WO2004103612A1

WO2004103612A1 - A brake pad and corresponding production process

Info

Publication number: WO2004103612A1
Application number: PCT/EP2004/050853
Authority: WO
Inventors: Bruno Greppi; Riccardo Crosa
Original assignee: Braking Italia S.R.L.; Hoganas A.B. (Publ.)
Priority date: 2003-05-20
Filing date: 2004-05-19
Publication date: 2004-12-02
Also published as: ITMI20030995A1

Abstract

A brake pad comprising a base (2) and a friction part (1) obtained by simultaneous sintering of two layers of powder of different composition previously pressed in a single pre-form. Said pad can be produced by successively charging a mould (6) of appropriate form with two layers of powder of appropriate composition, pressing said powder in said mould, and baking the pre-form obtained at an adequate temperature.

Description

A brake pad and corresponding production process

FIELD OF THE INVENTION

The present invention relates to a production process for brake pads, in particular for the production of brake pads comprising a part made of friction material and a supporting part, more in particular made of metal material.

The invention also relates to a brake pad produced using said method. PRIOR ART

Commonly used as friction elements in braking systems, in particular for disk brakes such as those for automobiles and motor vehicles in general, are sintered brake pads, i.e., brake pads in which the friction material is constituted by powder, in general comprising metal powder, which have undergone sintering in an appropriate baking step.

The brake pads of the sintered type are commonly formed by a part made of friction material and a metal support (or base). In general, the friction material is pre-formed in a press, after it has been mixed with an appropriate material in the form of metal powder (copper, iron, bronze, etc.) to obtain the so-called pre-form. This is applied to the support (or base), previously coated with an appropriate material that will be able to guarantee bonding of the two parts (for example, a thin layer of copper) on the side on which the pre-form will be applied. A subsequent treatment in an oven at appropriate temperatures will bring about sintering of the materials constituting the pre-form and adhesion of the pre-form to the base. Grinding and branding of the pad on the side of the friction material complete the process; in particular, grinding is rendered necessary to prevent any deformation that the pad undergoes during the step of baking in the oven. There are various problems connected to the method of production outlined above. The process is burdensome on account of the number of different operations to be carried out for the production of the base (blanking, deburring), for the production of the pre-form separately from the base, and for the assembly described above. Furthermore, it is generally necessary to obtain, on the brake pad, structures that are required for proper operation of the product, for example grooves for cooling on the outer surface of the base, such as to enable circulation of a cooling fluid. In a blanked metal base, said grooves must be obtained, for example, by milling, with the consequent addition of another operation in the production process. Furthermore, the adhesion between base and pre-form takes place during the sintering of the latter, thanks to a layer made of material set between the two elements. This creates a marked discontinuity of composition, and hence of physico-mechanical characteristics between the parts, and may consequently adversely affect resistance to the marked shearing stresses to which the piece is subjected during use. There may moreover exist problems of planarity of the base due to the temperatures of adhesion.

It would consequently be desirable to be able to produce brake pads by means of a simpler and more economical process with a smaller number of operations, which will guarantee an end product that possesses good characteristics of resistance in operating conditions. SUMMARY

The problems set forth above have now been solved by means of a brake pad comprising a base and a friction part, said pad being characterized in that said base and friction part are both made of sintered material. Preferably, said parts are sintered together in a single operation. In this way, the brake pad obtained will be monolithic.

The invention also relates to a production process for making brake pads, which comprises the following operations:

- introduction, in any order, in a mould of appropriate form, of at least one layer made of sinterable material in the form of powder having a composition suitable for constituting, once sintered, the base of a brake pad and of at least one layer made of sinterable material in the form of powder having a composition suitable for constituting, once sintered, the friction part of said pad;

- pressing of the material introduced in said mould; and

- baking of the material at an adequate temperature to bring about sintering of the material.

LIST OF FIGURES The present invention will now be illustrated by means of the detailed description of preferred, but non-exclusive, embodiments, provided purely by way of example, with the aid of the attached figures, in which:

- Figure 1 shows a rear view (on the friction side) of a brake pad according to .the present invention;

- Figure 2 shows a side view of a brake pad according to the present invention;

- Figure 3 shows a front view (on the side of the base) of a brake pad according to the present invention; and

- Figure 4a and 4b are schematic illustrations of an apparatus for charging a mould with powder and pressing the material that is to undergo sintering in two different charging steps.

DETAILED DESCRIPTION OF AN EMBODIMENT

With reference to Figure 1 , a brake pad is illustrated which comprises a friction part 1 and a base 2 made of appropriate sintered material. The two parts are indicated with a different background to distinguish the two different materials that make them up. The pad can be obtained by charging a mould with two layers of different materials in the form of powder and subsequent pressing, by means of one or more pistons. Subsequent baking in an oven causes sintering of the materials, with the formation of the finished piece. It is to be noted that in this way there can be obtained structures on the surfaces directly during the step of pressing thanks to appropriate forms of the mould or of the pistons, for example, the cooling channels 3 on the outer surface of the base or else the grooves 4 obtained on the surface of the friction part (which have, among other things, the purpose of collecting any material that is formed by degradation of the pad during braking, i.e, material which would otherwise accumulate on the surface of the friction part, with consequent deterioration of the braking capacity). The material constituting the base can be of any appropriate type; for example, it may be made of iron and carbon powder, with possible addition of other materials, such as, for example, copper. The same applies to the friction part, which may comprise, according to a possible example of embodiment, iron, graphite and copper, in addition to other materials, such as manganese oxide. Pressing can be carried out in a machine comprising a die or mould and one or more pistons. According to a preferred aspect of the invention, pressing takes place in a mould with two mobile pistons, a bottom one and a top one. The mould can present a through cavity, which can have, for example, a cross section having a perimeter corresponding to the perimeter 5 of the pad to be produced. If the cavity is positioned with walls set vertically, it can be closed at the bottom by a bottom piston. On the surface of the bottom piston there can be charged in succession, and in any order deemed appropriate, two (or more if it were deemed necessary) layers made of material to be sintered, corresponding, for example, to a base and a friction part. According to a particular embodiment, it is possible to charge the cavity of the mould with a first layer, with the bottom piston set in such a way that the cavity will have a depth corresponding to the desired thickness of said layer. In this way, a passage of appropriate mechanical members on the top surface of the mould will have the function of levelling the material charged. At this point, the bottom piston can be displaced downwards for releasing the portion of mould designed to receive the next layer. Charging is thus repeated with a layer of powder that can be different from the first. Finally, a top piston can be introduced in the mould for carrying out pressing.

