GB2169987A - Crank axle for bicycles - Google Patents

Abstract

A crank axle (11) for a bicycle has, in its intermediate part, a cavity with a greater cross section area than that of the screw-threaded holes provided in its ends (7) to receive screws for securing the pedal cranks. The axle may be formed from steel by extrusion and subsequent cold forging or from a length of tubing (defining the cavity) and end members welded to the ends of the tubing. The ends of the axle have an octagonal profile to engage in octagonal openings in the pedal cranks. <IMAGE>

Description

SPECIFICATION Crank axle for bicycles The present invention concerns a crank axle for sports and competition bicycles, with improved characteristics of weight, strength and stiffness, and apt to be obtained at far lower costs than the known type crank axles now in use.

The crank axle is that bicycle component which connects the two pedal cranks and onto which bear the weight and muscular strength of the cyclist. This component is hence subjected to considerable cyclic bending, shearing and torsional stresses which require at present, for the production of said axle, the use of very strong materials - such as casehardening or hardening and tempering steels, or special alloys (even titanium alloys) - and different types of processings, as turning from a drawn bar, forging or cold forging, normally followed by a finishing operation.

Considering that, as known, a better use of the material - for what concerns the advantages in respect of weight, strength and stiffness - is obtained by moving the material itself away from the neutral axis, and that in conventional crank axles such a result is achieved only through a piercing of diameter at the most equal to that of the screws locking the pedal cranks, it has been thought to overstep this limit by constructing the crank axle with the technique of impact extrusion, known per se but never applied in manufacturing products of this kind.

This technique allows to obtain, according to the present invention, a crank axle for bicycles which is lighter and, at the same time, stiffer and stronger than those of known technique.

Said crank axle is essentially characterized by having, in its intermediate part, a cavity with a wider cross section area than that of the threaded holes provided at its ends for the screws locking the pedal cranks, and thus a wider cross section area than that of the normal lightening holes of known crank axles.

Said axle is produced of steel by extrusion and subsequent cold forging and, more particularly, by cold extrusion of a steel chip, up to forming a blank in the shape of a shank, and by subsequent forging of said blank into a moving die.

Said axle can be realized as a monolithic product, or two elements can be obtained with the above process, which are then either mutually welded, or else welded to an intermediate tubular element of normal production.

The axle according to the invention is also preferably formed with its ends like a prism, having an octagonal instead of a square cross section, which advantageously increases the resisting section of the ends.

The invention will now be described in further detail, by mere way of example, with reference to the accompanying drawings, in which: Figure 1 shows the extrusion of a monolithic crank axle according to the invention; Figure 2 shows the forging of the axle of figure 1; Figures 2A and 2B are enlarged lateral section views, showing one of the octagonal prism-like ends of the axle of figures 1 and 2; Figure 3 is a view, with some parts in section, of the axle produced according to the technique of figures 1 and 2; and Figures 4 and 5 are similar views, showing two modified embodiments of the crank axle according to the invention, produced in two pieces or in three pieces respectively, with techniques similar to those used for the monolithic axle of figure 3.

The crank axle of figure 3, according to the invention, is produced by carrying out the two successive steps shown in figures 1 and 2: the starting material 1 - substantially in the form of a steel chip - is first of all deformed by being forced through the extruder 2, by means of a plunger 3 and a counterplunger 4 having an inner cavity or impression 5. It is substantially an extrusion process, which starts on the left of figure 1 and finishes on the right of the same figure. The blank obtained at the end of this first step appears as a cylindrical shank 6, with one profiled end 7 partly corresponding to the profile of the axle being produced, and with the other end comprising a thicker crown 8, from which will be derived the material being deformed in the second step of the process.

In this second step, the blank obtained through the first step is introduced (left of figure 2) into an appropriate die 9. The crown 8, projecting from an end thereof, is then buckled (right of figure 2) through a moving die 10, in order to obtain the axle 11 which is shown finished in figure 3. As the die 10 is opened again, the axle 11 is expelled by the ejecting plunger 12.

Subsequently, the necessary finishing operations are carried out on the blank axle 11, to obtain the finished product.

It can be seen at once, from figure 3, that the intermediate cavity of the axle thus produced has a positively wider cross section area than that of the threaded holes provided at its ends for the screws locking the pedal cranks (and therefore wider than the cross section areas of the normal lightening holes of known crank axles). It can also be seen, from the same figure, how the special geometry of the axle 11 allows to optimize the distribution of the working stresses on the material, so as to achieve objects of lightness and stiffness.

