WO2024099916A1

WO2024099916A1 - Crank arm and crank arm assembly for a bicycle as well as production process for said crank arm

Info

Publication number: WO2024099916A1
Application number: PCT/EP2023/080749
Authority: WO
Inventors: João Filipe Tavares Miranda
Original assignee: Miranda & Irmão Lda.
Priority date: 2022-11-10
Filing date: 2023-11-06
Publication date: 2024-05-16
Also published as: DE102022129801A1

Abstract

The invention relates to a crank arm for a bicycle and a production process for said crank arm. The invention also relates to a crank arm assembly comprising said crank arm. The crank arm (1) has a first through hole (2) at a first end portion (3) for coupling to a spindle of a bicycle, and a second through hole (4) at a second end portion (5) for coupling to a pedal, wherein the crank arm (1) is formed of a carbon composite material. The object of the present invention is to provide a crank arm and a crank arm assembly that are light weight and durable. In case of an attachment of the crank to a metallic interface, the crank should enable a long-lasting and detachable coupling. The object is solved in that multiple carbon fiber layers are arranged concentric to a central axis (7, 13) of at least one of the first and second through hole (2, 4).

Description

CRANK ARM AND CRANK ARM ASSEMBLY FOR A BICYCLE AS WELL AS PRODUCTION PROCESS FOR SAID CRANK ARM

The invention relates to a crank arm for a bicycle and a production process for said crank arm. The invention also relates to a crank arm assembly comprising said crank arm.

Crank arms are known in the prior art. A crank arm has a first through hole at a first end portion for coupling the crank arm to a spindle of a bicycle, and a second through hole at a second end portion for coupling the crank arm to a pedal. Known crank arms may be formed of metal or of a carbon composite material, wherein carbon composite materials have recently received special attention.

Crank arms are also referred to as cranks, arms, or crankarms in practice. A crank arm is part of a crank set of a bicycle. A crank set is also referred to as chain set in practice. The crank set of a bicycle is used to convert a reciprocating motion of a rider's legs into a rotational motion used to drive a chain or belt of the bicycle. For this purpose, crank sets comprise at least one crank arm and at least one sprocket, connected to each other in a rotationally fixed manner. For example, the sprocket may be connected to the crank arm using an adhesive connection, an interlocking connection, and/or a frictional connection. In practice, one-piece crank sets are also known that combine the crank arm and the sprocket to a single piece. In the case of electric bikes, the sprocket may be fixed to a spindle of the electric drive while the crank is fixed to an interface of the spindle of the electric drive.

The crank arm and the sprocket (i.e. the crank set) are coupled to a spindle, in the case of regular bicycles the spindle of the bottom bracket. The bottom bracket on a bicycle connects the crank set to the bicycle frame and allows the crank set to rotate freely. The connection of the crank set to the bottom bracket may be realized in that a metallic interface of a spindle of the bottom bracket is inserted in the first through hole at the first end portion of the crank arm to establish an interlocking connection. The crank arm extends in the direction orthogonal to a central axis of the interface. In other words, a male-female connection may be established. The interface of the spindle is also referred to as a mount. Several geometries of mounts (interfaces) are used in practice, e.g. square taper interfaces or splined interfaces such as the so-called 'International Splined Interface Standard' (ISIS Drive). The interface and the corresponding geometry of the first through hole at the crank arm may be tapered in the axial direction of the interface.

[Rectified under Rule 91, 09.01.2024]
Crank arms formed of a carbon composite material with a first through hole at a first end portion and a second through hole at a second end portion are known in the prior art.

EP 2 006 199 A2 discloses a crank arm for a bicycle, comprising an elongated body having, at a first free end portion thereof, a first seat for coupling with a bicycle pedal and, at a second free end portion thereof, a second seat for coupling with a first free end of a shaft of a bottom bracket assembly. The crank arm comprises front coupling means at an annular portion of said elongated body extending circumferentially about said second seat. The crank arm may be made from carbon fiber composite material. If the crank arm is made from carbon fiber composite material, the crank arm comprises an insert made from metallic material in which the front coupling means are formed.

Metal inserts are usually used in carbon composite crank arms to provide a long-lasting connection of the crank arm to the metallic interface of a spindle. In this context, reference is made to US 2021/0387694 A1, which discloses a bicycle crank arm that may be made of a composite material. The crank arm has one or more inserts at the ends of the crank arm, where it is connected to the pedal or bottom bracket. According to US 2021/0387694 A1, the metal insert is useful because carbon composite materials used to form crank arms are not well suited to connect to or support a rotatable connection mechanism.

