The present invention relates to an engine starter and, particularly, to an improvement of such starter having a planet type reduction gear mechanism.
FIG. 1 shows a typical example of a conventional engine starter having a planet type gear reduction mechanism. In FIG. 1, the engine starter comprises an electric d.c. motor having an armature 1 rotatably supported by a shaft 2 and a yoke 3 in the form of cylinder having a wall 31 and a front bracket 5 on an inner surface of which a component of a planet reduction gear mechanism is formed integrally. The component comprises an internal gear member in the form of a flange including an axial annular tooth portion extending toward the yoke 3 and having an inner surface formed integrally with gear teeth 4, and an annular projection 41 which is coaxial with the annular tooth portion in the opposite direction and defines a center hole. A solar gear 6 is formed on a front end portion of the armature shaft 2, with which planet gears 7 are meshed. The planet gears 7 are supported by bearings 8 which are, in turn, supported by support pins 9 implanted in a flange 10 of an output shaft 11. The output shaft 11 is supported by a sleeve bearing 12 which is supported by an inner periphery of the center hole of the annular projection 41 of the internal gear member. The front end portion of the armature shaft 2 is supported by a sleeve bearing 13 fixedly provided in an axial hole formed in an inner peripheral surface of a rear portion of the output shaft 11. A reference numeral 14 depicts a steel sphere arranged between ends of the armature shaft 2 and the output shaft 11 and functions to receive thrust load. A reference numeral 15 denotes a permanent magnet fastened on an inner peripheral surface of the yoke 3. The flange 31 functions to provide a thrust support for the planet gears and seals the planet reduction gear mechanism from the motor portion to thereby provide a dust-free structure.
In operation, the armature 1 is energized by closing an engine key switch (not shown) and produces a rotational force under the influence of the magnetic field of the permanent magnet 15. The rotational force of the armature 1 is transmitted through the solar gear 6 on the shaft 2 to the planet gears 7 to rotate the latter to thereby rotate the output shaft 11 at a reduced speed through the flange portion 10. The rotation of the output shaft 11 is transmitted to a ring gear of an internal combustion engine (not shown) through a pinion gear of an over running clutch (not shown) which is fitted on the output shaft 11 and housed in the front bracket 5.
Since, in the conventional device, the internal gear member having internally projecting teeth which constitute a portion of the planet reduction gear mechanism is provided separately from the yoke and assembled thereto, the number of parts constituting the mechanism is relatively large, the structure thereof is relatively complicated and the assembly thereof is relatively difficult.
Further, the partitioning effect of the flange portion of the yoke arranged between the motor and the planet reduction gear mechanism is not complete due to the existence of the center hole of the flange. It is thus difficult to assure a dust-proof structure for shielding one of them with respect to the other.
A starter having a center bracket which is formed integrally with a yoke of a motor so that it covers the latter is disclosed in U.S. Pat. No. 4,454,437.
U.S. Pat. Nos. 4,488,073 and 4,520,285 disclose a starter in which an internal gear member is formed integrally with a center bracket.
In any of these prior arts, it has been found that, although the number of parts is reduced, the rotational force of the armature shaft under shock load is transmitted to the yoke, causing the operational reliability of the starter to be degraded. Further, in order to provide a dust-proof structure, these prior arts propose the use of specially designed shielding member arranged between the motor and the planet reduction gear mechanism.