The fact that these objects are fully achieved - with the considerable advantages deriving therefrom - is confirmed by the torsional tests carried out on conventional type crank axles and on axles of the type according to the invention, produced by the Applicant, such tests being summed up in the following table:: TABLE Torsional test on crank axles fixed at one end and subjected at the other end to a constant moment (arm 170 mm) Type of axles Weight Load Shifting from point (gas) (Newton) where force is applied (mum) A) Conventional axle 141.44 1450 16.65 having cavity with cross sec tion area equal to end holes B) Axle of invention 138.53 1450 13.27 having cavity with cross sec tion area wider than end holes As can be seen from this table, an equal standard load produces, in a crank axle according to the invention, a torsional strain which is about 20% less than that produced in a conventional axle, the first axle being moreover about 2 grams lighter than the second.

It should also be noted that the special geometry of the axle according to the invention facilitates piercing thereof, said piercing no longer being required throughout its length - as in conventional crank axles - but only at the two ends, with considerable advantages from the point of view of construction and cost reduction. Further advantages derive from the fact that a same blank can be used to obtain axles of various standard sizes, by simply changing the type of machining.

The axle of figure 3 is finally formed with its ends 7 of octagonal instead of square cross section - see details of figures 2A and 2B - which improve its capacities of resistance. The dashed area of Figure 2A shows very clearly how this shape of the axle ends is apt to notably increase the resisting section, as compared to the conventional square-shaped ends. Furthermore, the actual shape of the axle ends also improves the engagement with the pedal cranks.

Figures 4 and 5 illustrate - as already said - an axle 13 according to the invention, in two mutually welded pieces and, respectively, an axle 14 according to the invention, in three mutually welded pieces: these two constructive solutions can form an alternative to the previous solution and they differ therefrom for the presence of one or more welding stages.

In the case of axle 13 - formed in two elements 15 and 16, mutually connected by a welding 17 (figure 4) - the extrusion process is carried out for each of said elements, following exactly the same sequence adopted for the monolithic axle 11, with the only variant that the blank does not comprise the projecting crown 8. The two elements 15 and 16 are then connected by the welding 17, so as to obtain the axle 13 ready to be trimmed by the machine tools.

The advantages of this embodiment lie in the initial reduced cost for the equipment and in the reduced management cost of the parts before their assembly.

Moreover, the pieces thus produced will have, from the start, higher qualities of dimensional precision and a higher finishing degree in respect of the monolithic axle. Also the standardization of the various lengths is easier to operate.

In the case of axle 14 - formed in three elements 18, 19 and 20, mutually connected by two weldings 21 and 22 (figure 5) - the process is even simpler than in the previous cases. In fact, the two end parts 18, 19, cold forged with a high precision and finishing degree, are obtained with a single die (in that, also in this case, one takes advantage of the perfect symmetry of these parts), while the central part 20 is obtained from commercial tube, in a material of the same type as that used to obtain the blanks 18 and 19, and/or anyhow compatible with the welding operations having to be carried out in-correspondence of 21, 22, to connect the three elements of the crank axle.

Said solution provides a great number of advantages: in addition to those already listed for the axle 13 welded in two pieces, there is a further reduction in the costs deriving from the use of commercial tube for the central part 20 of the axle. Furthermore, the actual section of the tube can be varied each time, depending on the use of the crank axle.

The invention of course covers also other embodiments of the crank axle for bicycles, of which only some examples of construction have been described and illustrated. For instance, the protection scope of the invention also includes crank axles, wherein the intermediate part comprises a cavity and the end parts are without holes and have different means for locking the pedal cranks.

Claims (6)

1. A crank axle for bicycles, characterized by having, in its intermediate part, a cavity with a wider cross section area than that of the threaded holes provided at its ends for the screws locking the pedal cranks.

2. A crank axle as in claim 1), produced of steel by extrusion and subsequent cold forging.

3. A crank axle as in claim 1), obtained by cold extrusion of a steel chip, up to forming a blank in the shape of a shank, and by subsequent forging of said blank into a moving die.

4. A crank axle as in claim 1), obtained as a monolithic body.

5. A crank axle as in claim 1), obtained as a body in two or three extruded and forged elements, connected by welding.

6. A crank axle as in claim 1), having ends with an octagonal cross section.