Thrust compensation for flat-commutator starter

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a starter of a flat commutator type for starting a vehicle engine or the like.

2. Prior Art

FIG. 1 shows a conventional starter of a flat commutator type, which comprises the armature 1 of a DC motor; a rotary shaft 2 of the armature; flat commutator 3 fitted behind the armature and connected to an armature coil 1a; brushes 4 provided in a rear cover 5 made of plastic and pushed onto the surface of the flat commutator 3 by brush springs 6; a bearing 7 secured to the rear cover 5 and supporting the rotary shaft 2; fixed contacts 8 of an electromagnetic switch 9 which are attached to the rear cover 5 by nuts 10; movable contacts 11 of the electromagnetic switch which are attached with interposed electric insulators 12 to a plunger rod 13; a core 14; a driving coil 15 wound on a plastic-made bobbin 17 and secured to a case 18 to drive a plunger 16; an internal opening 19 provided in the plunger; a rod 21 urged forward by a spring 20; a yoke 22 of the DC motor; a permanent magnet 23 provided on the inside circumferential surface of the yoke to act as the field of the DC motor; a front cover 25 secured to the end face of the yoke 22 and formed with an internal gear 24 located on the inside circumferential surface of the rear portion of the front cover and constituting a planetary speed reducer; a spur gear 26 formed as the sun gear of the planetary speed reducer at the front end of the rotary shaft 2 of the armature 1 and engaged with a planet gear 27; a bearing 28 fitted on the inside circumferential surface of the planet gear and supported by a support pin 30 secured to the rear portion of an overrunning clutch 29 composed of an outer member 29a, an inner member 29b and rollers 29c located therebetween; a bearing 30 provided between the inner member 29b of the overrunning clutch 29 and a recess 25a of a front cover 25 and carrying as radial load; a rotary output shaft 31 which receives the action of the plunger 16 through the rod 21 and a steel ball 32 and is fitted to the inside surface of the inner member 29b of the overrunning clutch 29 so that the rotary output shaft is slidable back and forth; a sleeve bearing 33 supporting the rotary output shaft 31 at the rear end thereof; a bearing 34 supporting the rotary shaft 2 of the armature 1 at the front portion of the rotary shaft and fitted on the inside surface of the end portion of the yoke 22; and a pinion 35 which is fitted on a straight spline 31a formed on the outside circumferential surface of the rotary output shaft 31 and is engaged with a stopper 36 so as not to move forward.

The operation of the conventional starter of the flat commutator type is now described. When an ignition switch not shown in the drawing is turned on, electricity is applied to the driving coil 15 of the electromagnetic switch 9 to move the plunger 16 forward to put the movable contacts 11 into contact with the fixed contacts 8. As a result, electricity is applied to the armature coil 1a through the fixed contacts 8, lead wires not shown in the drawings which connect the contacts 8 and the brushes 4, the brushes 4 and the flat commutator 3 so that a turning force acts to the armature 1. In the meantime, the motive power of the moved plunger 16 is transmitted to the rotary output shaft 31 through the rod 21 to move the rotary output shaft forward so that the pinion 35 fitted on the rotary output shaft 31 is engaged with a ring gear for an engine not shown in the drawing. As a result, the engine is started.

However, since the flat commutator 3 is always pushed forward by the brushes 4 urged by the brush springs 6, there is a problem that the rotary shaft 2 of the armature 1 always receives a forward force to cause a torque loss at a thrust bearing.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a starter of the flat commutator type, in which torque loss caused by thrust is reduced.

The magnetic center of the field of the motor of the starter is displaced from the center of the armature of the motor in the axial direction thereof in the direction inverse to that of the pushing force of brushes, so that a force oriented toward the magnetic center of the field is caused when the armature is rotated under the application of the electricity thereto. As a result, the pushing force of the brushes to flat commutator of the starter is offset so that the thrust load upon the rotary shaft of the armature is decreased. For that reason, the torque loss is reduced while the starter is in action.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of a conventional starter of the flat commutator type;

FIG. 2 shows a sectional view of a starter of the face commutator type according to an embodiment of the present invention; and

FIG. 3 shows a sectional view of a starter of the flat commutator type according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be hereafter described in detail with reference to the drawings attached hereto.

