DESCRIPTION

The present invention relates to the mechanical sector, and in particular to mechanisms for the conversion of a rotary motion into a rectilinear reciprocating motion, and vice versa.

Connecting rods find application in a vast range of production sectors, such as for example in reciprocating motors, pumps, machine tools, presses, etc .

In some cases, the speeds involved are low, whilst in others, such as for example in reciprocating motors, the speeds, and the stresses deriving therefrom, may even be very high. In these cases, the moving masses play a very important role on account of the inertial forces associated to the motion of such masses.

Prior art

It is well known that a connecting rod, preferably made of special steels but also of light metal alloys, is used to convert a rotary motion into a reciprocating motion, and vice versa, and is subject to tensile loads, compressive loads, bending, Eulerian instability, and fatigue.

On account of the continuous change of direction of its motion and of the presence of additional balancing masses, there are constantly produced losses of kinetic energy linked to the value of the weight of the connecting rod.

In order to improve the overall efficiency of the mechanism, reducing the value of the weight of the connecting rod, many attempts have been made to produce lighter connecting rods given the same performance, as has been done by attempting to use metals lighter than steel, such as light alloys, titanium, and other materials. In the last few decades, following upon the continuous improvement and the increased understanding of the mechanisms of operation of fibrous composite materials, many attempts have been made to use such materials for the production of connecting rods that, appropriately designed, could exploit fully the very high values of the mechanical properties together with the low values of the density of large part of the composite materials, even though these materials present characteristics of anisotropy and of non- homogeneity, instead of being isotropic and homogeneous like metals and their alloys.

The above characteristics of anisotropy and non- homogeneity of the composites, however, are precisely the reason why these attempts of using fibrous composites for connecting rods have been discouraging and have not so far produced results of any great importance in the production of lighter connecting rods .

Amongst the main drawbacks encountered in connecting rods made of composite material, the following may be listed:

■ It is well known that the tensile strength of a fibre along its longitudinal axis is certainly higher than its compressive strength along the same axis; consequently, also the composite material constituted by those fibres englobed in any matrix presents mechanical characteristics of tensile strength, in the direction of the fibres, higher than those of compressive strength in the same direction. In all the attempts to produce connecting rods made of composite material it has so far been envisaged to position the fibres along the longitudinal axis of the connecting rod, which thus - during operation - are necessarily also subjected to compressive loads.

^■ On account of the anisotropy and non-homogeneity of the composite material with oriented fibres, it is found that these materials are very subject to concentrated local compressive loads, in particular in the proximity of the ends of the fibre on account of the low possibilities of withstanding the loads, including pluriaxial loads, that are present in these areas. And this case of concentrated pluriaxial load certainly arises close to the big head and small head of the connecting rod during compression of the connecting rod, in the fibres set with their own axis in the longitudinal direction, and these areas of concentrated loads initiate failure of the matrix of the composite.

Both of the problems referred to above could be overcome by increasing the amount of composite material present, but this would markedly reduce the advantages of use of these materials.

Description of the innovation

In order to overcome the drawbacks referred to above, the present invention envisages a connecting rod having a structure configured in an innovative way that uses to a major extent composite materials, altogether eliminating, however, precisely thanks to its structural configuration, the drawbacks that characterize composite materials both during tensile loading and compressive loading of the connecting rod. In this way, it is possible to obtain a low final weight given the same performance, as compared to a similar connecting rod made of metal.

In fact, in a totally innovative way, as compared to any other structural configuration so far known, according to the present invention it is envisaged that the different portions of composite material with reinforcement fibres present in the connecting rod are loaded exclusively by a tensile force along its own axis (i.e., in such a way that they can provide the maximum characteristics of mechanical strength that are not available in the case of compressive loads), both when the connecting rod is subjected - as a whole - to compression and when it is subjected to tensile loading.

The above and other purposes of the invention will be better understood from the ensuing detailed description and with reference to the attached drawings, which illustrate, purely by way of non- limiting example, a preferred embodiment.

In the drawings:

Figure 1 is a schematic illustration of the main parts that constitute a preferred embodiment of a connecting rod according to the present invention;

Figure 2 is a schematic illustration of a detail of the connecting rod of Figure 1, highlighted in which is the winding for hooping the central body; and Figure 3 shows, by way of illustration, how the polar winding is set in the case where it is wound on or underneath the hooping (not illustrated for reasons of greater clarity) .

