This invention relates to an engine starter motor for starting an engine and particularly to an engine starter motor having a speed reduction gear between a unidirectional clutch and a pinion gear.
An engine starter motor is known, in which the rotational speed of an electric motor is reduced by one or two stages, an electric motor unit is reduced in size and weight, and in which an unidirectional (over-running) clutch is mounted to a speed-reduced pinion shaft. In the engine starter motor of this structure, the effect of the inertia moment of the electric motor on the unidirectional clutch increases by the square of the speed reduction ratio as it increases. Therefore, a disadvantageously large capacity unidirectional clutch is required for an engine starter motor having no speed-reduction mechanism, in which the rotation of the armature rotary shaft is directly transmited to the unidirectional clutch.
Therefore, there has been proposed an engine starter motor such as disclosed in Japanese Patent Laid-Open No. 52-73315 in which an over-running clutch is disposed on an armature rotary shaft as well as an engine starter motor such as disclosed in Japanese Patent Laid-Open No. 60-2895 in which an over-running clutch is disposed on an idle shaft in order to eliminate the above-discussed disadvantages of the conventional design.
FIG. 1 illustrates the engine starter motor disclosed in Japanese Patent Laid-Open No. 52-73315, in which reference numeral 1 indicates a d.c. motor, 2 indicates an armature of the d.c. motor 1, and 3 indicates an armature rotary shaft. An over-running (unidirectional) clutch 4 is mounted to the armature rotary shaft 3, and a clutch outer member 5 of the over-running clutch 4 is secured to the armature rotary shaft 3 by splines 6 and a clutch inner member 9 having a pinion gear 8 of a speed reduction gear 7 is mounted on the armature rotary shaft 3 rotatable relative to it. Rollers 10 are interposed between the clutch outer member 5 and the clutch inner member 9, and 11 indicates a cover. Reference numeral 12 indicates a larger gear which is in mesh with the pinion gear 8 and which has a pinion shaft 13 centrally engaged through helical splines 14 on the pinion shaft 13. A front end portion of the pinion shaft 13 has mounted thereon a pinion 16 which is enagageable with an engine ring gear 15, and a rear end portion of the pinion shaft 13 has mounted thereto a stopper 17 for positioning the forward position of the pinion shaft 13 and a return spring 18 disposed between the stopper 17 and the large gear 12. A lever 19 transmits a drive force from a plunger 20 of the solenoid.
In the engine starter motor with the above structure, when the plunger 20 is actuated by an electromagnetic attraction, the lever 19 rotates in the clockwise direction as viewed in the figure, the pinion shaft 13 is driven in the forward direction against the spring action of the return spring 18 to cause the pinion 16 to mesh with the engine ring gear 15. Also, the rotation of the armature rotary shaft 3 is transmitted through the unidirectional clutch 4, the smaller gear 8, the larger gear 12, the splines 14 and the pinion shaft 13 to the engine ring gear 15 to start the engine. The torque applied to the unidirectional clutch 4 is the maximum torque generated by the electric motor 1, and the torque at the pinion gear 16 is equal to the torque generated by the electric motor 1 multiplied by the speed reduction ratio between the small gear 8 and the large gear 12. Also, while the effect of the inertia moment of the rotating armature 2 only directly acts on the uni-directional clutch 4 which is directly connected to the armature 2, it acts on the pinion 16 as increased by a factor of the square of the speed reduction ratio.
After the engine is started and the plunger 20 is released, the action of the return spring 18 causes the shift lever 19, the pinion shaft 13 and the pinion 16 to return to their original positions, whereby the operation of the engine starter motor is stopped.
