Patent Publication Number: US-6662446-B2

Title: Method for manufacturing a torque converter

Description:
This application is a divisional application of Ser. No. 09/971,665, now U.S. Pat. No. 6,594,895 filed Oct. 9, 2001, which is a divisional application of Ser. No. 09/429,170, now U.S. Pat. No. 6,474,062filed Oct. 28, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a torque converter used for an automatic transmission mechanism of a vehicle and, more particularly, relates to components constituting an outer hull of a torque converter. 
     The torque converter of an automobile is equipped with a lock-up clutch which functions to improve fuel consumption of an engine while giving two operation modes, with one operation mode corresponding to a torque converter state and the other operation mode corresponding to a lock-up state. In the first operation mode (torque converter state), an output element is rotated with increasing torque by a reaction force of a stator while circulating a hydraulic oil to an impeller of the output element (generally a turbine runner) and a reaction element (generally a stator) by the rotation of an impeller of an input element (generally a pump impeller) driven by the engine. And in the second operation mode (lock-up state), the engine turns the output element by coupling the input and output element by fixing the lock-up clutch. 
     The Japanese Examined Patent Publication No. HEI 7-33861 of the same application discloses conventional examples of a front cover and a pump impeller, which are constructional elements of a torque converter equipped with a lock-up mechanism of the type mentioned above. 
     In such conventional examples, rotating power from the engine is transmitted to a flange portion, which performs an inertia function, and a starter gear is meshed with an annular ring gear. The flange and the annular ring gear are formed on an outer peripheral surface of the torque converter. The ring gear is meshed with a pinion of a starter motor at a starting time of the engine operation for transmitting the rotating force of the starter motor to a crank shaft of the engine. 
     FIG. 1 shows a structure of the torque converter provided with the conventional lock-up mechanism mentioned above. 
     With reference to FIG. 1, the torque converter  1  is composed of a pump impeller  3  formed by integrally coupling an impeller  2   a  to an impeller shell  2 , a turbine runner  4  and a stator  5 . The impeller shell  2  of the pump impeller  3  is integrally coupled to a front cover  6  by welding. The front cover  6  is formed on an outer peripheral surface with an annular flange portion  7 , and a drive plate  9  coupled to an engine crank shaft  8  is fastened to the flange portion  7  by a bolt. A ring gear  10  for the stator  5  is provided at an outer peripheral surface of the impeller shell  2 . 
     The detailed structure of the conventional torque converter will be further explained hereunder with reference to FIG.  1 . 
     The turbine runner  4  is mounted to a turbine hub  12  by rivets, for example, and an input shaft  13  is connected to the turbine hub  12  via a spline coupling engagement. The stator  5  is supported by a hollow fixed shaft  17  through an outer race  14 . A sprag  15  and an inner race  16  constitute a one-way clutch to be rotatable in one direction. The hollow fixed shaft  17  is fastened to a housing  19  of an oil pump  18  by bolts and is coupled to the inner race  16  via a spline engagement. 
     An oil pump driving shaft  20  in the shape of a sleeve is provided at a shaft core portion of the impeller shell  2  of the pump impeller  3 , and an inner rotor  21  of the oil pump  18  is coupled to the oil pump driving shaft  20  so as to be driven rotatably. 
     One example of such coupling structure of the inner rotor  21  to the oil pump driving shaft  20  of the impeller shell  2  is disclosed in the Japanese Utility Model Laid-open Publication No. HEI 5-34348). That is, a flat portion is formed on an outer periphery of a front (tip) end portion of the oil pump driving shaft  20 , and another flat portion is formed on an inner surface of the inner rotor  21  of the oil pump  18 , and when both flat portions are engaged, the oil pump driving shaft  20  and the inner rotor  21  of the oil pump  18  are coupled to be driven in a rotational direction. 
     Further, an outer periphery of a cylindrical portion of the oil pump driving shaft, at an approximately axial central portion of the oil pump, is rotatably supported by an inner peripheral surface of a bush  22  fixed to the housing  19  of the oil pump  18  so as to bear the driving load of the oil pump  18  at this portion. Accordingly, a driving load in proportion to a discharge pressure (line pressure) of the oil pump  18  is applied to the flat portion of the oil pump driving shaft  20  and the portion thereof supported by the bush  22 . 
     Stator collars  23   a  and  23   b  are mounted on both sides of the sprag  15  of the one-way clutch, and thrust bearings  24   a  and  24   b  are interposed between a pilot boss portion  32 , as described hereinafter, and the turbine hub  12  and between the stator collar  23   b  and the oil pump driving shaft  20 , respectively. 
