WO2005064190A1

WO2005064190A1 - Mechanical device for the transmission of tractive and for braking forces in vehicles

Info

Publication number: WO2005064190A1
Application number: PCT/IT2004/000736
Authority: WO
Inventors: Riccardo Cesarini; Simone Turani; Raffaello Cornolti
Original assignee: Freni Brembo S.P.A.
Priority date: 2003-12-30
Filing date: 2004-12-29
Publication date: 2005-07-14
Also published as: ITMI20032627A1

Abstract

A mechanical device for transmitting the drive force and/or braking force in vehicles is made of a nanocrystalline metal alloy comprising at least 10% wt of a material having a grain size less than 100 nm (100 x 10-9 m) .

Description

DESCRIPTION "Mechanical device for the transmission of tractive and for braking forces in vehicles" The object of the present invention is a mechanical device for transmitting the drive force and/or braking force in vehicles, particularly a wheel or a disc brake caliper. Typically, a disc brake caliper is fastened to the suspension of a vehicle wheel and extends astride a brake disc being fixed to the wheel hub. The caliper supports at least two pads being arranged on both sides of the brake disc, which pads can be clamped against the brake disc by means of suitable stress means, for example hydraulic cylinder/piston assemblies either being housed in the caliper body or connected thereto. When braking, the caliper body is subjected to high mechanical and thermal stress which also favour, with the passing of time, a high corrosion of the material, typically metallic, of the caliper. The mechanical stress acting on the caliper is of a dynamical nature and comprises: - vibrations ; - almost permanent high stress conditions lasting for a few seconds, for example in the case of a controlled braking at red light, up to tens of minutes, for example in the case of a continuous braking during a long sloping tract on a mountain road; - sudden stress and very short strokes with very high stress peaks occurring for example with brakes with ABS or in the case of excessive braking force or in the case of emergency braking. These mechanical stresses are almost always accompanied by a strong increase in the temperature due to the dissipation, by means of friction, of the kinetic energy of the braked vehicle. While braking, the wheels of a vehicle transmit the braking force from the wheel hub through the tyre to the road surface and are further subjected to secondary mechanical stress, such as the side thrust due to the effect of the centrifugal force generated while driving round a corner as well as the impacts and vibrations due to the unevenness of the road. While braking, the wheels are further subjected to a temperature increase due to their close proximity to the brake. While driving without braking, on the other hand, the brake is deactivated and the caliper is only subjected to gravitational and inertial forces, the wheel rim transmits the drive force to the road surface and therefore is permanently subjected to stress. It is therefore required to manufacture the brake calipers and the rims in a material having high breaking strength and high toughness over a wide temperature range . It if further desired that the same material be also sufficiently resistant to corrosion and available at such a cost that would allow it to be widely used in the manufacture of disc brake calipers and performance rims. In order to obtain, in the case of the brake caliper, an even wear of the pads and limit the size and overall weight of the caliper, it is further desired that the caliper material has a high elastic module, i.e. reduced deformability. With reference to the wheel, reducing weight and size, the resistance being equal, or preferably with an increase in the elastic module, is desired in order to decrease the moment of inertia of the wheel and make passage openings, which are useful for brake cooling. On the other hand, it is less important that a disc brake caliper or a wheel has either a high resistance to abrasion or a high surface hardness, since no caliper/wheel outer surface (except the tyre) is intended to frictionally interact either with other components of the brake or vehicle or with the road surface. It is known the use of aluminium alloys in the manufacture of calipers and wheels by means of casting and subsequent mechanical finishing of the semifinished product or through mechanical processing starting from a solid block of material. The mechanical characteristics of the semifinished product are further improved by means of thermal and mechanical treatments, such as quenching and forging. The final caliper is usually subjected to a final anodizing process, for example by means of nickel-plating, in order to make the outer surface resistant to corrosion and scratches. While the disc brake calipers and the wheels manufactured of said aluminium alloys have a high break resistance and a sufficient elastic module at room temperature, yet these properties worsen as temperature increases . The corrosion resistance of the prior art calipers is to be considered as being satisfactory only in case of suitable, and consequently, expensive anodizing by means of a noble metal, such as nickel. Furthermore, the alloy volume changes upon casting and this causes considerable size tolerances of the semiprocessed products, thereby entailing a quite expensive subsequent mechanical processing and a non negligible amount of waste material. The object of the present invention is accordingly to provide a mechanical device for transmitting the drive and/or braking force in vehicles, particularly a disc brake caliper or a wheel with improved mechanical performance, while overcoming the drawbacks mentioned with reference to the prior art . This object is achieved by means of a mechanical device for transmitting the drive and/or braking force in vehicles, which is made of a nanocrystalline metal alloy comprising at least 10% wt of material having a grain size less than 100 nm (100 x 10^"9 m) . The use of nanostructured crystalline materials is known from the technical fields of cutting tools and tools for abrasive processing, their ability being exploited of providing surfaces with extremely high hardness and abrasion resistance. Compared to the conventional crystalline materials that have a grain size, i.e. crystal size, in the range of a few microns (thousandths of millimetres) , the nanostructured material is different in that it comprises at least one part of crystalline material, with a grain size less than 100 nanometres (100 x 10^"9 m) . Due to the small size of the individual grains or crystals, the number of atoms forming the surface, i.e. the contour of a crystal or the crystal interface is considerably increased compared to the total number of crystal atoms, to the extent that the amount of these contour atoms either reaches or exceeds the amount of atoms within the