FIG. 2 illustrates an engine starter motor disclosed in Japanese Patent Laid-Open No. 60-2895. In the FIGURE, an idle shaft 21 is disposed on an axis different from those of an armature rotary shaft 3 and a pinion shaft 13, and the unidirectional clutch 4 is rotatably supported on this idle shaft 21. On the outer circumference of the clutch outer member 5 of the unidirectional clutch 4, a speed reducing gear 5a is formed so that it engages a gear 3a formed in the armature rotary shaft 3. Also, a small gear 8 integral with the clutch inner member 9 is in engagement with a large gear 12 formed on a tube 22, thereby forming a speed-reduction gear 7, and the tube 22 and the pinion shaft 13 are spline-engaged to each other through helical splines 14. Reference numerals 23, 24 indicate a housing and a center bearing support, respectively, for supporting the armature rotary shaft 3, the idle shaft 21 and the pinion shaft 13.
Reference numberal 25 indicates a solenoid switch, 26 indicates a plunger of the solenoid switch 25, 27 indicates a rod constructed to move together with the plunger 26, 28 indicates a movable contact, 29 indicates stationary contacts one of which is connected to a battery (not shown) and the other of which is connnected to a d.c. motor 1. Reference numeral 30 indicates a ball interposed between the rod 27 and the pinion shaft 13. In the figure, the same reference numerals used also in FIG. 1 indicate identical or corresponding components, so that their explanation is omitted.
In the engine starter motor as above constructed, the starting operation is achieved in a manner similar to that of the starter shown in FIG. 1, and the transmission torque of the unidirectional clutch 4 equals to the torque of the d.c. motor 1 multiplied by the speed-reduction ratio between the gear 3a and the speed reducing gear 5a, and further even though the effect of the inertia moment of the d.c. motor 1 acting on the unidirectional clutch 4 is increased by a factor of the square of the speed-reduction ratio, this effect is smaller than that where the unidirectional clutch 4 is mounted on the pinion shaft 13.
In the conventional engine starter motor, it has been attempted to reduce the effect of the speed-reduction ratio of the unidirectional clutch 4 and to use the unidirectional clutch 4 in common with the engine starter having no speed reduction gear.
However, since the pinion shaft 13 of the conventional engine starter is engaged with the speed-reduction gear components by the helical splines, a return force is generated on the pinion shaft 13 when the pinion 16 is driven by the engine immediately after the engine is started or when the engine starter motor cannot quickly follow the change in rotation of the engine. That is, when a drag torque which causes the unidirectional clutch 4 to rotate in the idling direction is considered as to the pinion shaft 13, this drag torque is increased by an amount corresponding to the speed-reduction ratio between the unidirectional clutch 4 and the pinion shaft 13. As shown in FIG. 3, the return force F can be expressed by: EQU F=tan (.theta.-.rho.)T/r.sub.H
where,
T: drag torque PA0 r.sub.H : mean diameter of the helical splines 14 PA0 .theta.: twist angle of the helical splines 14 PA0 N: drag PA0 .tau.: friction angle of the helical splines 14. PA0 .alpha.: angle defined between the frictional surfaces of the clutch outer an inner members PA0 .gamma.: angle of direction of a spring force with respect to a tangent at the point of contact of the roller to a frictional surface of clutch inner member PA0 n: number of rollers PA0 r: outer diameter of the clutch inner member PA0 .mu.: friction coefficient of the roller contacting surfaces.
Accordingly, when a unidirectional clutch similar to that of an engine starter with no speed reduction mechanism is used as in the above conventional engine starter, the drag torque as considered from the pinion shaft 13 is increased by an amount corresponding to the speed-reduction ratio and the return force F is also increased accordingly. Therefore, the return force F exceeds the electromagnetic attractive force on the plunger 20 or 26, causing the pinion 16 to return and disengage from the ring gear 15, and if the engine starter motor is being energized at this time and plunger 20 or 26 is being electromagnetically actuated, the pinion 16 is again driven into mesh with the engine ring gear 15. Even when the pinion 16 did not completely disengage from the engine ring gear 15, the pinion 16 moves fiercely, resulting in large mechanical impacts which may destroy or damage the ring gear 15 and other components and generate noise.