1.  Field of the Invention
The present invention relates to a fluid-type torque transmission device having a lockup clutch e.g., a torque converter or a fluid coupling.  More specifically, the present invention relates to a fluid-type torque transmission device having a sliding contact surface formed on its front cover for sliding against the lockup clutch. 
2.  Background Information
In general, a torque converter is designed to facilitate smooth acceleration and deceleration because it transfers power using a fluid.  However, slippage of the fluid causes energy loss to occur and fuel economy to be poor. 
Therefore, conventional torque converters are equipped with a lockup device that mechanically couples a front cover on an input side to a turbine on an output side.  The lockup device is disposed in a space between the front cover and the turbine.  The lockup device chiefly has a circular disk-shaped piston, a driven plate, and a torsion spring.  The circular disk-shaped piston can couple to the front cover.  The driven plate is mounted to the rear surface of the turbine.  Further, a torsion spring elastically couples the piston and the driven plate together in a rotational direction.  An annular facing part is fixed to the piston in a position opposite a flat sliding contact surface of the front cover. 
In the conventional lockup device just described, the operation of the piston is controlled by the change in hydraulic pressure within a main body of the torque converter.  More specifically, when the lockup engagement is released, operating oil is delivered from the external hydraulic circuit to the space between the piston and the front cover.  This operating oil flows radially outward through the space between the front cover and the piston and enters the main body of the torque converter from a radially outward location.  When the lockup device is engaged, the operating oil between the front cover and the piston is drained from a radially inward location and the hydraulic pressure difference causes the piston to move toward the front cover.  As a result the facing part provided on the piston is pressed against the sliding contact surface of the front cover.  Thus, the torque of the front cover is transmitted to the turbine via the lockup device. 
A conventional front cover is a disk-shaped member made of sheet metal formed by press molding.  The front cover chiefly has a radial flange part, an axial flange part, and an inside cylindrical portion.  The radial flange part has a sliding contact surface corresponding to the facing part of the lockup clutch.  The axial flange part extends axially from the outer circumferential edge of the radial flange part and is fixed by welding to the pump impeller.  The inside cylindrical portion is located radially inward of the radial flange part.  A center boss is welded to the inner circumferential edge of the front cover, i.e., the inner circumferential edge of the inside circumferential portion. 
The front cover requires a certain degree of strength in order to withstand the hydraulic pressure inside the torque converter and the internal pressure caused by rotation.  In order to secure the required strength, it is necessary conventionally to use raw material that has a large thickness.  A front cover made with such material is heavy and expensive.  Thus, it has been difficult to reduce the weight of the front cover while also maintaining sufficient strength in conventional front covers. 
The shells of the front cover and impeller are fixed together by welding.  More specifically, a cylindrical axial flange part that extends in the axial direction is formed on the outer circumferential edge of the front cover and the outer circumferential edge of the impeller shell is welded to the tip of the flange part.  The welding is accomplished using a plurality of torches that weld the two members in the circumferential direction at a plurality of locations. 
Since the welding is conducted over a certain amount of time, uneven stresses in the welding bead portion cause welding distortion to occur in the front cover and the sliding contact surface provided on the radial flange part becomes wavy in the circumferential direction.  Consequently, when the lockup device frictionally engages, the desired friction characteristic cannot be obtained and wheel vibration occurs. 
In view of the above, there exists a need for a fluid-type torque transmission device with a lockup device that overcomes the above mentioned problems in the prior art.  This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure. 
An object of the present invention is to reduce the weight of the front cover while also maintaining sufficient strength. 
Another object of the present invention is to make it difficult for distortion of the front cover sliding contact surface to occur when the front cover and impeller shell are welded together. 
A fluid-type torque transmission device with a lockup clutch in accordance with a first aspect of the present invention has a front cover, a turbine, a pump impeller, and a lockup clutch.  The front cover is fixed to an input shaft.  The turbine is connected to an output shaft.  The pump impellers are connected by welding to the front cover.  The lockup clutch has a facing part for sliding against the front cover.  The front cover has a radial flange part that extends in the radial direction and an axial flange part that extends in the axial direction from the outside edge of the radial flange part.  Preferably, the flange parts are formed integrally as a single unit.  The radial flange part has a sliding contact surface for sliding against the facing part on an axially facing surface of the radial flange part.  The axial flange part has a connection part at one end where it connects to the radial flange part and a welding part at the other end where it is connected by welding to the pump impeller.  The axial flange part is formed such that the region containing the welding part has a lower rigidity than the region containing the connection part. 
Since the region containing the welding part has a lower rigidity than the region containing the connection part with this fluid-type torque transmission device, the region containing the welding part distorts relatively easily and the region containing the connection part does not distort easily when the welding part of the front cover is welded to the pump impeller.  As a result, it is difficult for welding distortion to occur at the sliding contact surface of the front cover. 
A fluid-type torque transmission device with a lockup clutch in accordance with a second aspect of the present invention is the device of the first aspect, wherein the rigidity of the region containing the welding part is made smaller than the rigidity of the region containing the connection part by making the wall thickness of the region containing the welding part thinner than the wall thickness of the region containing the connection part. 
