Starting electric motor

Abstract: In a starting electric motor (20) comprising: a movable pinion cylinder (22) slidably mounted on the extension (3) of the output shaft of the motor (20), the movable pinion cylinder (22) having the front end portion formed into a pinion part (22a), the rear end portion formed into a clutch inner part (7b) of an overrunning clutch (7), and the middle portion formed into a slidably supporting surface (22b) held by a first bearing (25) fitted in the front frame (1), the shaft extension (3) being supported by the front frame (1) through the movable pinion cylinder (22), the movable pinion clyinder (22) is slidably and rotatably supported on the shaft extension (3) by a second bearing (27) fixedly fitted in an inner cylindrical wall which is extended in the movable pinion cylinder (22) from the clutch inner part (7b) to the middle portion (22b) with a gap (26) relatively large between the inner cylindrical wall and the shaft extension (3).

Description: BACKGROUND OF THE INVENTION (Field of the Invention) This invention relates to a starting electric motor according to the preamble part of claim 1, and more particularly to an overhang type starting electric motor for starting the engine of a vehicle. (Prior Art) Such an over-hang type starting electric motor for starting the engine of a vehicle is well known in the art, as disclosed for instance by Japanese Utility Model Application Publication No. 6679/1986. The specific feature of the structure of an overhang type starting electric motor is, in general, as follows: As is apparent from FIGS. 1 and 2 showing the over-hang type starting electric motor disclosed by the aforementioned Japanese Utility Model Application Publication No. 6679/1986, a front frame 1 has an opening 2, through which an extension 3 of an armature rotary shaft of a DC motor (not shown) provided inside the frame is extended outwardly, and a movable pinion cylinder 4 is fitted through bearings 5 on the extension 3 in such a manner that the pinion cylinder 4 is movable axially and is rotatable around the extension 3. The pinion cylinder 4 is rotatably and slidably supported by a bearing 6 arranged inside the frame 1 and near the opening 2, thus supporting the extension 3 of the armature rotary shaft (hereinafter referred to as "a shaft extension 3", when applicable). In the starting electric motor, the outer wall of the front end portion (on the side of the opening in the frame) of the pinion cylinder 4 is formed into a pinion 4a, and the rear end portion is formed into a clutch inner 7a which is a part of an overrunning clutch device 7. The outer wall of the middle part of the movable pinion cylinder 4, which is located between the pinion 4a and the clutch inner 7a is a supporting surface 4b in slide contact with the bearing 6 so that the movable pinion cylinder 4 is supported while sliding on the bearing 6. In FIGS. 1 and 2, reference character 7b designates a clutch outer of the overrunning clutch device 7; 7c, rollers for transmitting torque from the clutch outer 7b to the clutch inner 7a; 8, an oil seal; 9, a dust protective cap detachably mounted on the front end of the movable pinion cylinder 4; 10, a stopper secured to the end of the shaft extension 3 to prevent the pinion cylinder 4 from coming off the shaft extension 3; and 11, a ring gear of the engine. The operation of the starting electric motor thus constructed will be briefly described. When the overrunning clutch device 7 is moved in the forward direction (or in the direction of the arrow 12) by a shift lever (not shown), the movable pinion cylinder 4 is slide on the shaft extension 3, as a result of which, as shown in FIG. 2, the pinion 4a comes outside the frame 1 through the opening 2 to engage with the ring gear 11. A power switch for the DC motor is turned on immediately before the pinion 4a engage with the ring gear 11. As a result, torque is transmitted from the armature rotary shaft through the clutch outer 7b of the overrunning clutch device 7 and the rollers 7c to the clutch inner 7a, so that the pinion cylinder 4 is rotated. The rotation of the pinion cylinder 4 is transmitted through the pinion 4a and the ring gear 11 to the engine to start the latter. After the engine is started in this way, the movable pinion cylinder 4 may be rotated at high speed by the engine before returning to the original position. In this case, the clutch inner 7a of the overrunning clutch device 7 is rotated at higher speed than the clutch outer 7b. Therefore, the function of transmitting torque in one way of the overrunning clutch device 7 is exercised so that the rollers 7c are disengaged from both of the clutch inner 7a and the clutch outer 7b. As a result, the high speed rotation of the pinion cylinder 4 will not transmitted to the DC motor. On the other hand, in the conventional