1. Field of the Invention
This invention relates to an improvement of a starting electric motor having a resin rear bracket.
2. Prior Art
A conventional starting electric motor with a resin rear bracket is shown in FIG. 1.
In FIG. 1, reference numeral 1 designates the armature of a DC motor; 1a, an armature rotary shaft; 1b armature coils; 2, magnets forming magnetic fields; 3, a yoke forming magnetic paths and serving as an outer wall; 4, a commutator having segments in the outer periphery; 5a, a positive brush for rectification; 5b, a negative brush for rectification; 6a, a brush holder holding the positive brush 51; 7, a lead wire connected between the positive brush 5a and a terminal section of an electromagnetic switch (not shown); 7a, a rubber member insulating the lead wire 7; 8, an insulating plate; and 9, a metal base to which the insulating plate is secured, the metal base 9 being engaged with a rear bracket 10 of resin with bolts 11.
Further in FIG. 1, reference character 6b designates a brush holder holding the negative brush 5b; 12, a lead wire having one end connected to the brush holder 6b and the other end connected to the earth (ground) terminal of a starter (not shown); 13, a bearing fitted on the armature rotary shaft 1a and inserted into the recess of the rear bracket 10; and 14, through-bolts joining the resin bracket 10, the yoke 3, and a front bracket (not shown) together.
The operation of the motor thus constructed will be described. When the electromagnetic switch (not shown) is closed, current is supplied through the lead wire 7 to the positive brush 5a and through the segment in the outer periphery of the commutator 4 to the armature coil 1b and the current is allowed to flow through the segment in the outer periphery of the commutator, the negative brush 5b and the lead wire 12 to the earth terminal of the starter (not shown).
As the current flows in the magnetic field formed by the magnets 2 in the above-described manner, the armature 1 produces torque.
The conventional starting electric motor using the resin rear bracket 10 for the purpose of reducing its weight, being constructed as described above, suffers from the following problems: It is necessary to provide the lead wire 12 on the earth side (hereinafter referred to as "an earth side lead wire 12", when applicable), and the earth terminal. This will make the manufacture of the motor intricate, and the number of components increase. On the other hand, during travel of the vehicle, the motor suffers heat generated by the engine, and the rear end face of the rear bracket 10 locally receives the axial tensions of the through-bolts 14 and the bolts 11. As a result, the rear bracket 10 is deformed; i.e., creeped, and the through-bolts 14 and the bolts 11 are loosened. Also, since the end face of the rear bracket 10 is in the form of a plate of resin, the end face is insufficient in bend resisting strength and in compression resisting strength so that it is difficult to tightly and stably mount the rear bracket 10 on the yoke 3.