Drive mechanism of engine ignition system

A drive mechanism of an engine ignition system has a housing fixed to an engine and a driven shaft of the engine ignition system rotatably supported by the housing. An engine provides a driving force for rotating a drive shaft, the rotation of which is transmitted to the driven shaft through screw gears having axes crossing each other. A screw gear provided on the driven shaft includes a main gear fixed to the driven shaft and an auxiliary gear opposed to an end face of the housing and rotatably and axially movably mounted on the driven shaft. The respective teeth of the main gear and auxiliary gear is staggered from each other when the main gear and auxiliary gear come in contact with each other. A resilient member is provided between the end face of the housing and the auxiliary gear. The screw gear on the drive shaft is adapted to push the auxiliary gear against the resilient force of the resilient member toward the end face of the housing so that the screw gears on the drive and driven shafts are engaged with each other. A sliding member may be provided between the end face of the housing and the auxiliary gear to be biased by the resilient member to come in direct or indirect contact with the end face of the housing, thereby causing sliding resistance of rotation.

BACKGROUND OF THE INVENTION 
The present invention relates to a drive mechanism for an ignition 
distributor, a crank angle sensor, etc. used in the ignition system of an 
engine. In particular, the present invention relates to a drive mechanism 
using a screw gear as a power transmission system. 
In the ignition distributor, the crank angle sensor, etc., the rotation of 
a crank shaft of the engine is transmitted to a rotating shaft of each of 
the ignition distributor and the crank angle sensor through a screw gear. 
A backlash of the screw gear of this kind tends to be structurally 
increased for reasons of manufacture of the screw gear. 
An angular acceleration of a crank shaft or a drive shaft of the engine is 
generated by fluctuations of rotation caused by an explosion in cylinders, 
etc. The rotating shaft which is a driven shaft has a rotational inertia. 
Therefore, tooth faces of the drive and driven shafts come into collision 
with each other by the above backlash, so that an ignition timing becomes 
unstable and noises, abrasion and damages are involved with respect to the 
screw gears. 
For example, Japanese Patent Examined Publication No. 51-27814 and Japanese 
Patent unexamined Publication No. 3-290059 (U.S. Pat. No. 5,088,459) 
disclose techniques for solving these problems. In these documents, a 
screw gear on a driven shaft comprises a main gear fixed to the driven 
shaft and an auxiliary gear rotatably and slidably fitted on the driven 
shaft. While the main and auxiliary gears are staggered from each other by 
an amount corresponding to backlash, a resilient spring force is applied 
to the auxiliary gear in a rotational direction thereof. Thus, teeth of a 
screw gear on the side of a drive shaft are caught by teeth of the main 
and auxiliary gears therebetween to remove the backlash. 
In Japanese Patent Examined Publication No. 60-24311 and Japanese Patent 
Unexamined Publication No. 59-63367, a sliding plate adapted to rotate 
with a driven shaft is biassed against an end face of a housing, by 
resilient spring force which housing supports this driven shaft. Thus, 
rotational friction force is produced on the driven shaft to increase 
rotational torque of the driven shaft, so that self-propelling of a driven 
shaft system caused by its rotational moment is restrained. 
Between the above measures to counter backlash, the former one is 
disadvantageous in that contact pressure between screw gears becomes great 
since a spring force (a spring force for biasing an auxiliary gear in a 
direction of rotation) required for removing backlash must be made great 
as an angular acceleration of a drive shaft and a rotational inertia of a 
driven shaft become great. 
The latter one naturally has a limit because a spring force is made great 
to increase surface pressures (friction force of rotator) on contact 
faces, thus involving a problem such as abrasion of screw gears and parts 
which generate friction forces. 
SUMMARY OF THE INVENTION 
In consideration of the above-mentioned problems, an object of the present 
invention is to provide a drive mechanism of an engine ignition system 
which is reasonable and simplified to enable accommodating an angular 
acceleration and a rotational inertia greater than those in the 
conventional backlash removing means without losing durability of the 
drive mechanism. 