A carbon composite crank arm which does not have a metal insert is known from DE 20 2014 103 455 U1. This document discloses a crank set with a spindle, a first crank arm connected to one end of the spindle and a second crank arm connected to the other end of the spindle. For connecting the spindle to at least one crank arm one end of the spindle has a tooth profile, and one crank arm has a counter-tooth profile. The counter-tooth profile of the crank arm is made of carbon fiber-reinforced plastic. Providing a counter-tooth profile made of carbon fiber-reinforced plastic is possible because the tooth profile of the spindle is also made from carbon fiber-reinforced plastic, which causes less wear on the counter-tooth profile than usually used metal spindles.

US 2012 / 0 064 284 A1 discloses a crank with endlessly wound fiber layers along the periphery of the crank. US 2005 / 0 012 298 A1 also discloses a crank with endlessly wound fiber layers extending along the periphery of the crank and, in an embodiment, crossing each other in the central region of the crank. FR 2 636 386 A1 and US 2007 / 0 199 403 A1 again show fiber bundles extending along the periphery of the crank. US 2022 / 0 250 711 A1 and DE 10 2017 128 691 A1 do not clearly disclose the position of the reinforcement fibers.

The object of the present invention is to provide a crank arm and a crank arm assembly that are light weight and durable. In case of an attachment of the crank to a metallic interface, the crank should enable a long-lasting and detachable coupling. Another object of the invention is to provide a production process for said crank arm.

The objects of the invention are solved by the crank arm, the production process and the crank arm assembly according to the independent claims.

The crank arm for a bicycle has a first through hole at a first end portion for coupling to a spindle of a bicycle, and a second through hole at a second end portion for coupling to a pedal. The crank arm is formed of a carbon composite material, wherein multiple carbon fiber layers are arranged concentric to a central axis of at least one of the first and second through hole.

In other words, the carbon composite material is a layered material, wherein the layers comprise woven and/or non-woven carbon fibers. The carbon fibers may be endless and/or unidirectional. The first end portion of the crank must transmit the torque generated by the rider of the bicycle to the spindle of the bicycle. This is the portion of the crank where the strongest forces and torques act on the material of the crank. In particular, when the crank is fixed on a metallic interface of the spindle, strong radial forces act on the first through hole. The best resistance against radial extension of the material around the through hole can be obtained by wrapping bands comprising unidirectional and endless carbon fibers around the through hole. The carbon fibers provide a reinforcement to the carbon composite material. Therefore, the layers formed of carbon fibers are also referred to as reinforcement layers. The reinforcement layers are embedded in the matrix formed by a plastic resin. The resin may be chosen from any suitable polymer material and most commonly consists of epoxy resin. The first end portion of the crank arm comprises multiple layers arranged around an axis that forms the central axis of the first through hole. Additionally or alternatively, the second end portion of the crank arm comprises multiple layers arranged around an axis that forms the central axis of the second through hole.

The arrangement of multiple carbon fiber layers concentric to the central axis of at least one of the first and second through hole enables the crank arm to distribute loads from a spindle, particularly from an interface of a spindle, effectively into the end portions. In particular, the concentric arrangement of the reinforcement layers allows to bear high tensile forces in the circumferential direction around the central axis of the through hole. Therefore, the crank arm may for example be pressed with a high force onto a metallic tapered interface of a spindle, wherein the force pressing the interface of the crank arm onto the tapered interface of the spindle causes said circumferential tension around the through hole. The high circumferential tension within the reinforcement layers causes a high clamping force that tightly fits the internal interface of the first through hole onto the metallic interface of the spindle. Even in cases of other connections between the crank and the spindle than pressing, such as a glue connection, the concentric arrangement of the reinforcement layers around the through hole leads to a strong and durable crank in the region of the first trough hole that is connected to the spindle. If the concentric arrangement of the reinforcement layers is arranged around the second through hole, it provides the desired rigidity and durability for the attachment of the shaft of a pedal.