This fluid-type torque transmission device employs a simple method to make it more difficult for welding distortion to occur at the sliding contact surface of the front cover. 
A fluid-type torque transmission device with a lockup clutch in accordance with a third aspect of the present invention is the device of second aspect, wherein the region containing the welding part occupies at least 30% of the entirety of the axial flange part. 
This fluid-type torque transmission device makes it even more difficult for welding distortion to occur at the sliding contact surface of the front cover. 
A fluid-type torque transmission device with a lockup clutch in accordance with a fourth aspect of the present invention is the device of the second aspect, wherein the axial length of the region containing the welding part is longer than the axial length of the region containing the connection part. 
This fluid-type torque transmission device makes it even more difficult for welding distortion to occur at the sliding contact surface of the front cover. 
A fluid-type torque transmission device with a lockup clutch in accordance with a fifth aspect of the present invention is the device of the second aspect, wherein the region containing the welding part occupies at least 80% of the entirety of the axial flange part. 
This fluid-type torque transmission device makes it even more difficult for welding distortion to occur at the sliding contact surface of the front cover. 
A fluid-type torque transmission device with a lockup clutch in accordance with a sixth aspect of the present invention is the device of the second aspect, wherein the region containing the welding part occupies most of the entirety of the axial flange part except for the connection part. 
This fluid-type torque transmission device makes it even more difficult for welding distortion to occur at the sliding contact surface of the front cover. 
A fluid-type torque transmission device with a lockup clutch in accordance with a seventh aspect of the present invention is the device of any one of the second to sixth aspects, wherein the wall thickness of the region containing the welding part is 20 to 90% of the wall thickness of the region containing the connection part. 
This fluid-type torque transmission device makes it even more difficult for welding distortion to occur at the sliding contact surface of the front cover. 
A fluid-type torque transmission device with a lockup clutch in accordance with an eighth aspect of the present invention is the device of the seventh aspect, wherein the wall thickness of the region containing the welding part is 30 to 80% of the wall thickness of the region containing the connection part. 
This fluid-type torque transmission device makes it even more difficult for welding distortion to occur at the sliding contact surface of the front cover. 
A ninth aspect of the present invention is a method of manufacturing the front cover of a fluid-type torque transmission device with a lockup clutch.  The device is provided with a front cover, a turbine, a pump impeller, and a lockup clutch.  The front cover is fixed to an input shaft.  The turbine is connected to an output shaft.  The pump impeller is connected by welding to the front cover.  The lockup clutch has a facing part for sliding against the front cover.  The manufacturing method has the following steps:
a step for forming a sliding contact surface for sliding against the facing part on an axially facing surface of the radially extending radial flange part of the front cover;
a step for forming a welding part for welding to the pump impeller on the axial flange part, which is connected to the outside edge of the radial flange part at the connection part and extends in the axial direction; and
a step for making the rigidity of the region containing the welding part lower than the rigidity of the region containing the connection part. 
With this method of manufacturing the resulting front cover of a fluid-type torque transmission device has a region containing the welding part that is made to have a smaller rigidity than the region containing the connection part.  Consequently, when the welding parts of the front cover are welded to the pump impeller, the region containing the welding parts distorts easily and the region containing the connection part does not distort easily.  As a result, it is difficult for welding distortion to occur at the sliding contact surface of the front cover. 
A tenth aspect of the present invention is a method of manufacturing the front cover of a fluid-type torque transmission device with a lockup clutch in accordance with the ninth aspect, wherein the method also has a step for making the wall thickness of the region containing the welding part thinner than the wall thickness of the region containing the connection part. 
The resulting fluid-type torque transmission device employs a simple method to make it more difficult for welding distortion to occur at the sliding contact surface of the front cover. 
An eleventh aspect of the present invention is a method of manufacturing the front cover of a fluid-type torque transmission device with a lockup clutch in accordance with the tenth aspect, wherein the step for making the wall thickness thinner involves transferring a portion of the wall thickness of the region containing the welding part to the region containing the connection part. 
The resulting fluid-type torque transmission device employs a simple method to make it more difficult for welding distortion to occur at the sliding contact surface of the front cover. 
A fluid-type torque transmission device in accordance with a twelfth aspect of the present invention has a front cover, a turbine, and a pump impeller.  The front cover is fixed to an input shaft.  The turbine is connected to an output shaft.  The pump impeller is connected to the front cover.  The front cover has a radial flange part that extends in the radial direction and an axial flange part that extends in the axial direction from an outside edge of the radial flange part.  The wall thickness of the radial flange part is 105 to 200% of the thickness of the sheet material from which the front cover is made.  Further, the wall thickness of an inside circumferential portion of the front cover is 50 to 95% of the thickness of the sheet material from which the front cover is made. 
In this fluid-type torque transmission device, the wall thickness of the radial flange part is larger than the thickness of the sheet material and the wall thickness of the inside circumferential portion of the front cover is smaller than the thickness of the sheet material.  Therefore, the pressure that the radial flange part can withstand, the withstand pressure, is increased while the overall weight is reduced.  Although the radial flange part requires a certain withstand pressure, the inside circumferential portion can perform sufficiently with a smaller withstand pressure.  Consequently, no problems regarding withstand pressure occur as a result of reducing the wall thickness of the inside circumferential portion.  In short, this front cover simultaneously achieves both sufficient withstand pressure and reduced weight. 