     The pump impeller  3  driven by the rotation of the crank shaft  8  of the engine acts to circulate the hydraulic oil filling a torque converter chamber  25 . The hydraulic oil circulates in passages of the respective impellers of the pump impeller  3 , the turbine runner  4  and the stator  5 , whereby the turbine runner  4  is rotated while increasing the torque by the reaction force due to the operation of the stator  5 , and the rotating force is transmitted to the input shaft  13  through the turbine hub  12  and then to an automatic transmission mechanism or non-stage transmission mechanism, not shown. 
     A lock-up clutch hub  27  of a lock-up clutch  26  is fastened to the turbine hub  12  by rivets, and the lock-up clutch hub  27  is provided with a torsional damper  28  for damping a shock at a time of clutch engagement and for absorbing vibrations or noises of the driving system. 
     The lock-up clutch  26  is provided with a lock-up piston  29  at a portion between the lock-up clutch hub  27  and the front cover  6 . And the lock-up piston  29  is fitted to the turbine hub  12  in a fashion that the inner peripheral portion of the lock-up piston  29  is slidable in an axial direction of the turbine hub  12  in a liquid tight manner with respect to the outer peripheral portion of the turbine hub  12 . The inside inner peripheral portion of the torsional damper  28  is spline-engaged with the outer peripheral portion of the lock-up clutch hub  27 . Further, the lock-up piston  29  is provided with a lock-up facing  30  on a side surface portion on the side of the front cover  6 . 
     A front hydraulic chamber  31  is defined between the lock-up piston  29  and the front cover  6 . The engine crank shaft  8  is formed with a center hole into which a pilot boss portion  32  protruding from the front cover  6  is fitted so as to achieve a rotational axis alignment of the torque converter  1 . The front hydraulic chamber  31  is communicated with an oil passage  33  formed within the input shaft  13  and is then communicated with a control valve of a hydraulic controller of the automatic transmission mechanism or non-stage transmission mechanism, not shown. 
     The torque converter  1  has an oil passage between the hollow fixed shaft  17  and the oil pump driving shaft  20 . The passage is communicated with a groove formed on the side surface of the thrust bearing  24   b  disposed between the stator collar  23   b  and the oil pump driving shaft  20 . This passage is then communicated with the hydraulic controller. 
     At the lock-up time, a hydraulic pressure is applied to the turbine-side side surface of the lock-up piston  29  from the inside of the torque converter  1  through the oil passage between the stator collar  23   b  and the oil pump driving shaft  20 , and at the same time, the pressurized oil in the front hydraulic chamber  31  is drained to thereto cause a pressure difference between the front and rear portions of the lock-up piston  29  (so as to make the hydraulic pressure in the front hydraulic chamber  31  smaller than that in the torque converter). Thus, the lock-up piston  29  is pushed to the front cover  6 , and via the lock-up facing  30  the lock-up state is achieved. 
     Further, when the lock-up state is to be released, the pressurized oil is supplied into the front hydraulic chamber  31 , and the hydraulic pressure in the oil passage from the inside of the torque converter  1  is controlled by the control valve of the hydraulic controller to cause a pressure difference between the front hydraulic chamber and the torque converter (so as to make the hydraulic pressure in the front hydraulic chamber  31  larger than that in the torque converter), whereby the lock-up state is released. 
     In the torque converter  1  of the structure mentioned above, the outer hull has the front cover  6  and the impeller shell  2 . As the outer hull is generally formed by a thin steel plate having projections (protrusions), thickened portions and thinned portions may be formed partially as required. 
     The thickness may be changed if a certain strength or rigidity is required, and the outer hull portion of the torque converter  1  has a thickened flanged portion  7  for receiving inertia. The outer hull has the drive plate  9  tightened thereto by bolts, or there is provided a pilot boss portion  32  adapted to align the crank shaft with the front cover  6 . These members are independently manufactured and integrally assembled together by welding, for example, as shown in FIG.  2 . 
     The impeller shell  2  will be manufactured such as shown in FIG.  3 . That is, the ring gear  10 , the sleeve-shaped oil pump driving shaft  20  and so on, which are independently manufactured, are welded to the outer peripheral portion and the inner peripheral portion, respectively, of the impeller shell  2  which is formed of a thin iron plate. 
     A centrifugal force is generated in accordance with the increase of the engine revolution, an axial thrust force is generated due to the circulation of the hydraulic fluid through the pump impeller  3 , the turbine runner  4  and the stator  5 , in this order, and a working force of the lock-up clutch  26  is also generated. These forces are applied to the outer structural members or portions constituting the outer hull of the torque converter  1 . For this reason, it is necessary for the outer structural members, such as impeller shell  2  and the front cover  6 , to be capable of withstanding such forces and suppressing deformation thereof, such as axial swelling, within an allowable predetermined value. In this regard, for example, it is preferred that the front cover  6  is formed of an iron plate having a relatively large thickness of about 4.5 to 6 mm and the impeller shell  2  is formed of an iron plate having a thickness of about 3 to 4 mm. 
