This invention relates to silent chains for use in power transmissions, for example, the timing drives of automobile engines, and in various kinds of industrial machinery, conveying mechanisms, and the like.
In recent years, the use of silent chains has increased, especially in the timing chain of automobile engines where high operating speeds and heavy load conditions are encountered.
In general, a silent chain is composed of a set of guide link plates, inner link plates having engagement teeth for meshing with sprockets, and connecting pins. The connecting pins are fixed to the guide link plates by fitting into pin holes formed in the guide link plates. The pins, however, extend with a clearance through pin holes formed in the link plates, thereby allowing articulation of the chain. In a silent chain, since the teeth of the inner link plates slide along the teeth of a sprocket from the time when they come into contact with the sprocket teeth to the time when they separate from the sprocket teeth, impact noise due to collision between the sprocket and the chain is reduced and the overall noise level produced during operation of the transmission is comparatively low.
In a conventional silent chain, alloy steel and high carbon steel have been used as the base materials of the connecting pins and link plates because of their availability and low cost. The surfaces of the connecting pins are subjected to heat treatment, such as carburization or carbonitriding, to suppress wear.
In addition to the heat treatment of the surface of the connecting pin, it has been proposed, in Japanese Laid-open Patent Publication 2000-249196, for example, to subject a surface of the link plate to a surface-hardening treatment, such as nitriding or diffusion coating, to suppress wear of the link plate.
However, it has been reported that some chains do not have the expected wear resistance, and exhibit abnormal wear in spite of the surface treatments of the connecting pins and the link plates. Elimination of this abnormal wear is urgently needed to improve engine reliability.
Excessive wear of the walls of the pin holes of the inner link plates is generated when the engine lubricating oil in the engine deteriorates through oxidation, and its pH falls to a level below 3. In addition, when the base materials of the connecting pin and the inner link plate are the same, the base materials are liable to bind together, further promoting wear of the chain. An object of the invention, therefore, is to provide a silent chain, which maintains its flexibility and operates smoothly, and without abnormal wear elongation, over a long period of time, even when used in lubricating oil which has deteriorated as a result of a high degree of oxidation.
In the silent chain in accordance with the invention, carbon steel inner link plates are polished by barrel tumbling before heat treatment so that their surface roughness is 0.5 μm Rz or less. After the links are polished, they are subjected to a chromizing treatment. A vanadium carbide layer is formed on the surfaces of the connecting pins, and the pins are then polished by barrel tumbling so that their surface roughness is also 0.5 μm Rz or less. In the assembled chain, the connecting pins are fitted into, and fixed to, the pin holes of the guide link plates, but extend through the pin holes of the inner link plates with a clearance.
A powdered, liquid, or gaseous plating metal can be made to diffuse into a heated base-metal surface. In the chromizing treatment of the invention, metallic chromium is diffusion-coated onto steel, preferably by a powder pack process, in which the inner link plates, composed of carbon steel, are heated in a diffusion material preferably composed of a mixture of chromium, alumina as an antisintering agent, and ammonium chloride (NH4Cl) as a reaction accelerator. The powder pack process forms chromium carbide on the surfaces of the inner link plates.
The chromizing treatment, to which the surfaces of the inner link plates is subjected, resists corrosion due to exposure to oxidized and deteriorated oil. Furthermore, the vanadium carbide layer, formed on the surface of the connecting pin, is disposed between the chromium on the surface of the link plate and the chromium contained in the connecting pin, so that binding resulting from mutual contact between similar elements is reduced.
Polishing the surfaces of the connecting pins to a low surface roughness improves the ability of the pins to slide relative to the surfaces of the holes in the inner link plates, thereby reducing wear. Smoothness of the surfaces of the sliding materials is important not only in order to reduce mechanical wear, but also to reduce the susceptibility of the surfaces to chemical attack. It has been determined experimentally that, when the surface roughness is 0.5 μm Rz or less, a significant reduction in wear and chemical deterioration is realized. The silent chain according to the invention is maintains its flexibly, and operates smoothly over a long period of time without abnormal wear elongation, even in the corrosive environment of deteriorated lubricating oil.
In
Although, in the embodiment depicted in
Each inner link plate ILP is composed of carbon steel, subjected to polishing by barrel tumbling before heat treatment, so that its surface roughness, including the surface roughness of the cylindrical walls of its pin holes, is 0.5 μm Rz or less. After polishing, the inner link plates ILP are subjected to chromizing treatment. The chromizing treatment produces a chromium carbide layer on the cylindrical walls of the pin holes as well as on the other surfaces of the inner link plates. Polishing of the inner link plates should be carried out before chromizing so that the surfaces of the link plates, especially the inner surfaces of the holes through which the connecting pins extend, are made sufficiently smooth to be chromized adequately.
Vanadium carbide layers are formed on the surfaces of the connecting pins, and the connecting pins are then polished, by barrel tumbling, so that their surface roughness is also 0.5 μm Rz or less.
A chain in accordance with the invention was compared to five other silent chains, namely, a conventional silent chain and four additional examples. In the conventional silent chain the connecting pins were subjected only to the formation of a vanadium carbide layer, but were not polished. The inner link plates were not subjected to polishing or chromizing. Reference example 1 is a silent chain in which the connecting pins were subjected only to polishing without the formation of a vanadium carbide layer and in which the inner link plates were subjected to both polishing and chromizing. Reference example 2 is a silent chain in which the connecting pins were subjected only to the formation of a vanadium carbide layer without polishing, and in which the inner link plates were subjected to both polishing and chromizing. Reference example 3 is a silent chain in which the connecting pin were subjected both to the formation of a vanadium carbide layer and to polishing, but in which the inner link plates were subjected only to chromizing without polishing. Reference example 4 is a silent chain in which the connecting pins were subjected both to the formation of a vanadium carbide layer and to polishing, and the inner link plate were neither polished nor subjected to chromizing treatment.
Elongation of the respective chains, after being operated in weak acid oil, was determined, and the results are shown in the right-hand column of
As apparent from
In accordance with the invention, when polishing and surface treatment of the base materials of the inner link plates and connecting pins are carried out as described, abnormal wear elongation in deteriorated oil is reproducibly avoided, and consequently the invention has broad and significant industrial applicability.
Number | Date | Country | Kind |
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2004-321202 | Nov 2004 | JP | national |