CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority from Italian patent application no. 102019000021813 filed on 21/11/2019, the entire disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The invention relates to blocks for wheel brakes with an improved resistance to wear.

BACKGROUND ART

In the railway industry, the aforesaid blocks for wheel brakes are usually used in trains with a medium/low speed (a maximum speed of 160 Km/h), with large masses and subjected to frequent braking, such as city trains.

One of the problems the industry is faced with concerns the wear of the blocks. A problem like this necessarily has strong consequences in terms of productivity. This problem is even more serious when dealing with city trains, which are subjected to many braking operations.

Therefore, the need was felt to have a brake block for wheels whose technical characteristics were such as to ensure a reduction in wear without, obviously, worsening the effectiveness of braking.

The inventors of this invention unexpectedly found out that, if the block comprises inserts made of a material having a compressibility modulus which is smaller than the one of the material making up the remaining part of the block, a significant wear reduction can be obtained. The technical solution described below offers the great advantage of significantly reducing the wear of the block without affecting the effectiveness of the braking at all, which is also demonstrated by the following analysis of the friction coefficients between the friction surface and the wheel.

DISCLOSURE OF INVENTION

The subject-matter of the invention is a block for wheel brakes comprising a matrix made of a first material and having one or more inserts included inside said matrix and made of a second material; said one or more inserts (3; 13) made of said second material being arranged in a symmetrical manner relative to a transverse middle plane (X); said block being characterized in that said second material has a compressibility modulus (pressure necessary to compress the material by 1mm) lower than that of said first material by at least a value equal to 10 MPa; said shoe having a friction surface composed of 95 to 60% of said first material and 5 to 40% of said second material; said first material being a sintered material.

Indeed, experiments have shown that, if the friction surface consists of the first material in a quantity smaller than 60%, the effectiveness of the braking is jeopardized.

Hereinafter by "friction surface" of the block we mean the surface of the block in contact with the wheel when the braking action is exerted.

Said first material preferably has a compressibility modulus ranging from 20 to 40 MPa and said second material preferably has a compressibility modulus ranging from 2 to 15 MPa. BRIEF DESCRIPTION OF THE DRAWINGS

Hereinafter there is a description of embodiments of the invention, by mere way of explanatory and non-limiting example, with reference to the accompanying drawings, wherein: - figures 1 and 2 are a side view and a bottom view, respectively, of a first embodiment of a block for wheel brakes according to the invention;

- figures 3 and 4 are a side view and a bottom view, respectively, of a second embodiment of a block for wheel brakes according to the invention;

- figures 5 and 6 are two diagrams showing the friction coefficients and the temperature changes of the friction surface detected for two respective blocks of figure 1 subjected to a plurality of braking operations; and figure 7 is a diagram showing the friction coefficients and the temperature changes of the friction surface detected for a comparison block subjected to the same plurality of braking operations as in figures 5 and 6.

BEST MODE FOR CARRYING OUT THE INVENTION In figure 1, number 1 generically indicates, as a whole, a block for wheel brakes according to the invention.

The block 1 comprises a matrix 2 made of a sintered material (first material) used in the prior art to manufacture the entire block. The block 1 further comprises six inserts 3 with a cylindrical shape, which are made of a material (second material) having a compressibility modulus which is smaller than the one of the first material making up the matrix 2.

The inserts 3 are arranged in a symmetrical position relative to a transverse middle symmetry plane X.

The block 1 comprises a friction surface 4, which is occupied by said first material for 93% and by said second material for 7%.

In particular, for manufacturing the inserts 3, two different materials (second material (a) and second material (b)) were alternatively used. In particular, the second material (a) is an organic material, whereas the second material (b) is a sintered material.

Table I shows the physical features of the materials.

Table

According to the data of Table I, the second material (a) differs from the first material in terms of compression, in terms of hardness and in terms of density; whereas the second material (b) differs from the first material only in terms of compression.

The materials with a different compressibility can also be obtained, besides from materials with a different chemical nature, by means of a different process of preparation of a same chemical type of material. For example, compressibility can be changed by acting upon the porosity of the resulting material.

The materials of Table I were used to manufacture three different blocks: block std (comparison block) entirely manufactured with the first material, block A of the type of block 1 of figures 1 and 2 and whose matrix 2 is made of the first material whereas the inserts 3 are made of the second material (a); block B of the type of block 1 of figures 1 and 2 and whose matrix 2 is made of the first material whereas the inserts 3 are made of the second material (b).

The blocks described above were subjected to a plurality of braking actions both under dry conditions and under wet conditions. For each one of said braking actions, the temperature change of the friction surface of the block, the friction coefficient and the wear of the material were measured. The results are shown in the diagrams of figures 5, 6 and 7. The difference between blocks A and B and the comparison block std lies in the presence of the six inserts 3. In other words, the comparison block std is entirely made of the same material making up the matrix in blocks A and B.

From an analysis of the diagrams it is evident that the presence of the inserts with a smaller compressibility modulus does not affect at all the temperature difference on the friction surface, the friction coefficient and, hence, the braking efficiency.

Tables II-IV show data concerning the wear of the materials following braking actions to which the respective blocks are subjected, specifically analysing, by way of example, three sample braking actions.

In particular, Tables II-IV show the energy applied in each single braking action, the wear caused by the single braking action in terms of material reduction volume in cm ³ and also the wear as a function of the energy applied to the single braking action.

Table

Table III

Table IV

The data shown in Tables II-IV clearly show how the blocks according to invention are subjected, given the same energy involved in the braking, to a wear that is significantly smaller than the one affecting the comparison block.

In particular, the data of Tables II - IV clearly show how both block A and block B, despite having inserts made of materials of a completely different nature (the second material (a) is organic, whereas the second material (b) is sintered), can ensure advantages in terms of wear resistance because of the sole fact that the inserts themselves fulfil the compressibility features claimed herein. As any person skilled in the art could confirm, the advantages in terms of wear deriving from the presence of inserts with a smaller compressibility modulus were completely unexpected.

To sum up, the presence of inserts with a smaller compressibility modulus in the blocks ensures a significant improvement both in terms of wear of the friction material of the block and in terms of friction coefficient and, hence, effectiveness of the braking. The advantages described above are even more evident when the hardness of the material making up the matrix and the one of the material of the inserts are comparable.

In figures 3 and 4, number 11 indicates, as a whole, a second embodiment of a block according to the invention.

The block 11 comprises a matrix 12 made of a sintered material used in the prior art to manufacture the entire block. The block 11 further comprises ten inserts 13 with a cylindrical shape, which are made of a material having a compressibility modulus which is smaller than the one of the first material making up the matrix 12.

The inserts 13 are arranged in a symmetrical position relative to a transverse middle symmetry plane X.

The materials making up the matrix 12 and the inserts 13 are the same as the ones making up the matrix 2 and the inserts 3.

The block 11 comprises a friction surface 14, which is occupied by said first material for 88.5% and by said second material for 11.5%.

In other words, the difference between the block 11 and the block 1 exclusively lies in the fact that the former comprises 10 inserts made of an organic material instead of 6.

Besides the embodiments shown in figures 1, 2 and 3, 4, the block according to the invention can also assume other configurations, provided that the requirement according to which the relative friction surface 4 consists, for 95 to 60%, of the material making up the matrix is fulfilled.