     In the prior art mentioned hereinbefore, the sleeve-shaped oil pump driving shaft  20  integrally coupled to the impeller shell  2  is formed at its rear end portion with a flat portion which is fitted to a flat portion formed on the inner peripheral portion of the inner rotor  21  of the oil pump  18 , to thereby rotate the pump. Furthermore, the cylindrical portions, other than the flat portions are also provided with high accuracy so as to achieve rotational alignment between the oil pump driving shaft  20  and the oil pump rotor, including the inner rotor  21  and outer rotor. 
     Furthermore, a hydraulic pressure of the automatic transmission mechanism (i.e. pump discharge pressure or line pressure) generated by the inner rotor  21  of the oil pump  18  is applied to a projected area of the outer periphery of the oil pump rotor, so that a load corresponding to the projected area acts on the fitting portion of the central cylindrical portion of the oil pump shaft  20  and the inner rotor  21 . The load acting on this fitting portion constitutes a moment load corresponding to a distance from the bush  22  supporting the central portion of the oil pump driving shaft  20  to the fitting portion of the inner rotor  21 , and the moment load hence acts on the bush  22 . The bush  22  axially supports the entire structure of the torque converter, so that, other than the above load, an eccentric load or swing load due to the misalignment at the mounting time of the torque converter assembly to the drive plate  9  can be supported. 
     The structure of the outer hull of the torque converter mentioned above and the manufacturing process thereof has provided the following problems. 
     That is, the front cover and the impeller shell comprise a plurality of parts which are respectively independently manufactured, and the parts are thereafter assembled integrally by welding, for example, so that many processes are required, involving much cost as well as low production efficiency. 
     Furthermore, since in general, the body of the front cover is manufactured through a press working operation, a soft steel plate is generally used because of its good formability (yieldability). However, it is necessary to increase the thickness of the soft steel plate in order to ensure the strength and rigidity thereof, when used with high revolution and high torque engines, because the front cover body bears the centrifugal hydraulic pressure generated in the rotating torque converter and is subjected to the thrust load from the turbine runner and the pressing load of the lock-up clutch. Accordingly, in spite of the fact that it is important to reduce weight and vibration while ensuring the inertia moment that is required for a rotating member, it is difficult to prevent the structure from increasing in weight, whereby problems remain unsolved. 
     Furthermore, it is required for the torque converter of the automatic transmission mechanism or non-stage transmission mechanism to cooperate with a lean combustion type engine or direct-injection type engine in place of a conventional engine from the viewpoints of environmental pollution and fuel consumption improvement. However, in the torque converter having the outer hull of the conventional structure, issues involved with the above viewpoints have been easily handled by making a member constituting the outer hull thin or mounting an inertial ring to a vacant space. Therefore, it is very difficult for the member constituting the outer hull of the torque converter to have the effective inertia moment against the strong accelerating force from a stopping state of the automobile, and the lightening of the engine due to the problem of vibration of the engine. Accordingly, the weight of the engine has been further increased, constituting further problems of preventing a realization of the reduction of weight or compact structure of the engine. 
     Further, the oil pump driving shaft, having a thin sleeve shape, which is welded to the pump impeller, is hardened by heat treatment to provide substantially a hardness of HRC18, because a transverse load due to the pump discharge pressure is applied to the fitting portion of the oil pump rotor and a load due to the pump driving torque is applied to the pump rotor driving portion. However, in general, the pump discharge pressure in an automatic transmission mechanism is about 1.5 to 1.8 Mpa, and in a metal-belt type non-stage transmission mechanism the pump discharge pressure is about 4 to 5 Mpa, and accordingly, when used for the automatic transmission mechanism, the drive shaft and the supporting bush of the oil pump are easily broken by fatigue. 
