Drum Brake

The invention refers to a drum brake of the type comprising two shoes with friction linings mounted in a slidable manner on a plate along two substantially opposite radial directions between respective facing guide wall pairs, integral with the plate, and driving means which push the shoes against a drum rotor in response to a braking command.

For a better understanding of the state of the art and the problems pertaining thereto, a drum brake of conventional design illustrated in figure 14 will first be described. Referring to figure 14, a drum brake of known type is indicated in its entirety with 10, comprising a disc-shaped plate 11 adapted to be fixed to a suspension arm of a vehicle, a primary brake shoe 12 and a secondary brake shoe 13 having respective ends 12a, 13a between which a driving cylinder 14 is interposed, and second ends 12b, 13b between which a reaction element 15 is interposed, integral with the plate 11. The brake moreover comprises a drum rotor, not shown, integral with the hub of the wheel and which surrounds the shoes 12, 13 with clearance.

Each brake shoe 12, 13 is composed of a rigid support 20 in half-moon shape, a band 21 welded or riveted on the radially outer part of the support 20 and a friction lining 22 applied on the radially outer part of the band 21, the lining being able to come into contact with the drum during braking. The second ends 12b, 13b of the shoes 12, 13 are maintained in abutment at the reaction element 15 by means of a connection spring 17.

The drum brake moreover comprises a return means 18 of the shoes towards the centre of the plate 11, formed in part by two elastic elements and a crossbar 19 arranged parallel to

the axis of the driving cylinder 14. Known means 22 for compensating of the lining wear are associated with the crossbar 19. The crossbar 19 is maintained in contact with the shoes 12, 13 by respective retaining springs (not shown).

The driving cylinder 14 is hydraulically controlled by a brake pedal for moving away the first ends 12a, 13a of the shoes, forcing them towards the outer drum.

During braking, with the drum in rotation in the direction indicated with the arrow F, four forces can be distinguished which are applied to each shoe. In brief, only the forces acting on the primary shoe 12 are described here. A first force called entrance force Fe is applied in the contact zone between the cylinder 14 and the first end 12a of the shoe 12, a second force called application force Fa is applied in the contact zone of the lining 22 with the drum, a third force called braking force Ff is applied at the contact point of the second end 12b of the shoe with the reaction element 15 and a fourth force called reaction force Fr is also applied in the contact point of the second end 12b of the shoe with the reaction element 15 (fixed point) .

The fact that the braking force Ff, the reaction force Fr are applied in a same point favours the so-called self-locking by the primary shoe's winding on the drum, which causes an early wear of the related lining and renders the braking torque unstable .

It is therefore an object of the present invention to provide an improved drum brake, mainly facing the problem of optimising the distribution of the forces and reactions which occur during braking in order to improve the reliability of the braking and to avoid the above mentioned self-locking.

These and other objects and advantages, which will be better understood below, are attained according to the present invention by a drum brake having the features set forth in the attached claims.

A feew preferred but not limiting embodiments will now be described of a drum brake according to the invention; reference is made to the attached drawings, in which: Figure 1 is a simplified, partially sectioned elevation view of a drum brake according to the invention; Figure 2 is an axial section view along line II-II in figure

1; Figure 3 is an elevation view of a component of the drum brake of figures 1 and 2;

Figure 4 is a section view along line IV-IV in figure 3; Figure 4A is a view, similar to that of figure 4, of a variant of a detail of the invention; Figure 5 is a side view of a component of the drum brake, in an alternative embodiment to that of figures 3 and 4; Figure 6 is a partial elevation view of a drum according to the invention with the component of figure 5, sectioned along line VI-VI in figure 5; Figures 7-9 illustrate variants of a connecting rod of the brake according to the invention; Figures 10 and 11 are perspective views of two blocks with friction lining for a brake according to the invention; and Figures 12 and 13 show an elastic element associable to the component of figure 3; and Figure 14 is a simplified elevation view of a drum brake of known type.

With reference now to figures 1 and 2, a drum brake according to the invention comprises a backing plate 10 fixable to a suspension arm (not illustrated) of a motor vehicle. Two

blocks 20, opposite according to a diameter direction X-X, are slidably mounted along this direction between two respective facing guide wall pairs 41a, 41b and 51a, 51b axially projecting from the plate 10. In the example of figures 1-4, the aforesaid guide wall pairs are formed by respective parallel columns 41, 51 whose bases are integral with the plate 10, for example by means of bolts 46 (figure 2), while their ends distant from the plate are rigidly connected to each other by means of a tie-rod 60, whose function will be explained below. Alternatively, as illustrated in figures 5 and 6, a pair of guide walls 41a, 41b can be formed from a body 45 (for example of metal plate) which forms in addition to the aforesaid facing walls also a base 42 with holes 43 for the fixing to the plate 10 and a tie-rod portion 60 which rigidly connects the ends of the walls 41a, 41b distant from the plate 10.

Each block 20 is equipped with a brake lining 21, preferably fixed by moulding or gluing on a respective rigid support 22 formed starting from a metal plate folded as a C so to form a pair of parallel lateral flanges 23, 24 which extend from a radially outer portion 25. This portion is convex and curved with a curvature corresponding to that of a drum 80, illustrated only partially and integral in rotation with the wheel of the vehicle along the direction of arrow F in figure 1. The linings 21 are fixed on external cylindrical surfaces of the curved support portions 25 (figures 1 and 6) . The lateral flanges 23, 24 have a pair of opposite parallel surfaces 26a, 26b (figures 10 and 11) adapted to be interposed and radially guided between the guide walls 41a, 41b and 51a, 51b, maintaining a radial translation freedom. Throughout the present description and in the claims, terms and expressions indicating positions and orientations such as "axial", "radial", are intended to refer to the central rotation axis of the drum 80.

