The present invention relates to metal-graphite brushes which are used in electrical motors for automobiles, etc, and in particular, Pb-less metal-graphite brush.
Metal-graphite brushes have been used as brushes for low-voltage operation, such as brushes for electrical motors in automobiles. They are produced by mixing graphite and a metal powder such as copper powder, molding and sintering the mixture. As operated at low voltages, their resistivities are lowered by adding a low resistance metal powder. A metal sulfide solid lubricant, such as molybdenum disulfide or tungsten disulfide, and Pb are added to metal-graphite brushes in many cases. For example, in brushes for heavy load such as brushes for starting motor, Pb and a metal sulfide solid lubricant are added in most of the cases.
In recent years, Pb has been attracting greater attention as one of materials damaging to the environment, and there is a growing demand for Pb-less brushes. Of course, brushes containing no lead have been available up to the present and they have been used in some motors other than starting motors. Even some brushes for starting motors can be used by simply eliminating Pb from them, provided that they are used under normal service environments. To improve the lubricating properties without Pb, Japanese Patent Opening Hei 5-226048 (U.S. U.S. Pat. No. 5,270,504) proposes that a metal having a melting point lower than that of copper is mixed in such a way that copper and the metal do not form an alloy. The present inventors, however, found that in metal-graphite brushes wherein a metal sulfide solid lubricant is added to copper and graphite, the elimination of Pb results in an increase in the lead connection resistance under high temperature or high humidity.
The initial object of the present invention is to control the increase in the lead connection resistance of a so-called Pb-less metal-graphite brush under high temperature or high humidity.
In the present invention, a metal-graphite brush comprising a copper-graphite brush body to which a metal sulfide solid lubricant is added and a lead embedded in the copper-graphite brush body is characterized in that the brush body includes Pb in different concentrations between a neighborhood of the lead and a portion with which a commutator of a rotational electric armature is to be in contact in the brush body and that a concentration of Pb in the neighborhood of the lead is higher than a concentration of Pb in the portion.
Preferably, the brush body is molded of different powder materials in the Pb concentrations and the Pb concentration in the neighborhood of the lead in the brush body is 0.4-10 wt %.
More preferably, the different powder materials comprise a first powder material including 0.4-10 wt % Pb for the neighborhood of the lead and a second Pb-less powder material for the portion in contact with the commutator and the different materials are shaped in a common mold so that a tip of the lead is embedded in the neighborhood of the lead.
Preferably, the lead is added with Pb at least in a second portion embedded in the brush body and the brush body is molded of a Pb-less material.
Preferably, the metal sulfide solid lubricant is at least a member of a group comprising molybdenum disulfide and tungsten disulfide, and a concentration of the metal sulfide solid lubricant in the portion in contact with the commutator is 1-5 wt %. The metal sulfide solid lubricant is used to improve sliding when the brush contacts the commutator, and the concentration of the metal sulfide solid lubricant in the neighborhood of the lead is discretionary.
Preferably, a copper concentration in the neighborhood of the lead is higher than a copper concentration in the portion in contact with the commutator.
According to the present invention, an unleaded state or a state of substantially containing no lead does not mean a state being free of lead even as impurities. And a leaded state means that Pb is added intentionally and the Pb concentration is higher than the impurity level. The impurity level of Pb is normally 0.2 wt % or under.
According to the experiments by the present inventors, the increase in the lead connection resistance under high temperature or high humidity is attributed to the influences of the metal sulfide solid lubricant. When the metal sulfide solid lubricant was not added, the lead connection resistance did not increase substantially even under high temperature or high humidity. This is related to the presence or absence of Pb. When Pb was added, the lead connection resistance hardly increased. In Pb-less brushes, in correspondence with the increase in the lead connection resistance, the copper powder and the lead embedded in the brush body showed a greater tendency to be oxidized under high temperature or high humidity.