crystal volume. This increase in the contour atoms compared to the inner atoms entails a fundamental change in the mechanical, electrical and physical properties of the material in general, which is due to the fact that the characteristics of the interface are more important than those of the continuous material. The most important characteristics of the nanostructured materials concern, precisely, the resistance to abrasion, hardness and, in some cases, electrical conductivity. It is known that the aluminium nanocrystalline alloys, besides having a high heat resistance, also have a high break resistance at room temperature, but a slightly lower ductility, a lower density and a lower elastic modulus than conventional materials. These characteristics have so far qualified the crystalline material for military, aerospace and aeronautical applications, which make use of its lightness and structural strength without however imposing rigid restrictions to deformations. In the present invention it has been found that several nanocrystalline alloys, particularly the aluminium alloys that will be described below, besides the known heat resistance, also have high resistance to deformation and small volume change upon forming the mechanical devices obtained therefrom. In order to better understand the present invention and appreciate the advantages of the same, some embodiments thereof will be described below, with reference to the annexed figures, in which: Figure 1 is a longitudinal sectional view of a disc brake caliper according to the present invention; Figure 2 is a perspective view of a disc brake caliper according to the present invention; Figure 3 is a perspective view of a wheel according to the present invention. With reference to Figures 1 and 2, a disc brake caliper 1 comprises either one individual body or more bodies that can be composed by means of suitable connecting means, for example two side walls 2 being connected to each other by means of transversal bridges 3 extending astride a space 4 for the brake disc (not shown) , wherein both side walls 2 can be connected either by means of connecting screws being arranged within the transversal bridges 3 or obtained as one piece by casting the entire caliper body. The caliper 1 further comprises fixing means for fixing the caliper 1 to a vehicle wheel suspension. These fixing means are embodied for example by two holes 5 being arranged in one of both side walls 2 and suitable to house respective clamping screws (not shown) that can be screwed to the vehicle suspension. At least one of both side walls 2 supports thrust means, for example one or more hydraulic cylinder/piston assemblies 6 or linear motors suitable to cooperate with at least two pads 7 being arranged on both sides of the disc brake in suitable housings 8 that are obtained in the side walls 2, such that said pads 7 can be clamped to the brake disc. The whole individual body or all the modular bodies of caliper 1 are obtained in a nanocrystalline metal alloy comprising at least 10% wt of material having a grain size less than 100 nm (100 x.10^"9 m) . Figure 3 shows a wheel 10 of the disc wheel type comprising a disc-shaped middle portion 11 provided with means for fixing the wheel to a vehicle wheel hub, for example a plurality of holes 12 suitable to house as many clamping screws (not shown) . This middle portion 11 supports, along an outer periphery 13 thereof, a rim 14 suitable to carry a tyre (not shown) or, more in general, an elastic covering. The wheel 10 can be made either as one piece or composed starting from single components, such as a middle disc to which there is fixed the outer rim by means of a plurality of screws. In accordance with an embodiment of the invention, the wheel is a motorcycle wheel, wherein the rim 14 is connected to the middle portion by means of a plurality of spokes . The whole individual body or at least one of its modular bodies are obtained in said nanocrystalline metal alloy comprising at least 10% wt of material having a grain size of less than 100 nm (100 x 10^"9 m) . In accordance with an embodiment, said metal alloy for the caliper 1 and the wheel 10 is an aluminium alloy, preferably an aluminium and lithium and/or magnesium alloy. Advantageously, the metal alloy further comprises alumina (Al₂0₃) and/or aluminium carbide (Al₄C₃) . In accordance with yet another embodiment, the metal alloy comprises : : - about 5% wt of Magnesium; 0, 1 to 1,5% wt of alumina (A1₂0₃) ; 0,1 to 1,5% wt of aluminium carbide (Al₄C₃) , in which the remainder is aluminium with optional impurities . In accordance with a further embodiment, the metal alloy comprises: about 5% wt of Magnesium; 0,1 to 2.0% wt of aluminium carbide (A1₄C₃) , in which the remainder is aluminium with optional impurities . In accordance with a still further embodiment, the metal alloy comprises: about 3% wt of Lithium; 0,1 to 1,5% wt of alumina (Al₂0₃) ; 0,1 to 1,5% wt of aluminium carbide (Al₄C₃) , in which the remainder is aluminium with optional impurities . In accordance with a still further embodiment, the metal alloy comprises: - about 3% wt of lithium; 0,1 to 2.0% wt of aluminium carbide (Al₄C₃) , wherein the remainder is aluminium with optional impurities. The mechanical device, particularly the caliper 1 or the wheel 10 is preferably obtained by means of a processing method comprising the following steps: high energy milling of a mixture of aluminium powder with lithium and/or magnesium, and adding alumina and/or aluminium carbide powders thereto, such that at for least 10%, preferably 30% to 70% wt of the resulting powder mixture, the grain size is less than 100 nm (100 x 10^"9 ) ; subsequent thermoforming of the powder mixture to form a semiprocessed product of the device 1, 10; - optionally, subsequent mechanical finishing of the device 1, 10. The mechanical device according to the present invention has a number of advantages. It is particularly resisting and shows a low deformability, i.e. a higher elastic modulus than that of similar prior art devices . The optimum mechanical characteristics of the device are substantially unchanged over a wide temperature range, particularly at high temperatures, for example around 200°C - 400°C. Manufacturing the device according to the invention is less expensive than manufacturing similar prior art devices . Due to the use of the nanostructured material described above, the size tolerance of the semiprocessed products results to be very reduced, the amount of waste material is lowered and expensive mechanical processing avoided. Obviously, to the mechanical device according to the present invention, those skilled in the art, aiming at satisfying contingent and specific requirements, may carry out a number of modifications and variations, all being however contemplated within the scope of protection of the invention, such as defined in the annexed claims .