     As a countermeasure to the above defects, there is considered a change to a material hardened by heat treatment, such as high-frequency wave (induction) heating, and/or increasing of the thickness of the oil pump driving shaft. There is also considered an entire change of a support structure supporting the pump rotor. However, such countermeasures will make large the entire structure of the apparatus, increase the weight thereof and increase the manufacturing cost, which will hence constitute barriers against improving the production efficiency and the lightening of the apparatus, thus providing problems. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art mentioned above and to provide a torque converter capable of reducing the manufacturing cost thereof, realizing a light weight requirement without damaging the strength and rigidity of members constituting the outer hull of the torque converter, and effectively reducing or eliminating fatigue failure of an oil pump driving shaft and damage to a bush rotationally supporting the oil pump driving shaft. 
     Other objects can be achieved according to the present invention by providing a torque converter which comprises an impeller shell, a pump impeller integrally formed with the impeller shell, a turbine runner, a stator and a front cover. At least one of a flanged portion, through which a driving power of an engine is transmitted, and a ring gear, for a starter, is formed on an outer peripheral portion of an outer hull, which outer hull includes the impeller shell and the front cover. A pilot boss portion is disposed at an axial portion of the front cover and is adapted to be fitted to a crank shaft of the engine, and an oil pump driving shaft in the shape of a sleeve is disposed at an axial portion of the impeller shell and is adapted to drive an oil pump. The pilot boss portion is formed integrally with the front cover through a plastic working operation, and the flanged portion is formed at a front portion of the outer periphery of the front cover through a plastic working operation, and is increased in thickness through the plastic working operation by forming the flanged portion as a lamination. 
     In a preferred embodiment, a liquid-tight brazing and a coating layer is formed on at least a portion of an inner peripheral side boundary portion of the flanged portion, which is in communication with a screw hole portion formed in the flanged portion. 
     The sleeve-shaped oil pump driving shaft is formed integrally with the impeller shell through plastic working, and the ring gear for the starter is also integrally formed through plastic working so as to form an increased thickness portion. 
     The ring gear for the starter can be integrally formed on a rear portion of the outer periphery of the front cover through a plastic working operation. The flanged portion and the ring gear for the starter are integrally formed through plastic working operations on the outer peripheral portion of the outer hull, which is composed of the front cover and the impeller shell, so that the integrally formed portions perform an inertia function and a power transmission function. 
     At least one of the front cover and impeller shell are formed, through a deformation process, of a steel plate material consisting of, in weight %, C: 0.2-0.6, Si: 0.01-0.1, Mn: 0.05-0.5, Ti: 0.01-0.1, B: 0.001-0.01, and Fe: remainder. An induction hardening is carried out on at least one of a portion of the oil pump driving shaft which is fitted to the rotor of the oil pump, and a bush fitting portion of the oil pump driving shaft that is supported relative to a housing of an oil pump through a bush. An induction micro-wave hardening is also carried out on an inside surface of the oil pump driving shaft, which is opposed to a disc surface of the stator through a thrust bearing. The induction micro-wave hardening is also carried out on at least one of an inner surface of a disc portion of the front cover, a surface of the front cover opposing a turbine hub through a thrust bearing, and a central protruded portion of the front cover. 
     According to the present invention of the characters and structures mentioned above, the front cover and the impeller shell constituting the outer hull of the torque converter are integrally formed, through the plastic working, with the associated members such as the pilot boss portion, the flanged portion, the ring gear and the oil pump driving shaft. Accordingly, in comparison with the conventional torque converter in which the associated members mentioned above are independently formed and then assembled with the front cover and the impeller shell, which are also independently formed, the productivity of the front cover and the impeller shell of the torque converter according to the present invention provided with the associated members mentioned above can be improved and the manufacturing cost can be reduced. In addition, as the respective members are integrally formed, unnecessarily thickened portions are eliminated, thereby realizing the light weight requirement of the entire structure of the torque converter. 
     Furthermore, welding of the ring gear for starting the engine, and welding of the flanged portion through which the engine driving power is transmitted, can be effectively eliminated. The elimination of such welding can prevent the formation of irregularly thickened portions due to the welding, and prevent the occurrence of defects which may be caused by the welding. Therefore, the adjustment of rotational balance after the assembling of the torque converter can be easily performed, and moreover, oil leakage due to welding defects will be prevented, whereby the quality of products and the productivity can be improved. 
