[Catalog:News] [Date:2019-10-16] [Hits:] [Return]

Brake pad compression is influenced by the compressive stiffness of the friction material, and theflexure of the backplate as explained in Chapter 5referring to friction interface pressure distribution. Modern brake pad friction material can be assumed to be linear in tension and compression over ±3% strain, and using a disc pad friction material compressive modulus of 3.5 GPa, assuming the pad is compressed uniformly by the actuation force, for a pad area of 3500 mm2, and thickness 12 mm, the compressive stiffness and compressibility are approximately 1.0 × 106 and 1.0 × 10−6 mm/N respectively. At a line pressure of 100 bar, the compression of the pad assembly, assumed uniform, is therefore 0.01 mm. The fluid consumption (for a 57 mm diameter piston) would be 17 mm3. Because of flexural effects in the brake pad assembly (Chapter 5), disc brake pads do not experience uniform compression even where there is more than one actuation piston on each pad; therefore, this is a conservative estimate, which may be doubled to 34 mm3 to provide a more realistic approximation. In sliding calipers, the outboard pad actuation force is usually applied via ‘fingers’ spaced apart to spread the load more uniformly. The process of wear causes the interface pressure distribution to tend towards uniform, but this does not necessarily reduce the flexure of the pad assembly when the actuation force is applied and removed because of the clearances generated between the friction material and the disc at the friction interface.

Drum brake shoes tend to run at larger clearances than disc brake pads, although sliding abutmentshoes enable some of the clearance to be taken up by translation of the shoe rather than actuation displacement. At high actuation forces, flexure of the brake shoe can affect wear and pressure distributions (see Chapter 5), which can increase fluid consumption and thus actuator travel, but on modern cars and light vans, drum brakes are only fitted to rear axles and the braking duty is relatively low.