Brush mounting as a factor of satisfactory operation

R. F. Franklin: I wish to discuss two points concerning Mr. Jones´ paper. One is the criterion of zero box pressure; the other, the mathematical analysis of the brush forces. The condition of zero box pressure which Mr. Jones strives to obtain, will not give satisfactory operation in practise since the slight variations of commutator friction for different points on the commutator will cause the brush to move back and forth in the box and chatter. Practical operation requires a brush angle which will insure the brush always bearing against one side of the box. However, the “box reaction” involved in this condition should not be great enough to prevent free axial movement of the brush in the box. The proper brush angle, therefore, is not the one that will give zero box pressure, but the one that will give a box pressure sufficient to hold the brush in contact with the box for all values of commutator friction and yet not so large as to prevent free axial movement of the brush.