Development and Progress of Molded Brake-Lining 290045
FIRST reviewing briefly the history of molded brake-lining, the author states that the introduction of molded lining has, until recently, met with considerable opposition. After the first volume-production adoption in 1924, there were no further adoptions of the strictly molded types in production until 1927, when the trend in brake design seemed to change suddenly from the external type to the internal type of brake. The present movement toward the use of molded brake-lining was brought about through the inability of woven lining to meet the exacting demands of some of the newer types of internal brake. In the author's opinion, the molded type of lining has more nearly fulfilled the present requirement of internal brakes than has any other type. He states that at least seven different brands of molded lining are now on the market, and that three of them are in large-volume production.
After discussing the characteristics of woven brake-linings, the author explains that the general procedure for the manufacture of molded lining is to mix the binding compound and the asbestos fibers thoroughly, press the mixture into plastic sheets and cut them into strips of the proper width and length, shape them, and cure them in molds which are subjected to pressure and heat. The complete inter-mixture of binding compound and fiber produces a homogeneous material that has a high degree of uniformity. The homogeneity of the material makes the kinetic friction nearly constant over a normal temperature-range. The material is practically incompressible and its consistency makes close machining possible.
Claims made by the author in favor of molded linings are that they are practically free from objectionable noises, are virtually incompressible, do not move on the rivets, and that it is possible to make them so that the coefficient of friction is uniform. There is only a small variation between the values of the kinetic and the static coefficients of friction, and molded linings add rigidity to the brakes.
In conclusion, the statement is made that, according to indications, a well-adjusted set of brakes equipped with molded lining will give a normal service of between 20,000 and 30,000 miles. The author is convinced that the surest way to improve upon the present brake is to add metal to the drums and use molded brake-linings. In his opinion, the brake-drum, and not the brake-lining, is the weakest part of the braking system.
In the discussion is presented an outline with regard to a mathematical analysis of braking forces in a motor-vehicle as a foundation on which efficient design of brakes can be based. By the means outlined, the dynamic braking-load can be computed.
Experiences of the operator of a brake-service station with regard to brake-testing and to brake-lining material of the molded and the woven types are related, and this operator is convinced that molded lining is not desirable on any brake operated by foot power unless the braking mechanism was specially designed for using molded brake-lining material. Ideal braking-requirements and the present status of brake difficulties are stated by another discusser who believes that a set of standards or designation numbers for brake mechanisms should be devised.
A discussion of the effect of changes of the coefficient of friction of linings used with self-energizing brakes is included, as well as of conditions relating to the shift of the center of gravity forward when making a quick stop.