The above object of the present invention can be achieved by a drive 
mechanism of an engine ignition system having a housing fixed to an 
engine, and a driven shaft of the engine ignition system rotatably 
supported by the housing, said engine providing a driving force for 
rotating a drive shaft, the rotation of which is transmitted to the driven 
shaft through screw gears having axes crossing each other, said driving 
mechanism further comprising a screw gear provided on said driven shaft 
and having a main gear fixed to said driven shaft and an auxiliary gear 
opposed to an end face of said housing and rotatably and axially movably 
mounted on said driven shaft, the respective teeth of said main and 
auxiliary gears being staggered from each other when the main and 
auxiliary gears come in contact with each other, and a resilient member 
provided between the end face of said housing and said auxiliary gear, 
said screw gear on said drive shaft pushing said auxiliary gear against 
the resilient force of said resilient member toward the end face of said 
housing so that the screw gears on said drive and driven shafts are 
engaged with each other.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments of a drive mechanism of an engine ignition system 
in the present invention will next be described in detail with reference 
to the accompanying drawings. 
FIGS. 1 to 3 show an ignition distributor, which builds therein an ignition 
signal generator, in an engine ignition system in accordance with a first 
embodiment of the present invention. In FIG. 3, a housing 3 of the 
distributor is fixed to an engine (not shown) by fastening thereto a 
flange 3b by bolts, etc. A shaft 4 of the engine ignition system is 
inserted into the housing 3 and is journaled by a plain bearing 14 to 
drivingly rotate a signal rotor of an ignition signal generator 5 built in 
the housing 3. The plain bearing 14 is press-fitted in and fixed to one 
end 3a of the housing. 
The rotation of a crank shaft 1 of the engine is transmitted to the shaft 4 
through screw gears 2 and 6 having axes crossing each other at a 
predetermined angle. 
A screw gear system and a backlash removing means in this embodiment will 
next be described in detail with reference to FIGS. 1 and 2. 
A screw gear 2 on a drive side of the screw gear system is fixed to one end 
of the crank shaft 1. A screw gear 6 provided on the shaft 4 on a driven 
side of the screw gear system comprises a main gear 7 and an auxiliary 
gear 8. As shown in FIG. 1, the main gear 7 is fixed to the shaft 4 such 
that its one end face toward the auxiliary gear 8 is located in the 
vicinity of an engaging point P of the screw gear 6 and the screw gear 2 
on a drive shaft side. The main gear 7 is fixed to the shaft 4 by riveting 
a sleeve 7A formed with the main gear 7 to the shaft 4 by a rivet 13. 
The auxiliary gear 8 is loosely fitted on the shaft 4 in a manner to face 
the housing end face 3a, and is formed at its one end with a sleeve 8A. 
Slits or guide grooves 15 are formed in this sleeve 8A in a thrust 
direction and are engaged with a pin 12 which is press-fitted and fixed to 
the shaft 4. Thus, the auxiliary gear 8 can be rotated together with the 
shaft 4 and can be guided along the slit 15 to be moved in the thrust 
direction. As shown in FIG. 2, teeth 17 and 18 of the main gear 7 and the 
auxiliary gear 8 are staggered from each other by a distance or stagger S 
when the opposite end faces of the main gear 7 and the auxiliary gear 8 
come in contact with each other. Such stagger S contributes to causing a 
thrust force to be applied to the auxiliary gear 8 so as to move the gear 
toward the housing 3 when the screw gear 2 on the drive side is engaged 
with the main gear 7 and the auxiliary gear 8. 
A compression spring 10 and a sliding plate 9 mounted on the shaft 4 are 
arranged between the auxiliary gear 8 and the housing end face 3a. The 
sliding plate 9 comes in indirect contact with the housing end face 3a by 
the resilient force of the compression spring 10 through the plain bearing 
14. The sliding plate 9 is formed integral with a sleeve 9a which has a 
slit 11 extending in the thrust direction. This slit 11 is positioned to 
align with the above slit 15 in the auxiliary gear 8 and to engage with 
the pin 12 so that the sliding plate 9 can rotate with the shaft 4 to 
cause sliding resistance of rotation. 
An operation of the drive mechanism in the first embodiment of the present 
invention will next be described. When the screw gear 2 on the crank shaft 
1 is engaged with the screw gear 6 on the shaft 4 which comprises the main 
gear 7 and the auxiliary gear 8, the auxiliary gear 8 on the shaft 4 is 
pushed against the resilient force of the compression spring 10 in a 
direction in which the stagger between the main gear 7 and the auxiliary 
gear 8 is reduced. This direction is a thrust direction directed toward 
the housing 3. Thus, the resilient force of the compression spring 10 is 
constantly applied to the auxiliary gear 8. Accordingly, one tooth face of 
the auxiliary gear 8 which is opposed to a face of the main gear 7 coming 
in contact with the screw gear 2 comes in contact with the screw gear 2 on 
the crank shaft 1. 
