Evaluations of the critical current densities of carbon brushes by thermal shock fracture testings

It is well known that a carbon brush for a rotating commutator collects a large electric current through a number of very localized contruction area as it is called ''a-spot'' on the sliding surface. In this paper, we pay paticular attention to that a-spot causes local Joule's heating then the thermal stress state is simulated by a model depicting the distribution of thermal stresses in a locally heated disk. The critical current densities of a carbon brush are expressed in two ways, each by the thermal shock fracture toughness Δ (=KICk/Eα) and, for the cracking at the adjacent periphery, by the thermal shockresistance Δ (=σ tK/Eα), where KIC is the mode I fracture toughness value, K is thermal conductivity, E is Young's modulus, α is thermal expansivity and σt is tensile strength. Equations of the critical current densities contain also PV-value on the sliding contact, exponent on the contact voltage drop between brush and commutator and so on. These thermal shock parameters were experimentally determined for four kinds of carbon by using a series of disk testing methods. Practical performances of carbon brushes made of these carbons are discussed in relation to the determinded parameters