Due to the concentric arrangement of the layers and the benefit associated with it, the crank arm described herein does not require a metallic insert for establishing a detachable, long-lasting connection of the first through hole to a spindle, particularly when the first through hole is connected to an interface of the spindle. Thus, the weight of the crank arm is reduced compared to other crank arms using inserts for the attachment to interfaces of metallic spindles. Furthermore, the crank arm described herein is completely recyclable as it is made of a single, recyclable material. Additionally, the crank arm described herein may be easily arranged on the spindle and removed from the spindle compared to crank arms with metal inserts because the carbon in the carbon composite material of the crank arm acts as lubricant on the interface of the metallic spindle. Crank arms with metal inserts embedded in the resin matrix in the first end portion having the first through hole are at risk of cold-welding to the metallic interface of a spindle which makes removal of these crank arms from the interface difficult or impossible. The metal inserts further require specific precautions so that they do not rotate within the plastic material of the crank. Metal inserts usually have hooks embedded in the plastic material that prevent rotation. The shape of the metal inserts with the hooks significantly influences the final shape of the crank. With a crank of the present design without embedded metal inserts, there are no such limitations to the shape of the crank. Embedded metal inserts also require special treatment in order to obtain a strong adhesion to the resin material of the crank. The crank of the present proposal without an embedded insert does not require such special treatment. The concentric arrangement of the reinforcement layers that guarantees the rigidity and stability of the end region of the crank around the through hole is itself penetrated by the plastic matrix, preferably the epoxy resin of the plastic matrix so that it is an integral part of the carbon composite material of the crank.

For obtaining the concentric arrangement of the reinforcement layers, the reinforcement layers may be wound around a core in the cavity of a mold for the crank that defines the through hole. In a preferred embodiment, so called prepregs can be used to constitute the concentric arrangement of the reinforcement layers around the core that defines the through hole. Prepregs are carbon fiber layers pre-impregnated with resin such as epoxy resin. The prepregs may be partly cured. Due to the adhesion of the resin, the prepregs may be precisely wound around the core of the mold. Other patches of carbon reinforcement layers may be placed in the cavity of the mold that forms the crank. These other patches may also be prepregs. They are preferably placed on the surface of the cavity of the mold. Multiple patches may be placed in the mold and may overlap one another in practice.

In practice, the crank may be bladder molded. For this purpose, an inflatable bladder is placed inside the cavity of the mold after the carbon fiber layers, e.g. in the form of prepregs, are applied to the cavity and before the mold is closed. Additional resin may be applied to the cavity of the mold. The pressure inside the bladder is increased with respect to the exterior of the mold so that the bladder is inflated and presses the resin and the reinforcement layers towards the walls and the cores of the mold. The bladder creates a hollow space inside the crank arm. This reduces the weight of the crank arm further. The bladder further presses the different reinforcement layers against each other which increases the rigidity of the crank arm. The bladder also helps removing air bubbles from the matrix of the carbon composite material.

The arrangement of the layers of carbon material in a middle portion of the crank arm between the first end portion and the second end portion may be chosen depending on the needs of the application. When the layers are applied to the cavity as described above, in the middle portion of the crank arm the layers of the carbon reinforcement material are arranged orthogonal to the axes of the first and second through hole along the surface of the crank arm and the middle portion may be hollow. Alternatively, a laminar structure of carbon fiber layers may be realized at least in a part of the middle portion by arranging multiple carbon fiber layers on top of each other.

In practice, the multiple carbon fiber layers arranged concentric to the central axis of at least one of the first and second through hole are circular or spiraling layers. A circular layer is a layer that is arranged with an essentially constant radius around the central axis of the through hole. Multiple circular layers may be arranged around the central axis of the through hole one adjacent to another in the radial direction to form the end portion. Such circular layers may be cut into lengths that correspond to the circumference of the circle on which they are located, or shorter lengths (i.e. patches). A spiraling layer has a varying radius around the through hole and consists of at least one sheet of carbon material wound around the axis of the through hole. The sheet may be one-piece or be created by adhering multiple carbon fiber patches to one another. The end portion may substantially be formed of a single spiraling layer that spirals around the axis of the through hole or two or three sheets that are wound around the axis of the through hole.

In practice, the crank arm is a one-piece crank arm formed of the carbon composite material. That is, the crank arm is a body with multiple carbon fiber layers or patches and a continuous resin matrix, wherein the matrix penetrates and surrounds all carbon fiber layers. A one-piece crank arm is particularly stable and easy to assemble to a bicycle. If the crank arm is produced in a bladder molding process, the crank arm is a hollow body.

In practice, the first through hole may have an internal splined interface for coupling to a complementary external splined interface of a spindle. In other words, the first through hole has a number of radial projections extending towards the central axis that match with grooves or flutes on an external splined interface of a spindle. Particularly, the interfaces may be designed according to the 'International Splined Interface Standard' (ISIS Drive). The combination of an internal splined interface in the first through hole and a complementary interface of a spindle transfers torque effectively and reliably. The splined interfaces may be tapered, in case of an ISIS-Interface by 1° with respect to the axial direction.