Since the wall thickness of the radial flange part is made larger than the thickness of the sheet material, a relatively thin sheet material is used.  Consequently the weight of the front cover can be reduced even further. 
A fluid-type torque transmission device in accordance with a thirteenth aspect is the device of the twelfth aspect, wherein the wall thickness of the radial flange part is 150 to 200% of the sheet material from which the front cover is made, and the wall thickness of the inside circumferential portion of the front cover is 50 to 75% of the thickness of the sheet material from which the front cover is made. 
A fluid-type torque transmission device in accordance with a fourteenth aspect of the present invention is the device of twelfth or thirteenth aspect with a lockup clutch, wherein the wall thickness of the inside circumferential portion of the front cover is 50 to 90% of the wall thickness of the radial flange part. 
This fluid-type torque transmission device achieves an even better balance between the withstand pressure and weight reduction. 
A fluid-type torque transmission device with a lockup clutch in accordance with a fifteenth aspect is the device of the fourteenth aspect, wherein the wall thickness of the inside circumferential portion of the front cover is 70 to 90% of the wall thickness of the radial flange part. 
This fluid-type torque transmission device achieves the best balance between the withstand pressure and weight reduction. 
A fluid-type torque transmission device with a lockup clutch in accordance with a sixteenth aspect of the present invention is the device of any one of the twelfth to fifteenth aspects, wherein the axial flange part has a connection part at one end where it connects to the radial flange part and a welding part at the other end where it is connected by welding to the pump impeller.  Further, the axial flange part is formed such that the wall thickness of the region containing the welding part is thinner than the wall thickness of the region containing the connection part. 
In this fluid-type torque transmission device, the wall thickness of the base portion of the axial flange part is larger than the wall thickness of the tip portion of the axial flange part.  As a result, the withstand pressure of the base portion is increased while the overall weight is reduced.  Although the base portion of the axial flange part requires a certain withstand pressure, the tip portion can perform sufficiently with a smaller withstand pressure.  Consequently, no problems regarding withstand pressure occur as a result of reducing the wall thickness of the tip portion.  In short, this front cover achieves both sufficient withstand pressure and reduced weight simultaneously. 
A seventeenth aspect of the present invention is a method of manufacturing the front cover of a fluid-type torque transmission device provided with a front cover fixed to an input shaft, a turbine connected to an output shaft, and a pump impeller connected to the front cover.  The method has the following step:
A step in which the wall thickness of the radial flange part is increased by transferring a portion of the wall thickness at a radially inner portion of the front cover to a radially outer portion of the front cover. 
In this front cover manufacturing method, the wall thickness of the radial flange part is made larger than the wall thickness of the inside circumferential portion of the front cover.  Therefore, the withstand pressure of the radial flange part is increased while the overall weight is reduced.  Although the radial flange part requires a certain withstand pressure, the inside circumferential portion can perform sufficiently with a smaller withstand pressure.  Consequently, no problems regarding withstand pressure occur as a result of reducing the wall thickness of the inside circumferential portion.  In short, this front cover achieves both sufficient withstand pressure and reduced weight simultaneously. 
Furthermore, raw material having a small thickness can be used because the wall thickness of the radial flange part is increased by transferring a portion of the wall thickness at a radially inner portion of said front cover to a radially outer portion of said front cover.  This allows the manufacturing cost to be reduced. 
An eighteenth aspect of the present invention is a method of manufacturing the front cover of a fluid-type torque transmission device in accordance with the seventeenth aspect having a step in which the wall thickness of the base portion of the axial flange part is increased by transferring a portion of the wall thickness of the tip portion of the axial flange part to the base portion.  The axial flange part extends in the axial direction from the outside edge of the radial flange part. 
In this front cover manufacturing method, the wall thickness of the base portion of the axial flange part is made larger than the wall thickness of the tip portion of the axial flange part.  As a result, the withstand pressure of the base portion is increased while the overall weight is reduced.  Although the base portion of the axial flange part requires a certain withstand pressure, the tip portion can perform sufficiently with a smaller withstand pressure.  Consequently, no problems regarding withstand pressure occur as a result of reducing the wall thickness of the tip portion.  In short, this front cover achieves both sufficient withstand pressure and reduced weight simultaneously. 
Furthermore, raw material having a small thickness can be used because the wall thickness of the base portion of the axial flange part, which extends in the axial direction from the outside edge of the radial flange part, is increased by transferring a portion of the wall thickness of the tip portion to the base portion.  This allows the manufacturing cost to be reduced. 
A nineteenth aspect of the present invention is a method of manufacturing the front cover of a fluid-type torque transmission device in accordance with the seventeenth or eighteenth aspect also having a step in which a cylindrical boss that protrudes toward the engine in the axial direction is formed integrally at the center of the inside circumferential portion. 
With this front cover manufacturing method, the boss of the front cover is also formed integrally, thus simplifying the manufacture of the front cover. 
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.