Chain for use in automobile engine

Abstract

Connecting pins in a roller chain are chromized by a diffusion penetration treatment, that results in inner and outer chromium carbide layers on the steel base material. The inner layer contains Cr7C3, and the outermost layer, which is thinner than the inner layer, contains Cr23C6. The surface roughness of the outer layer is reduced by barrel polishing, without removal of all of the Cr23C6-containing layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cut away perspective view showing a part of a roller chain according to the invention;

FIG. 2 is a graph showing the results of elongation tests in deteriorated lubricating oil for roller chains according to the invention and a conventional chain; and

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

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be described with reference to a roller chain. However, it should be understood that the beneficial effects of the invention are also applicable to a bushing chain.

The roller chain 10, shown in FIG. 1, is composed of inner plates 11, outer plates 14, bushings 13, pins 15 and rollers 13. Both ends of each bushing 12 are press-fit into bushing holes 11a of a pair of opposed inner plates 11 of the chain. Similarly, both ends of a pin 15, which fits loosely in a bushing 12, are press-fit into pin holes 14a of a pair of opposed outer plates 14, which are disposed on the outer sides of the inner plates 11 in overlapping relationship. Rollers 13 are rotatably mounted on the bushings 12. When used as an engine timing chain, the chain is formed into an endless loop consisting of alternating pin links and bushing links, each pin link being composed of two outer plates 14 and two pins 15, and each bushing link being composed of two inner plates 11, two bushings 12 and two rollers 13.

The bushing 12 is composed of alloy steel. The base material of the pin 15 can be high carbon steel or low carbon steel. In the case of low carbon steel, a high carbon surface layer is formed on the surface 15a of the pin 15. The high carbon surface layer can be formed by a wide variety of known methods. However, the formation of the high carbon surface layer is preferably carried out by a carburizing treatment, in which the pin 15 is heated to approximately 900° C. to 950° C. in a carburizing agent to diffuse carbon into in the surface of the pin. If high carbon steel is used as the base material of the pin 15, the carburizing treatment is not needed.

A chromizing layer, that is, a diffused chromium layer is formed on a surface of the pin 15 by a diffusion penetration treatment known as a “powder pack” treatment. In this treatment, the periphery of the pin 15 is filled with a chromium powder or a chromium alloy powder, and is heat-treated at a temperature in the range from 900° C. to 1200° C. for 5 to 25 hours. When this powder pack method is carried out, an anti-sintering agent such as alumina or the like, and a reaction promoting agent such as ammonium chloride or the like, are added in appropriate amounts.

Another known option for forming the chromizing 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 chromium 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 chromizing 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 1000° C. Carbon contained in the high carbon surface layer formed as a surface layer of the pin 15, or from the pin base material itself in the case of a pin formed of high carbon steel, penetrates by diffusion into the chromizing layer formed on the surface of the pin to combine with chromium in the chromizing layer. As can be seen from a cross-sectional photograph of FIG. 3, taken by an electron microscope, the chromizing layer is composed of two parts: an inner chromium carbide layer, formed on the steel base material, which becomes a Cr₇C₃-containing layer, and an outermost chromium carbide layer, which becomes a Cr₂₃C₆-containing layer. Cr₇C₃ is the main component of the inner chromium carbide layer, and Cr₂₃C₆ is the main component of the outer chromium carbide layer.

Based on measurements taken on microphotographs corresponding to the photograph in FIG. 3, the thickness of the inner, or base material-side, Cr₇C₃-containing layer is in the range from 8 to 20 μm, and the thickness of the outermost, or Cr₂₃C₆-containing layer is in the range from 1 to 6 μm. The mechanism by which the chromizing layer of the surface becomes divided into two different sublayers is not fully understood at the present time. However, it appears that the chromium, which is supplied as a powder in the chromizing treatment, combines with carbon supplied from the high carbon pin, or from the high carbon surface layer of the pin, to form a Cr₇C₃ layer, i.e. a layer having a high carbon to chromium ratio in the vicinity of the base material, and a Cr₂₃C₆ layer, i.e., a layer having a lower carbon to chromium ratio, farther from the base material. It is believed that, in the process of formation of the solid phase chromium carbide, a balancing takes in which the chromium carbide becomes divided into the two different layers.