After pressing, the material pressed, or pre-form, is expelled from the mould, for example pushed outside by the bottom piston. According to a particular aspect of the invention, the surfaces of the pistons will have a form such as to impart an appropriate shape to the surfaces of the pad (for example, as has been seen above, such as to form directly the cooling channels and the grooves).

It is to be noted that baking and cooling of the pre-form obtained by means of pressing of the powder do not cause in general any distortion such as to require subsequent treatments such as grinding of the surface of the friction part, unlike what occurs with a friction part to be sintered that is fixed, during baking, to a base pre-formed by blanking.

Additives of an appropriate type can also be introduced, for example volatile liquids of appropriate composition, according to the knowledge of the person skilled in the art, if particular structures of the product formed are required.

The present invention relates likewise to an apparatus for pressing brake pads comprising:

- a mould, comprising a through cavity;

- means for charging said mould with at least two different solids in the form of powder at different times;

- a bottom piston, which is designed for closing said through cavity at the bottom and is mobile therein; and

- a top piston designed for closing said through cavity at the top.

Preferably, said bottom piston can be moved downwards after charging of one of the two solids.

Figure 4a is a schematic view of an apparatus as outlined above. It comprises a mould 6, a bottom piston 7, and a top piston 8. Furthermore, it comprises a structure 9 that can be translated (in the direction of the arrow A) with respect to said mould 6, by means of appropriate means for movement, for example the fluid-dynamic arms 10. Said structure comprises the hoppers 11 and 12 for the two different types of solid in the form of powder and the pipes 13 and 14 for discharge of the solids. The pipes 13 and 14 can be set facing alternately the chambers 15 and 16 of the slide 17, from which the solids are charged in the mould 6. In the position of Figure 4a, the solid is discharged from the hopper 11. Then, the structure is translated into the position of Figure 4b, with discharge from the hopper 12. Finally, the structure 9 is displaced in a position intermediate between the positions of Figure 4a and Figure 4b, with closing of both of the pipes. A subsequent alternating translation of the slide 17 will set alternately the chambers 15 and 16 facing the cavity of the mould 6, causing charging in succession of the powder and smoothing of the surface of the mould. Preferably, the bottom piston 7 is displaced downwards as mentioned above between charging of one powder and charging of the other. Movement of the piston will be synchronized to that of the slide. At this point, the top piston 8 can intervene for pressing. In synchronism with the descent of the top piston 8, there can occur a partial return upwards of the bottom piston 7, in order to improve the uniformity of the pressure applied to the material. Subsequently, the top piston 8 is extracted, and a movement upwards of the bottom piston 6 causes expulsion of the pre-form ready for baking. These latter movements occur preferably simultaneously. The pressure applied by the pistons during pressing will be chosen so as to guarantee that an adequate resistance is imparted to the pre-form, also in view of the subsequent handling that it will undergo; for example, the pressure applied may be comprised between 400 and 800 MPa.

Sintering of the pre-form takes place by means of baking. The temperatures are chosen according to the composition of the pre-form and the characteristics desired for the end product. According to a possible way of operating, the baking takes place at a temperature comprised between 1050°C and 1150°C with permanence at the maximum temperature for at least 15 minutes. The sintering operation can be carried out in a single heating step or else, if deemed appropriate, in a number of steps. There may likewise be present other heating operations if there are particular requirements. According to a preferred aspect of the invention, the compositions of the powder both of the base and of the friction part include iron as base material; they preferably comprise other elements, such as copper and carbon. Purely by way of example, indicated in Table 1 is a series of intervals of weight percentages for various components present in iron-based compositions both for the base and for the friction part. Table 1

Claims

1. A brake pad comprising a base (2) and a friction part (1 ), said pad being characterized in that said base and friction part are both made of sintered material.

2. The brake pad according to Claim 1 , in which said base and friction part are sintered together in a single operation.

3. The brake pad according to Claim 1 or Claim 2, in which said base and friction part comprise iron, copper and carbon.

4. A production process for making brake pads, comprising the following operations:

- pressing, in said mould, of the material introduced; and

- baking of the material at an adequate temperature to bring about sintering thereof.

5. A pressing apparatus for making brake pads, comprising:

- a mould (6), comprising a through cavity;

- means (11 , 12, 13, 14, 15, 16) for charging said mould with at least two different solids in the form of powder at different times;

- a bottom piston (7), which is designed to close said through cavity at the bottom and is mobile therein; and

- a top piston (8), designed to close said through cavity at the top.

6. The equipment according to Claim 5, comprising a structure (9) that can be translated with respect to said mould (6), said structure comprising at least two hoppers (11 , 12) and discharge pipes (13, 14) of said hoppers that can be alternately set in communication with chambers (15, 16) of an appropriate perforated slide (17) via translation of said structure (9). The apparatus according to Claim 6, in which said perforated slide can translate so as to set said chambers alternately in communication with the cavity of said mould.