1. Field of the Invention
This invention relates to a multilayer brush for electric rotating machines which is used in DC motors, e.g., a multilayer brush used in starting motors of automobiles.
2. Description of the Prior Art
Recently available DC motors are made high-speed and high-current-density so as to be made compact and light-weight. However, in the present state of things, the motors of this type may greatly lower in commutation performance, output characteristics and so forth, and may much suffer the wear of brushes, resulting in a short service life. In order to solve such problems, the structure of brushes is devised to cope with the matter because there is a limit to mere improvements in performance of brush materials. As one means therefor, the problems are solved by providing a multilayer brush devised from the form of a brush alone (see Japanese Patent Publication No. H06-007505, pages 1 to 3, FIGS. 1 and 2).
In the multilayer brush, a brush is divided into two or three portions so that short-circuit current can be restrained and commutation can be improved by making the resistance on the outlet side larger than that on the inlet side in respect to a commutator.
However, in such a multilayer brush as well, the surface of the commutator may blacken as a result of the driving of a motor for a long time, so that not only sparks may become uncontrollable but also the commutator may come to have an uneven surface to cause an increase in wear of the brush, and its durability is affected.
Brushes for motors of automobiles are also required to have durability, wear resistance, corrosion resistance and small electrical loss. They also come to have high temperature when the motor interiors have a high temperature and a high brush resistivity. Accordingly, for the purpose of lowering resistivity, a metal graphite brush is used which contains copper powder, graphite, lead, molybdenum disulfide, a novolak phenolic resin and a furfural resin (see Japanese Patent Application Laid-open No. 07-213022, pages 1 to 5).
Brushes for motors of automobiles also include brushes containing copper powder in a large quantity. Such brushes may come to have a high resistance upon oxidation of the copper when they come to have high temperature and high humidity, so that problems may arise such that electrical loss increases to cause a lowering of the performance of motors (a lowering of output). As a countermeasure therefor, a brush to which lead or lead oxide is added is devised (see Japanese Patent Publication No. 58-029586, pages 1 to 3).
However, the lead or lead oxide used as an additive is harmful, and has come to be prohibited from use in view of environment.
An object of the present invention is to provide a multilayer brush having a superior durability, which can prevent the performance of motors from lowering, without use of the harmful substance such as lead, and may less undergo any wear due to mechanical and electrical sparkling of the brush.
To achieve the above object, the present invention provides a multilayer brush composed chiefly of copper and graphite and incorporated therein with a solid lubricant, which brush consists essentially of two types of brushes, a high-copper-content part brush containing the copper in a large quantity and a low-copper-content part brush containing the copper in a small quantity, wherein;
at least the high-copper-content part brush contains zinc in an amount of from 0.1% by weight to 5% by weight, and the zinc and the copper form an alloy.
In the above multilayer brush, the low-copper-content part brush may further contain zinc in an amount of from 0.1% by weight to 3% by weight, and the zinc and the copper may form an alloy.
In the above multilayer brush in which the low-copper-content part brush further contains zinc, the high-copper-content part brush may further contain at least one of manganese and nickel in an amount of from 0.1% by weight to 3% by weight.
In the above multilayer brush in which the low-copper-content part brush further contains zinc and the high-copper-content part brush further contains at least one of manganese and nickel, the low-copper-content part brush may further contain at least one of manganese and nickel in an amount of from 0.1% by weight to 3% by weight.
In any one of the above multilayer brush, the high-copper-content part brush contains the copper in an amount of from 30% by weight to 80% by weight and the low-copper-content part brush contains the copper in an amount of from 10% by weight to less than 45% by weight.