In practice, the splined interface may extend over only a portion of the axial length of the first through hole. Adjacent to the splined interface there is an internal thread for applying an extractor tool, the so-called crank tool or crank puller. The standard dimensions for the internal thread for the crank tool is an M22 x 1 mm female thread.

In practice, a threaded sleeve having an internal thread made of metal for coupling to a pedal may be detachably inserted in the second through hole. Loads from the shaft of the pedal introduced to the second through hole are often multiaxial loads. Multiaxial loads can lead to increased wear in the second through hole if no precaution is taken. A precaution may be the use of a metallic threaded sleeve inserted in the second through hole. During the intended use of the crank arm and the threaded sleeve, the threaded sleeve is in contact with the second through hole of the crank arm and connected to the shaft of a pedal. Hence, loads from the shaft of the pedal are exerted on the threaded sleeve and not directly on the carbon composite material of the crank arm. Since the threaded sleeve is detachably inserted in the second through hole, it may be replaced when it is worn while use of the crank arm may be continued. This reduces the amount of waste over the lifetime of the crank arm. For recycling of the crank arm, the metal sleeve may simple be removed.

In practice, the second through hole may have an internal polygonal portion and the threaded sleeve may have a complementary external polygonal portion. The internal polygonal portion extends along the axial direction of the through hole over a part of the length of the through hole and has a polygonal cross section. The internal polygonal portion of the second through hole and the external polygonal portion of the threaded sleeve form an interlocking connection, which suppresses a rotation of the threaded sleeve in the second through hole. The internal polygonal portion and the external polygonal portion may be hexagonal portions with hexagonal cross sections. Alternatively, the polygonal portions may have less than six sides, for example the polygonal portions may be triangular portions or rectangular portions. It is also possible to use a polygonal portion with more sides, for example octagonal portions.

At one end of the polygonal portion of the second through hole comprises a ring-shaped projection that projects radially towards the center of the second through hole. A circular end portion of the threaded sleeve having a small diameter projects into the inner diameter of this ring-shaped projection. The ring-shaped projection fixes the threaded sleeve inside the second through hole in the axial direction.

Further, the threaded sleeve may have a circular collar and the second through hole may have a complementary circular end portion in the axial direction for accommodating this collar. This circular end portion has a diameter that is equal to or larger than the maximum diameter of the polygonal portion. The circular end portion and the polygonal portion of the threaded sleeve may be arranged adjacent to one another in the axial direction of the second through hole in which the threaded sleeve is inserted. The circular collar and the end portion of the through hole may be arranged on a side of the crank arm opposite a pedal that is connected to the crank arm. Thus, when the threaded shaft of a pedal is screwed into the threaded sleeve, the circular collar and the pedal form an interlocking connection in the axial direction of the second through hole. In this case, the circular collar faces the chain of a bicycle to which the crank arm is connected. The circular collar is therefore quickly soiled by chain grease but covers the rest of the through hole against such soiling. The circular shapes of the collar and the end portion are easy to clean because they have no corners.

The threaded sleeve may also be formed of carbon composite material, but generally, the threaded sleeve will be formed of metal. When the threaded sleeve is formed of carbon composite material, removal of the threaded sleeve from the crank arm at the end of the lifetime of the crank arm is not required before recycling of the crank arm. The complete crank arm together with the threaded sleeve can be recycled. When the threaded sleeve is formed of metal, it has a better wear resistance but must be removed from the crank arm before recycling.

The invention also relates to a production process for the crank arm described above. The production process comprises the steps of

arranging carbon fiber layers in a mold for the crank arm;
adding synthetic resin to the mold;
curing the synthetic resin.

In the process, multiple carbon fiber layers are arranged concentric to an axis that forms the central axis of at least one of the first and second through hole. Thus, the concentric carbon fiber layers form at least part of the first and/or second end portion.

The multiple carbon fiber layers may be wound around a core of the mold defining the through hole. The carbon fiber layers may be applied as prepregs that are pre-impregnated with resin so that they adhere to the core. Further carbon fiber layers may be applied to the cavity of the mold before the mold is closed and the resin matrix is cured. The crank may be bladder molded. In this case, a bladder is placed inside the mold and expanded during curing.