FIG. 2 shows a sectional view of a starter of the flat commutator type which is one of the embodiments. The portions (whose reference symbols from 1 to 36) of the starter are the same as those (whose reference symbols are from 1 to 36) of the conventional starter shown in FIG. 1 and therfore are not described in detail hereinafter. In FIG. 1, a permanent magnet 23 is secured to the inside circumferential surface of the yoke 22 of the starter to act as the field of the DC motor thereof. The magnetic center of the permanent magnet 23 in the axial direction of the starter is located between the center of the armature 1 in the axial direction thereof and the axial location of each brush 4 thereof.

When electricity is applied to the electromagetic switch 9 of the starter shown in FIG. 2, the armature 1 is rotated and the rotary output shaft 31 is moved forward so that the pinion 35 is engaged with a ring gear for an engine to start the engine, in the same manner as the conventional starter shown in FIG. 1.

Since the magnetic center of the permanent magnet 37 of the starter shown in FIG. 2 is displaced from the center of the armature 1 in the axial direction thereof toward the brushes 4, the armature and the rotary shaft 2 thereof are always attracted backward during the rotation of the armature 1. For that reason, the forward pushing forces of the brush springs 6 are offset by the backward attractive force on the armature 1 and the rotary shaft 2 thereof so that the thrust load upon a thrust bearing is decreased. As a result, the torque loss of the starter is reduced.

FIG. 3 shows another embodiment which is also a starter of the flat commutator type, in which the field of a DC motor is composed of main magnetic poles 38a and auxiliary magnetic poles 38b located adjacently to the main magnetic poles 38a in the direction inverse to that of the rotation of the armature of the DC motor. The main magnetic poles 38a are constituted by a permanent magnet. The auxiliary magnetic poles 38b are constituted by iron pieces secured adjacently to the permanent magnet. The portions (whose reference symbols are from 1 to 36) of the starter shown in FIG. 3 are the same as those (whose reference symbols are from 1 to 36) of the conventional starter shown in FIG. 1 and therefore not described in detail hereinafter. The magnetic center lines of the main magnetic poles 38a extend through the center of the armature 1 of the starter in the axial direction thereof shown in FIG. 3, but the magnetic center lines of the auxiliary magnetic poles 38b extend off the center of the armature 1 in the axial direction thereof and are displaced therefrom toward the brushes 4. As a result, when the total load upon the starter is heavy, an electrical current flowing through the armature coil 1a is also heavy and the magnetic flux of the field of the starter is therefore large in quantity so that a force attracting the armature 1 toward the brushes 4 is caused and offsets the pushing forces of the brush springs 6. For that reason, the forward thrust load upon the starter is decreased so that the torque loss thereof is reduced.

When the total load upon the starter is light, the electrical current flowing through the armature coil 1a is small and the magnetic flux of the field of the starter is therefore small in quantity. Accordingly, the backward attractive force caused by the auxiliary magnetic poles 38b is weak so that the forward thrust load upon the starter is relatively heavy. For that reason, a braking action is caused so that the starter is prevented from excessively rotating. In other words, the rotative loss of the starter is decreased when the total load thereupon is heavy, and the excessive rotation of the starter is prevented by the braking action of the thrust load thereupon when the total load thereupon is light.

Although the field in each of the above-described embodiments is constituted by the permanent magnet, the present invention is not confined thereto but may be otherwise embodied to provide a field constituted by a core and a coil wound thereon, to produce the same effect as the embodiments.

As described above, according to the present invention, the magnetic center of the field of the motor of the starter is displaced from the center of the armature of the motor in the axial direction thereof in the direction inverse to that of the pushing force of the brushes, so that the thrust load is reduced when the armature is rotated. Accordingly, the torque loss is reduced while the starter is in action.

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