Mechanical assembly with incompatible metallic materials

Abstract: Various parts in an assembly of parts are made of differing metals having incompatible ionization characteristics. Any two parts which have incompatible ionization characteristics are separated from one another such that corrosion inducing contact therebetween is eliminated. Therefore, the possibility of electrolytical corrosion can be reduced. For example, in one embodiment a spool assembly in a spinning reel includes a magnesium-alloy first part (12) and a second part (20) formed of another metal coupled to the magnesium alloy part at a clearance (20b). The other metal and the magnesium-alloy have ionization characteristics that are not compatible, leading to possible electrolytic corrosion therebetween. However, an electronically insulating filling agent (20c) is injected into a clearance between the first (12) and second (20) parts.

Description: The present invention relates to mechanical assemblies of metals, and in particular to mechanical assemblies of metal parts having incompatible ionization energies. Mechanical assemblies according to the present invention may be employed in a variety of mechanisms such as fishing reels and bicycle components. The present invention is directed to building certain machines that require contact of parts made from metals having incompatible ionization energies. If the ionization energies of metals in contact are sufficiently dissimilar, electrolytic corrosion may occur when current flows between them in the presence of an electrolyte such as, for example, saltwater. Human-powered rotary machines optimally are lightweight for ease of operation and at the same time rigid to withstand over time the irregular stress and strain that human rather a mechanical application of power to the machines entails, Steel, for example, while rigid in having a high modulus of elasticity, is often undesirably heavy for such applications. In addition to lightness of weight and rigidity, reducing bulk is also desirable in manufacturing human-powered rotary machines, so that they are not unwieldy. Manufacturing parts for such machines of synthetic polymers, for example, may require unacceptable bulk in order to achieve sufficient rigidity. In some applications, furthermore, synthetic-polymer manufactured parts lack the desired texture and cannot lend sought-after quality to the appearance. Aluminium alloys provide desirable texture and high-grade appearance, and have moduli of elasticity higher then synthetic polymers, but lower than steel. Likewise is the case with zinc alloys, titanium and titanium alloys. Magnesium alloys are lightweight and highly rigid, but of course lack the strength of steel. Employing magnesium alloys in combination with iron or steel in manufacturing machine components can serve to reduce weight while maintaining operatively sufficient strength and rigidity. Iron and steel have ionization energies incompatible with magnesium alloys, to the extent that electrolytic corrosion may occur between them if they are in direct contact and come into the presence of an electrolyte such as saltwater or even air. An object of the present invention is to prevent or reduce electrolyic corrosion in machine parts manufactured of magnesium alloy and employed in combination with.machine parts made of materials having ionization energy incompatible with magnesium alloy. The present invention in one aspect is a mechanical assembly of a magnesium alloy first part and a second part of another metal coupled to the magnesium alloy part at a clearance therewith, wherein the clearance between said first and second parts is occupied by an injectable filling agent, said filling agent being electrolytically insulating. The second part may be an aluminium or zinc alloy; alternatively, the second part may be titanium or titanium alloys, or stainless steel. The magnesium alloy first part may be anodized. Where the second part is made of aluminium or zinc alloy, it may be coupled to and in direct contact with a steel part in a mechanical assembly according to the present invention. The filling agent may be a liquid for injection into the clearances by capillary action. In addition, the filling agent may be liquified adhesive that solidifies after insertion. The filling agent improves anti-corrosion properties of the mechanical assembly by acting as an insulator against electrolytic corrosion. In another aspect, the present invention is a mechanical assembly that includes: a magnesium alloy first part; a lower ionisation-energy stainless steel or titanium/titanium alloy second part coupled to the magnesium alloy first part; an insulating material interposed between and in direct contact with the first and second parts; and a filling agent injected into clearances between the first part and the second part or the insulating material. The insulating material electrically insulates the magnesium alloy from the lower ionization-energy stainless steel or titanium/titanium alloy to make electrolytic corrosion unlikely. A further aspect of the invention provides a method, as defined in claim 9, for assembling such a mechanical assembly. From the following detailed description in conjunction with the accompanying drawings, the foregoing and other objects, features, aspects and advantages of the present invention will become readily apparent to those skilled in the art. Fig. 1 is a section view of a dual-bearing reel spool mounted on a shaft shown fragmentarily, in one embodiment of the present invention; Fig. 2 is an enlarged fragmentary view of a portion of the spool and shaft depicted in Fig. 1; and Fig. 3 is a fragmentary partly in section view of a bicycle crank assembly in another embodiment of the invention. Embodiment 1 The spool 1 depicted in Fig. 1 is manufactured from magnesium alloy by injection molding in a die-casting process. A boss 1c is formed integrally with and centrally along the inner cylindrical surface of a line-winding barrel section 1b of the spool 1. A sleeve 2 made of an aluminium alloy is pressure-inserted into the bore of the boss 1c. Interposition of the sleeve 2 prevents electrolytic corrosion of the magnesium alloy spool 1. The spool 1 is shown serration-coupled to a shaft 3 passing through the sleeve 2. The shaft 3 is a rod of, for instance, a stainless steel. A slight clearance B exists between the spool 1 and the sleeve 2. An adhesive agent A injected by capillary action fills the clearance B. The liquid filler is not limited to the adhesive agent A, and other liquids, such as synthetic resin polymers and oils may be used so long as the liquid has excellent permeability and anti-corrosion properties. As further depicted in Fig. 2, an oxide film C is formed on the surfaces of the spool 1 and the sleeve 2 to enhance their anti-corrosion properties. The oxide film C is formed by anodizing, i.e, by an anodic oxidation or conversion coating process. Disposing the sleeve 2 between the magnesium alloy spool boss 1c and the stainless steel shaft 3 is toward preventing electrolytic corrosion. The ionization energy of the aluminium alloy sleeve 2 is intermediary with respect to the ionization energies of the magnesium alloy spool 1 and the stainless steel shaft 3. The method interposing the aluminium alloy part between the magnesium alloy and the steel parts is not limited to pressure insertion; applicable methods include insertion molding. But wherein clearances B remain between, in the present example, the magnesium alloy spool 1 and the aluminium alloy sleeve 2, atmospheric air, salt water or like corrosive fluids acting as electrolytes would be likely to enter the clearances B and lead to corrosion. To eliminate the possibility thus of corrosion, a filler such as the adhesive agent A is filled into the clearance B. Embodiment 2 Fig. 3 illustrates a bicycle crank assembly 10 that includes right and left magnesium-alloy crank arms 11 and 12 fixed to either end of the aluminium alloy crank shaft 13. An aluminium alloy sleeve 14 having a threaded bore 15 is non-rotatably pressure-inserted into the distal end of a crank portion 11a of the right crank arm 11. A stainless steel pedal shaft 16 for a pedal 17 is screwed into the threaded bore 15 of the sleeve 14. Likewise, another aluminium-alloy sleeve 14 for another pedal shaft 16 is pressure-inserted into the distal end of a crank portion 12a of the left crank arm 12. Similarly to the first embodiment, an adhesive agent is filled into clearances between the sleeve 14 and the distal end of the crank portions 11a and 12a. Further, an oxide film (not indicated) is formed on the surfaces of the right and left crank arms 11, 12. In the present mechanical assembly of bicycle parts, the magnesium-alloy crank arms 11 and 12 coupled to the stainless steel pedal shafts 16 is not likely to be electrolytically corroded. Further, since the adhesive agent is filled into the clearance between the sleeves 14 and each of the crank portions 11a and 12a, corrosion is not likely to start in the clearances. Modifications In the embodiments just described, the magnesium alloy part is in contact with an aluminium alloy part intervening and of intermediate ionization energy between the magnesium alloy part and a stainless steel part. Therein, a zinc alloy part may do instead of the aluminium alloy part. Other Embodiments In a mechanical assembly in accordance with the present invention a magnesium alloy part may be directly coupled to a stainless steel, titanium or titanium alloy part, with an insulating material interposed between and in direct contact with the magnesium alloy part and the stainless steel, titanium or titanium alloy part. The insulating material may be a synthetic polymer such as nylon, or may be an insulating paint, for example. Interposing the insulating material between the magnesium alloy part and the stainless steel, titanium or titanium alloy part, reduces the flow of electrons between the parts, making electrolytic corrosion of the magnesium alloy part unlikely. Herein as well, a filling agent is injected into clearances between the parts of the mechanical assembly. That is, filler is filled into clearances between the magnesium alloy part and the stainless steel, titanium or titanium alloy part, as well as between the magnesium alloy part and the insulating material. While only selected embodiments have been chosen to illustrate the present invention, to those skilled in the art it will be apparent from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing description of the embodiments according to the present invention is provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims.

