STARTING ELECTRIC MOTOR

Abstract: A starting electric motor comprising: an armature rotary shaft (1a) on which an armature (1) is mounted; a commutator (4) mounted on the rear end portion of the armature rotary shaft (1a); power supplying brushes (5) slidably held on the commutator (4); brush holders (16a; 16b) which hold the brushes (5a; 5b), respectively; a rear bracket (21) of resin which rotatably supports the armature rotary shaft (1a) in association with a front bracket; a yoke (3) having magnetic field means (2) which is fixedly held between the rear bracket (21) and the front bracket; a conducting base (19) of electrically conductive material on which the brush holders (16) are mounted, and which is held between the yoke (3) and the rear bracket (21); through-bolts (14) inserted into the rear bracket (21) and the front bracket; tightening the yoke (3) and the conducting base (19) together; and metal plates (22) provided between the seats of the through-bolts (14) and the outer surface of the periphery of the rear bracket (21).

Description: This invention relates to an improvement of a starting electric motor having a resin rear bracket. A conventional starting electric motor with a resin rear bracket is shown in Fig. 1. In Fig. 1, reference numeral 1 designates the armature of a DC motor; 1a, an armature rotary shaft; 1b, armature coils; 2, magnets forming magnetic fields; 3, a yoke forming magnetic paths and serving as an outer wall; 4, a commutator having segments in the outer periphery; 5a, a positive brush for rectification; 5b, a negative brush for rectification; 6a, a brush holder holding the positive brush 5a; 7, a lead wire connected between the positive brush 5a and a terminal section of an electromagnetic switch (not shown); 7a, a rubber member insulating the lead wire 7; 8, an insulating plate; and 9, a metal base to which the insulating plate is secured, the metal base 9 being engaged with a rear bracket 10 of resin with bolts 11. Further in Fig. 1, reference character 6b designates a brush holder holding the negative brush 5b; 12, a lead wire having one end connected to the brush holder 6b and the other end connected to the earth (ground) terminal of a starter (not shown); 13, a bearing fitted on the armature rotary shaft 1a and inserted into the recess of the rear bracket 10; and 14, through-bolts joining the resin bracket 10, the yoke 3, and a front bracket (not shown) together. The operation of the motor thus constructed will be described. When the electromagnetic switch (not shown) is closed, current is supplied through the lead wire 7 to the positive brush 5a and through the segment in the outer periphery of the commutator 4 to the armature coil 1b, and the current is allowed to flow through the segment in the outer periphery of the commutator, the negative brush 5b and the lead wire 12 to the earth terminal of the starter (not shown). As the current flows in the magnetic field formed by the magnets 2 in the above-described manner, the armature 1 produces torque. The conventional starting electric motor using the resin rear bracket 10 for the purpose of reducing its weight, being constructed as described above, suffers from the following problems: It is necessary to provide the lead wire 12 on the earth side (hereinafter referred to as "an earth side lead wire 12", when applicable), and the earth terminal. This will make the manufacture of the motor intricate, and the number of components increase. On the other hand, during travel of the vehicle, the motor is exposed to heat generated by the engine, and the rear end face of the rear bracket 10 locally receives the axial tensions of the through-bolts 14 and the bolts 11. As a result, the rear bracket 10 is deformed; i.e., creeped, and the through-bolts 14 and the bolts 11 are loosened. Also, since the end face of the rear bracket 10 is in the form of a plate of resin, the end face is insufficient in bend resisting strength and in compression resisting strength so that it is difficult to tightly and stably mount the rear bracket 10 on the yoke 3. An object of this invention is to eliminate the above-described difficulties accompanying a conventional starting electric motor. Another object of the invention is to provide a starting electric motor in which it is unnecessary to provide the earth side lead wire and the earth terminal which form a particular earthing circuit. A further object of this invention is to provide a starting electric motor in which the creep deformation attributed to the heat generated by the engine and to the axial tensions of the through-bolts and the brush holder mounting bolts is prevented. A still further object of this invention is to provide a starting electric motor in which the rear bracket can be suitably mounted on the yoke with the through-bolts. These objects are solved according to the invention by a starting electric motor having the features of claim 1. In a starting electric motor according to the invention, a conducting metal plate means or base, on which the brush holders are mounted, is fixedly held between the rear bracket of resin and the yoke, and a metal plate is provided between the seat of the through-bolt and the rear end face of the rear bracket. Also, an axial tension receiving means or built-up portion is formed on the rear bracket at the parts through which the through-bolt is extended in such a manner that the end of the built-up portion on the side of the yoke is abutted against the conducting base. Fig. 1 is a sectional view partially showing a conventional starting electric motor with a resin rear bracket; Fig. 2 is a sectional view partially showing a starting electric motor according to one embodiment of this invention; Fig. 3A is a front view of a metal plate employed