Patent Application
20070186719
The present invention relates to a bicycle crank arm.
The invention also relates to an intermediate product and to a core used for manufacturing such a crank arm. The invention further relates to a method for manufacturing the aforementioned crank arm.
A constant requirement of manufacturers of bicycles and/or of bicycle components, above all of racing bicycles, is to reduce the overall weight of the bicycle and, therefore, of the various components of the bicycle to as low as possible, at the same time ensuring, if not even improving, the characteristics of structural strength of such components.
In order to satisfy this requirement various attempts have been made, e.g., hollow crank arms and/or crank arms made of composite materials have been designed. However, in order to obtain a favorable balance between lightness and structural strength, the structure of the crank arm is often complex and therefore expensive to realize.
The applicant has manufactured a bicycle crank arm having an elongated body extending along a longitudinal axis X-X and having:
a first end portion comprising a first seat for coupling with a bicycle bottom bracket assembly;
a second end portion comprising a second seat for coupling with a pedal; and
an intermediate portion having an inner portion without structural strength, a first reinforcement structure and a second reinforcement structure arranged between said inner portion and said first reinforcement structure and comprising at least one bundle of structural fibers having at least in part a first orientation;
wherein said intermediate portion has a section perpendicular to said longitudinal axis X-X in which said inner portion occupies a space defined by a first area having a first surface extension and by a second area having a second surface extension and in which said second reinforcement structure occupies a space having a surface extension equal to the difference between said first and second areas.
Further characteristics and advantages of the crank arm of the present invention shall become clearer from the following description of some preferred embodiments thereof, made hereafter, for indicating and not limiting purposes, with reference to the attached drawings. In such drawings:
a are perspective views of alternative embodiments of the core of
a is a top view of a preferred embodiment of the core of
b is a view of a detail of the core of
Introduction
The present invention therefore relates, in a first aspect thereof, to a bicycle crank arm comprising an elongated body extending along a longitudinal axis X-X and having:
a first end portion comprising a first seat for coupling with a bicycle bottom bracket assembly;
a second end portion comprising a second seat for coupling with a pedal;
an intermediate portion extending between the first end portion and the second end portion and comprising an inner portion without structural strength, a first reinforcement structure and a second reinforcement structure arranged between the inner portion and the first reinforcement structure and comprising at least one bundle of structural fibers having at least in part a first orientation;
wherein the intermediate portion has a section perpendicular to the longitudinal axis X-X in which the inner portion occupies a space defined by a first area having a first surface extension and by a second area having a second surface extension and in which the second reinforcement structure occupies a space having a surface extension equal to the difference between the first and second areas.
Advantageously, the crank arm of the present invention achieves a favorable balance between lightness and structural strength. Indeed, the second reinforcement structure is arranged at the areas of the crank arm most stressed in operation, the other areas being hollow or made from material without structural capability. Moreover, in such a crank arm, advantageously, when a stress is transmitted from the first reinforcement structure to the second reinforcement structure through their mutual contact surfaces, the second reinforcement structure is free to deform at the surface thereof which faces the inner portion of the crank arm. Indeed, since the inner portion of the crank arm does not have structural capability, at the interface surface between the second reinforcement structure and the inner portion no constraint tension is generated that, acting in contrast to the deformation stress of the second reinforcement structure, would cause the detachment of the second reinforcement structure from the first reinforcement structure. Advantageously, in the case in which the inner portion is defined by a cavity, the constraint tensions are completely zero.
Preferably, the second reinforcement structure is arranged in the perpendicular section so that each surface portion of the second reinforcement structure that faces the first reinforcement structure is opposite a respective surface portion of the second reinforcement structure that faces the inner portion.
Preferably, the fibers of the second reinforcement structure are unidirectional continuous structural fibers extending, at the intermediate portion, substantially parallel to the longitudinal axis X-X. Such fibers, advantageously, give the crank arm a structural resistance to traction stresses.
Preferably, the fibers of the first reinforcement structure are unidirectional continuous structural fibers extending substantially along at least one direction inclined with respect to the longitudinal axis X-X by a first predetermined angle. More preferably, the first predetermined angle is equal to about 45°. The applicant has indeed found that this inclination is optimal for giving the crank arm characteristics of resistance to traction, bending, and torsion stresses.
The fibers of the first reinforcement structure can, however, be pieces of fibers or small sheets of fabric having a random arrangement.
Preferably, the aforementioned structural fibers are incorporated in a composite material, preferably polymeric.
Preferably, the first and the second end portion of the crank arm comprise a composite material comprising structural fibers arranged randomly and incorporated in a polymeric material. Advantageously, the structural fibers arranged randomly, for example in the form of small sheets of fabric or pieces of fibers, are capable of flowing when subjected to pressure in a mold and therefore are able to optimally fill the end areas of the cavity of the mold so as to form the end portions of the crank arm. Such end portions, being in any case made from a material comprising structural fibers, have the desired characteristics of structural strength.