With reference to the figures, the purposes described above have been obtained, according to a peculiar characteristic of the invention, by providing a connecting rod, the central body of which is configured in such a way that, when the connecting rod is subjected to compressive force Fc, it tends to undergo deformation by pushing its two arms outwards, in other words, so that said arms tend to undergo deformation bending outwards so as to move away from to one another. By so doing, the two arms subject the fibres wound around them (i.e., the fibres that constitute the hooping of said central body) to a tensile state. This is possible precisely by virtue of the direction of winding of these fibres with respect to the axis of the connecting rod: in fact, the fibres that constitute the hooping are deposited around the arms of the central body of the connecting rod by winding them in the direction orthogonal to the longitudinal axis of the connecting rod itself.

This innovative structural configuration of the connecting rod advantageously makes it possible to have the fibres for reinforcement of the composite material that are subjected just to tensile loading along their axis (hence presenting the maximum characteristics of mechanical strength) precisely when the connecting rod - as a whole - is subject to compressive loads.

A further and significant peculiar characteristic of the present invention, determined precisely by the innovative structural configuration of the connecting rod described, lies in the fact that, when the force acting on the connecting rod as a whole is a tensile force Ft, the composite material of the hooping does not participate significantly, whereas said state of stress is instead withstood directly by the portion of composite material that has the fibres wound in a longitudinal direction externally (or internally ) and orthogonally to the fibres of the hooping, along the entire perimeter of all the connecting rod thus obtaining a polar winding: in this way, the tensile load acting on the entire connecting rod is discharged on the fibres of the polar winding, arranged in a longitudinal direction with respect to the connecting rod itself, so that they work in tension along their axis .

With reference to the figures, in Figure 1:

A - inner core or central body, comprising the big-end bushing 12 and small-end bushing 13 of the connecting rod, which are connected to one another by two lateral arms 16 slightly curved outwards;

B - hooping, made of composite material deposited by winding or with some other technique around the two arms 16 so that the fibres are transverse to the main axis of the connecting rod. This hooping covers the entire length of the two arms 16 excluding the big end 12 and the small end 13 of the connecting rod; and

C - polar area, constituted by composite material deposited with polar winding, or with some other technique, on the entire outer perimeter of the core or central body A and set on top (or underneath) alongside the hooping B.

The quantitative and qualitative distribution of the fibres of the composite in the hooping area and in the polar area is a function of the loads actually present on the mechanism, and it is consequently possible to tailor the quality and quantity of fibres in the hooping area B separately in order to withstand the compressive stresses present on the connecting rod, and in the polar area C in order to withstand the tensile stresses.

It should be noted that, in particular, carbon/graphite fibres are preferable for the hooping B in so far as, since they have a high elastic modulus, they are subjected to a high tensile stress even for the extremely small strains that are allowed, whereas glass fibres, aramide fibres, basalt fibres, or the like are preferable for the polar area C, where the tensile strength is the most important characteristic and the elastic modulus has a lesser effect.

It is easy to identify how, in the present invention, the action of the two arms 16 of the inner core A, made of metal material or other suitable material, which are appropriately curved outwards, exerts - during compression of the connecting rod - a lateral thrust that is parallel to the axis of the unidirectional fibres wound in the hooping B, which are thus subjected exclusively to tensile stress thanks to the fact that the aforesaid two arms 16 tend to move away from one another as illustrated schematically in Figure 2.

Even more evident and understandable, instead, is the function and efficiency of the unidirectional fibres of the composite material wound in the area of polar winding C, when the connecting rod is subjected to the tensile force acting on the connecting rod (see Figure 3), in which the fibres are clearly subjected to tensile load alone.

The present invention also envisages that, in the case where for the connecting rod marked compressive loads are envisaged, on both sides between the inner core A and the composite material of the hooping B there is present a reinforcement layer D made of composite material with high elastic modulus, typically a composite with carbon/graphite fibres of high modulus arranged in a longitudinal direction with respect to the connecting rod, as emerges purely by way of example in Figure 1, said reinforcement D being rigidly connected to the core A itself and to the hooping B.

Advantageously, this reinforcement layer D is a further contribution to the flexural stiffness in the principal plane of the connecting rod, i.e., the Eulerian elastic stability of the inner core A and hence of the entire connecting rod.