in the starting electric motor of this invention; Fig. 3B is a perspective view showing the metal plates mounted on the rear bracket of the motor; Fig. 4 is a sectional view partially showing a starting electric motor according to another embodiment of this invention; Fig. 5A and 5B are sectional views showing a built-up portion of the rear bracket in Fig. 4, respectively; Fig. 6 is a sectional view partially showing a starting electric motor according to a further embodiment of this invention; Figs. 7 and 8 are side views showing a part of an axial tension receiving part forming a part of a rear bracket in the starting electric motor shown in Fig. 6, respectively; and Fig. 9 is a sectional view partially showing a starting electric motor according to a further embodiment of this invention. In the starting electric motor of the invention, the conducting base held between the rear bracket and the yoke provides an earth circuit, and no bolt is required for positioning the conducting base, which eliminates the difficulty that the rear bracket is creeped by such a bolt when tightened. Furthermore, the metal plates provided on the rear end face of the rear bracket eliminate the difficulty that the axial tensions of the through-bolts might otherwise act locally on the rear bracket when tightened, and the pressure is exerted on a larger area, i.e., the rear bracket is scarcely stressed. Therefore, the rear bracket is free from the creep deformation even when held at high temperature for a long period. Thus, the starting electric motor of the invention is high in reliability. On the other hand, the axial tensions of the through-bolts are exerted on the conducting base through the built-up portions of the rear bracket which surround the through-bolts completely or partially. Moreover, the metal plates disperse the axial tensions of the through-bolts. Therefore, when the axial tensions of the through-bolts are applied to the conducting base, as described above, which forms a part of the earth circuit, the built-up portions will not be bent. Thus, the rear bracket is stable when under axial tension, and is prevented from being creeped. When, in the starting electric motor of the invention, the through-bolts are tightened to secure the resin rear bracket to the rear open end of the yoke, the axial tensions thereof act through the axial tension receiving parts integral with the end wall of the rear bracket on the metal plate, and then on the yoke. The negative brush holder is mounted directly on the metal plate, thus providing an earth circuit extending from the brush holder through the metal plate to the yoke. Accordingly, it is unnecessary to connect the lead wire to the negative brush holder to ground it. The preferred embodiments of this invention will be described with reference to the figures. In Fig. 2, reference numeral 16a designates a brush holder holding a positive brush 5a; 17, a lead wire connected between the positive brush 5a and a terminal of an electro-magnetic switch (not shown); and 17a, a rubber member insulating the lead wire 17. The brush holder 16a is secured through an insulating plate 18 to a conducting metal plate means or base 19 of electrically conducting material. The conducting base 19 is fixedly interposed between a rear bracket 21 of resin and a yoke 3. Further in Fig. 2, reference character 16b designates a brush holder holding a negative brush 5b, the brush holder 16b being secured directly to the conducting base 19; 13, a bearing mounted on an armature rotary shaft 1a and fitted in the recess of the rear bracket 21; 14, through-bolts which are inserted through metal plates 22 into the rear bracket 21, the conducting base 19, the yoke 3, and a front bracket (not shown) to securely join these components together; and 20, a spacer of electrical conductive material, the spacer 20 being interposed between the conducting base 19 and the negative brush holder 16b. Each of the metal plates 22 is shown in Fig. 3A. The metal plates 22 are engaged with ribs 21a of the rear bracket 21 so that they may not be displaced when the through-bolts 14 are tightened, as shown in Fig. 3B. It is preferable that the metal plates 22 are formed by using a plate such as a quenched band steel plate so that they may not be deformed by the axial tensions of the through-bolts 14 when tightened. In the starting electric motor thus constructed, the conducting base 19 is securely held between the rear bracket 21 and the yoke 3, as a result of which the negative brush 5b has an earth circuit extending as follows: the negative brush 5b -- the spacer 20 -- the conducting base 19 -- the yoke 3. Thus, in the starting electric motor of the invention, unlike the conventional one, it is unnecessary to provide the earth side lead wire and to use the bolts or screws for fixing the conducting base 19. Accordingly, the starting electric motor of the invention is free from the difficulty accompanying the conventional starting electric motor that the rear bracket 21 is deformed by the bolts adapted to securely fix the brush holder mounting base. Furthermore, in the motor of the invention, the metal plates 22 are provided between the rear end face of the rear bracket 21 and the seats of the bolts 14, and therefore the tightening forces, in the axial direction, of the through-bolts 14 will not be locally applied to the rear end face of the rear bracket 21, and the rear bracket 21 will not be creeped even if it is exposed to high temperatures for a long period of time. Furthermore, in the motor of the invention, the ribs 21a are formed on the rear end face of the rear bracket 21, while the metal plates 22 have slits which are engageable with the ribs 21a. Therefore, the metal plates 22 will never be displaced during the