Preferably, the structural fibers in the composite material, whether continuous or in pieces arranged randomly, are selected from the group consisting of: carbon fibers, glass fibers, aramid fibers, boron fibers, ceramic fibers, and combinations thereof, carbon fibers being most preferred.
Preferably, the polymeric material in the composite material is a thermosetting material. More preferably, the thermosetting material comprises an epoxy resin. However, the possibility of using a thermoplastic material is not excluded.
Preferably, the elongated body of the crank arm has a first face adapted to face a bicycle frame and a second face opposite the first face, and the second reinforcement structure of the intermediate portion of the crank arm is closer to the second face than to the first face.
In a first embodiment of the crank arm of the present invention, in particular in the case of a left crank arm, the second reinforcement structure also extends at at least one of the end portions and, at the at least one of the end portions, is closer to the first face than to the second face.
In a second embodiment of the crank arm of the present invention, in particular in the case of a right crank arm, such a crank arm comprises at least one reinforcement structure also at at least one of the end portions and the reinforcement structure, at the at least one of the end portions, is closer to the second face than to the first face.
In both of the embodiments discussed above, advantageously, the second reinforcement structure is arranged at the area of the crank arm most stressed during pedaling. Such an area, in the left crank arm, is the one at the area for coupling with the bottom bracket assembly, on the side facing towards the bicycle frame, whereas in the right crank arm it is the face of the crank arm facing outwards.
In the case of the left crank arm, the reinforcement structure of the end portion of the crank arm can be an extension of the second reinforcement structure of the intermediate portion of the crank arm. Preferably, however, the reinforcement structure of the end portion of the crank arm is distinct from the reinforcement structure of the intermediate portion of the crank arm, as shall be described more clearly hereafter.
Preferably, the intermediate portion of the elongated body has a section perpendicular to the longitudinal axis X-X in which the second reinforcement structure has a height equal to about half the height of the inner portion. In this way a hollow, and therefore light, crank arm is obtained, in which the second reinforcement structure is arranged at the part of the crank arm most stressed in operation.
Preferably, the aforementioned inner portion is defined by a first cavity. In alternative embodiments, the inner portion can be made from material without structural capability, for example, expanded polyurethane.
Preferably, the second reinforcement structure extends longitudinally along the elongated body and winds in a loop around the first and second end portions and the intermediate portion. Advantageously, a reinforcement structure made in this way gives the crank arm a high resistance to traction loads.
More preferably, the elongated body of the crank arm further comprises, at at least one of the end portions, at least one holding element of a portion of the second reinforcement structure. Advantageously, such a holding element, during the molding process of the crank arm, contributes to keep tensioned and in position the fibers of the second reinforcement structure at the end portions of the crank arm. The presence of structural fibers is therefore ensured at the areas of the crank arm most stressed during pedaling, among which there is in particular the end portion of the crank arm for coupling with the bottom bracket assembly of the bicycle. Even more advantageously, the holding element ensures that, during molding, due to the flowing of the filling material, the fibers do not shrivel in the end portions of the crank arm. The applicant has indeed found that the structural fibers, if bent excessively, can break. A possible shriveling would thus generate weakening points in the structure of the body of the crank arm.
Preferably, the holding element has a small thickness at an outer surface portion thereof facing the inner portion of the intermediate portion of the elongated body. Advantageously, this allows the core adapted to form the lightened inner portion of the crank arm to be brought closer towards the end area of the crank arm itself and therefore allows a crank arm to be obtained in which the lightened inner portion has greater longitudinal extension.
For example, the holding element can be defined by a ring nut adapted to be screwed into an outer threading formed on a forming element of the end portion of the crank arm, but any other type of coupling is not excluded.
Preferably, the holding element, given that it is intended to remain within in the finished crank arm, is made from a light material, such as a composite material or a metal alloy. The weight of the crank arm thus remains low.
More preferably, the holding element is made from phenolic resin loaded with glass fiber, the content of the glass fibers preferably being equal to 40-50%. The applicant has found that a holding element thus made gives excellent performance results combined with a particularly low cost.
Preferably, at least one of the first and second end portions comprises, at the seat, a respective coupling hole.
More preferably, the crank arm further comprises, at the coupling hole, a plurality of centering elements of a forming element of the end portion.
Such centering elements are preferably made from the same material as the crank arm and, more preferably, extend cantilevered from the coupling seat inside the coupling hole.
Preferably, the crank arm further comprises a second cavity extending around the coupling hole. The cavity around the coupling hole advantageously produces an even lighter crank arm.
Preferably, an insert adapted to allow the coupling of the crank arm with the bottom bracket assembly of the bicycle or with a pedal is inserted into the coupling hole.
Preferably, the second cavity is an extension of the first cavity formed at the intermediate portion of the crank arm. Nevertheless, the possibility that the cavity formed in the end portion of the crank arm and the cavity formed in the intermediate portion of the crank arm can be distinct and separate is not excluded.
