Brake Tests

260 pass off with the sparks, a proportional amount will produce a rise in the temperature of the shoe as a whole. 506. However, it is not reasonable to look for evidences of the effect of the temperature of the whole brake shoe on the coefficient of friction, when it is appreciated that the temperatures at the working surfaces greatly exceed the maximum temperatures ever reached by the shoe as a whole. Consequently for a correct understanding of the relation between temperature and coefficient of friction it is neces- sary to examine minutely the phenomena which occur during the development of brake shoe friction and to study the action of the materials immediately concerned in this process. 507. To illustrate how the slight inequalities of the two surfaces in contact interlock and resist relative movement, the following sketch has been made to an enlarged scale. If the top surface is regarded as stationary any movement of the lower surface in the direction indicated by the arrow will be resisted by a force made up of two components:— First—Tearing or abrasion of some of the surface projections which are interlocked. In other words, the motion of one surface with respect to the other can be accomplished by shearing off the interlocking pro- jections as indicated by the dotted lines in the sketch. Second—By the lifting or unlocking of some of the surface projections which would require forcing the surfaces apart against the normal pressure until the minute projections on the surface succes- sively cleared. This resistance component involves the performance of work in separating the surfaces against the normal pressure through the medium of the (for this purpose) inefficient surface angles of the minute projections. 508. The first component, namely, tearing or abrasive action between the minute interlocked portions of the bearing surfaces is the important component of the two. This is particularly true in the case of the friction developed between a cast iron shoe forced against a