Regarding the production process, reference is also made to the preceding description of the crank arm, which discloses steps of the production process. The concentric carbon fiber layers arranged in the mold may be multiple circular layers or at least one spiraling layer. Again, if the concentric fiber layers are made of endless carbon fibers, the material around the through hole is best protected against radial extension.

In practice, during the production of the crank arm, material may be removed from the cured crank arm by machining from the first end portion with the concentric carbon fiber layers to form the first through hole. In other words, the crank arm is first formed as an integral block without through holes or with through holes having small diameters and then cured. In one or more subsequent steps, the final internal shape of the first through hole is obtained by machining. Preferably, the volume of the cured crank arm that is removed to obtain the final shape of the first through hole does not, or only locally, contain carbon reinforcement fibers. That is, substantially no carbon fibers are arranged in the area, where the final shape of the first through hole will be. Alternatively, carbon fibers are only arranged in the outer region of this area so that the projections after machining the first through hole contain carbon fibers.

When material is cut from the first end portion, for example by milling or broaching, in practice, the final shape of the first through hole may be formed complementary to an external interface of a spindle of a bicycle. Particularly, a cutting tool with a shape corresponding to the splined interface of a spindle may be inserted so as to produce the final shape of the first through hole. This final shape of the first through hole may correspond to an internal interface according to the 'International Splined Interface Standard' (ISIS Drive).

In practice, material may also be removed by machining from the second end portion of the cured crank arm to form the second through hole. The machining of the second end portion may be executed in the same manner as the machining of the first end portion.

As an alternative, the cavity of the mold may be shaped to define as many structural elements and shapes of the crank arm as possible, e.g. including the tapered interface and/or the internal thread of the first through hole and the polygonal portion, the ring shaped projection and/or the circular end portion of the second through hole. In this case, the most of the machining can be omitted and the production of the crank arm is less costly. It may also be possible to define some of the structural elements and shapes of the crank arm during the molding process and then add or further define the rest of the structural elements and shapes by machining.

The invention also relates to a crank arm assembly comprising a crank arm according to the above description, preferably having a sprocket, a bottom bracket with a spindle having a metallic interface, and a pedal. In other words, the crank arm assembly preferably comprises a crank set, a bottom bracket, and a pedal. In case of an electric bike, the bottom bracket may be part of an electric drive and the sprocket is fixed to the electric drive. The pedal is detachably connected to the crank arm and the crank arm is detachably connected to the metallic interface of the spindle via the first through hole. Further details of the crank arm assembly are described in the above description of the crank arm and its production process. The sprocket may directly be connected to the crank arm, for example using screws. Additionally or alternatively, the sprocket may be connected to the spindle. If the sprocket is connected to the spindle, the connection may be an interlocking connection using a standard interface. It is also possible to connect multiple sprockets to the crank or the spindle by using a so-called spider.

Further practical embodiments and advantages of the invention are described below in connection with the drawings.

shows the crank arm according to the invention in a first side view with a threaded sleeve inserted in the second through hole.

shows the crank arm of in a second side view opposite to the first side view.

shows a sectioned view of the crank arm of on plane A-A indicated in .

shows a magnification of portion Z of the sectioned view of .

shows the threaded sleeve in a side view.

shows the threaded sleeve in a top view.

shows a flow diagram of a production process for producing the crank arm.

shows a crank arm assembly according to the invention.

In the figures, identical parts are marked with identical reference numbers.

Referring to Figs. 1 to 3, the crank arm 1 for a bicycle is a flat, elongated body. The crank arm 1 has a first through hole 2 at a first end portion 3 for coupling to an interface of a spindle of a bottom bracket connected to the bicycle. The crank arm 1 also has a second through hole 4 at a second end portion 5 for coupling to a pedal. The first and second through hole 2, 4 extending through the flat crank arm 1 have parallel axes. At the first and second end portions 3, 5, the crank arm has round endings. Between the first and second end portion 3, 5 an elongated and hollow middle portion 6 extends. The hollow middle portion 6 is formed by inflating a bladder inside the mold before and during curing of the resin. A concave recess is formed in the surface of the elongated middle portion of the crank arm 1.

