Friction Lining Requirements
In the book “Brake Friction Products for Road Vehicles” issued by Textar it states: “Friction lining formulations always represent a compromise between desirable and achievable characteristics”. Regarding the formulation it is therefore imperative to find an acceptable compromise between the customers demands and the respective production technology.
Experience and instinct are required to solve this problem. In light of this, it is understandable that such a formulation ranks among the best kept secrets of any manufacturer.
However, the “stuff”, from which brake friction products are made, is no secret. It can be composed of the following raw materials: bonding agents (in the form of resin and rubber), organic and anorganic fillers (like chalk or iron oxide), lubricants (of graphite or coke powder) and metals (of steel wool or powder).
Thus only now do the various additives and the matrix, with a total of up to 25 substances, produce the actual
brake friction material.
It can of course be done in a more simple manner. A brake friction product, with a high friction coefficient, can be produced from just three components, 30% steel wool, 55% petroleum coke and 15% binding resin. Such brake friction products are however void of any comfort. This comfort can only be achieved by making the complex calculations required for the ideal brake friction formula. This requires a high degree of specialist knowledge.
The practical process of blending, pressing and hardening also places high demands on know-how. Apart from this, an even distribution and compaction of the raw materials during the blending and pressing processes is of vital importance for the brake friction characteristics. The choice of a suitable mixing machine as well as the blending sequence are amongst the manufacturers best guarded “secrets”.
Here are some of the criteria, which are of vital importance when manufacturing brake friction products:
Friction materials are split into five main groups. Each has its own history, stands for an individual product philosophy and satisfies the needs of a particular market.
TMD offers material groups A) to C) for the passenger and commercial vehicle sectors, depending upon application area and market requirements.
A) Semi-metallic friction materials are mainly used in the US American domestic market. As the name implies, these contain a high proportion of steel fibers. The friction coefficient is approx. 0.35, is not very heat resistant but has a low wear value, which preserves the disc. The low friction coefficient normally produces a good noise characteristic. The high proportion of steel fibers guarantees a favorable pricing.
B) Steel-free friction materials are typically for the Japanese market. The characteristics of this category are comparable to category A. The material does not, however, contain any steel fibers and the material mix is
C) NAO (Non-Asbestos Organics) conforms with the European friction material philosophy. Depending upon the area of application, these materials have a friction coefficient of between 0.35 and 0.5 and contain a small percentage of steel fibers. With regard to the manufacturing costs, the materials lie between categories A) and B) but offer the highest performance braking characteristics. Unlike categories A) and B), European legal standards are always fulfilled by NAO materials.
D) Ceramic friction materials contain a certain proportion of ceramic fibers, as the name implies but do not normally contain steel fibers. The materials tested by us displayed an extreme lack of pedal feel and braking power and for this reason were rejected by our customers. Over and above this, they would not fulfil the legal requirements in Europe and already had trouble gaining acceptance of the US American standard FMVSS135. TMD will no longer pursue this family of materials.
E) Sintered friction materials are manufactured for special applications like motor cycles, industrial applications and rail vehicles.
1. Bedding properties
2. Cold friction coefficient
3. Speed friction coefficient
4. Friction coefficient under temperature load
5. Friction coefficient after temperature load
6. Friction coefficient under wet conditions
7. Friction coefficient under the effects of thawing salt
8. Friction coefficient under pressure
9. Static friction coefficient
10. Friction coefficient when reversing
11. Expansion and shrinkage
13. Heat transfer
14. Strength (cracks, tear-off)
16. Corrosion resistance
17. Bending strength
19. Vehicle vibrations
20. Pedal feel
22. Smoke development
23. Application force
24. Wheel fouling
25. Environmental pollution by abrasive wear
For economic efficiency:
26. Lining wear
27. Counter material wear
29. Production costs