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

Chain: Roller chain having a pitch of 8 mm

Number of teeth on the sprockets: 18×36

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.

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 Cr₂₃C₆. In this case, the surface roughness of the outermost layer of the pin, i.e., the layer containing Cr₂₃C₆, was decreased by barrel polishing. And the Cr₂₃C₆-containing outermost layer in example 2 was porous so that recesses are 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 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. On the other hand, 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.

In the conventional case, used for comparison, the pins were subjected to a full barrel polishing treatment, so that the outermost Cr₂₃C₆ 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.

As shown by the results of the chain elongation tests, as depicted in FIG. 2, when the roller chain of Example 1 is compared with a conventional roller chain after a test time of 50 hours, the elongation of the chain of Example 1 was about 60% of the elongation of the conventional chain. The slopes of the curves depicting chain elongation become more gradual after 10 hours from the start of the test, indicating a higher rate initial wear in all three cases. However, it can be seen that the initial wear is less in Examples 1 and 2 than in the conventional case. The initial wear in Example 1, which has two chromium carbide layers on the surface of the pin, is lower than the initial wear in the conventional case, where the pin has only one chromium carbide layer. In example 2, elongation during the initial 10 hours of the test was still further reduced by virtue of the fact that the reduced surface roughness of the pin resulted in a reduction in wear due to the attackability of the bushing.

From the results of the tests, if the elongation of the conventional chain after 50 hours is set at 100, the elongation of the chain of Example 1 was about 60, and the elongation of the chain of Example 2 was about 40. Thus, a clear improvement was realized as a result of the reduction in the surface smoothness of the outermost chromium carbide (Cr₂₃C₆) layer. This improvement is due to suppression of the attackability of the bushing during the initial stage of the test, and also to reduction of abrasive loss of the bushing. Both results are achieved by reducing the surface roughness of the pin while still ensuring that the outermost layer, i.e. the layer containing Cr₂₃C₆ is present. The target thickness of the outermost layer of the pin of Example 2 is in the range from 1 to 6 μm.

When the barrel polishing treatment is fully carried out, so that the outermost layer is completely removed, the result corresponds to the conventional case, and the function of the outermost layer is lost, and as seen in FIG. 2, the elongation of the chain in the conventional case is greater than that of the chains of Examples 1 and 2, both at the 10 hour point and at the 50 hour point, due in part to wear of the bushing during the initial stage of the test.

It has also been found that the abrasive loss in the pins of Examples 1 and 2 was only about 1/15 of the abrasive loss in the conventional case. Furthermore it was determined that the abrasive loss of the bushing in the case of Example 1 was improved to about 90% and the abrasive loss of the bushing in the case of Example 2, was improved to about 65%, as compared with the bushing in the conventional case.

INDUSTRIAL APPLICABILITY

The invention has significant industrial applicability in that it reduces abnormal wear elongation of a chain for use in an automobile engine due to operation in deteriorated lubricating oil and the reduction in abnormal wear elongation can be achieved reproducibly and without the need for special production facilities or expensive materials.

Claims

1. A power transmission chain for use in an automobile engine comprising: pairs of inner plates and pairs of outer plates in alternating, overlapping relationship along the length of the chain;a pair of bushings for each pair of inner plates, ends of the bushings being press-fit into bushing holes in the inner plates;a pair of pins for each pair of outer plates, ends of the pins being press-fit into pin holes in the outer plates, each pin having an outer surface, and extending through, and fitting loosely in, one of said bushings;wherein the base material of each of the pins is steel, and an outer portion of each pin, extending from the base material to an outer surface thereof, contains chromium carbide and is formed by diffusion penetration, and said outer portion is composed of an inner layer containing Cr7C3, and an outermost layer containing Cr23C6.

2. A power transmission chain according to claim 1, in which said inner layer, containing Cr7C3 is thicker than said outermost layer containing Cr23C6.

3. A power transmission chain according to claim 1, in which the outermost layer of each of said pins, containing Cr23C6, is subjected to treatment to reduce the surface roughness of the pin.

4. A power transmission chain according to claim 3, in which said inner layer, containing Cr7C3 is thicker than said outermost layer containing Cr23C6.

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

6. A power transmission chain according to claim 5, in which said inner layer, containing Cr7C3 is thicker than said outermost layer containing Cr23C6.