It should also be noted that the composite materials used in the present invention are preferably made up of composite materials with unidirectional fibres .

In the preferred embodiment described, the operating temperature of the mechanism is envisaged in the range from -50°C to +200°C, and the preferred matrix for englobing the fibres is of the polymeric type, either of a thermosetting type TI, such as epoxy matrix, phenolic matrix, or the like, or of a thermoplastic type TP, such as PPS, PEI, PEEK, or the like.

Furthermore, it is preferable to use composites in which these fibres are present in the composite in a proportion of between 30 vol% and 75 vol% constituting a composite with elastic tensile modulus E comprised between 20 000 MPa and 400 000 MPa and ultimate tensile strength of between 300 MPa and 3 000 MPa, these fibres being of the types Glass E, Glass S, Glass R, or other types of Glass, or carbon/graphite of any type, or basalt, or aramide, said properties being obtained - as has already been mentioned - thanks to the use of composites with or without a matrix of a polymeric type either of the thermosetting type TI (such as epoxy matrices, polyester matrices, phenolic matrices for recommended maximum temperatures not higher than 180 °C approximately) or else of the thermoplastic type TP (such as PPS, PEI, PEEK or the like for maximum temperatures even higher than 250°C approximately) , or again to the use of matrices of a different type, such as metal or ceramic matrices, the aforesaid composite having a density or volumic mass of between 0.7 g/cm ³ and 2.2 g/cm ³ .

The above arms 16 have variable dimensions and geometry both as a function of the characteristics of the mechanism of which the connecting rod forms part, in particular r.p.m., power, and maximum torque of the mechanism itself and in any case along their extension, where these dimensions are comprised: as regards the thickness transverse to the plane of the connecting rod between 1 mm and 50 mm, and as regards the thickness of the arms 16 in the plane of the connecting rod between 1 mm and 60 mm, these dimensions being also linked to the length L of the connecting rod and to the distance h between centres of the big-end area 12 and small-end area 13 of the connecting rod itself, where this distance h is preferably between 40 mm and 2 000 mm.

It should in any case be pointed out that all the aforesaid dimensions may exceed the values referred to in the cases of connecting rods to be inserted in particular large-sized mechanisms.

As regards the hooping B, constituted by the aforesaid unidirectional composite material deposited around the two arms 16 by transverse winding of these fibres with respect to the arms 16 and around the axis of the connecting rod, or with some other technique, it is to be recalled that this hooping covers exclusively the entire length of the arms 16 of the connecting rod, and that the layer of composite material of the hooping B has a dimension in the longitudinal direction that is dictated by the length of the arms 16, whilst the thickness has a value comprised between 1 mm and 30 mm, the dimensions possibly exceeding the values referred to in the cases of connecting rods to be inserted in particular large-sized mechanisms.

The polar-winding area C, constituted by the aforesaid unidirectional composite material deposited by polar winding, or some other technique, on the entire outer perimeter of the arched arms 16 in a longitudinal direction with respect to the connecting rod and hence transverse to the hooping B, is set on top of (or underneath, i.e., wound underneath) the latter and has a dimension in the longitudinal direction equal to the total length L of the connecting rod, whereas in the thickness transverse to the plane of the connecting rod it has a value of between 1 mm and 50 mm, and in the thickness of the aforesaid polar winding C in the plane of the connecting rod it has a value comprised between 1 mm and 60 mm, the dimensions possibly exceeding the values referred to in the cases of connecting rods to be inserted in particular large- sized mechanisms.

It should be noted that to wind the polar winding C underneath the hooping B it is sufficient to wind it on the central body A of the connecting rod prior to carrying out the hooping operation.

The reinforcement layer D, which is designed to increase further the flexural stiffness and Eulerian instability of the connecting rod in its principal plane during compression, is set between said central stem or core A and said hooping B. As has already been mentioned, it is constituted by a layer of composite material with fibres set longitudinally, preferably a composite with carbon/graphite fibres of high modulus, in which the material has an elastic tensile modulus E comprised between 80 000 MPa and 500 000 MPa, and a thickness comprised between 0.5 mm and 5 mm.

Finally, it should be noted that the unidirectional fibres of the composite of the hooping A and of the polar winding C are wound using simple conventional wet winding technologies subjecting them just to tensile stress along their own axis.