Preferably, the second cavity has a height at least in part shorter than the height of the first cavity, so that the crank arm, at the coupling areas with the bottom bracket assembly and with the pedal, has areas of increased thickness suitable for ensuring the desired structural strength.
Nevertheless, alternative embodiments in which the second cavity has a height equal to that of the first cavity, or to that of the insert inserted into the respective coupling seat formed in the end portion of the crank arm, are not excluded.
In a specific embodiment of the crank arm of the present invention, the first and the second end portion both comprise a respective second cavity extending around the respective coupling seat.
As already stated, the crank arm of the present invention can be a left crank arm or a right crank arm. In the case in which it is a right crank arm, it comprises, at the end portion thereof for coupling with the bottom bracket assembly of the bicycle, a plurality of spokes that, preferably, comprise respective cavities that branch from the second cavity formed around the coupling seat provided in the end portion of the crank arm. A particularly light right crank arm is thus made.
Preferably, the bundle of unidirectional fibers that constitutes the second reinforcement structure of the intermediate portion of the crank arm extends along the spokes around the respective cavities and follows the profile thereof, so as to ensure that the crank arm has structural strength also at the spokes.
Preferably, the height of the cavities of the spokes is greater than that of the cavity around the coupling seat formed in the end portion of the crank arm. A thickened area around the seat for coupling with the bottom bracket assembly is thus obtained in the crank arm; such thickening ensures the desired characteristics of structural strength in this area of the crank arm that is particularly stressed during pedaling. Such an area, in particular, is the area for coupling with the bottom bracket assembly, on the side facing outwards.
Preferably, the crank arm of the present invention comprises a layer of continuous structural fibers wound in a spiral around the coupling seat. This is to enhance the characteristics of resistance to stress of the crank arm at the aforementioned area for coupling with the bottom bracket assembly.
In a second aspect thereof, the invention relates to an intermediate product for manufacturing a bicycle crank arm, such an intermediate product comprising a core having an elongated body extending along a longitudinal axis X-X, a first reinforcement structure and a second reinforcement structure comprising at least one bundle of structural fibers having at least in part a first orientation. The intermediate product further comprises a section perpendicular to the longitudinal axis X-X in which the core occupies a space defined by a first area having a first surface extension and by a second area having a second surface extension and in which the second reinforcement structure occupies a space having a surface extension equal to the difference between the first and second areas.
Advantageously, the intermediate product described above can be used to manufacture the crank arm of the present invention, and thus results in the advantages mentioned above with reference to such a crank arm.
Preferably, the intermediate product of the present invention comprises, individually and/or in combination, all of the preferred structural characteristics described above with reference to the crank arm and correlated to the presence, composition, and arrangement of unidirectional structural fibers incorporated in a composite material, possibly a filling material.
Preferably, the second reinforcement structure is arranged in the perpendicular section in such a way that each surface portion of the second reinforcement structure that faces the first reinforcement structure is opposite a respective surface portion of the second reinforcement structure that faces the core.
Preferably, the fibers of the bundle of fibers of the second reinforcement structure are unidirectional continuous structural fibers extending substantially parallel to the longitudinal axis X-X and the fibers of the first reinforcement structure are unidirectional continuous structural fibers extending substantially along at least one direction that is inclined, preferably by 45°, with respect to the longitudinal axis X-X.
Even more preferably, the aforementioned intermediate product also comprises at least one first layer of filling material capable of flowing under pressure and arranged between the core and the second reinforcement structure. More preferably, at least one second layer of filling material is associated with the first reinforcement structure at the outside thereof. Such a filling material is intended to form the end portions of the crank arm following a pressure molding operation of the intermediate product thus arranged.
Preferably, the elongated body of the core comprises opposite longitudinal sides provided with respective housing seats of respective portions of the fibers of the bundle of fibers.
Preferably, each housing seat is defined by a side surface of the elongated body of the core and by a projecting surface that projects laterally from the side surface. Advantageously, such a projecting surface, when the intermediate product is inserted into the cavity of the mold to manufacture the crank arm, is provided so as to be arranged between the pressure element of the mold and a portion of the second reinforcement structure defined by the aforementioned bundle of fibers so that, when the pressure element is actuated, the projecting surface exerts a uniform pressure on such a portion of second reinforcement structure keeping tensioned the fibers of the structure and thus avoiding the shriveling thereof due to the flow of the filling material.
This is particularly advantageous if the bundle of fibers of the second reinforcement structure also embraces two forming elements of the end portions of the crank arm. The tension generated by the thrusting of the aforementioned projecting surface of the core indeed prevents the flowing of the filling material on two opposite sides of the aforementioned forming elements causing a shriveling of the fibers of the reinforcement structure at the forming elements, such shriveling generating weakening points in the finished crank arm.
