Bicycle crank and method for manufacturing same

A crank arm for a bicycle includes a crank arm body having a pedal attachment hole on a first end thereof and a spindle attachment hole on a second end thereof. The crank arm body defines an elongated interior cavity surrounded by a shell, wherein the interior cavity is open to an exterior of the crank arm body. The opening can be used to access the cavity during and after manufacturing. The shape of the cavity may be varied to produce a lightweight yet strong structure. In the method used to form the crank arm body, a mold core formed by a shell containing a sand-like filler material is positioned into a casting mold so that a melt space is formed around the mold core, molten metal is poured into the casting mold, and the molten metal is solidified to form a crank billet. The filler material may be removed through an opening in the crank billet. This may be accomplished by drilling the crank billet to form the pedal attachment hole in a location that communicates with the filler material and then removing filler material through the pedal attachment hole.

BACKGROUND OF THE INVENTION
 The present invention is directed to bicycle crank arms and, more
 particularly, to a bicycle crank arm that includes a shell to define an
 interior cavity or to hold a filler material.
 It is desirable for a bicycle to be as lightweight as possible, so the
 bicycle parts should be reduced in weight as much as possible. This is
 true of bicycle cranks as well. A bicycle crank that is made lightweight
 by being manufactured in the form of a hollow tube is known from Japanese
 Patent Publication 2-18652, for example. Furthermore, a method for forming
 an internal cavity in a solid material by extrusion forging has also been
 proposed in Japanese Laid-Open Patent Application 5-116670. This hollow
 crank is obtained by the welding or plastic deformation of a pipe or crank
 billet, but this method affords little freedom in the design of the crank
 shape. The shape is further restricted because molding is impossible
 without certain portions that are otherwise unnecessary in terms of
 material dynamics, among other reasons. Another drawback is that a high
 quality appearance is difficult to achieve because of limitations in the
 machining process, despite demand for certain types of designs.
 Methods for manufacturing a bicycle crank from a light alloy by casting are
 also known from Japanese Laid-Open Patent Application 58-93554. The shape
 restrictions noted above are eliminated with these casting methods, but
 forming a cavity on the inside is difficult with a crank because of the
 small size of the part and the hollow interior can degrade the mechanical
 strength of the product Accordingly, it has been proposed that a pipe or
 the like be integrally cast in the interior as shown in Japanese Laid-Open
 Utility Model Applications 48-7948 and 61-131391. Integrally embedding a
 pipe or other such member with high strength in the crank does indeed
 preserve the strength of the crank, but a problem remains in terms of
 making the product lightweight and strong at the same time.
 SUMMARY OF THE INVENTION
 The present invention is directed to a hollow bicycle crank that can be
 manufactured by casting while providing substantial design freedom. Such a
 crank can be lightweight while also being strong. In one embodiment of a
 bicycle crank according to the present invention, a crank arm for a
 bicycle includes a crank arm body having a pedal attachment hole on a
 first end thereof and a spindle attachment hole on a second end thereof
 The crank arm body defines an elongated interior cavity surrounded by a
 shell, wherein the interior cavity is open to an exterior of the crank arm
 body. The opening can be used to access the cavity during and after
 manufacturing. The shape of the cavity may be varied to produce a
 lightweight yet strong structure. For example, the interior cavity may
 have a substantially semicircular cross sectional shape in proximity to
 the spindle attachment hole and a substantially rectangular shape in
 proximity to the pedal attachment hole, and a cross sectional diameter of
 the cavity may decrease from a central portion of the crank arm body to
 the first and second ends of the crank arm body. The cavity may be filled
 with a material having a lower specific gravity than the metal forming the
 crank arm to provide strength while still saving weight.
 In one embodiment of a method used to form the crank arm body according to
 the present invention, a mold core formed by a shell containing a
 sand-like filler material is positioned into a casting mold so that a melt
 space is formed around the mold core, molten metal is poured into the
 casting mold, and the molten metal is solidified to form a crank billet.
 The filler material may be removed through an opening in the crank billet.
 This may be accomplished by drilling the crank billet to form the pedal
 attachment hole in a location that communicates with the filler material
 and then removing filler material through the pedal attachment hole.
