Chain for use in automobile engine

Abstract

In a silent chain for an automobile engine timing drive, the pins which interconnect the link plates of the chain have a vanadium carbide layer formed on their surfaces by diffusion penetration. The vanadium carbide layer is composed of two parts: an inner part, on the steel base material of the pin, being a V8C7 layer, and an outermost part being a V2C layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially broken-away perspective view of a part of a silent chain according to the invention;

FIG. 2 is a graph depicting the results of chain elongation tests, of a conventional silent chain and chains in accordance with the invention, carried out in deteriorated lubricating oil; and

FIG. 3 is a microphotograph showing the cross-sectional structure of a pin in a silent chain according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Although the invention will be described with reference to a silent chain, it should be understood that similar effects can be achieved in a roller chain and also in a rollerless bushing chain. In the case of a silent chain, the connecting pins are in sliding contact with the inner walls of the pin holes in the link plates of the chain. In the case of a roller chain or a rollerless bushing chain, the connecting pins are in sliding contact with the inner walls of the bushings. In all of these kinds of transmission chains, the invention can prevent deterioration due to sliding contact between the connecting pins and their opposed surfaces, especially in a deteriorated lubricating oil environment.

In FIG. 1, two guide link plates GLP are partly cut away to show a link plate having a pair of teeth 12a. The silent chain 10 is an endless chain formed by interleaved rows of link plates interconnected by pins Q. Each row comprises a plurality of inner link plates ILP, and every second row also comprises a pair of guide link plates GLP. The silent chain 10 shown in FIG. 1 comprises guide rows B, each consisting of two inner links ILP and two guide link plates GLP, and joint rows A, each consisting of three inner link plates ILP. The connecting pins Q are press fit into holes in the guide link plates GLP, which having no teeth. The inner link plates ILP, are rotatable on the pins.

The link plates are composed of carbon steel. The base material of the pins can be steel or a low carbon steel. A high carbon surface layer is formed on a surfaces Qa of the pins. The method of forming the high carbon surface layer is not particularly limited, but the formation of the high carbon surface layer is preferably carried out by a carburizing treatment. In the carburizing treatment, the pin 12 is heated to approximately 900EC to 950EC in a carburizing agent in order to diffuse carbon into the surface of the pin and thereby increase the carbon content in the surface. If high carbon steel is used as the base material of the pin Q, carburizing treatment is not needed.

A vanadium carbide layer is formed on a surface of the pin Q by a diffusion penetration treatment, preferably carried out by the “powder pack” method, in which pins are placed in vanadium powder or a vanadium alloy powder. Preferably, an anti-sintering agent such as alumina or the like, and a reaction promoting agent such as ammonium chloride or the like, are added to the vanadium or vanadium alloy powder. The pins are then heat-treated at a high temperature in the range from about 900EC to 1200EC, for about 5 to 25 hours, to form a vanadium carbide layer on the surface of each of the pins.

Another known option for forming the vanadium carbide layer is to utilize a molten salt method, also known as the “Toyota Diffusion” method, in which the pin is treated in molten salt. Still another known option for forming the chromizing letter is to utilize the so-called “A application” method, in which a vanadium powder and a suspending agent are applied as a coating material to the pin, and the coated pin is then dried and heated in an inert gas atmosphere or in a vacuum. The “powder pack” method, however, is preferred because it is inexpensive and especially suitable for treatment of small articles such as connecting pins for timing chains.

As the vanadium carbide layer is formed on the high carbon surface layer of the pin by the diffusion penetration treatment, the treatment temperature is preferably set to about 1000EC. Since vanadium is a strong carbide-forming element, carbon contained in the high carbon surface layer formed of the pin, or from the pin base material itself in the case of a pin formed of high carbon steel, penetrates by diffusion into the vanadium layer formed on the surface of the pin to combine with the vanadium. As can be seen from a cross-sectional photograph of FIG. 3, which was taken by an electron microscope, the vanadium carbide layer is composed of two parts: an inner layer, formed on the steel base material, which becomes a V₈C₇ containing layer, and an outermost layer, which becomes a V₂C containing layer. V₈C₇ is the main component of the inner layer, and V₂C is the main component of the outer layer.

In the preferred embodiment, the thickness of the inner layer, i.e., the V₈C₇-containing layer, is in the range from 8 to 12 μm, and the thickness of the V₂C-containing outermost layer is in the range from about 1 to 4 μm.

The mechanism by which the vanadium carbide layer on the surface of the pin becomes divided into two different sublayers is not fully understood at the present time. However, it appears that the vanadium, supplied as a powder, combines with carbon supplied from the high carbon pin, or from the high carbon surface layer of the pin, to form a V₈C₇ layer, i.e. a layer having a high carbon to vanadium ratio in the vicinity of the base material, and a V₂C layer, i.e., a layer having a lower carbon to vanadium ratio, farther from the base material. It is believed that, in the process of formation of the solid phase vanadium carbide, a balancing takes in which the vanadium carbide becomes divided into the two different layers.

Wear elongation tests were carried out to determine the properties of the pin according to the invention under the following test conditions:

Chain: Silent chain having a pitch of 6.35 mm

Number of teeth on the sprockets: 18 and 36

Rotation speed: 6500 r/min

Lubricating oil: Deteriorated engine oil

Amount of oil: 1 L/min

The tests were carried out using a testing apparatus and method generally used by the art. However, the same general results can be expected even if a different test method is used.