Claims: A mechanical assembly comprising: a magnesium alloy first part (1); and a second part (2) of another metal coupled to the magnesium alloy part at a clearance (B) therewith; characterised in that the clearance (B) between said first (1) and second (2) parts is occupied by an injectable filling agent (A), said filling agent (A) being electrolytically insulating. The mechanical assembly as set forth in Claim 1, wherein said second part (2) is one selected from the group consisting of aluminium alloys and zinc alloys. The mechanical assembly as set forth in Claim 2, wherein said second part (2) is coupled to and in direct contact with a stainless steel part (3). The mechanical assembly as set forth in Claim 1, wherein said second part (2) is one selected from the group consisting of titanium, titanium alloys and stainless steel. The mechanical assembly as set forth in Claim 1, wherein said filling agent (A) is a liquid for injection into the clearance (B) by capillary action. The mechanical assembly as set forth in Claim 1, wherein said filling agent (A) is a liquified adhesive solidifying after injection into the clearance (B). The mechanical assembly as set forth in Claim 1, wherein said magnesium alloy first part (1) is anodized. A mechanical assembly comprising: a magnesium alloy first part (1); a second part (3) coupled to and at a clearance (B) with said magnesium alloy first part (1), said second part (3) having lower ionization energy than said first part (1) and selected from the group consisting of stainless steel, titanium and titanium alloys; an insulating material (2) interposed between, in direct contact and at a clearance (B) with said first (1) and second (3) parts; and a filling agent (A) injected into the clearances (B) between said first (1) and second (3) parts, and between said first part (1) and said insulating material (2). A method for assembling a mechanical assembly comprising: coupling a magnesium alloy first part (1) and a second part (2) of another metal such that there is a clearance (B) therebetween; injecting an electrolytically insulating filling agent (A) into the clearance (B) between said first (1) and second (2) parts. The method of Claim 9 further comprising: selecting said second part (2) from the group consisting of aluminium alloys and zinc alloys. The method of Claim 9 further comprising: selecting said second part (2) from the group consisting of titanium, titanium alloys, and stainless steel. The method of Claim 9 further comprising: selecting a liquid filling agent (A) for injection into the clearance (B) by capillary action. The method of Claim 9 further comprising: selecting a liquid adhesive filling agent (A) that solidifies after injection into the clearance (B). The method of Claim 9 further comprising: anodizing said magnesium alloy first part (1).

Inventors: HITOMI YASUHIRO, KAWASAKI KEN ICHI, NISHIMURA YASUSHI, HITOMI, YASUHIRO, KAWASAKI, KEN'ICHI, NISHIMURA, YASUSHI
Applicants: SHIMANO KK, SHIMANO INC.
Application year: 1998
Main classification: C23F15
IPC codes: A01K89, B62M3, C23F15