tightening of the through-bolts 14. That is, the motor can be assembled easily. In the above-described embodiment, the permanent magnets 2 are used for generation of magnetic fields; however, coils wound on iron cores may be used instead of the permanent magnets. In another embodiment shown in Fig 4, axial tension receiving means or built-up portions 21b of the rear bracket 21 are so formed as to cover the through-bolts 14. The end faces of the built-up portions 21b are abutted against the conducting base 19. Fig. 5A and Fig. 5B are sectional views showing the built-up portion 21 as viewed from the yoke 3. The built-up portion 21b shown in Fig. 5A is so formed that it completely surrounds the through-bolt 14, and the built-up portion 21b in Fig. 5B is so formed that it partially surrounds the through-bolt 14. In the starting electric motor of the invention as described above, the rear bracket 21 has the built-up portions 21b through which the through-bolts 14 are extended. Each of the built-up portions 21b is so formed that it completely or partially surrounds the through-bolt 14, and its end face, on the side of the yoke 3, is abutted against the conducting base 19. Therefore, during the tightening of the through-bolts 14, the axial tensions of the through-bolts act on the rear bracket 21 to compress the latter, and the resulting pressure is exerted on the conducting base 19. Thus, the axial tensions of the through-bolts 14 acting on the rear bracket are further dispersed; that is, the creep resisting strength and the mechanical strength of the rear bracket 21 is increased as much. In this embodiment, the metal plates 22 may be included in the rear bracket 21 when the latter is molded. This method will result in a reduction of the number of components and in a reduction of the number of assembling steps. In a starting electric motor shown in Fig. 6, the resin rear bracket 21 of a cup-shape has, as shown in Fig. 7, at least two cylindrical axial tension receiving parts 28 which are formed integral with the rear bracket 21 in such a manner that the axial tension receiving parts 28 extend along a cylindrical wall 21c of the rear bracket in the axial direction. The front end faces of the axial tension receiving parts 28 are flat surfaces which are flush with the open end of the cylindrical wall 21c, and the rear ends thereof merge with an end wall 21d of the rear bracket 21. A bolt insert hole 23 is formed in each of the axial tension receiving parts 28 at the center in such a manner that it penetrates the axial tension receiving parts 28 from the end wall 21d. The cylindrical wall 21c of the rear bracket 21 has an in-low part 25a at the open end thereof. The in-low part 25a is engaged with a mating in-low part 25b provided at the rear open end of the yoke 3, when the rear bracket 21 is engaged with yoke 3. In this engaging operation, a disk-shaped metal plate 26 is interposed between the in-low parts 25a and 25b. The disk-shaped metal plate 26 has an opening 26a at the center into which the commutator 4 is inserted. As the metal plate 26 is held between the engaging parts of the rear bracket 21 and the yoke 3 in this manner, the front end faces of the axial tension receiving parts 28 of the rear bracket 21 is brought into close contact with the metal plate 26. The brush holders 16a and 16b are mounted on the metal plate 26. In this connection, it should be noted that the positive brush holder 16a is mounted through an insulating member 18 on the metal plate 26. That is, for the negative brush 5b, an earth circuit extending from the negative brush holder 16b through the metal plate 26 to the yoke is provided. Thus, in the DC motor, unlike the conventional one, it is unnecessary to connect the negative brush holder to the earth terminal with the lead wire. In the starting electric motor, the resin rear bracket 21 is engaged with the rear open end of the yoke 3 through the metal plate 26 on which the brush holders 16a and 16b are mounted. Under this condition, the front threaded parts of the through-bolts 14, which are inserted into the end wall 21d of the rear bracket 21, are inserted into a front bracket (not shown) and tightened with nuts, so that the rear bracket is securely fixed to the yoke. In this operation, the axial tensions of the through-bolts 14 act through the axial tension receiving parts 28 on the metal plate 26 and then on the yoke 3. Accordingly, the rear bracket 21 is never deformed or bent by the axial tensions of the through-bolts 14 so that the rear bracket 21 is rigidly and tightly secured to the yoke 3. In order to make the axial tension receiving parts 28 strong enough to withstand the axial tensions, it is preferable that the outside diameter D of the axial tension receiving parts 28 is larger than the diameter d of the seats of the heads of the through-bolts 14. In the starting electric motor as described above, the axial tension receiving parts 28 are in the form of a cylinder. The axial tension receiving parts may be modified as shown in Fig. 8; that is, axial tension receiving parts 29 C-shaped in section may be employed, providing the same effect. Furthermore, in the above-described starting electric motor, the metal plate 26 is not a part of the yoke 3. Therefore, if a part of the yoke is bent to provide the metal plate 3a as shown in Fig. 9, then the number of components can be decreased as much. As was described above, in the starting electric motor of the invention, with the brush-holder mounting conducting base held between the rear bracket and the yoke, and with the metal plates provided on the rear end face of the rear bracket, the metal plates, the rear bracket, the conducting base, and the yoke are tightened against the front bracket with the through-bolts, with the