HOLLOW CRANK ARM FOR A BICYCLE AND PROCESS FOR MANUFACTURING THE SAME
A hollow crank arm for a bicycle comprising a body of elongated shape, at the ends of which are formed, respectively, an area for connection of the bottom bracket spindle and an area for connection of the pedal. The body includes a cavity of elongated shape and two solid end portions which are the aforesaid connection areas for the bottom bracket spindle and for the pedal. The crank arm is preferably formed through a forging process.
Latest CAMPAGNOLO S.R.L. Patents:
This application is a division of U.S. patent application Ser. No. 10/391,705, filed Mar. 19, 2003, which is incorporated by reference as if fully set forth.
FIELD OF INVENTIONThe present invention relates to a hollow crank arm for a bicycle and a process for its fabrication.
Crank arms for bicycles traditionally consist of a monolithic body made of metal material (typically aluminium or its alloys) provided at its ends with holes for attachment of the pedal and of the bottom bracket spindle.
Various solutions have already been proposed which have the purpose of creating a cavity inside the body of the crank arm in order to reduce its weight.
BACKGROUNDThe Japanese document No. 5116670 describes a process for forming an inner cavity in the solid body of a crank arm by extrusion. According to this proposal, the cavity extends up to one end of the crank arm. Consequently, in order to obtain the threaded hole for fixing of the pedal it is necessary to provide an insert fixed in an end area of the crank arm. This solution presents structural strength problems in so far as it gives rise to a basically tubular structure opened at one end. This presents poor resistance to torsional and bending stresses, in particular owing to the fact that the point of application of a load, represented by the area of connection to the pedal, is provided in a hollow part. The U.S. Pat. No. 6,058,803 describes a process for producing a hollow crank arm by casting, in which a sand core is positioned in a die and the molten metal is poured into the die and solidified in order to form a crank arm body with an elongated cavity having a blind bottom and an end that opens to the outside of the hole which is provided for fixing the pedal. Also this solution presents strength problems because the cavity opens into the hole for connection of the pedal and thus weakens that area.
SUMMARYThe purpose of the present invention is to provide a hallow crank arm for a bicycle and a process for its fabrication which will overcome the problems of the prior art.
The invention provides a forged hollow crank arm for a bicycle comprising a first end area for connection to a bottom bracket spindle, a second end area for connection of a pedal and an intermediate area having a defined longitudinal cavity that extends between the ends along a longitudinal axis with an opening that faces to the outside of the crank arm and is offset with respect to the longitudinal axis.
BRIEF DESCRIPTION OF THE DRAWINGSThe characteristics and advantages of the present invention will emerge clearly in the course of the ensuing detailed description with reference to the annexed drawings, in which:
FIGS. 18 to 23 illustrate another embodiment of the invention which is formed by folding over flaps or extensions to form an internal cavity; and
FIGS. 24 to 26 illustrated the embodiment of FIGS. 18 to 23 with the further addition of a weld bead along the folds forming the internal cavity.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Designated by 20 in
The crank arm preform 26 undergoes a third forging step, at the end of which the semi-finished product illustrated in
By means of a further forging step the semi-finished product 26 assumes the shape illustrated in
The semi-finished product 26 then undergoes a fifth and final fabrication step of hot plastic deformation, which results in a crank arm body 36 having the final shape illustrated in
The finished crank arm body 36 comprises a solid area 28 for connection to the bottom bracket spindle and a solid area 40 for connection of the pedal, in which respective holes 42, 44 are formed by machining away material. The hole 44 for connection of the pedal 44 is usually threaded, whereas the hole 42 for connection to the bottom bracket spindle has a shape, for example square, designed to transmit torque when coupled with a complementary part of the bottom bracket spindle. The crank arm body has a hollow section 50 and a solid section 52 which extend between the aforesaid areas 28 and 40 for connection to the bottom bracket spindle and connection of the pedal. The solid section 52 includes the solid area 40 for connection of the pedal. The hollow section is adjacent to the solid area 28 for connection to the bottom bracket spindle. In said hollow section, the walls are thin and the cavity has a predominant dimension with respect to the thickness of the walls.
During the last formation step, the projecting portion 34 is bent and shaped so that the hollow section 50 and the solid section 52 are substantially aligned on the longitudinal axis 60 of the crank arm body. Alternatively, the solid section 52 may be slightly inclined towards the inner side of the crank arm, as illustrated in the variants of
With respect to the forging process the temperatures can range between 260 and 480 degrees Celsius. For the aluminium alloy of the present embodiments, temperatures between 390 and 400 degrees Celsius render the metal plastically deformable in all steps. As will be recognized by those skilled in the art, the actual temperatures and formation pressures will depend on the selected material, tooling and equipment used in the process.
Again with reference to
The cavity 22 has an elongated shape with an axis that substantially coincides with the longitudinal axis 60 of the crank arm body. From the point of view of the structural strength of the crank arm, it is particularly important that the areas for connection of the bottom bracket spindle and of the pedal should be made in solid areas that do not communicate with the cavity 22. This provides a greater structural strength in the connection areas. The cavity 22 is substantially closed, except for the opening 24, and this provides high stiffness and high resistance to torsional and flexural stresses to be obtained.
The opening 24 may be closed, to prevent any penetration of dust, water or dirt into the cavity 22, with a closing element that need not make any contribution to the structural strength of the crank arm.