Preferably, the core is made from a metallic material having a low melting point to allow it to be removed by melting when the crank arm has been manufactured. The material having a low melting point is in particular a material capable of melting at a temperature slightly higher than the cross-linking temperature of the composite material. In this way the core allows very high pressures to be applied that optimally compact the composite material and the filling material, inducing an intense degasification.
Preferably, in the case in which one wishes to make a crank arm having a cavity around the seats for coupling with the bottom bracket assembly and/or with the pedal, the core comprises a central body portion and two rings integrally associated with the elongated body at opposite ends of the central body portion. The two rings create the cavities in the end portions of the crank arm whereas the central body portion of the core creates the cavity at the intermediate body portion of the crank arm.
Preferably, a ring of the pair of rings has a height at least in part shorter than the height of the central body portion. Such a ring of shorter height is adapted to create the cavity at the end portion for coupling with the bottom bracket assembly of the bicycle. Advantageously, the fact that such a ring has a shorter height than that of the central body portion of the core makes it possible to not exert a pressure on the portion of the bundle of structural fibers supported by the holding element during the molding of the crank arm, thus ensuring correct tensioning of such fibers.
Preferably, at least part of the ring has a height shorter than the height of the projecting surface of the central body portion of the core. The ring can also have at least one body portion having a height greater than that of the remaining body portion.
The other ring of the pair of rings can have a height substantially equal to the height of the projecting surface or equal to that of the other ring.
Preferably, the aforementioned rings comprise respective positioning elements extending towards the inside of the rings. Advantageously, such positioning elements, pressing in the molding step against the forming inserts of the end portions of the crank arm, prevent the core from moving.
In a preferred embodiment thereof, the core comprises, at one of the two rings, a central hole and a plurality of spokes that extend radially outwards from the central hole. Such a core is advantageously used in the manufacture of a right crank arm, where the spokes of the crank arm are used to support the gear wheels of the sprocket assembly of the bicycle.
Preferably, the spokes of the core have a height greater than the height of the ring at the central hole. Advantageously, the crank arm obtained using such a core therefore comprises, around the seat for coupling with the bottom bracket assembly of the bicycle, a thickened area that ensures a high structural strength in an area where there is notoriously a high concentration of tensions.
Preferably, a layer of continuous structural fibers is wound in a spiral around the central hole. More preferably, the aforementioned bundle of structural fibers extends along the spokes, following the profile thereof. The structural strength of the finished crank arm is thus increased at the spokes.
Preferably, in the case in which one wishes to make a right crank arm, the projecting surface defined on the sides of the elongated body of the core extends parallel to the axis X-X for a part having a length shorter than the length of the central body portion of the core. This promotes a thickening of the body of the crank arm at the joining area between the intermediate portion and the end portion for coupling with the bottom bracket assembly of the bicycle, i.e. where there is notoriously a high concentration of tensions.
In a third aspect thereof, the invention relates to a core for manufacturing a hollow bicycle crank arm, such a core comprising an elongated body extending along a longitudinal axis X-X. The elongated body comprises, at opposite longitudinal sides thereof, a side surface and a projecting surface that projects laterally from the side surface.
Advantageously, the core described above can be used to manufacture the crank arm of the present invention, and therefore achieves the advantages mentioned above with reference to such a crank arm.
Preferably, the core of the present invention comprises, individually and/or in combination, all of the preferred structural characteristics described with reference to the core of the intermediate product for manufacturing the crank arm of the present invention described above.
In a fourth aspect thereof, the invention relates to a method for manufacturing a bicycle crank arm, comprising the steps of:
providing a core adapted to define an inner cavity of an intermediate portion of a crank arm, the core comprising an elongated body extending along a longitudinal axis X-X;
providing two forming elements of opposite end portions of the crank arm;
providing an intermediate product comprising the core, a first reinforcement structure and a second reinforcement structure arranged between the core and the first reinforcement structure and comprising at least one bundle of structural fibers having at least in part a first orientation, the intermediate product having a section perpendicular to the longitudinal axis X-X in which the core occupies a space defined by a first area having a first surface extension and by a second area having a second surface extension and in which the second reinforcement structure occupies a space having a surface extension equal to the difference between the first and second areas;
associating the forming elements with the intermediate product, the forming elements being arranged between the core and the second reinforcement structure at opposite free ends of the core;
introducing the intermediate product with the forming elements into a cavity of a mold together with at least one mass of filling material comprising a polymeric material and capable of flowing under pressure, the mold being equipped with a pressure element and the cavity having the shape of the crank arm;
applying a predetermined pressure to the pressure element to make the filling material flow in the cavity;
heating the mold to a temperature corresponding to the cross-linking temperature of the polymeric material; and
opening the mold and removing the formed crank arm.
Through this method it is advantageously possible to manufacture the crank arm of the present invention.
Preferably, the second reinforcement structure is housed in a pair of housing seats formed on opposite longitudinal sides of the core.