 Since the crank of the present invention, and the manufacturing method
 therefor, have a core on the inside of the crank, the crank is lightweight
 and yet rigid. Furthermore, since the crank is manufactured by casting, it
 can be designed in any shape, allowing a greater degree of latitude in
 design, so it is easier to achieve a high-quality appearance. If the cast
 blank is also mold-forged, the product is lightweight and also has
 sufficient strength. Also, since a core containing a filler is positioned
 on the inside of the crank arm, the shape of the core is not flattened
 during mold forging.

DETAILED DESCRIPTION OF THE EMBODIMENTS
 FIG. 1 is a rear view of a particular embodiment of a left side bicycle
 crank 1 according to the present invention. The left crank 1 is made from
 an aluminum alloy and, as shown in FIG. 1, is formed such that its cross
 section is narrower on the pedal spindle end 4 side where the pedal
 spindle (not shown) is attached and thicker on the crank spindle end 2
 side where the crank spindle (not shown) is attached. Thus changing the
 cross sectional area by varying the thickness of the left crank 1 in
 different locations is intended to enhance strength such that the stress
 to which the left crank is subjected is more or less the same everywhere
 in the cross section. A chamfered section 11 (see FIG. 2) is formed along
 both edges on the back side of the left crank 1.
 A pedal attachment hole 6 for attachment of a pedal spindle (not shown) is
 formed in the pedal spindle end 4 on the pedal attachment side of the left
 crank 1. A crank spindle attachment hole 5 for attaching the left crank 1
 to the crank spindle by inserting the spindle into the crank spindle
 attachment hole 5 is formed at the crank spindle end 2 of the left crank
 1. More specifically, a flange 8 protrudes inward from the inner surface
 of the crank spindle attachment hole 5, and a male serration 9 is provided
 integrally on the rear surface side of this flange 8. In this example, the
 serration 9 has eight teeth, as shown in FIG. 1. If there are too few
 teeth, the strength of the rotational bond will be inadequate. On the
 other hand, if there are too many teeth the machining will be difficult,
 the cost will be higher, and there will be a higher incidence of errors in
 the positioning of indexing in the rotational direction during assembly.
 The portion of the crank spindle attachment hole 5 to the rear of the
 serration 9 is structured as a centering component (also called a guide
 component) 10 that is a concentrically tapered hole. The centering
 component 10 is in the form of a cylindrical tapered hole that widens to
 the rear, and, in this example, it is formed at a taper angle of 2.degree.
 to 3.degree.. The taper surface of the centering component 10 is snugged
 up against the taper surface of the centering component of the crank
 spindle (not shown), which accurately aligns the two centers and also
 links them together integrally and securely.
 As shown in FIGS. 1, 3 and 5(a)-(d), a pipe shell 7 made from pure aluminum
 is formed along the pedal spindle end 4 side and the crank spindle end 2
 side centering on the crank center 3, wherein the cross sectional
 structure of the pipe shell 7 is shown in FIGS. 5(a)-(d). More
 specifically, the cross sectional structure of the pipe core 7 is such
 that the shape is semicircular on the crank spindle end 2 side, and this
 shape flattens out to a rectangular shape on the pedal spindle end 4 side.
 The cross sectional area of the pipe shell 7 continuously decreases, and
 the height is at a minimum at the two ends. In other words, the shape of
 the pipe shell 7 approximates that of a ship hull. The weight of the left
 crank 1 is reduced by the pipe shell 7 on the interior of the crank center
 3. The pipe shell 7 also contributes to flexural and other aspects of
 mechanical stress. The metal of the pipe shell 7 may be the same as the
 metal that makes up the left crank 1, but preferably should have as low a
 specific gravity as possible, be resistant to heat, and be able to
 withstand the pressure of forging as discussed below.
 There is an opening 15 on the pedal spindle end 4 side of the pipe shell 7.
 The opening 15 communicates with the internal cavity 14 of the pipe shell
 7, as discussed below. The internal cavity 14 is temporarily filled with
 sand or another filler. The filler packed into the pipe shell 7 prevents
 the pipe shell 7 from being crushed by the pressure of hot forging. The
 filler need not be taken out as discussed below, and may instead remain
 packed inside the finished product.
 The left crank 1 may be manufactured by the following method. FIG. 6 is a
 cross sectional view of the molding apparatus during initial casting. A
 melt space 22, into which the molten metal is allowed to flow between the
 metal mold 20 and the metal mold 21, is demarcated within the metal mold
 20 and the metal mold 21. The melt space 22 is demarcated in a shape
 roughly corresponding to the left crank 1, but the melt space 22 is
 slightly larger to accommodate the shrinkage of the molten metal. The pipe
 shell 7 is inserted into the melt space 22.