The crank arm 1 is formed of a carbon composite material that comprises multiple layers of carbon fibers embedded in a matrix. In the first end portion 3, multiple carbon fiber layers are arranged concentric to a central axis 7 of the first through hole 2 as spiraling layers 8. Alternatively, multiple carbon fiber layers may be arranged concentric to the central axis 7 of the first through hole 2 as circular layers (not shown). In the second end portion 5 and in the middle portion 6, the carbon fiber layers are generally arranged in a flat, layered manner in planes substantially orthogonal to the central axis 7 to form a laminar structure. In the hollow middle portion 6, the layers are located near the surface, whereas in the bulky second end portion 5, multiple layers are arranged throughout the entire thickness of the crank arm 1. The flat, individual layered carbon fiber layers are not shown in the figures, only the individual spiraling layers 8 in the first end portion 3 are shown (not to scale). The polymer matrix, in which all carbon fiber layers are embedded, is for example made of epoxy resin and penetrates the carbon composite material of the crank arm continuously. Hence, the crank arm 1 is made of only one piece.

The first through hole 2 of the crank arm 1 has a portion extending in the direction of the central axis 7 with an internal spline interface. The internal spline interface comprises a number of teeth 9 or projections 9 projecting inwardly towards the central axis 7 and being shaped complementary to flutes or grooves on a matching interface of a spindle. Thus, the crank arm 1 may be coupled to an external spline interface of a spindle. In the figures, the first through hole 2 has fourteen projections 9, which extend substantially parallel to the central axis 7. As best seen in , the projections 9 are slightly tilted towards the central axis 7 so that the through hole 2 is slightly tapered in the portion with the projections 9 in the direction of the central axis 7. The representation of the tapered portion in the figures is not to scale, that is the tapering may be more or less pronounced in practice than shown in the figures. The taper can also be waived completely in a practical embodiment (not shown). Also, in practice, the first through hole 2 may have more or less than fourteen projections 9. In particular, the first through hole 2 may be formed to match with an external splined interface according to the so-called 'International Splined Interface Standard' (ISIS Drive). Adjacent to the tapered portion in the axial direction of the first though hole 2, the first though hole 2 has a circular portion with an M22 x 1 mm female thread. In use of the crank arm, a bolt head (cf. ) may be fixed inside this circular portion. A complementary thread of a bolt of an extractor tool may be screwed into the M22 x 1 mm thread for removing the crank arm from a spindle.

Referring to Figs. 3 to 6, a metallic threaded sleeve 10 is detachably inserted in the second through hole 4. As best seen in , the threaded sleeve 10 has an internal thread 11 for coupling the inserted threaded sleeve 10 to a shaft of a pedal. To suppress rotation of the threaded sleeve 10 in the second through hole 4, the threaded sleeve 10 and the second through hole 4 have complementary geometries with polygonal portions 12, 14. In particular, the second through hole 4 has an internal polygonal portion 12 that extends parallel to a portion of a central axis 13 of the second through hole 4. The term 'polygonal portion' refers to a polygonal cross section of the through hole 4 extending over a portion of the axial length of the through hole 4. Complementary to the internal polygonal portion 12, the threaded sleeve 10 has an external polygonal portion 14 which also extends in the axial direction of the central axis 13 of the second through hole 4 in , when inserted in the second through hole 4. The external polygonal portion 14 of the threaded sleeve 10 is shown in detail in Figs. 5 and 6. In these figures, the external polygonal portion 14 is a hexagon. Alternatively, the threaded sleeve 10 may have a polygonal portion of another cross section, such as a triangle, a rectangle or an octagon (not shown in the figures). The internal polygonal portion 12 of the through hole 4 is always formed complementary to the external polygonal portion 14 of the threaded sleeve.

Adjacent to the external polygonal portion 14 in the direction of the central axis 13 of the second through hole 4, the threaded sleeve 10 has a circular collar 15. The circular collar 15 matches with a complementary portion of the second through hole 4. This complementary portion is a circular end portion 16 in the axial direction of the second through hole 4 and is located on a side of the crank arm 1 that faces away from the pedal during the intended use of the crank arm 1. The terms 'circular collar' and 'circular end portion' refer to circular cross sections of the collar 15 and the end portion 16 orthogonal to the central axis 13. When a threaded bolt of a shaft of a pedal is screwed into the internal thread 11 of the threaded sleeve 10 from a side opposite the circular end portion 16, the shaft of the pedal and the circular collar 15 clamp the second through hole 4 of the crank arm 1.

In the embodiment of the crank arm 1 described herein, the second through hole 4 has a ring-shaped projection 17. The ring-shaped projection 17 is formed as a shoulder and is located at the end of the polygonal portion 14 opposite the circular end portion 16 in the direction of the central axis 13. The threaded sleeve 10 has a complementary circular end portion 18 on the opposite side of the collar 15 in the direction of the central axis 13. The complementary circular end portion 18 of the threaded sleeve 10 has a reduced diameter compared to the external polygonal portion 14 and is accommodated inside the inner diameter of the ring-shaped projection 17. Thus, the ring-shaped projection 17 is clamped between the polygonal portion 14 of the threaded sleeve 10 and a collar of the shaft of the pedal so that the threaded sleeve 10 is fixed within the second through hole 4.