More preferably, each housing seat is defined by a side surface of the core and by a projecting surface that projects laterally from the side surface and the intermediate product is inserted in the cavity of the mold so that the pressure element acts upon a first portion of the second reinforcement structure through the projecting surface.
Preferably, the method of the present invention further comprises the step of providing a holding element of a second portion of the second reinforcement structure on at least one of the forming elements so that the holding element acts upon the second reinforcement structure on the opposite side to the projecting surface of the core.
Preferably, the step of providing an intermediate product comprises the step of winding at least one bundle of unidirectional continuous structural fibers in a loop around the two forming elements and the core.
Preferably, the core consists of a metallic material having a low melting point, and the method of the present invention further comprises, after the step of heating the mold, the steps of:
forming at least one hole on a surface of the crank arm;
heating the crank arm up to the melting point of the metallic material having a low melting point in order to allow it to flow out.
In this way, advantageously, a cavity is obtained at the space occupied by the core thus manufacturing a crank arm having the desired characteristics of lightness.
Preferably, the melting point is higher than the cross-linking temperature of the polymeric material by a value ΔT of between about 1° C. and about 25° C. More preferably, the melting point is higher than the cross-linking temperature of the polymeric material by a value ΔT equal to about 5° C.
Preferably, the first reinforcement structure comprises unidirectional continuous structural fibers that extend along a preferential direction of extension and the step of providing the intermediate product comprises the step of arranging the at least one layer of structural fibers so that the preferential direction of extension is inclined with respect to the longitudinal axis X-X by a predetermined angle, preferably equal to about 45°.
Preferably, the method further comprises, before the step of insertion of the intermediate product and of the mass of filling material in the cavity of the mold or after the aforementioned insertion step and before the step of application of the predetermined pressure, the step of preheating the intermediate product and the at least one mass of filling material bringing it to a predetermined preheating temperature for a predetermined preheating time. This preheating step is used to make the composite material of the first reinforcement structure and possibly of the layer of structural fibers that wind in a loop around the core and the aforementioned forming elements more easily malleable during molding and the filling material more easily capable of flowing under pressure.
The parameters of the preheating step are selected so as to achieve homogeneous heating without the polymeric components aging excessively. A first preferred range of values is given by a temperature of between about 60° C. and about 100° C., maintained for a time of between about 5 and about 70 minutes. An even more preferred range of values is given by a temperature of between about 80° C. and about 100° C. maintained for a time of between about 5 and about 60 minutes. A particularly preferred range of values is given by a temperature of between about 85° C. and about 90° C. maintained for a time of between about 35 and about 55 minutes. In this way it is possible to serve a mold with a limited number of preheating ovens.
Preferably, the step of inserting the intermediate product into the cavity of the mold together with the at least one mass of filling material comprises the step of providing at least one first layer of filling material between the core and the first reinforcement structure.
More preferably, at least one second layer of filling material is also inserted outside of the first reinforcement structure.
In this way it is possible to obtain an optimal distribution of the filling material in the cavity of the mold and therefore an end product of high quality both in terms of structure and appearance.
Preferably, the aforementioned filling material is a composite material comprising portions of structural fibers, preferably pieces or small sheets of fibers arranged randomly and incorporated in a polymeric material.
Preferably, the predetermined pressure exerted by the pressure element during the molding of the crank arm is between about 5 bar and 400 bar, more preferably between 40 bar and 300 bar and even more preferably between 200 bar and 300 bar.
The structural fibers, continuous or in pieces, and the polymeric material of the composite material are selected as described above with reference to the crank arm of the present invention.
The present invention also relates, in a further aspect thereof, to a bicycle crank arm, comprising an elongated body extending along a longitudinal axis X-X and having:
a first end portion comprising a first seat for coupling with a bicycle bottom bracket assembly;
a second end portion comprising a second seat for coupling with a pedal; and
an intermediate portion extending between the first end portion and the second end portion;
wherein at least one of the first and second end portions comprises, at the seat, a coupling hole and a cavity extending around the coupling hole.
Preferably, at least one of the first and second end portions is at least in part made with a cross-linkable composite material capable of flowing under pressure before cross-linking.
Advantageously, such a crank arm comprises end portions provided with respective cavities around respective coupling holes; such portions are formed through molding using a cross linkable composite filling material capable of flowing under pressure around the inserts. The filling material is distributed homogeneously in the cavity of the mold without creating points of non-homogeneity that would constitute points of concentration of tension and, therefore, weakening points of the structure.
Preferably, the cavity continuously surrounds the coupling hole.
Preferably, such a crank arm comprises all of the preferred structural characteristics discussed above.
In a further aspect thereof, the invention relates to a core for manufacturing a hollow bicycle crank arm, comprising an elongated body extending along a longitudinal axis X-X. The elongated body comprises a central body portion and two rings integrally associated with the elongated body at opposite ends of the central body portion.
