"BRAKE DISC FOR A DISC BRAKE"

[0001] The present invention relates to a brake disc for a disc brake having a braking band made in cast iron.

[0002] The present" invention finds particular but not exclusive application in the automobile sector. In particular, the present invention finds application in high performance vehicles and also on heavy goods vehicles having the need to dissipate, through the brake disc, large quantities of kinetic energy and therefore, subject to elevated mechanical and thermal stresses.

[0003] As is known, in the automobile sector, and in particular for high performance vehicles, the need is felt to make brake discs in cast iron which can resist mechanical and thermal stresses of high intensity.

[0004] Such stresses cause high temperatures to be reached on the braking bands which may lead to reductions in braking performance and even to the formation of cracks on the braking band.

[0005] The need is also felt to make brake discs which have a high resistance to corrosion so as to increase their duration.

[0006] The brake disc solutions of the prior art envisage the use of cast irons. The cast irons used in the art for the production of brake discs are for example described in JP 7305138 A and in KR 20030075972 A.

[0007] In the art types of cast iron which are suitable for using for the realisation of brake discs able to simultaneously ensure high performance in terms of mechanical resistance, thermal resistance and corrosion are not known of.

[0008] The purpose of the present invention is to make a brake disc which overcomes the drawbacks mentioned with reference to the prior art, that is which guarantees high braking performance in terms of mechanical, thermal resistance and resistance to corrosion, in demanding working conditions, without incurring in detectable cracking phenomena.

[0009] According to a preferred embodiment, such drawbacks are resolved by a brake disc comprising a braking band made from a cast iron comprising carbon, silicon, manganese, molybdenum, chrome wherein the percentage in weight of carbon C% is from 3.70 to 3.85%, the percentage in weight of silicon Si% is from 1.30 to 1.60%, the percentage in weight of manganese Mn% is from 0.40 to 0.70%, , the percentage in weight of molybdenum Mo% is less than 0.10%, the percentage in weight of chrome Cr% is from 0.10 to 0.30% and wherein the percentage in weight of carbon equivalent C _eq is from 4.13 to 4.38 , where C _eq = C%+ Si%/3. [0010] According to one embodiment, the percentage in weight of carbon is from 3.70 to 3.80%

[0011] According to one embodiment, the percentage in weight of silicon is from 1.30 to 1.40%,

[0012] According to one embodiment, the percentage in weight of manganese is from 0.60 to 0.70%.

[0013] According to one embodiment, the percentage in weight of manganese is approximately 0.66%.

[0014] According to one embodiment, the percentage in weight of molybdenum is less than 0.05%,

[0015] According to one embodiment, the percentage in weight of chrome is approximately 0.20%.

[0016] According to one embodiment, the cast iron comprises copper in a percentage in weight from 0.20 to 0.35%.

[0017] According to one embodiment, the matrix of said cast iron is of the pearlitic, fine lamellar type.

[0018] According to one embodiment, the cast iron of the braking band comprises a percentage of ferrite of less than 5%.

[0019] Further characteristics and advantages of the present invention will be more clearly comprehensible from the description given below of its preferred and non-limiting embodiments.

[0020] According to one embodiment, the brake disc according to the present invention comprises a braking band made in a cast iron comprising carbon, silicon, manganese, molybdenum, chrome. The braking band may be of any type, shape and size, it may for example be fitted with cooling holes or grooves and is preferably of the self-ventilating type. Said braking band, in the known manner, may be associated to a bell, of any type, material, shape and size, so as to form a brake disc associable for example to a wheel. The bell may be made of the same material as the braking band or in different material. Preferably, the braking band and the bell are made in different material and are coupled by suitable coupling means, such as bushes or pins, or by means of simultaneous melting of the bell and the braking band. Alternatively, the braking band and the bell are made in the same material.

[0021] The braking band is made in cast iron.

[0022] Advantageously, the cast iron used to make the braking band comprises carbon, silicon, manganese, molybdenum, chrome.

[0023] According to a preferred embodiment, the percentage in weight of carbon C% is from 3.70 to 3.85%. The percentage in weight of silicon Si% is from 1.30 to 1.60%. The percentage in weight of manganese Mn% is from 0.40 to 0.70%. In addition, the percentage in weight of molybdenum Mo% is less than 0.10% and the percentage in weight of chrome Cr% is from 0.10 to 0.30%. Preferably, the cast iron of the braking band has a percentage in weight of carbon equivalent C _eq which is from 4.13 to 4.38, where C _eq = C%+ Si%/3. Such formula for calculating C _eq is normally used in the field of metallurgy .