The crank arm described above may be manufactured according to the fabrication process shown in in the form of a flow chart. In a first fabrication step f1, a crank mold and carbon fiber layers or patches of carbon fiber layer in the form of prepregs are provided.

In a second fabrication step f2, the carbon fiber layers are arranged concentrically around a core that is arranged in a first end portion of the cavity of the crank arm mold. When prepregs are used, the layers adhere to the core and to one another. The first end portion of the mold defines the external geometry of the first end portion of the crank arm. The mold also has a second end portion that defines the external geometry of the second end portion of the crank arm, and an elongated middle portion between the first and second end portions of the mold that defines external geometry of the middle portion of the crank arm.

In a third fabrication step f3, flat carbon fiber layers are placed in the middle portion and the second end portion of the mold. In the hollow middle portion, the layers extend parallel to the surface of the crank arm. In the second end portion, the layers may be applied in accordance to the required rigidity of the different regions of this end portion. For example, reinforcement layers may be located around a core if a core is used to define the second through hole.

In a fourth fabrication step f4, a synthetic resin is added to the mold that fills possible voids between the carbon layers and penetrates the carbon fiber layers. The most commonly used resin is an epoxy resin.

In a fifth fabrication step f5, the crank arm is cured and subsequently removed from the mold. After this step, the cured crank arm may already have the final external shape. Alternatively, the crank arm may experience a machine finishing, e.g. by milling and/or grinding, to obtain the final external shape. For example, recesses may be produced in volumes of the crank arm that are subject to low loads, thus reducing the weight of the crank arm.

In a sixth fabrication step f6, material may be removed from the first and/or second end portion of the cured crank arm by machining, in particular by drilling or milling, to form a circular through hole. Subsequently, special geometries may be formed in the circular through hole. For example, material may be removed from the first circular through hole in the first end portion by broaching so that projections are formed between the broached volumes. In particular, a geometry with an internal splined interface may be formed in the first through hole that is complementary to an external splined interface of a spindle. Additionally, or alternatively, an internal polygonal portion and circular end portions of the second through hole may be formed by the machining.

shows a crank arm assembly with an embodiment of the crank arm 1. The crank arm assembly comprises the crank arm 1, a spindle 19 and a pedal 20. The spindle 19 generally is a substantially cylindrical body and has a metallic external interface 21 at a first end portion. This end portion also has a hole in the axial direction of the spindle 19 with an internal thread 22. The external interface 21 of the spindle 19 has a tapered spline geometry with grooves and corresponds to the internal interface of the crank arm 1. The interface 21 of the spindle 19 is inserted in the tapered portion of the first through hole 2 of the crank arm 1, forming an interlocking connection with the crank arm 1. The spindle 19 protrudes from the first through hole 2 towards a first side of the crank arm 1 in the direction of the central axis 7 of the spindle 19. A bolt 23 is inserted into the first through hole 2 and screwed into the internal thread 22 of the spindle 19 from a second side of the crank arm 1 opposite to the first side. The bolt 23 pulls the interface 21 of the spindle 19 into the internal interface of the crank arm 1. Due to the taper of the interfaces of the spindle and the crank arm, the bolt induces a strong radial pressure on the interfaces.

The pedal 20 of the crank arm assembly is detachably connected to the second through hole 4 of the crank arm 1. The pedal may be a click pedal, but any other type of pedal may be used. The pedal 20 has a main body 24 that freely rotates around the central axis 13 of the second through hole 4. Further, the pedal 20 has a shaft with a cylindrical end portion 25, a shoulder 26 and an external thread 27. The external thread 27 of the shaft of the pedal 20 is screwed into the threaded sleeve 10, which is inserted in the second through hole 4 from the opposite side. The shoulder 26 of the cylindrical end portion 25 rests on the ring-shaped projection 17 inside the second through hole 4. Thus, the crank arm 1 is clamped between the shoulder 26 of the pedal shaft and the threaded sleeve 10 in the direction of the axis 13 of the second through hole 4 in detachable manner.

