Machine element or mechanism – Elements – Cranks and pedals
Reexamination Certificate
1999-01-25
2001-11-13
Luong, Vinh T. (Department: 3682)
Machine element or mechanism
Elements
Cranks and pedals
C074S594200
Reexamination Certificate
active
06314834
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to structural machine components subjected to flexural and torsional loads, and more specifically, to a hollow bicycle crank arm of improved strength-to-weight ratio and reliability.
When a bicycle rider exerts a force on a crank arm, he applies a bending moment that tends to flex it into an arc. A transverse shear (beam shear) force also results from the pedaling force. Because it is laterally offset from the arm, the pedaling force also tends to twist the arm about its long axis. The applied bending moment varies from near-zero at the pedal to a maximum value at the crank spindle. This can cause reliability problems for any joints or welds that are near the crank spindle.
The need to reduce the weight of bicycle components, including crank arms, has resulted in several crank designs that are tubular, or hollow in the central portion. The optimal configuration may be seen to comprise a central, thin-wall tubular portion monolithic with substantially solid end portions that serve as mounting-bosses for a crank spindle and a pedal. The transitions from the solid end portions to the tubular portion are gradual and there are no sharp edges or comers. Sharp edges, corners, abrupt changes in material thickness and other geometric discontinuities induce stress concentrations. Stress concentrations significantly increase stress levels over nominal values, requiring extra material thickness to ensure reliability. A crank arm that is largely free of stress concentrations may have reduced material thickness and weight without any reduction in strength or reliability.
The optimal bicycle crank arm configuration is readily apparent in biological systems subjected to similar structural loads. For example, a human femur is continually subjected to flexure and torsion during walking, running, lifting objects, etc. A cross-section of a femur reveals dense, compact bone at the hip and knee condyles and around the outer perimeter of the central shaft, with porous cancellous bone and non-structural marrow in the shaft's interior region. The optimal crank arm configuration is analogous to a femur, and thus biomimetic. There has been great interest lately, in a wide range of applications, to adapt prima facie optimal biological systems to man-made biomimetic articles. There have been various attempts to economically produce an optimal hollow crank arm.
DISCUSSION OF PRIOR-ART HOLLOW BICYCLE CRANK ARMS
Bezin (U.S. Pat. No. 4,811,626) teaches a hollow crank arm comprised of three components: a tube and two end-lugs. The end-lugs have protrusions that fit within the tube, ready for bonding or welding. The end-lugs are solid to absorb concentrated loads from a crank spindle and pedal. In order to achieve a lightweight assembly, the tube is of thin-wall section or made from fiber composite material. The crank arm has at least one joint proximate to a region of maximum bending moment, where the crank arm mounts to the spindle shaft. Locating a joint near a region of maximum bending moment increases the likelihood of failure unless additional material thickness is provided. Bezin's crank arm also suffers from an abrupt transition where the thin tube meets the solid lug, producing a stress concentration. There is no provision for optimizing the configuration of the components to reduce the stress concentration. If the crank arm is fabricated using a welding process, it is difficult to obtain proper weld penetration in the heavy lug without overheating the thin tube. Instead, it would be preferable to weld metals of similar thickness.
Girvin (U.S. Pat. No. 5,179,873) teaches a crank arm comprised of four components: a tube, two end-lugs and a plate-like redundant doubler which is used to reinforce a weld proximate to a region of maximum bending moment. Girvin discusses configuring the redundant doubler plate to reduce stress concentration at the upper welded interface, but does not address the abrupt transition where the tube otherwise meets the lug. He discusses using a tapered tube to increase the section modulus in regions of high stress, but does not provide for a varying wall thickness. Girvin states that his crank arm is to be made from high strength 4130 steel, permitting a very thin tube to reduce weight. Girvin does not address the issue of satisfactorily welding a thin-wall tube to a more massive lug. He provides no way to adapt the invention to lightweight alloys. Girvin states that the doubler is used to safeguard against tensile failure at the weld. A Girvin crank arm made from a lightweight, low-modulus metal, such as aluminum alloy, could be expected to experience localized, Brazier-type compressive buckling failure prior to any tensile rupture failure in the tube. If Girvin's crank arm were feasible in lightweight alloy, it would not be necessary to specify heavy steel as the preferred material to create a lightweight bicycle component. It would be preferable to provide a crank arm that could be readily adapted to a wide range of materials, and did not require redundant doubling of material to compensate for inherently weak joints proximate to a region of maximum bending moment.
Yamanaka (U.S. Pat. Nos. 5,819,599 and 5,819,600 and 5,845,543) depicts a hollow crank arm comprised of two parts: a forged crank arm with a central, longitudinal groove, and a long cap that is welded over the groove to form a box-beam. The welded surfaces are substantially parallel to the long axis of the crank arm, which results in a relatively large weld area, long processing time, and high fabrication expense. The cross-section of the resulting box-beam is substantially rectangular, which is structurally less efficient than a circular or ovoid cross-section. The sharp interior corners and abrupt changes in wall thickness cause stress concentrations, and the flat sides do not efficiently transmit torsional shear forces. There is relatively little space enclosed by the box-beam, owing to the relatively thick walls, which results in a heavy, stiff and rigid crank arm. Yamanaka discusses shaping the long axis of the groove into a “ship hull shape” to better distribute the stresses in the crank arm, but provides no other means of optimizing the crank arm. Yamanaka's patents claim a novel means of mounting a crank spindle to a crank arm, with no discussion of adapting, modifying or improving the hollow crank arm depicted. It would be desirable to provide a hollow crank arm that was largely free of stress concentrations, had relatively thin walls in the tubular portion, enclosed a relatively large volume of space to reduce weight, comprised minimal welded area, and could be readily designed and fabricated in the most efficient configuration for an optimal degree of flexibility.
OBJECTS AND ADVANTAGES OF THE PRESENT INVENTION
One of the objects of the present invention is to provide a bicycle crank arm of improved reliability and strength, while simultaneously reducing weight. Another object of the present invention is to provide a hollow crank arm that may be readily optimized with respect to material configuration, strength and flexibility, resulting in a biomimetic component of superior performance. Another object of the present invention is to produce a crank arm that is largely free of stress concentrations, and thus highly resistant to fatigue-type failure. Still another object of the present invention is to provide a crank arm that may be readily and economically manufactured to an optimal design, with minimal compromises and allowances for fabrication process limitations. Yet another object of the present invention is to provide a crank arm that may be readily fabricated from a wide variety of common materials. A further object of the present invention is to provide a crank arm that does not have joints located near regions of high bending moment. Another object of the present invention is to provide for joining of components at regions of substantially equal wall thickness. Still another object of the present invention is t
Kozuschek Michael A.
Nelson David A.
Smith Garrett A.
Luong Vinh T.
Truvativ International Co., Ltd.
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