[ 0024 ] According to one embodiment, the percentage in weight of carbon is from 3.70 to 3.80%

[ 0025] According to one embodiment, the percentage in weight of silicon is from 1.30 to 1.40%,

[ 0026] According to one embodiment, the percentage in weight of manganese is from 0.60 to 0.70%; preferably said percentage in weight of manganese is approximately 0.66%.

[ 0027 ] According to one embodiment, the percentage in weight of molybdenum is less than 0.05%,

[ 0028 ] According to one embodiment, the percentage in weight of chrome is approximately 0.20%.

[ 0029] According to one embodiment, the cast iron comprises copper in a percentage in weight from 0.20 to 0.35%.

[ 0030 ] Preferably, the matrix of said cast iron is of the pearlitic, lamellar type. Such type of matrix behaves so as to oppose the formation and subsequent propagation of cracks due, for example, to a combined effect of mechanical and/or thermal stresses.

[0031] Preferably, the cast iron according to the invention comprises a percentage in weight of ferrite of less than 5%.

[0032] Preferably, the cast iron according to the invention comprises a percentage in weight of cementite and free carbons of less than 1%.

[0033] The braking band according to the present invention makes it possible to contain and in part overcome the drawbacks presented in relation to the prior art. In particular such braking band makes for a considerable reduction in the formation of thermal- mechanical cracks compared to the discs of the prior art.

[0034] In addition the large quantity of graphite gives the cast iron and the disc improved heat conductivity which enables elevated heat exchange during the braking shock .

[0035] The risk of formation of thermal stress cracks is thereby significantly reduced even in conditions of heavy use of the braking system.

[0036] In addition, the braking band of the present invention has a high damping capacity which helps to increase resistance to the formation and propagation of cracks .

[0037 ] Thanks to the percentages of silicon provided for in the cast iron which the present invention relates to, a graphitising effect is produced, avoiding the formation of cementite and carbons.

[ 0038] In particular, the cast iron used to produce the braking band has a percentage of carbon equivalent Ceq of 4.13 to 4.38.

[ 0039] Such Ceq value is contained compared to the percentages of carbon equivalent of the cast irons of the prior art. It should be noted that such a limited value is obtained by means of a relatively high percentage of carbon, of 3.7 to 3.85 and, at the same time a limited percentage of silicon, of 1.3 to 1.6.

[ 0040 ] The limited C _eq value ensures that a material with a good graphitising effect is obtained, at the same time avoiding the formation of cementite and carbons.

[ 0041 ] This aspect is particularly advantageous given that the cementite and the carbons constitute particularly fragile inclusions which could easily trigger the formation and propagation of cracks.

[ 0042 ] At the same time, the graphitising effect is obtained with a particularly low percentage of silicon. The low percentage of silicon improves the fatigue behaviour of the cast iron, and thereby of the braking band, given that silicon is an element which increases the rigidity of the material and thereby its fragility.

[ 0043] At the same time the relatively high percentage of carbon, from 3.7 to 3.85 which would tend to increase the fragility of the material is offset by the low percentage of manganese.

[ 0044 ] In other words, the combined use of a low percentage of manganese and of a low percentage of silicon offsets the relatively high percentage of carbon.

[ 0045] As a result, with the percentages of carbon, silicon and manganese envisaged by the present invention a low value of carbon equivalent can be achieved, which permits a high graphitisation and good thermal and mechanical resistance. Such advantages are obtained by maintaining a low value of silicon and a relatively high quantity of carbon.

[ 0046] Moreover, the low percentage of manganese and of silicon improves the ductility of the cast iron offsetting the fragility caused by a relatively high percentage of carbon.

[0047 ] From some experimental tests carried out it was seen that the braking band made with a cast iron having the aforesaid percentages of components, has a resistance to breakage by fatigue substantially double that which can be achieved with cast irons of the prior art .

[0048] Moreover, the chrome and the molybdenum stabilise the formation of lamellar pearlite in the metal matrix.

[0049] Thanks to the present invention brake discs can be designed having high performance geometries and with frequency levels on average lower than the discs of the prior art and therefore improved in terms of driving comfort .

[0050] In fact it has been observed that compared to the discs of the prior art the discs according to the present invention show a lowering of the frequencies of over 5%.

[0051] The present invention is particularly advantageous for applications to brake discs of high performance vehicles but also to brake discs of heavy goods vehicles which similarly to high performance vehicles needs to dissipate large quantities of kinetic energy and are thereby particularly subject to the formation of cracks as a result of repeated thermal and/or mechanical stresses.

[0052] A person skilled in the art may make numerous modifications and variations to the brake discs described above so as to satisfy contingent and specific requirements while remaining within the sphere of protection of the invention as defined by the following claims .