result that the current path on the earth side extending from the conducting base to the yoke is formed. Therefore, in the motor of the invention, it is unnecessary to provide the particular earth circuit. Furthermore, the metal plates provided on the rear end face of the rear bracket decentralize the axial tensions of the through-bolts which, in the conventional starting electric motor, are centralized at the rear bracket; that is, the stress on the rear bracket is eased. Therefore, the rear bracket will never creep even if it is exposed to high temperatures for a long period. Thus, the starting electric motor provided by the invention is high in reliability. Also, according to the invention, the rear bracket has the built-up portions through which the through-bolts are extended, and the built-up portions are abutted against the conducting base. Therefore, the axial tensions of the through-bolts act on the rear bracket to compress the latter, and the resultant pressure is exerted on the conducting base. Furthermore, the metal plates will disperse the axial tensions, and prevent the parts of the rear end face of the rear bracket which are adjacent to the through-bolts from being bent. Thus, the invention can provide a starting electric motor having a resin rear bracket which is improved in mechanical strength and in creep resistance. Furthermore, according to the invention, the rear bracket of the DC motor is made of resin, and has the axial tension receiving parts, and the metal plate is interposed between the rear bracket and the yoke. Therefore, when the rear bracket is secured to the yoke with the through-bolts, the axial tensions of the through-bolts act directly on the yoke, and therefore the end wall of the rear bracket is never deformed, or bent, by the compression; that is, the rear bracket is suitably secured to the yoke. Furthermore, in the starter device of the invention, the brush holders are mounted on the metal plate which is in contact with the yoke. Therefore, it is unnecessary to connect the lead wire to the negative brush holder to provide an earth circuit for the negative brush. This will contribute to a reduction of the number of components, and to simplification of the starter device manufacturing work.

Claims: Starting electric motor, comprising: an electric motor having a commutator (4), an armature (1), brush means (5) slidably held on the commutator (4), and brush holder means (16) for holding the brush means (5); yoke means (3) forming a magnetic circuit and a housing of the electric motor, the yoke means (3) having a rear open end thereof and a cylindrical wall; through-bolt means (14) having a head with a seat; and a rear bracket (21) made of resin having a cup-like shape and being engaged with the rear open end of the yoke means (3) and secured to the yoke means (3) by means of the through-bolt means (14); wherein the electric motor further comprises conducting metal plate means (19; 26) held between the rear open end of the yoke means (3) and the rear bracket (21), the conducting metal plate means (19; 26) having a through-hole (26a) at the center thereof into which the commutator (4) is inserted and the brush holder means (16) being mounted on the conducting metal plate means (19; 26); and the rear bracket (21) is provided with axial tension receiving means (21b; 28; 29) formed as built-up portions integrally with the rear bracket (21) along the inside of the cylindrical wall (21c) thereof, the axial tension receiving means (21b; 28; 29) surrounding at least a part of the through-bolt means (14) in the circumferential direction thereof and having one end which is flush with the end face of the cylindrical wall (21c) for contacting the conducting metal plate means (26), the other end merging with the end wall (21d) of the rear bracket (21). Starting electric motor according to claim 1, characterized in that the conducting metal plate means (26) is integrally formed with the yoke means (3). Starting electric motor according to claim 1 or 2, characterized in that the axial tension receiving means (21b; 28; 29) has a diameter (D) which is greater than the seat diameter (d) of the head of the through-bolt means (14). Starting electric motor according to one of the preceding claims, characterized in that the electric motor further comprises an armature rotary shaft means (1a) upon which the armature (1) is mounted, the commutator (4) being mounted on the rear end portion of the armature rotary shaft means (1a). Starting electric motor according to claim 4, characterized by further comprising a front bracket, wherein the rear bracket (21) rotatably supports the armature rotary shaft means (1a) in association with the front bracket. Starting electric motor according to one of the preceding claims, characterized in that the yoke means (3) includes magnetic field means (2). Starting electric motor according to one of the preceding claims, characterized by at least one metal plate (22) being provided between the seat of the through-bold means (14) and the outer surface of the rear bracket (21). Starting electric motor according to claim 7, characterized in that the rear bracket (21) is provided with ribs (21a) at the rear end thereof, the metal plate (22) being engaged with the ribs (21a) for preventing the displacement of the metal plate (22). Starting electric motor according to claim 7 or 8, characterized in that the metal plate (22) is formed of quenched band steel. Starting electric motor according to claim 7, 8 or 9, characterized in that the metal plate (22) is integrally molded with the rear bracket (21).

Inventors: ISOZUMI SHUZOO C O MITSUBISHI, ISOZUMI, SHUZOO C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1988
Main classification: H02K5
IPC codes: H02K5, F02N11