On the Failure of Carbon Brush for the Main Motor of an Electric Car

Statistical investigations of carbon brushes in use for the main motor of an electric car have shown that rejections on account of failure amount to 30% of the total number in service on electric cars, electric locomotives or on the private as well as the Government Railways. As a result of a close observation of the failure mechanism, it has been disclosed that cracks inherent in pitch coke, base material of brush, are responsible t for such failure leading to scaling; pitch coke has a layer structure easily coming to failure under a negligible pressure or developing cracks, if not failure. Then, the form of pitch coke powder, form factor and frictional coefficient has been determined and it has been found the larger form factor m is accmpanied by the higher frictional coefficient so that failure percentage of such powder rises correspondingly. It follows from this that a brush block made of coke powder with smaller form factor will have a layer structure to such small extent that scaling will not occur.