The angular extension A of each block 20 can be in the range of 40° - 80°, this numeric example not being limiting. The angular extension A is advantageously less than that of a classic drum brake block, which permits considerably diminishing the volume of the friction lining 21, preserving however a considerably equivalent lifetime. This volume reduction of the lining with regard to a traditional drum brake can reach approximately 60%.

To exert a braking action, forcing the blocks towards the inner cylindrical surface of the drum, a driving group is provided comprising a hydraulic cylinder 30 and a pair of leverages 31 interposed between the hydraulic cylinder and the respective blocks 20. Each leverage comprises a lever 32 and a connecting rod 33. The hydraulic cylinder, when driven, moves a pair of pistons 34 away from one another in opposite directions, the pistons acting on first ends 32a of the levers 32 which have second ends 32b articulated on axial pins 35 integral with the plate 10.

In an intermediate position between the pin 35 articulation points and the contact points with the pistons 34, and preferably in a position comprised halfway between the levers and 1/3 of the distance with respect to the pins 35, each lever

32 has a pin 36 for the articulation of the related connecting rod 33 thrustmgly coupled to the related block 20. In the examples illustrated in the drawings, the connecting rods

33 have curved radially outer heads 33a which abut against radially inner surfaces 25a of the curved portions 25 of the supports 22 in order to force the blocks 20 into substantially radial directions. The connecting rods can be bound to the blocks 20 by means of one or more pins 37, as illustrated for example in figures 7 and 9, or with elastic retaining elements (not shown) .

A command lever 70 of the hand brake is pivoted about an axis 71 centred on the lever 32 near its end 32a. The end 72 of the lever 70 opposite the axis 71 is folded to facilitate the coupling of a traction cable (not shown) which passes on a guide 73 fixed on the plate 10.

A crossbar 74 is provided between the lever 70 and the two levers 32, 32 for limiting the return stroke of the ends 32a towards each other under the action of a first elastic return means 75 taut between zones close to these ends. The crossbar 74 is preferably associated to an automatic wear compensating device 76 of the brake lining adapted to automatically modify the length of the crossbar itself in order to compensate the wear of the brake lining.

Operation of the drum brake of figures 1 and 2 is described herein after. It is assumed that the drum 80 turns in the direction of the arrow F. By driving the brakes, liquid under pressure is sent into the cylinder 30 between the pistons 34, which move away from each other, pushing the ends 32a of the levers 32. These levers respectively rotate on the pins 35 and exert a predominantly radial thrust by means of the connecting rods 33 against the supports 22 of the associated blocks. Each block slides radially between the respective guide wall pairs 41a, 41b, 51, 51b and comes into abutment against the inner surface of the drum 80 to perform the braking.

A reaction of friction acting on the lining 21 applied on the outlet edge, in the direction of the arrow F, pushes the supports 22 in abutment against the guide walls 41a and 51a, respectively. The block 22 then applies a braking force Ff against the guide wall 41a, in the direction of the arrow Ff. A reaction force Fr is then applied along the arrow Fr on the end 32b of the lever 32 against the pin 35. Consequently, the

braking force Ff and the reaction force Fr are applied in two separate points, considerably increasing the stability of the braking torque.

Due to the lever 32, and according to the distance of the pin 36 from the fixed pin 35, the force applied by the piston 34 to the end 32a of the lever 32 is doubled or tripled at the level of the pins 36 and support surfaces 25a of the connecting rods 33 against the blocks 20. The compression work of the two blocks permits reducing the volume of the friction material with respect to a classic drum brake in which one of the blocks is compressed and the other is drawn or taut.

According to the invention, the forces are better divided on the plate 10 and the stability of the braking torque is improved due above all to the fact that the facing guide walls 41a, 41b and 51a, 51b of each pair are rigidly connected to each other at their ends distant from the plate 10, by means of the respective tie-rod 60 capable of transferring part of the braking forces Ff from the wall 41a, 51a arranged downstream (considering the rotation direction of the braking wheel) to the wall 41b, 51b arranged upstream.

With reference to figures 12 and 13, an elastic element is indicated with the number 44 which is adapted to be snap- mounted (or fixed in another manner) on one of the columns 41; the elastic element has one or more flexible portions 44a adapted to elastically force the block 20 towards the opposite column which has the other guide surface 41b, in order to cancel tangential clearances between the blocks and the surfaces 41a and 41b (or 51a and 51b) which could generate noise due to the vibrations to which the vehicle is subjected.

In the example of figure 4A, the tie-rod 60 is fixed to the

free ends of the columns 41 by means of conical head screws 41c which engage in conical seats 41d and 6Od formed on the columns 41 and through the tie-rod 60. This arrangement yields a centring action that guarantees an accurate distance between the facing walls 41a, 41b and 51a, 51b for guiding the blocks accurately.

It is intended that the invention is not limited to the embodiments described and illustrated here, which are to be considered as embodiments of the drum brake. Rather, the invention is susceptible to modifications related to shape and arrangement of parts, structural and operational details, as well as employed materials, as will be apparent to those skilled in the art.