 The pipe shell 7 may be made by baking foamed volcanic glass. FIG. 7 is an
 oblique projection of the mold core mold. Volcanic glass or another
 sand-like filler 32 is poured into the internal hole 30 of the pipe shell
 7. The filler 32 may be made of any material that is resistant to the heat
 of the molten metal and is able to withstand the compression load created
 by forging (discussed below). After the pipe shell 7 has been filled with
 the filler 32, it is pressed and sealed by the provision of flattened
 components 33 and 34 at both ends. One of the flattened components 33 is
 integrally cast and fuses with the molten metal, thus constituting the
 pedal spindle end 4 of the left crank 1.
 The pipe shell 7 is positioned within the melt space 22 as shown in FIG. 6.
 In order for the pipe shell 7 to be accurately positioned within the melt
 space 22, spacers 26 made from foamed styrene or the like are used to
 position the pipe shell 7. The melt space 22 communicates with a sprue 28
 via a runner 27.
 A molten aluminum alloy is poured into the sprue 28, goes through the
 runner 27, and enters the melt space 22. The molten metal applies pressure
 to the melt space 22 by gravitational force. This casting method is a
 metal mold casting method, also called metal mold gravity casting, in
 which ordinary casting is performed without any pressure being applied to
 the molten metal, using only gravitational pressure.
 With casting alone, blowholes and the like can occur in the metal texture
 in the interior. Therefore, the casting is subjected to mold forging while
 the pipe shell 7 is still inside the crank billet 29. With mold forging,
 the casting is placed in a semi-closed metal mold 35 that is used for
 semi-closed forging, and is hot forged therein.
 FIG. 8 is a cross section of the state when the crank billet 29 has been
 put into a lower metal mold 36. The crank billet 29 is then heated to a
 specific temperature and placed in the lower metal mold 36, after which
 pressure is applied from an upper metal mold 37 to perform forging. As a
 result of this hot forging, the length, overall thickness, wall thickness,
 and surface of the cast crank billet 29 are precisely worked, the material
 of the crank billet 29 is tempered and homogenized, and the mechanical
 strength is increased. Because the pipe shell 7 produced by this hot
 forging is still inside the crank billet 29 during the forging, the pipe
 shell 7 is not crushed, and its shape is instead preserved.
 As shown in FIG. 9, the filler 32 may be taken out from the opening 15 by
 drilling a lower hole in the pedal attachment hole 6 of the crank billet
 29. The extra portion of the flattened component 33 is also cut off. After
 this, the casting is worked into the shape of the left crank 1 by cutting,
 grinding, polishing, or other such machining.
 The following was used for the filler 32:
 Trade name: "Terra Balloon" made by Calseed (8-2 Minami Kaigan, Itsui,
 Ichihara-Ichi, Chiba Prefecture)
 Components (wt %): SiO.sub.2 (77.3), Al.sub.2 O.sub.3 (12.8), Fe.sub.2
 O.sub.3 (1.7), CaO (1.0), MgO (0.1), Na.sub.2 O (3.2), K.sub.2 O(2.9),
 TiO.sub.2 (0.2), other (0.9)
 Particle size (.mu.m): 9.41 (10%), 27.71 (50%), 71.32 (90%).
 Density: 0.30 light charge density, 0.46 heavy charge density, 1.14
 particle density
 Heat resistance temperature (.degree. C.): 1100.
 While the above is a description of various embodiments of the present
 invention, further modifications may be employed without departing from
 the spirit and scope of the present invention. For example, the size,
 shape, location or orientation of the various components may be changed as
 desired. The functions of one element may be performed by two, and vice
 versa. In the above embodiment, the filler 32 inside the pipe shell 7 was
 taken out after forging, but it need not be taken out, and may instead be
 left packed inside the pipe shell 7 as shown in FIG. 10. Also, volcanic
 glass was used for the filler, but other filler materials may also be used
 as long as they have a lower specific gravity than the matrix metal, have
 heat resistance against the molten metal, and have enough compression
 strength to withstand forging, such as natural pumice or foamed gypsum.
 Thus, the scope of the invention should not be limited by the specific
 structures disclosed. Instead, the true scope of the invention should be
 determined by the following claims.