As a result, the auxiliary gear 8 accommodates and removes a backlash of 
the screw gears 2 and 6. 
When the auxiliary gear 8 is pushed by the screw gear 2 of the drive shaft 
as mentioned above, the resilient force of the compression spring 10 is 
applied toward the housing end face 3a to push the sliding plate 9 toward 
the plain bearing 14, thereby causing a contact face pressure of 
therebetween. Thus, frictional force of rotation is generated due to 
friction coefficients of contact portions of the sliding plate 9 and the 
plain bearing 14 when the shaft 4 is rotated. Accordingly, when the 
sliding plate 9 is rotated together with the screw gear 6 on the driven 
shaft, sliding resistance is produced which can reduce the rotational 
inertia of the driven shaft 4 and the screw gear 6 as a whole. 
In this first embodiment, the auxiliary gear 8, the spring 10 and the 
sliding plate 9 are mechanically associated with one another in a manner 
to provide two functions in which a frictional force of rotation is 
generated on the side of the driven shaft 4 and a backlash of the screw 
gears 2 and 6 is removed. Thus such reasonable and simplified mechanism 
can adequately mitigate a collision between the screw gears caused by the 
backlash. Each of a sliding resistance and a pressure on teeth franks 
required to mitigate this collision can be reduced by half in comparison 
with a conventional single function of each of a backlash removing system, 
a sliding resistance system, etc. In other words, if the sliding 
resistance and the pressure on teeth franks are equal to those in the 
conventional case, the drive mechanism can accommodate two times an 
angular acceleration or rotational inertia of a rotator in the 
conventional case. In the first embodiment, the spring or resilient member 
10 can serve as removing the backlash and generating the sliding 
resistance, so that the number of parts can be reduced and the drive 
mechanism can be made reasonable and simplified. 
A drive mechanism of an engine ignition system in accordance with a second 
embodiment of the present invention will next be described with reference 
to FIG. 4. In FIG. 4, the same reference numerals as in the first 
embodiment respectively designate the same or common constructional 
elements. 
The second embodiment is different from the first embodiment in a binding 
structure of a sliding plate 29, a main gear 27 and an auxiliary gear 28. 
Namely, in the second embodiment, guide pin receiving grooves 20 and 21 
respectively extend along inner peripheries of the main gear 27 and the 
auxiliary gear 28 on the shaft 4 in a thrust direction. A guide pin 22 is 
placed to connect the guide pin receiving grooves 20 and 21 to each other. 
Thus auxiliary gear 28 is engaged with the main gear 27 to enable rotating 
with the main gear 27 and the shaft 4 and moving on the shaft 4 in the 
thrust direction. A sleeve 29a is formed integral with the sliding plate 
29 and is adapted to be fitted on the shaft 4 with a compression spring 10 
arranged about the outer periphery thereof. A projection 29b is formed on 
the sleeve 29a and is adapted to be engaged with a slit 25 formed in a 
sleeve 28A of the auxiliary gear 28. The other constructional portions are 
similar to those in the first embodiment. 
Like the first embodiment, the auxiliary gear 28 in this second embodiment 
accommodates the backlash of the screw gear 2 on the drive shaft 1 and the 
sliding plate 29 produces sliding resistance. Accordingly, effects similar 
to those in the first embodiment can be obtained in this second 
embodiment. Further, it is possible in the second embodiment to dispense 
with the processing work of an attachment hole for attaching the guide pin 
12 to the shaft 4 in the first embodiment and a press fitting work of the 
guide pin, so that the drive mechanism can be simply assembled. 
As mentioned above, in accordance with the present invention, the auxiliary 
gear, resilient member and the sliding plate are mechanically associated 
with one another to provide a drive mechanism of an engine ignition system 
which is reasonable and simplified to enable accommodating an angular 
acceleration and a rotational inertia greater than those in the 
conventional backlash removing means without losing durability of the 
drive mechanism.