starting electric motor, both end portions of the movable pinion cylinder 4 are used as the clutch inner 7a and the pinion 4a, and the middle portion is employed as the slidably supporting surface 4b, and therefore the pinion cylinder 4 is relatively long. In order to allow the long pinion cylinder 4 to stably slide on the shaft extension 3 and to sufficiently support the shaft extension 3, two bearings 5 are positioned on the inner cylindrical wall of the pinion cylinder 4 near both ends. Recently, in order to increase the torque of the ring gear 11, it is intended to make the pinion 4a smaller in the art. If the tooth form of the pinion 4a is made smaller or the number of teeth is reduced, then the thickness m of the teeth base of the pinion 4a is reduced to less than the necessary value, because the bearing 5 is disposed on the inner cylindrical wall of the pinion 4a. Therefore, the pinion 4a may be damaged. Furthermore, in the conventional starting electric motor, the grease applied for the bearings provided on the inner cylindrical wall of the movable pinion cylinder 4 is removed by the reciprocation of the movable pinion cylinder, and the service lives of the bearings are shortened as much. Furthermore, in the conventional starting electric motor, the cap 9 is put on the front end of the movable pinion cylinder 4 as shown in FIG. 1 to prevent the entrance of water or dust, along the cylindrical surface of the shaft extension 3, into the movable pinion cylinder 4. However, the cap 9 secured to the end of the movable pinion may be damaged by the centrifugal force, or it may be damaged or broken being struck against the ring gear 11 of the engine when the starting electric motor is engaged with or disengaged from the engine. On the other hand, the length of the cap 9 is such that the cap 9 surrounds the front end portion of the shaft extension 3 when the movable pinion cylinder 4 is at the rearmost position as shown in FIG. 1. Therefore, when the movable pinion cylinder 4 is moved to the foremost position as shown in FIG. 2, the total length of the starting electric motor becomes considerably long, which lowers the efficiency of installation of the starting electric motor on the engine. On the other hand, since one of the bearings 5 is positioned in the pinion cylinder 4 at the front end, the distance L between the bearing 5 and the cut-up part 3a of the helical spline of the shaft extension 3 is remarkably long when the pinion cylinder 4 slides. Therefore, a great bending moment is applied to the cut-up part 3a, so that it may be broken. Furthermore, because the front bearing is positioned on the inner cylindrical surface of the pinion 4a which is the front end portion of the movable pinion cylinder 4, the thickness of the teeth base of the pinion is limited, and accordingly the tooth form of the pinion or the number of teeth is limited. Furthermore, EP-A-0 115 852 discloses a an electric starter with only one bearing to support a short pinion cylinder onto the shaft extension SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to provide a starting electric motor in which the tooth form of the pinion is made smaller or the number of heath is reduced without lowering the strength of the pinion of the movable pinion cylinder. The solution of this object is achieved by the features claim 1. The starting electric motor according to the present invention is able to prevent the grease applied to the inner cylindrical wall of the movable pinion cylinder from being removed. Moreover, although it is possible to provide dust proctive means the length of the starting electrical motor according to the present invention is not increased. Furthermore, if a bending moment is applied to the extension of the armature rotory shaft, said bending moment can be made small to prevent the extension from being broken. Furthermore, as a specific advantage the excentricity of the overrunning clutch device of the clutch inner can be prevented because bearing means are fitted in the inner periphery of the clutch inner part at the rear end such that as a result the durability of the clutch device is improved. In the starting electric motor according to the invention, the movable pinion cylinder is sufficiently slidably and rotatably supported by the second bearing fitted in the relatively large gap which is formed between the shaft extension and the first inner cylindrical wall extended from the clutch inner part to at least a part of the middle part, while the second inner cylindrical wall extended in the pinion part is made smaller in diameter than the first inner cylindrical wall so that the clearance formed between the second inner cylindrical wall and the shaft extension is smaller than the relatively large gap mentioned above. Therefore, even if the tooth form of the pinion is made smaller or the number