If desired the opening 24 may be closed with the purpose of improving the structural strength of the crank arm. A way of so closing the opening 24 is illustrated in
The crank arm according to the present invention can be produced with processes different from the one described previously. For example,
A crank arm having the shape illustrated in FIGS. 12 to 17 may be obtained by crosslinking, in a die, of a structural-fibre-based fabric embedded in a matrix of plastic material. The fibres may be chosen in the group comprising carbon fibres, glass fibres, aramidic fibres, boron fibres, ceramic fibres, or any combination thereof. In this case, the cavity 22 may be formed by providing an expandable core on which the fabric englobed in a non-crosslinked matrix is wound. During crosslinking of the material, the core applies a forming pressure on the inner surfaces of the cavity and is extracted from the crank arm body at the end of crosslinking of the material. The expandable core may consist of a thermally dilatable material such as Teflon or the like. Alternatively, the cavity may be formed by providing an inflatable element for forming the inner surfaces of the cavity.
With reference to FIGS. 12 to 17, the crank arm according to the present invention, irrespective of the process and of the material used for its fabrication, is characterized by certain dimensional ratios that enable optimal results to be obtained in terms of mechanical strength and lightness. In the figures, the letter l indicates the useful length of the crank arm, defined as the distance between the centres of the holes 42 and 44; the letter l1 indicates the distance between the outer end of the opening 24 and the centre of the hole 42; l2 indicates the distance between the outer end of the opening 24 and the hole 44; l3 indicates the distance between the inner edge of the opening 24 and the centre of the hole 42; and l4 indicates the distance between the outer edge and the inner edge of the opening 24, all these distances being measured in the direction of the longitudinal axis 60 of the crank arm. According to the present invention, the ratio l2/l (which expresses the length of the solid section in proportion to the useful length of the crank arm) is comprised between 0.2 and 0.5, and preferably between 0.3 and 0.4; the ratio l3/l (i.e. the ratio between the length in the “closed” hollow section and the useful length of the crank arm) is comprised between 0.3 and 0.8, and preferably between 0.5 and 0.7; the ratio l4/l (i.e. the length of the opening in proportion to the useful length) is comprised between 0.1 and 0.25, and preferably between 0.15 and 0.22; and the ratio l1/l (i.e. the ratio between the overall length of the hollow section and the useful length of the crank arm) is comprised between 0.3 and 0.8, and preferably between 0.6 and 0.7.
As is illustrated in
A further particularly important ratio of dimensions is that of the thicknesses, sc (thickness of the hollow section 50), sp (thickness of the solid section 52) and sl (thickness of the wall surrounding the cavity 22), indicated in
The foregoing description refers to a process for the production of what is commonly recognized as a left-hand crank arm. It will understood that the process applies equally to a right-hand crank arm.
The crank arm according to the present invention and the process for its fabrication may be varied. For example, the cavity 22 could open onto the outside in two different points, for instance on both of the wide sides, 54 and 56, of the crank arm. Furthermore, the crank arm 136 may also be obtained from a U-shaped forging as illustrated in
Claims
1. A process for the production of a hollow crank arm for a bicycle having an area for connection to the bottom bracket spindle and an area for connection of the pedal, wherein it forms a cavity and two solid end portions containing said areas located on opposite sides of said cavity.
2. A process according to claim 1, wherein it comprises the step of forming the cavity with an opening that opens onto the area between the solid portions.
3. A process according to claim 1, wherein it comprises the steps of forming holes for fixing the bottom bracket spindle and the pedal which do not communicate with the cavity.
4. A process according to claim 2, wherein it comprises the step of closing the opening.
5. A process according to claim 4, wherein it comprises the steps of forming a flap for integral closing which extends in the proximity of said opening and of fixing said flap along at least one part of an edge of the opening.
6. A process according to claim 5, wherein the fixing of said flap is carried out by welding or bonding.
7. A process according to claim 6, wherein it comprises the steps of: providing a separate closing element having a shape suited for fitting to the opening and fixing said closing element to the body.
8. A process according to claim 1, wherein the cavity is obtained by means of hot drawing with a punch.
9. A process according to claim 1, wherein the cavity is formed by fixing a closing element to a body in which a lowered area has been formed.
10. A process according to claim 9, wherein the closing element is fixed to the crank arm body by means of welding or bonding.
11. A process according to claim 9, wherein the lowered area is formed by machining with removal of material or plastic deformation.
12. A process according to claim 1, wherein the body is formed by casting in a die and in that the cavity is formed by a filling element positioned in the die prior to casting of the molten material.
13. A process according to claim 12, wherein the filling element is a core of porous material which remains inserted in the crank arm body.
14. A process according to claim 12, wherein the filling element is extracted from the body of the crank arm after solidification of the molten material.
15. A process according to claim 1, wherein the body is formed by the crosslinking, in a die, of a structural-fibre-based fabric embedded in a matrix of plastic material, in which the fibres are chosen in the group comprising carbon fibres, glass fibres, aramidic fibres, boron fibres, ceramic fibres, or any combination thereof.
16. A process according to claim 15, wherein the cavity is formed by providing an extractable expandable core.
17. A process according to claim 16, wherein the expandable core is made of thermally dilatable material.
18. A process according to claim 16, wherein the cavity is formed by providing an inflatable element.
Type: Application
Filed: Aug 10, 2007
Publication Date: Jan 10, 2008
Applicant: CAMPAGNOLO S.R.L. (Vicenza)
Inventors: Maurizio Valle (Vicenza), Paolo Pasqua (Vicenza)
Application Number: 11/837,209
International Classification: B21D 53/86 (20060101);