kept clean and firm. This precaution includes the brush shunts the flexible cables by means of which the brushes are connected to the electrical circuit. The terminals should be firmly secured to the brush holders or their mounting at the points provided for this purpose. Loose electrical connections are a frequent source of trouble on electrical equipment

*(fopytiuhl 1949 by National Carbon Company. Inc. Reprintvd with permission of the copyright owners,

but one that is easily avoided by a moderate degree of attention.

The grade of brush used exerts a marked innuence on the results obtained. On the commutators that are not undercut, that is, those on which the mica is fiush with the copper, it is generally desirable to use a brush having a mild abrasive action to keep the mica worn down even with the metallic surface. If the mica is allowed to project above the surface, the brushes will not make firm contact with the copper and the re sultant sparking will quickly develop burned bars and Hat spots, necessitating regrinding of the commutator. On undercut commutators, which are the general rule today, nonabrasive electrographitic brushes give excellent results on most generating equipment used in the projection room. This type of brush has low friction and excellent commutating characteristics and is hard enough to maintain a good commutator polish under most operating conditions. Sometimes a slight spotting or streaking of the commutator occurs when the arc is struck which the nonabrasive electrographitic brush cannot entirely remove before striking the are again. This condition can be remedied by polishing the commutator surface occasionally with smooth sandpaper. However, to avoid the need for this attention, some operators prefer to use a brush with a slight polishing action, enough to remove this incipient burning but not sufficiently abrasive to cause appreciable commutator wear. Graphite or graphite-carbon grades are usually preferred for this purpose. The use of artificially lubricated brushes or the application of lubricant to the commutator surface is not recommended when the mica is undercut and is unnecessary when the proper grade of brush is used.

The grade of brush supplied by the manufacturer of the equipment will generally give satisfactory performance since careful attention is given to brush selection by designing engineers. Distributors of iiNational" Projector Carbons also carry in stock uNational Pyramid" Brushes in suitable grades and sizes for all popular makes of motion picture theatre current converting equipment. Replacement brushes of correct grade can thus be obtained without delay from the same source as other theatre supplies. There are cases, however, where operating conditions or other causes indicate the need for a brush of special characteristics. In such cases an outline of your difficulties, mailed to the nearest Division Office of National Carbon Company, Inc., or a discussion of the trouble with a representative of the Company will secure the recommendation of a grade to meet the requirements of your specific case.

1948-49 Theatre Catalog, 7th Edition, Page 485