     Additionally, according to the present invention, the front cover and the impeller shell are formed of a material including boron (B) in addition to carbon (C) of 0.5 weight %, so that the induction hardening can be effectively applied to the members as required, thereby improving the strength and rigidity of the fitting portion between the oil pump drive shaft and the oil pump rotor and the bush rotation support portion, and the gear teeth of the ring gear to prevent the portions or members from being damaged, worn and deformed. 
     Additionally, the front cover and the impeller shell can be formed from a steel plate having a thickness thinner than that used for the conventional structure. Moreover, in a case where the engine is operated with a revolution number increased in a red zone under the lock-up condition, a compound force of the centrifugal hydraulic pressure acting on the torque converter and the lock-up clutch engaging load can be generated and at least a permanent deformation can be prevented from occurring, even if a thin steel plate is used, by performing, as required, the micro-wave induction hardening, or partially increasing the thickness of the outer peripheral bent portion at which large bending stress is caused. 
     Additionally, in spite of the use of the thin steel plate while maintaining the high strength and rigidity, the flanged portion formed on the outer periphery of the front cover is increased in thickness and the ring gear disposed on the outer periphery of the impeller shell is also increased in thickness by a roll spinning process. This ensures that the inertial mass can be most effectively ensured on the outer peripheral portion of the torque converter without arranging an inertia ring as an additional member, as is done in the conventional structure; thus realizing the light weight requirement. In particular, even in a case where it is required to improve an environmental pollution and fuel consumption, as with the use of a lean combustion engine or direct-injection combustion engine, the inertia can be ensured and the light weight can be realized while maintaining the improved productivity and reduced manufacturing cost. 
     The nature and further characteristic features of the present invention will become more clear from the following descriptions made with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a partial sectional view of a main portion of a torque converter of a conventional structure; 
     FIG. 2 is a sectional view showing a front cover of the torque converter of FIG. 1 for the explanation of a manufacturing process thereof; 
     FIG. 3 is a sectional view showing an impeller shell of the torque converter of FIG. 1 for the explanation of a manufacturing process thereof; 
     FIGS. 4A to  4 H are sectional views showing a manufacturing process of the present invention in the illustrated order; 
     FIG. 5 is a sectional view of the front cover of the torque converter of the present invention; 
     FIG. 6 is a sectional view corresponding to FIG. 4C showing another example of a front cover; 
     FIGS. 7A to  7 F are sectional views showing a manufacturing process of an impeller shell of the present invention; 
     FIG. 8 is a partial sectional view of an essential portion of the torque converter according to the present invention; 
     FIGS. 9A and 9B are front views showing patterns formed on the front cover by micro-wave induction hardening; and 
     FIG. 10 shows a sectional view of a front cover of the torque converter according to another embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A torque converter according to the present invention is entirely shown in FIG. 8, which is provided with an improved impeller shell  102  and front cover  106  as compared with those shown in FIG. 1, and preferred embodiments thereof will be described hereunder with reference to FIGS. 4 to  10 , in which Figures reference numerals are applied to members or portions corresponding to those shown in FIGS. 1 to  3 . 
     FIGS. 4A to  4 H are views explaining plastic working steps of the front cover  106  in this order. 
     FIG. 4A shows a punch-out step for punching out a disc-shaped material  41  of a predetermined size, FIG. 4B shows a step for forming a swelled or protruded portion  42  at a central portion of the material  41  through a press working operation, and FIG. 4C shows a step for plastically forming a boss portion  45 , constituting a pilot boss portion  132  of the front cover  106 , by placing the swelled portion  42  on a mold  43  composed of a pin  43   a  and a die  43   b  and then press-forming the swelled portion  42  from the upper side thereof by using a forming roller  44 . In these steps, the material volume of the swelled portion  42  is larger than that of the boss portion  45 , and accordingly, the thickness of the boss portion  45  is made larger than a plate thickness of the material  41 . That is, the thickness of the boss portion  45  is increased. 
     In the next step shown in FIG. 4D, a cylindrical portion  46  is formed from an outer peripheral portion of the material  41  through a press working operation, and a protrusion or projection  48  is formed by a roller  47 , so as to project outward in an annular shape from an inside portion of the cylindrical portion  46  to a portion that is to constitute a flanged portion  107 . 