Claims

CLAIMS 1. A mechanical device for transmitting the drive and/or braking force in vehicles, which is made of a nanocrystalline metal alloy comprising at least 10% wt of material having a grain size less than 100 nm (100 x 10^"9 m) . 2. The device according to claim 1, comprising a disc brake caliper provided with: - either one individual body or more bodies that can be composed by means of suitable connecting means, fixing means for fixing the caliper to the vehicle suspension, thrust means cooperating with at least two pads and suitable to clamp said pads against a brake disc, wherein the whole individual body or at least one of its modular bodies are obtained in said nanocrystalline metal alloy comprising at least 10% wt of material having a grain size less than 100 nm (100 x

10^"9 m) . 3. The device according to claim 1, comprising a wheel provided with: either one individual body or more bodies that can be composed by means of suitable connecting means, fixing means for fixing the wheel to a vehicle wheel hub; support means suitable to carry a tyre, wherein the whole individual body or at least one of its modular bodies are obtained in said nanocrystalline metal alloy comprising at least 10% wt of material having a grain size less than 100 nm (100 x

10^"9 m) . 4. The device according to any preceding claim, wherein said metal alloy is an aluminium alloy. 5. The device according to any preceding claim, wherein said metal alloy comprises aluminium and lithium. 6. The device according to any preceding claim, wherein said metal alloy comprises aluminium and magnesium. 7. The device according to any preceding claim, wherein said metal alloy comprises alumina (Al₂0₃) . 8. The device according to any preceding claim, wherein said metal alloy comprises aluminium carbide

(A1₄C₃) . 9. The device according to claim 1, 2 or 3, wherein said metal alloy comprises: about 5% wt of Magnesium; - 0,1 to 1,5% wt of alumina (Al₂0₃) ; 0,1 to 1,5% wt of aluminium carbide (Al₄C₃) , wherein the remainder is aluminium with optional impurities . 10. The device according to claim 1, 2 or 3, wherein said metal alloy comprises: about 5% wt of Magnesium; 0,1 to 2, 0% wt of aluminium carbide (Al₄C₃) , wherein the remainder is aluminium with optional impurities. 11. The device according to claim 1, 2 or 3, wherein said alloy comprises: about 3% wt of Lithium; 0,1 to 1,5% wt of alumina (Al₂0₃) ; 0,1 to 1,5% wt of aluminium carbide (Al₄C₃) , wherein the remainder is aluminium with optional impurities. 12. The device according to claim 1, 2 or 3 , wherein said alloy comprises: about 3% wt of lithium; - 0,1 to 2,0% wt of aluminium carbide (Al₄C₃) , wherein the remainder is aluminium with optional impurities . 13. The device according to any preceding claim, wherein said nanocrystalline metal alloy comprises 30% to 70% wt of a material having a grain size lower than 100 nm (100 x 10^"9 m) . 14. The device according to any preceding claim, obtained by means of the following steps: high energy milling of a mixture of aluminium powder with lithium and/or magnesium, and adding alumina and/or aluminium carbide powders thereto, such that for least 10%, preferably 30% to 70% wt of the resulting powder mixture, the grain size is less than 100 nm (100 x 10^"9 m) ; - subsequent thermoforming of the powder mixture to form a semiprocessed product of the device; subsequent mechanical finishing of the device.