Such a core can be used to manufacture the crank arm described above and preferably has all of the structural characteristics described above.
With reference to
The crank arm 1 comprises an elongated body 2 extending along a longitudinal axis X-X and has a first end portion 5 comprising a first seat 38 for coupling with a bottom bracket assembly (not shown) of the bicycle, a second end portion 10 comprising a second seat 39 for coupling with a pedal (not shown), and an intermediate portion 15 extending between such end portions 5, 10. On the crank arm 1 a front face 91, which, in conditions of assembly onto the bicycle, faces outwards, and a rear face 93, which, in conditions of assembly onto the bicycle, faces towards the frame of the bicycle are identified.
The elongated body 2 is made in a single piece of composite material, comprising structural fibers incorporated in a polymeric material. Typically, the structural fibers are selected from the group consisting of carbon fibers, glass fibers, aramid fibers, ceramic fibers, boron fibers and combinations thereof, carbon fibers being preferred. The polymeric material is typically a thermosetting material, preferably comprising an epoxy resin. However, the possibility of using a thermoplastic material is not excluded.
The structural fibers are suitably arranged inside the elongated body 2, so as to form different structures that react to different types of stresses.
In particular, the intermediate portion 15 comprises a non-structured inner portion which, in the illustrated preferred embodiment, is a cavity 20.
The outer reinforcement structure comprises a tubular structure 35, arranged around the cavity 20, that extends longitudinally less than the longitudinal intermediate portion 15 (illustrated in
The tubular structure 35 functions to provide adequate resistance to bending and torsion stresses. The tubular structure 35 has at least one sheet 36 of composite material wound multiple times around the longitudinal axis X-X, as made clearer hereafter with reference to the method for manufacturing the crank arm 1. In the preferred embodiment illustrated here, the sheet 36 comprises two layers 33, 34 of unidirectional continuous structural fibers having a respective preferential direction of extension inclined with respect to the longitudinal axis X-X (
The elongated body 2 preferably also comprises an intermediate reinforcement structure. In the preferred embodiments described here, such an intermediate reinforcement structure has a bundle 25 of unidirectional continuous structural fibers that extend longitudinally throughout the elongated body 2, winding in a loop around the end portions 5, 10 and the intermediate portion 15 and enclosing the coupling seats 38, 39. The bundle 25 serves the function of resisting overall the traction loads, but a minimum contribution to resist other loads is also not excluded.
At the intermediate portion 15, the bundle 25 crosses the cavity 20 twice and is arranged between it and the tubular structure 35.
What has been described can be seen in detail in the cross section of the intermediate portion 15 shown in
The two branches of the bundle 25 have a substantially rectangular section with a height preferably equal to about half the height of the cavity 20. In particular, the cavity 20 occupies a space defined by a first bottom area 20a having a certain surface extension and by a second top area 20b having a smaller surface extension (in
In general, the section of the two branches of the bundle 25 can be varied; nevertheless, it is important that each portion of the surfaces 26 of each branch that faces the outer reinforcement structure, however as configured, is opposite a respective surface portion 27 facing the cavity 20, or rather, in general, facing the inner portion substantially without structural strength of the intermediate portion 15.
Preferably, the intermediate portion 15 has a substantially rectangular section and the start and end of the sheet 36 wound to form the tubular structure 35 are arranged at a central area of one of the sides of the section, unlike what is illustrated in
Going towards the outside layers 30 and 30′ of filling material at the interface between cavity 20 and tubular structure 35, preferably, a layer of filling material is also provided at the interface between cavity 20 and intermediate reinforcement structure 25 and between intermediate reinforcement structure and tubular structure 35, so that the bundle 25 is in contact with the filling material and not with the tubular structure. Furthest towards the outside there is finally a layer 40 of filling material, which ensures uniformity of the outer surface of the crank arm 1.
The aforementioned filling material is preferably a composite material comprising structural fibers with random distribution, for example in the form of pieces of fiber or small sheets of fabric, incorporated in polymeric material, as described in patent application US 2005/0012298 to the same applicant, incorporated herein for reference as if fully set forth. The filling material must in any case have the characteristic of flowing under pressure, so as to be able to reach all of the desired points of the crank arm 1 during its manufacture through molding, as shall be described in detail hereafter. The same filling material preferably also forms the end portions 5, 10.
As an alternative to what has been stated above, the bundle 25 can be arranged outside the tubular structure 35, or incorporated between its spirals.
In alternative embodiments it is also possible to foresee respective cavities not communicating with the cavity 20 at one or both of the end portions 5, 10.