     In a step shown in FIG. 4E, the cylindrical portion  46  is pressed in an axial direction thereof, and according to this pressing, buckling is formed at the protrusion  48  so as to provide a substantially Ω-shape section. In a step shown in FIG. 4F, the outer shape of the protrusion  48  is shaped by a roller  49 , and the protrusion  48  is then pressed and folded by upper and lower rollers  50   a ,  50   b  so as to further form a portion, that is to constitute the flanged portion  107 , having a semi-circular front end shape in a step shown in FIG.  4 G. Thereafter, in a step shown in FIG. 4H, the front end is shaped by a roller  51  so as to provide a flat end shape, i.e. cylindrical shape of the flanged portion  107 . Further, in the steps shown in FIGS. 4F to  4 H, a back-up die  52  is disposed against an inside portion of the cylindrical portion  46 . Accordingly, after completion of the steps as depicted in FIGS. 4A-4H, the front cover  106  includes the flanged portion  107 , which is a monolithic lamination of the material of the front cover. 
     With reference to FIG. 5, the material  41  is thereafter entirely press-worked to provide the front cover shape, and a screw hole  107   a  is formed in the flanged portion  107  through drilling. In this step, if the screw hole  107   a  is communicated with an abutment portion  53  formed by folding the protrusion  48  in the step shown in FIG. 4G, an oil leakage may result. In order to obviate such a defect, a brazing or coating treatment will be applied to thereby form a coating layer  53   a , thus performing a fluid-tight sealing of that portion. 
     FIG. 6 shows another example for forming the pilot boss portion  132  from the swelled portion  42  formed from the material  41  of the front cover  106 . In this example, the swelled portion  42  is formed so as to provide a shape conforming with the shape of a mold  43  by virtue of a roller  44   a  having a small diameter. 
     FIGS. 7A to  7 F represent steps for the plastic working operations for forming the impeller shell  102 . 
     In a step shown in FIG. 7A, a disc-shaped material  55  of a predetermined size is punched out, in a step shown in FIG. 7B the material  55  is shaped via a press working operation so as to provide a semi-spherical shape, and in a step shown in FIG. 7C a swelled or protruded portion  56  is formed via a press working operation in a central portion of the disc-shaped material  55 . In a step shown in FIG. 7D, the swelled portion  56  is subjected to a further press working operating so as to provide a sleeve-shaped shaft portion, and an upper end (front end) of this sleeve-shaped shaft portion is cut away, thereby plastically forming an oil pump driving shaft  120 . 
     Thereafter, in a step shown in FIG. 7E, the material  55  is entirely press-worked so as to form an impeller shell  102  having a flanged portion  57  at an outer peripheral edge. In a step shown in FIG. 7F, the flanged portion  57  is increased in thickness by performing a roll spinning method, so as to form a thickened portion  58 . A ring gear  110  for starting the engine is formed through a rolling method or a machining operation. 
     FIG. 8 shows the torque converter  101  provided with the impeller shell  102  and the front cover  106  having the structures mentioned above formed in accordance with the steps also mentioned above. The impeller shell  102  is integrally coupled to an impeller  102   a  to form a pump impeller  103 . 
     Hereunder, there will be described a manner for reducing the weight of the torque converter, and improving the strength and rigidity of the members constituting the torque converter, while ensuring the inertial mass. 
     A conventional front cover  6  and impeller shell  2  such as shown in FIG. 1 are formed of a material of SPC or SPHC and have plate thicknesses of 4.9 mm and 3.2 mm, respectively. On the other hand, according to the preferred embodiment of the present invention, the SPC material is replaced by a material which is made by adding a small amount of boron (B) to an alloy material having a steel plate base containing 0.25 weight % carbon (C), and on which an induction hardening process is effectively performed. The SPC material has the best press formability, and although a material containing 0.5 weight % C with B added can provide a high strength, it will provide less press formability. However, by reducing the plate thicknesses of the front cover  106  and the impeller shell  102 , a working stress of a conventional extent can be maintained, so that the above replacement of the material does not result in an adverse effect with regard to the production of the torque converter. 
     Furthermore, as the flanged portion  107  and the ring gear  110  are formed with the increased thicknesses, the reduction of the weight can be realized while ensuring the necessary inertial mass because the portion  107  and the gear  110  are formed on the outer peripheral portions of the torque converter, which are the most effective portions for increasing the inertial mass. 
     In consideration of the above matters, as the materials  41  and  55  for the front cover  106  and the impeller shell  102 , materials of S25CTiB containing titanium (Ti) and B and having thicknesses of 4.5 to 4.2 mm and 3.0 to 2.8 mm are utilized, respectively, in place of steel plate materials (SPC  1 ) having thicknesses of 4.9 mm and 3.2 mm (JIS standard). The respective components of these S25CTiB and SPC1 materials are shown in the following Table 1. Wt. % means weight %. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Kind of 
                   