The crank arm assembly described above may be assembled by pressing the first through hole 2 onto the external interface of the spindle 19 by means of the crank screw. Due to the tapered interface of the spindle and the corresponding geometry of the first through hole of the crank arm, a pressing force exerted on the crank arm in the direction of the central axis of the first through hole causes a frictional connection between the crank arm and the spindle. Additionally, the complementary geometries of the external splined interface of the spindle (grooves) and of the internal splined interface of the first through hole (projections) form an interlocking connection. Consequently, torque is transferred effectively from the crank arm to the spindle. The threaded sleeve according to the above description with an internal thread is inserted into the second through hole from the first side of the crank arm oriented towards the spindle. Then, a threaded end portion of the shaft of a pedal is screwed into the internal thread of the threaded sleeve from a second side of the crank arm opposite to the first side.

The features of the invention disclosed in the present description, in the drawings as well as in the claims may be essential, both individually and in any combination, for the realization of the invention in its various embodiments. The invention is not limited to the embodiments described. It may be varied within the scope of the claims and taking into account the knowledge of the person skilled in the art.

1 crank arm
2 first through hole
3 first end portion
4 second through hole
5 second end portion
6 middle portion
7 central axis of the first through hole
8 spiraling layers
9 teeth, projections
10 threaded sleeve
11 internal thread
12 internal polygonal portion of the second through hole
13 central axis of the second through hole
14 external polygonal portion of threaded sleeve
15 circular collar of the threaded sleeve
16 circular end portion of the second through hole
17 ring-shaped projection of the second through hole
18 circular end portion of threaded sleeve
19 spindle
20 pedal
21 external interface of the spindle
22 internal thread of the spindle
23 bolt
24 main body of the pedal
25 cylindrical end portion of the pedal shaft
26 shoulder of the end portion of the pedal shaft
27 thread of the end portion of the pedal shaft

f1 first fabrication step
f2 second fabrication step
f3 third fabrication step
f4 fourth fabrication step
f5 fifth fabrication step
f6 sixth fabrication step

Claims

Crank arm (1) for a bicycle with a first through hole (2) at a first end portion (3) for coupling to a spindle of a bicycle, and a second through hole (4) at a second end portion (5) for coupling to a pedal, wherein the crank arm (1) is formed of a carbon composite material,
characterized in that multiple carbon fiber layers are arranged concentric to a central axis (7, 13) of at least one of the first and second through hole (2, 4).
Crank arm according to claim 1, characterized in that the multiple carbon fiber layers arranged concentric to a central axis (7, 13) of at least one of the first and second through hole (2, 4) are at least one of:
circular layers;

spiraling layers (8);

made of endless carbon fibers;

made of unidirectional carbon fibers.
Crank arm according to claim 1 or 2, characterized in that the crank arm (1) is a one-piece crank arm formed of the carbon composite material.
Crank arm according to any one of claims 1 to 3, characterized in that the first through hole (2) has an internal splined interface with projections (9) for coupling to a complementary external splined interface of a spindle.
Crank arm according to any one of claims 1 to 4, characterized in that a threaded sleeve (10) having an internal thread (11) for coupling to a pedal is detachably inserted in the second through hole (4).
Crank arm according to claim 5, characterized in that the second through hole (4) has an internal polygonal portion (12) and the threaded sleeve (10) has a complementary external polygonal portion (14).
Crank arm according to claim 6, characterized in that the threaded sleeve has a circular collar (15) and the second through hole (4) has a complementary circular end portion (16) for accommodating this collar (15).
Crank arm according to any one of claims 5 to 7, characterized in that the threaded sleeve (10) is formed of carbon composite material.
Crank arm according to any one of claims 5 to 7, characterized in that the threaded sleeve (10) is formed of metal.
Production process for the crank arm according to any one of claims 1 to 9, comprising the steps of
- arranging carbon fiber layers in a mold for the crank arm (f2, f3);

- adding synthetic resin to the mold (f4);

- curing the synthetic resin (f5);
characterized in that multiple carbon fiber layers are arranged concentric to an axis that forms the central axis of at least one of the first and second through hole (f1).
Production process according to claim 10, characterized in that the multiple carbon fiber layers are wound around a core of the mold defining the through hole.
Production process according to claim 10 or 11, characterized in that material is removed by machining from at least one of the first and second end portion of the cured crank arm (f6).
Production process according to claim 12, characterized in that at least a portion of the first through hole is formed complementary to an external interface (f6).
Crank arm assembly comprising a crank arm according to one of claims 1 to 9, a spindle having a metallic interface, and a pedal, wherein the pedal is detachably connected to the crank arm and the crank arm is detachably connected to the metallic interface via the first through hole.