The metal sulfide solid lubricant such as molybdenum disulfide or tungsten disulfide is added by the designer of the brush, but the metal sulfide solid lubricant is indispensable to brushes so as to have a long service life. Without metal sulfide solid lubricant, an excessive wear may be generated. In particular, this phenomenon is conspicuous in starter brushes to which Pb has been added. When Pb and the metal sulfide solid lubricant are eliminated simultaneously, the service life of the brush will be reduced significantly. Hence in many cases, the metal sulfide solid lubricant can not be eliminated from Pb-less brushes.
The present inventors estimated the mechanism by which the metal sulfide solid lubricant accelerates the oxidization of the copper powder and the embedded lead under high temperature or high humidity as follows: At the time of sintering the brushes, sulfur is liberated from the metal sulfide solid lubricant added to the brush and sulfur adsorbs on the surface of copper to produce copper sulfide. If moisture acts on copper sulfide under high humidity, strongly acidic copper sulfate will be produced to corrode severely the copper powder and the lead. Although the behavior of copper sulfide under high temperature is not certain in some aspects, it is estimated that copper sulfide is oxidized to increase the electrical resistance.
The mechanism by which Pb prevents the oxidization of the copper powder in the brush and the embedded lead is not known exactly. The present inventors estimate that Pb contained in the brush partially evaporates at the time of sintering and coats the surface of copper in the form of a very thin Pb layer. And this Pb layer protects the inner copper from sulfate ion, etc.
According to the present invention, the Pb concentration in the neighborhood of the lead is higher than the Pb concentration in the remaining portion of the brush body, hence the increase in the lead connection resistance due to the metal sulfide solid lubricant under high temperature or high humidity can be prevented. Moreover, as the Pb concentration is lower in the portion of the brush body in contact with the commutator, the amount of Pb which is to be released into the environment can be reduced even when the brush body is worn down by contact and sliding against the commutator.
Such a brush can be produced easily by using two powder materials of different Pb concentrations, one for the neighborhood of the lead and the other for the remaining portion of the brush body, to mold the brush body. When the Pb concentration in the neighborhood of the lead is from 0.4 to 10 wt %, the increase in the lead connection resistance under high temperature or high humidity can be prevented effectively, and the initial value of the lead connection resistance will not increase.
In particular, when the brush body is formed out of two powder materials, one for the portion of the brush body in contact with the commutator and the other for the neighborhood of the lead, and they are molded integrally with the top end of the lead being embedded in the neighborhood of the lead, the production of the brush can be much more simplified. Furthermore, when the portion of the brush body in contact with the commutator is unleaded, the amount of Pb to be released into the environment can be reduced much more.
Instead, Pb may be added to at least the neighborhood of the lead to be embedded in the brush body so as to supply Pb from the lead to the interface between the embedding portion and the lead. Then the increase in the lead connection resistance can be prevented by Pb which is supplied by the lead to the interface.
The metal sulfide solid lubricant is, for example, molybdenum disulfide or tungsten disulfide, and when the addition of the metal sulfide solid lubricant in the portion of the brush body in contact with the commutator is from 1 to 5 wt %, good lubrication can be obtained.
When the copper concentration in the neighborhood of the lead is higher than the copper concentration in the commutator side portion, the lead connection resistance can be reduced.
It should be noted that even in Pb-less brushes, Pb is contained in electrolytic copper, which is normally used in metal-graphite brushes, as an impurity related to production, in many cases. Moreover, in the production process of brushes, if Pb-less brushes and leaded brushes are produced by using the same facilities, a small amount of Pb will enter, as a contamination, into the Pb-less brushes. However, when Pb is not added intentionally to a brush, the Pb concentration in the brush body will not generally exceed 0.2 wt %. Similarly, when a metal sulfide solid lubricant such as molybdenum disulfide or tungsten disulfide is added, contamination in the production process like that of Pb cannot be avoided, and a trace of the metal sulfide solid lubricant will be contained in some cases. However, in the case of contamination, the concentration of the metal sulfide solid lubricant will be 0.1 wt % or under in general.