It should be noticed that the protrusions generated in the cavity 20 by the bundle 25, 225 are ribs that cross the cavity along its entire length. This has the effect of stiffening the crank arm while maintaining a clean design of its external surface. In order to achieve this result it is not necessary to have two ribs, as in
The intermediate product 60 comprises a core 45, the bundle 25 of unidirectional continuous structural fibers that extends longitudinally around the core 45, the tubular structure 35 with at least one sheet 36 of composite material wound axially many times (e.g., 4-5 times) around the core 45 and the bundle 25, the two layers of filling material 30, 30′ (only one of which can be seen in
The core 45 (
a illustrate alternative embodiments of the core 45, particularly suitable for manufacturing a crank arm 1 in which the cavity 20 also extends to the end portions 5, 10. In these embodiments, the core 45 comprises a central elongated body 145 and two rings 146 and 147 connected thereto at the opposite longitudinal ends thereof. Such rings are adapted to surround the forming elements 50, 51 and thus to form cavities around the coupling seats 38, 39 in the finished crank arm.
The elongated body 145, like the elongated body 47 in the embodiment described above, comprises, at the two opposite longitudinal sides thereof, respective projecting surfaces 148 that project laterally.
The rings 146, 147 preferably have at least in part a height shorter than the height of the central body 145, so as not to compress the bundle 25 wound around the forming elements 50, 51 during the molding operations.
The height of the rings 146, 147 can be the same as the height of the projecting surfaces 148 or different. In the embodiment of
a, on the other hand, shows an embodiment in which the ring 147 has a portion 170 of different thickness, and in particular greater thickness, with respect to that of the remaining portion of the ring 147. A similar configuration could also be present in the ring 146.
One or both of the rings 146, 147 can comprise positioning elements, which, pressing against the forming elements 50, 51 in the molding step, prevent the core 45 from moving. In the embodiments illustrated here the positioning elements consist of a pair of nodes 150 extending radially towards the inside of the ring 146, 147.
Irrespective of the embodiment of the core 45, to allow the removal by melting of the crank arm 1 when this has been formed, the core 1 is made from a metal alloy having a low melting point (eutectic), preferably tin, antimony, and lead. The alloy has a composition such as to have a melting point slightly higher Tf than the cross-linking temperature Tc of the polymeric material in the various composite materials used to make the body of the crank arm 1. In particular it is preferable to respect the following relationship:
Tf=Tc+ΔT
where Tf is between about 65° C. and about 185° C. and in any case is not more than about 200° C. so as not to damage the composite material; Tc is between about 60° C. and about 180° C.; ΔT is preferably between about 1° C. and about 25° C., about 5° C. being the preferred value.
The forming elements 50, 51 can either be profile-creating inserts, which, when the crank arm is formed, are removed and replaced with respective coupling inserts 78 and 80, respectively, for coupling with the bottom bracket assembly and with the pedal, or else they can consist of the coupling inserts 78, 80 themselves. In this last case the coupling inserts 78, 80 are co-molded with the crank arm as described in European patent application no. 05425576.5 to the same Applicant, incorporated herein by reference as if fully set forth.
One or both of the forming elements 50, 51 are associated with a holding element 63, so that, during the molding of the crank arm, it acts on the bundle 25 on the opposite side with respect to the projecting surface 48b (or 148) of the core 45. As holding element 63 it is possible for example to use a ring nut or a washer screwed onto an external threading 65 formed on the forming elements 50, 51, but the possibility of any other type of coupling is not excluded. In the embodiment of
In the case of a left crank arm, like the one illustrated in
Since, preferably, the holding element 63 remains trapped in the finished crank arm, it is desirable for it to be of low weight, therefore it is preferably made from a composite material, of a type selected from those described for manufacturing the rest of the crank arm, or from a metal alloy, like an aluminum alloy. It has been found that a ring nut made from phenolic resin loaded with 40-50% glass fiber gives excellent performance results combined with a particularly low cost.
In this embodiment the core 45 comprises an elongated body 245 having an end defining a star 260 with four spokes 270, adapted to form respective cavities in the support spokes of the finished crank arm.
The elongated body 245 has projecting surfaces 248 that project laterally from the two opposite longitudinal sides thereof. However, differently from what has been described for the embodiments of the core 45 adapted to the manufacture of the left crank arm 1, the projecting surfaces 248 do not extend for the entire length of the elongated body 245, but rather they are interrupted before they join with the star-shaped portion 260. This promotes the thickening of the body made from composite material of the finished crank arm at such a join, where there is notoriously a concentration of tensions.