of teeth thereof is reduced, the thickness of the teeth base can be made sufficiently large. And the minute clearance between the second inner cylindrical wall and the shaft extension prevents the removal of the applied grease which may otherwise be caused. The seal member fitted in the relatively large gap prevents the entrance of water or dust through the small gap. In the starting electric motor of the invention, the movable pinion cylinder is slidably and rotatably supported by one bearing fixedly fitted in the inner cylindrical surface large in diameter which is extended in the movable pinion cylinder from the clutch inner part to at least a part of the middle portion, whereby, when the movable pinion cylinder is moved, the distance between the bearing and the cut up part of the helical spline of the shaft extension is shortened, and accordingly the bending moment applied to the cut-up part through the bearing by the movable pinion cylinder is reduced. The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view showing essential components of a conventional starting electric motor; FIG. 2 is a sectional view showing a conventional starting electric motor in which a movable pinion cylinder is moved to cause a pinion to engage with a ring gear of an engine; FIG. 3 is a sectional view showing starting electric motor according to an embodiment of this invention; FIG. 4 is a sectional view showing a starting electric motor according to another embodiment of this invention in which a seal member 29 is provided; and FIG. 5 is a sectional view used for explaining a starting electric motor according to this invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One example of a starting electric motor according to the invention is shown in FIG. 3, in which those components which have been described with reference to FIGS. 1 and 2 are therefore designated by the same reference numerals or characters. The starting electric motor 20 includes a DC motor 21 whose armature rotary shaft 21a has a shaft extension 3 which is extended in the forward direction (to the right in FIG. 3). More specifically, the shaft extension 3 is extended out of a front frame 1 through a opening 2 formed therein. An overrunning clutch device 7 is mounted on the shaft extension 3 in such a manner that it is located next to the DC motor 21. The overrunning clutch device 7 is a conventional one. Its cylindrical part 7d integral with a clutch outer 7a is fitted on the shaft extension 3 in such a manner that it is slidable back and forth while engaging with a spline formed on the shaft extension 3. The rear end portion of a movable pinion cylinder 22, which is slidably mounted on the shaft extension 3, is formed into a clutch inner 7b. Therefore, the cylindrical part 7d, the overrunning clutch device 7, and the movable pinion cylinder 22 can be slidably moved, as one unit, on the shaft extension 3. In this operation, the force of moving these components axially (hereinafter referred to as "an axially acting force", when applicable) is provided through a shift lever 24 having one end engaged with a plunger 23a of an electro-magnetic switch device 23 mounted on the frame 1 and the other end in the form of a two-prong fork engaged with the cylindrical part 7d in such a manner that the cylindrical part 7d is held between the two prongs. As was described above, the rear end portion of the movable pinion cylinder 22 is converted into the clutch inner 7b. Therefore, for convenience in description, the rear end portion will be referred to as "a clutch inner part", when applicable. On the other hand, a pinion 22a is formed on the outer cylindrical wall of the front end portion of the movable pinion cylinder 22, and therefore the front end portion will be referred to as "a pinion part", when applicable. The middle part between the pinion part and the clutch inner part has a slidably supporting surface 22b. The slidably supporting surface 22b is fitted in the inner lace of a ball bearing 25 fixedly fitted in the front frame 1, inside the opening 2 formed in the front frame so that the movable pinion cylinder 22 is slidably supported. The inner cylindrical wall of the major part of the movable pinion cylinder 22, from the clutch inner part to the middle part having the slidably supporting surface 22, is made larger in diameter than that of the remaining part so as to provide a relatively large gap 26 between the inner cylindrical wall and the cylindrical surface of the shaft extension 3. A bearing 27 is provided in the gap 26 to support the shaft extension 3 through the movable pinion cylinder. The bearing 27 singly provided is positioned essentially on the inner cylindrical wall of the clutch inner part of the movable pinion cylinder 22 so that it together with the movable pinion cylinder 22 is slidably movable on the shaft extension 