Three examples were compared, a conventional chain and two chains according to the invention.

The chain of Example 1 is a chain according to the invention in which no treatment was applied to reduce the surface roughness of the pins. The surface roughness of the chain of Example 1 had a ten point mean roughness (Rz) value in the range of 0.4 to 0.8 μm.

The chain of Example 2 is a chain which is the same as the chain of Example 1, except that a treatment was applied to reduce the surface roughness of the pins, while still leaving an outermost layer containing V₂C. In this case, the surface roughness of the outermost layer of the pin, i.e., the layer containing V₂C, was decreased by barrel polishing. In barrel polishing, friction between a pin and an abrasive material is generated, and polishing of small articles such as pins can be carried out efficiently. The ten point mean surface roughness (Rz) of the pin of Example 2 had a value in the range from 0.2 to 0.3 μm. The V₈C₇-containing outermost layer in Example 2 was porous, so that recesses were formed in the exposed surface of the layer. The recesses function as basins that maintain improved lubricity over a long period of time, so that the endurance of the roller chain is improved.

In the conventional case, used for comparison, the pins were subjected to a full barrel polishing treatment, so that the outermost V₂C layer was completely removed. Thus, the surface roughness of the pin of the conventional case was improved and was substantially the same as the surface roughness of the pin Example 2.

From the results of the chain elongation tests shown in FIG. 2, it can be seen that, after 100 hours of operation, the elongation of the chain of Example 1 was only about 70% of the elongation of the conventional chain. The curves in FIG. 2, are relatively steep until about twenty hours of testing, at which time, the slope of the curves becomes more gradual. The steeper slopes at times up to twenty hours are due to initial wear. It can be seen, however, that the initial wear is smaller in Examples 1 and 2 than in the case of a conventional silent chain. The initial wear in Example 1, which had two vanadium carbide layers on the surface of the pin, was less than in the conventional case where the connecting pins had only one vanadium carbide layer. Furthermore, in Example 2, wear due to the attack by the pin on a surface of the pin hole through which it extends was reduced as a result of the reduced surface roughness of the pin.

In the roller chain of Example 2, the surface roughness of the outermost layer of the pin Q, containing V₂C, was decreased by barrel polishing treatment. The barrel polishing treatment resulted in superior lubricity that could be maintained over a long period of time. Thus, the endurance of the silent chain was improved. Barrel polishing generates relative friction between a pin to be worked and an abrasive material. Thus, polishing of a small article such as a connecting pin can be carried out efficiently by barrel polishing.

From the results of the chain elongation tests shown in FIG. 2, it can be seen that, at 100 hours, the elongation of the silent chain of Example 2, in which the surface roughness was reduced, was only about 40% of the elongation of the conventional silent chain. Thus, when the thickness of the outermost layer of the pin is within the range of about 1 to 4 μm, so that the outermost layer is not completely removed, a much lower wear elongation of the chain is achieved compared to that of the conventional chain.

In the silent chain of Example 2, in which the surface roughness of the pins is reduced while the presence of an outermost V₂C layer is maintained, attack by the connecting pins on the inner walls of the pin holes in the inner link plates is reduced, and abrasive loss of the inner circumferential surfaces of the pin holes is suppressed.

INDUSTRIAL APPLICABILITY

The invention has significant industrial applicability in that the V₂C layer, which was previously considered a useless impurity, reduces abnormal wear elongation of the chain due to operation in deteriorated lubricating oil, and improves shock resistance, heat resistance, and lubricity. Moreover, when the surface roughness of the V₂C layer is reduced, the affinity of the outer surfaces of the pins for the walls of the pin holes in the inner link plates is reduced by reducing the surface roughness of the outermost layers of the pins. These effects can be achieved reproducibly and without the need for special production facilities or expensive materials.

Claims

1. A chain for use in an automobile engine comprising links interconnected by connecting pins, wherein the base material of each of the connecting pins is steel, and an outer portion of each pin, extending from the base material to an outer surface thereof, contains vanadium carbide formed by diffusion penetration, and said outer portion is composed of an inner layer containing V8C7, and an outermost layer containing V2C.

2. A chain according to claim 1, in which said inner layer, containing V8C7, is thicker than said outermost layer containing V2C.

3. A chain according to claim 1, in which the outermost layer of each of said pins, containing V2C, is subjected to treatment to reduce the surface roughness of the pin.

4. A chain according to claim 3, in which said inner layer, containing V8C7, is thicker than said outermost layer containing V2C.

5. A chain according to claim 3, in which said treatment to reduce surface roughness is a barrel polishing treatment.

6. A chain according to claim 5, in which said inner layer, containing V8C7, is thicker than said outermost layer containing V2C.

7. A chain according to claim 1, in which the links are composed of link plates arranged in interleaved rows, and in which the interleaved rows are interconnected by said connecting pins.

8. A chain according to claim 7, in which said inner layer, containing V8C7, is thicker than said outermost layer containing V2C.

9. A chain according to claim 7, in which the outermost layer of each of said pins, containing V2C, is subjected to treatment to reduce the surface roughness of the pin.

10. A chain according to claim 9, in which said inner layer, containing V8C7, is thicker than said outermost layer containing V2C.

11. A chain according to claim 9, in which said treatment to reduce surface roughness is a barrel polishing treatment.

12. A chain according to claim 11, in which said inner layer, containing V8C7, is thicker than said outermost layer containing V2C.