A REPORT on the investigation of brake-lining materials by the Bureau of Standards was made by the author in 1922. The present paper gives information on work done in this field since that time. It places on record a summary and discussion of various test-methods and equipment at present employed by brake-lining manufacturers and others in the automotive industry. The difficulties connected with this work, resulting from the varying characteristics of brake-lining materials, are brought out. It is shown that some of the test methods in use do not furnish a basis for ready or fair comparison of different brake-linings. Other test procedures are so limited as to give only an incomplete picture of the characteristics of the brake-linings under conditions met in service; therefore, the test schedules generally require readjustment and amplification because a full and satisfactory knowledge of these materials can be obtained in this manner only. Finally, the essential features to be given consideration on the basis of present knowledge are reviewed and proposals are made which it is believed should, with reasonably limited tests, give a satisfactory picture of the character of brake-linings and a comparison of their relative merits.
The discussion of the paper consisted wholly of written contributions. Brake-maintenance information relative to noise and to the effect of grease on brake-linings, together with an outline of the causes of friction variation and the effect of water on brake-lining, the lubrication of brake linkage, and details relating thereto are included in the first portion. One discusser argues the desirability of using one type of testing machine. Another maintains that the temperature of the test is the keynote. A third maintains that the nature of the particular problem in hand influences laboratory tests of brake-lining. These and various other features are commented upon by the author.
Other features of the discussion include a statement of the advantages of the Carson type of machine, comments on the relative importance of the variation of the coefficient of friction, the advantage of intermittent tests, and the placement of emphasis on the desirability of agreement as to methods of testing brake-linings.