3. On the other hand, the inner cylindrical wall of the remaining minor part of the movable pinion cylinder 22, from the pinion part to the small part of the middle part having the slidably supporting surface 22b, is made smaller in diameter than that of the major part thereof so as to provide a minute clearance or small gap 28 between the inner cylindrical wall of the minor part of the movable pinion cylinder 22 and the cylindrical surface of the shaft extension 3. The minute distance 28 is formed by supporting the movable pinion cylinder 22 on the shaft extension 3 through the bearing 27 which is fitted in the relatively large gap 26 provided inside the rear end portion of the movable pinion cylinder 22. Therefore, even if the tooth form of the pinion is made smaller, or the number of teeth is reduced, the thickness of the teeth base can be made sufficiently large. As the minute clearance 28 is provided inside the pinion part of the movable pinion cylinder, the labyrinth dust protective effect is obtained. This effect in association with the use of a dust protective cap 9 similar to that in the conventional starting electric motor provides an excellent dust protective effect. Furthermore, the provision of the minute clearance 28 sufficiently prevents the removal of the applied grease which may otherwise be caused by the sliding motion of the bearing 27. In order to prevent the entrance of water or dust through the minute clearance 28, the starting electric motor shown in FIG. 4 is provided with a seal member 29 disposed in the large gap 26 at the front end. The seal member 29 is preferably an oil seal. The position of the seal member 29 relative to the bearing 27 fitted in the gap 26 should be taken into consideration. This will be described in more detail. As was described before, when the overrunning clutch device 7 slides on the shaft extension 3 in the forward direction from its rear rest position shown in FIG. 4, the bearing 27, together with the pinion cylinder 22, is moved to its front stop position indicated at 30, where it is rotated. In this operation, the seal member 29 is also moved forwardly together with the movable pinion cylinder 22. The bearing 27 is slid from the rear rest position to the front stop position, where it is turned, and therefore the cylindrical surface of the shaft extension 3 between the two positions is liable to become relatively rough. Therefore, if the rear rest position of the seal member 29 is in the range of slide of the bearing 29, then the sealing effect is greatly lowered by the rough surface of the shaft extension 3. Accordingly, it is preferable that the rear rest position of the seal member 29 is out of the range of side of the bearing 27. Further, in the starting electric motor 20 thus constructed, when the pinion cylinder 22 is slid in the forward direction as shown in FIG. 5, the bearing 27 supporting the movable pinion cylinder 22 on the shaft extension 3 is also slid. Since, in this case, the bearing 27 is arranged in the movable pinion cylinder 22 at the rear end, the distance ℓ between the bearing 27 thus slid and a cut-up part 3a of a helical spline 3b of the shaft extension 3 is much shorter than that in the case of the conventional starting electric motor in which the bearing is located at the foremost position. Accordingly, in the starting electric motor of the invention, the bending moment applied to the cut-up part 3a of the helical spline 3b is small. Furthermore, since the bearing 27 is fitted essentially in the inner cylindrical wall of the clutch inner part 7b; i.e., the rear end portion of the movable pinion cylinder as was described before, the clutch inner part 7b is scarcely off-centered in the overrunning clutch device 7, with the result that the overrunning clutch device is improved in durability. Furthermore, in the starting electric motor 20 of the invention, the inner cylindrical wall of the rear end portion of the movable pinion cylinder is made larger in diameter than that of the front end portion, and the bearing 27 is fitted in the large inner cylindrical wall to support the shaft extension; that is, the inner cylindrical wall of the pinion part 22 is smaller in diameter to provide the minute clearance 28 between the inner cylindrical wall and the extension shaft. Therefore, even if the tooth form of the pinion 22a is made smaller, or the number of teeth is reduced, the thickness of the teeth base can be sufficiently large; that is, the pinion formed is high in mechanical strength. As was described above, in the starting electric motor of the invention, the inner cylindrical wall of the major part of the movable pinion cylinder, from the clutch inner part to the middle part having the slidably supporting surface, is made larger in diameter than that of the remaining minor part, so as to provide the relatively