The spokes 270 project radially around a hole 266 having a diameter such as to allow the passage of the forming element 50 (
a shows a preferred embodiment of the core 45 of
Such centering elements 271, advantageously, are made from a composite material, of a type selected from those described for manufacturing the rest of the crank arm. For example, they can be made from phenolic resin loaded with 40-50% glass fiber, or a metal alloy, like an aluminum alloy. The centering elements 271 are to be lost, in the sense that they shall constitute part of the finished crank arm. Their function is that of keeping the forming element in centered position inside the central hole 266 during the molding of the crank arm, at the same time avoiding the core from being able to go into abutment against such a forming element (a similar function is carried out by the holding element 63 in a right crank arm). Typically, indeed, the forming insert of the end portion of the crank arm has an outer threading for making a corresponding internal threading in the hole of the coupling seat made in the end portion of the crank arm adapted to couple with the bottom bracket assembly of the bicycle. A possible abutment of the core against such a threading would indeed lead to the generation of areas of discontinuity in the thread formed on the finished crank arm. On the other hand, the fact that the centering elements 271 are made from the same material as the crank arm ensures that, during molding, they behave like the rest of the material that shall constitute the finished crank arm, contributing to the generation of a continuous and uniform profile in the seat for coupling with the bottom bracket assembly of the bicycle.
As better illustrated in
In the embodiment illustrated in
In the case of the right crank arm, the holding element 63 associated with the forming element 50 is superfluous and can be omitted. Indeed, it has been noted that, due to the star-shaped configuration, during molding the filling material flows radially around the forming element 50 and therefore does not generate the shriveling of the bundles 225 and 285, which can however typically occur during molding of the left crank arm 1.
The absence of holding elements at the end portions of the right crank arm ensures that, due to the thrusting of the projecting surface 148, 248 of the core 45 during the molding of the crank arm, the bundle 25 is arranged along the entire elongated body of the crank arm on the side facing outwards, right at the area of maximum stress of the right crank arm, which is indeed located at the area for coupling with the bottom bracket assembly, on the side facing outwards. Advantageously, also the bundle 285 wound in a spiral illustrated in
In an alternative preferred embodiment illustrated in
With particular reference to
In a first step of the method a core 45 is provided having a shape corresponding to the cavity 20 that is intended to be obtained in the finished crank arm.
Then the two forming elements 50,51 adapted to form the coupling seats 38, 39 at the end portions 5, 10 of the finished crank arm are provided.
Then the intermediate product 60 is formed (
The bundle 25 of unidirectional continuous structural fibers is wound in a loop around the core 45, in the housing seats 46, and around the forming elements 50, 51, arranged at the opposite longitudinal ends of the core 45. The layers of filling material 30, 30′ are rested on the core 45 on which the sheet 36 is wound so that the preferential directions of extension of the unidirectional continuous structural fibers of the layers 33, 34 form an angle of about 45° with respect to the longitudinal axis X-X of the core 45 (
The man skilled in the art will note that the layers of filling material 30, 30′, 55, 55′ can have whatever arrangement, their purpose being to flow when subjected to pressure towards the areas of a mold 70 at which the end portions 5 and 10 of the crank arm 1 will be formed. Amongst the various possibilities it is also possible to provide the necessary filling material, instead of in layers, in masses positioned in the aforementioned areas.
In a subsequent step of the method, the intermediate product 60 thus obtained is preheated bringing it to a predetermined preheating temperature for a predetermined preheating time which are sufficient to make the polymeric material of the composite material capable of flowing under pressure. Preferably, the intermediate product 60 is kept at a preheating temperature of between about 80° C. and about 100° C. for a sufficient time to heat it homogeneously. More preferably it is kept at a value of between about 85° C. and about 90° C. for a time of between about 35 and about 55 min, so that the core has the time to reach such a temperature before the most outer layer of composite material ages excessively.
When the intermediate product 60 is sufficiently hot it is inserted into the cavity of a mold 70 (
As an alternative to what has been described, the intermediate element 60 can be preheated directly inside the mold 70.
In a subsequent step of the method a pressure of between about 5 and about 350 bar is applied through the pressure element 72. Preferably, the pressure applied is selected in the upper area of the aforementioned range, to promote compacting of the material and degasification. In particular, the preferred pressure value is between about 5 bar and about 400 bar, more preferably between about 40 and about 300 bar and even more preferably between about 200 and about 300 bar. The pressure exerted makes the filling material flow until it fills the areas corresponding to the end portions 5, 10 of the crank arm 1. During the flowing of the filling material, the bundle 25 is kept under tension by the pressure exerted by the projecting surfaces 48 of the core 45, counteracted by the holding elements 63, in this way preventing shriveling.
When the molding step has ended the mold 70 is brought to a temperature corresponding to the cross-linking temperature Tc of the polymeric material present in the composite material forming the crank arm 1 for a sufficient time.
In a subsequent step of the method the finished crank arm 1 is removed from the mold 70.
After a subsequent cooling step, one or more holes 61 are formed in the crank arm 1, preferably on the rear face 93 intended to face towards the frame of the bicycle, and taking care not to cross the bundle 25. The hole 61 is illustrated with a broken line in
Optionally, a washing of the cavity 20 with oil at a predetermined temperature is carried out to remove the melting residues, possibly left stuck to the inner walls, and to avoid them generating undesired noises by detaching during use of the crank arm. The hole 61 is then closed preferably by applying a cap of phenolic resin.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06425086.3 | Feb 2006 | EP | regional |