                   
                   
                   
                   
                   
                   
                 Plate 
               
               
                 steel 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Ti 
                 B 
                 Thickness 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 S25CTiB 
                 0.25 
                 0.06 
                 0.39 
                 0.01 
                 0.008 
                 0.06 
                 0.004 
                 3.2 
               
               
                   
                 (wt. %) 
                   
                   
                   
                   
                   
                   
                 (mm) 
               
               
                 SPC1 
                 ≦0.12 
                 ≦1.2 
                 ≦0.5 
                 ≦0.04 
                 ≦0.045 
                 — 
                 — 
                 4.9 
               
               
                   
                 (wt. %) 
                   
                   
                   
                   
                   
                   
                 (mm) 
               
               
                   
               
            
           
         
       
     
     Further, although the respective components, other than phosphorous (P) and sulfur, (S) of the steel plate mentioned above usable for the present invention are different with regard to the shapes and functions to be required, a steel of the components in the range described in the following Table 2 will be suitably used, and further, P and S are substantially the same as those in the above S25CTiB. Wt. % means weight %. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
             
            
               
                 C 
                 Si 
                 Mn 
               
               
                   
               
               
                 0.20 ˜ 0.6% (wt. %) 
                 0.01 ˜ 0.1% 
                 0.05 ˜ 0.5% 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Ti 
                 B 
                 Elongation 
               
               
                   
               
               
                 0.01 ˜ 0.1% 
                 0.001 ˜ 0.01% 
                 ≧30% 
               
               
                   
               
            
           
         
       
     
     The front cover  106  and the impeller shell  102  after being plastically formed are strengthened as compared with the conventional ones by effecting induction hardening (including underwater induction hardening) of surfaces of materials containing 0.25 weight % C with B added, such as the portion of the oil ring driving shaft  120  that fits with the inner rotor  21  of the oil pump  18 , the portion of the oil ring driving shaft  120  that is supported by the bearing bush  22 , the outer peripheral bent portion of the front cover  106  and the inside surface portion of the front cover  106  that is pressed by the lock-up piston  29 . This strengthening reduces damage that may be caused by the plate thickness reduction. 
     In such an operation, the induction hardening is not a full-surface hardening, and it is preferable that the hardening is performed so as to provide a pattern of radial lines as shown in FIG. 9A or pattern of rings as shown in FIG.  9 B. 
     In the above embodiment, although the ring gear  110  for the starter is located on the side of the impeller shell  102 , the ring gear  110  may be formed through plastic working to the outer peripheral portion of the front cover  106 , together with the flanged portion  107  as shown in FIG.  10 . 
     While the presently preferred embodiment of the present invention has been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.