large gap between the inner cylindrical wall thereof and the cylindrical surface of the shaft extension, and with the aid of only one bearing arranged in the relatively large gap, the movable pinion cylinder is supported by and stably slid on the shaft extension. On the other hand, the inner cylindrical wall of the pinion part of the movable pinion cylinder is made smaller in diameter than that of the major part thereof so as to provide the minute clearance between the inner cylindrical wall thereof and the shaft extension. Therefore, even if the tooth form of the pinion is made smaller, or the number of teeth is reduced, the thickness of the teeth base can be made sufficiently large, with the result that the pinion will not be damaged or broken. The small clearance inside the front end portion of the movable pinion cylinder provides the labyrinth dust protective effect, and can positively prevent the removal of the applied grease which may otherwise be caused. Furthermore, in the starting electric motor of the invention, the seal member is fitted in the gap in which the bearing is arranged, thus preventing the entrance of water or dust through the small gap between the extension shaft and the pinion part. Therefore, in the starting electric motor of the invention, unlike the conventional one, it is unnecessary to use the dust protective cap, and the total length of the starting electric motor is reduced as much. Thus, the starting electric motor can be readily installed on the engine. As was described above, in the starting electric motor of the invention, the inner cylindrical wall of the major part of the movable pinion cylinder, from the clutch inner part to the middle part, is made larger in diameter, and only one bearing is fitted in the inner cylindrical wall, whereby the bending moment applied to the cut-up part of the helical spline of the shaft extension is considerably small. This will permit a reduction of the diameter of the shaft extension. Therefore, the thickness of the teeth base of the pinion can be increased; that is, the mechanical strength thereof can be increased.

Claims: A starting electric motor of an overhung type, comprising: an electric motor (21) for generating torque to start an engine, said electric motor (21) having an armature rotory shaft (21a) with a shaft extension (3) thereof; an overruning clutch device (7) having a clutch inner part (7b); a front frame (1) of said electric motor (21); a first bearing (25) fitted in said front frame (1); a movable pinion cylinder (22) with an inner periphery thereof slidably mounted on said shaft extension (3), which includes a pinion part (22a) at a front end thereof, said clutch inner part (7b) at a rear end thereof, and a middle portion (22b) between said pinion part (22a) and said clutch inner part (7b) with a slidably supporting surface (22b) held by said first bearing (25), said shaft extension (3) being supported by said front frame (1) through said movable pinion cylinder (22); and second bearing (27) slidably mounting said movable pinion cylinder means (22) on said shaft extension (3); being characterized in that a relatively large gap (26) is provided between said inner periphery of said movable pinion cylinder (22) and said shaft extension (3) which extends over said clutch inner part (7b) and at least a part of said middle pinion cylinder (22b); that said second bearing (27) is fixedly fitted in said relatively large gap (26) in which said movable pinion cylinder (22) is slidably and rotatably supported on said shaft extension (3) by only said second bearing (27) and that a minute clearance (28) smaller than said relatively large gap (27) is provided between the inner periphery of the front pinion-part (22a)-portion of the movable pinion cylinder (22) and said shaft extension (3). A starting electric motor as claimed in claim 1, further comprising a seal member (29) arranged in said relatively large gap (26) in front of said second bearing means (27). A starting electric motor as claimed in claim 2, wherein a range in position where said seal member (29) is slidably moved is out of a range in position where said second bearing (27) is slidably moved. A starting electric motor as claimed in claim 2, wherein said seal member (29) is formed by an oil seal. A starting electric motor as claimed in claim 3, wherein said seal member (29) is formed by an oil seal. A starting electric motor as claimed in claim 1, wherein at least a part of said second bearing (27) is positioned in said inner periphery of said clutch inner part (7b) provided at the rear end of said movable pinion cylinder (22).

Inventors: ISOZUMI SHUZOO, MORISHITA AKIRA, TANAKA TOSHINORI, ISOZUMI, SHUZOO, MORISHITA, AKIRA, TANAKA, TOSHINORI
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1988
Main classification: H02K7
IPC codes: F02N15