Patent GB-1415193-A (GB)
Title: Disc brakes
IPC: F16D55/14
Filed: 1972-03-23
Abstract: 1415193 Friction elements; disc brakes GIRLING Ltd 20 March 1973 [23 March 1972 23 Jan 1973] 13562/72 and 3303/73 Heading F2E A spot-type or a multi-plate type disc brake has a friction element 6 on a first brake member engageable with a second brake member, the friction element 6 comprising friction material subject to a change in its coefficient of friction relative to the second member during bedding of the friction element, and having an initial friction surface 16 which is adapted to engage the second brake member and to be worn away before the whole friction surface of the element 6 is engaged, the initial friction surface 6 having an area less than the area of the whole friction surface and having an effective radius larger than the effective radius of the whole friction surface if the coefficient of friction increases during bedding but smaller if the coefficient of friction decreases during bedding. The effective radius is considered to be the radial distance of the centre of pressure of the friction element 6 from the axis of the brake. Embodiments of the invention as illustrated relate to a multi-plate disc brake comprising two pressure plates 1, 2 and two rotatable discs 3, 4 carrying friction elements 6 slidably mounted on splines 7 of a rotatable shaft. To actuate the brake the pressure plates 1, 2 are rotated in opposite directions and urged apart axially to engage the discs 3, 4 which are moved to respectively contact on end wall 8 of a housing 5 and and an end cover.  The means for angularly moving the plates 1, 2 may comprise a pull rod connected to divergent links respectively connected to the plates 3, 4 or they may comprise a cam on an angularly movable shaft. Balls 9 working in complementary inclined ramps 10 act to cause axial movement of the plates 1, 2 and as shown the balls 9 are also located in axial grooves 11 in the housing 5 so limiting angular movement of one pressure plate; in another arrangement the balls are located wholly between the plates and the angular movement of one plate is arrested by engagement of a lug on the plate with a stop abutment on the housing.  Each disc 3, 4 is centrally stiffened by a plate 12 and the friction elements 6 are spaced radially outwardly of plates 12. Radial grooves 15 for cooling purposes divide the friction element into segments. As shown in Figs. 1, 3 each friction element 6 is secured to its plate by staggered rivets 13 and is of stepped form defining the initial friction surface 16, a second friction surface 17 and a third friction surface 18. During bedding in, the initial friction surface 16 and then the second friction surface 17 wear, the coefficient of friction increases and the braking torque will act at a decreased effective radius. In the embodiment of Fig. 4 the second surface 16 is formed by four concentric triangular section portions 21 and as the initial surface wears away the effective area of the second surface 17 increases. There can be any number of portions 21 and the surface 16 can itself consist of a plurality of concentric bands. The portions 21 can be in spiral configuration. In further embodiments there is no third friction surface as such. Fig. 5 (not shown) illustrates the friction element with a continuously decreasing axial thickness from the initial friction surface at its outer periphery to its inner periphery whilst in Fig. 6 (not shown) the radial dimension of the initial surface is about one-quarter the radial dimension of the element. In Fig. 7 (not shown) the initial friction surface is set in by a small amount from the outer periphery of the element and in Fig. 8 (not shown) the element is stepped to give an initial friction surface and a second friction surface, the radial dimension of the initial surface being substantially half the radial width of the element. In Fig. 9 the friction element is angularly stepped and surfaces 16, 17 are defined by regions of generally segmental outline, but other shapes may be used. In yet further embodiments the initial friction surface can comprise a plurality of concentric annular surfaces spanning the radial width of the element, and the friction element can be crowned to provide the initial braking surface by adding friction material the same as or different from the material of the element. The surfaces may be formed by moulding or machining.