Patent Application
20080009386
This invention relates to a toroidal-type continuously variable transmission and a method for producing torque a transmitting member thereof.
In a toroidal-type continuously variable transmission for an automotive vehicle etc., a plurality of rollers are rotatably sandwiched between raceway portions of an input disc and an out put disc opposing to each other by a predetermined force. Such a toroidal-type continuously variable transmission transmits torque between the discs by a traction force between the circumference of the roller and each raceway portion of the disc. This toroidal-type continuously variable transmission can increase and decrease the number of the revolution of the output disc against the input disc (transmission ratio) by adjusting the position of the roller with respect to each disc.
In the toroidal-type continuously variable transmission, each of rolling contact portions of the input disc, the output disc and the rollers, which are torque transmitting members, is suffered to high temperatures and high surface pressure in operation. For this reason, not only a black structure caused by conventional fatigue but also a white structure causing white layer flaking is occurred at the rolling contact portions. This causes a problem that its life is shortened. Some technologies trying to solve such a problem are known as follows:
(1) In order to increase fatigue resistance, the production process comprises performing carburization treatment on a mechanical structural steel containing Cr so that the material member has a surface carbon density within a range of 0.6-1.5% by weight; precipitating carbide by holding the carburized material member at an upper limit temperature which is calculated based on the alloy content; and quenching after holding at a temperature at which austenite phase is formed, and the mechanical structural steel includes a matrix having at least one of martensite structure and bainite structure. The matrix contains carbide, which includes M23C6 type carbide, having a mean particle size of 3 μm or less, dispersed and precipitated in form of at least one of generally spheres and pseudo-spheres (see, for example Japanese Patent Laid-Open Publication No. 2001-98343).
(2) In order to reduce the occurrence of the structural change, the amount of residual austenite in steel is set to 10% or less (see, for example Japanese Patent Laid-Open Publication No. 1996-210360).
(3) In order to reduce a temperature rise by reducing rolling friction, the amount of residual austenite in steel is set to not less than 0.05% and less than 10%. This also tries to prevent a plastic deformation caused by the decomposition of residual austenite at a rolling contact surface (see, for example Japanese Patent Laid-Open Publication No. 1997-105415).
(4) An alloy steel material, which contains C: 0.15-0.5 wt %, Si: 0.1-1.5 wt %, Mn: 0.1-1.5 wt %, Cr: 0.5-3.0 wt %, and Mo: 0.1-3.0 wt %, a content of 0 in the alloy steel material being set to be not more than 9 ppm, which surface after carbonitriding, hardening and tempering treatments is set to contain C: 0.8-1.2 wt % and N: 0.05-0.20 wt %, and which surface hardness is Hv 720 or more, and in which Mo or V carbide/carbonitride, the average particle size of which is in a range of from 50 nm to 500 nm, is dispersed and precipitated on a race surface or in a range of from the race surface to a deep position where a maximum shearing stress is generated, is used (see, for example Japanese Patent Laid-Open Publication No. 2001-32900).
However, there is a problem that any of the technologies mentioned above does not sufficiently achieve the effect and its cost is high.
It is an object to provide a toroidal-type continuously variable transmission and a method for producing torque transmitting member thereof capable of long life at low cost.
A toroidal-type continuously variable transmission according to the present invention comprises an input disc having a concavely curved raceway portion in one side, an output disc having a concavely curved raceway portion opposing to the raceway portion of the input disc in one side, and a roller rotatably sandwiched between the raceway portions of the discs for transmitting torque between the discs by a traction force between the circumference of the roller and the raceway portion of each of the discs, wherein, at least rolling contact portion of at least one transmitting member of the input disc, the out put disc and the roller is formed of a bearing steel which contains: C: 0.8-1.5 wt %; Si: 0.5-2.5 wt %; Mn: 0.3-2.0 wt %; Cr: 1.9-2.5 wt %; Mo: 0.3-1.0 wt %; and a total of 1.0 wt % or more of Si and Mo; with the balance being iron and unavoidable impurities; wherein, a residual austenite in a range of depth Z=1.0L, where L is the major axis of a contact ellipse of the traction contact portion, from the surface of the rolling contact portion is 15 wt % or less, and the hardness of the range is HRC 58-62.
In the toroidal-type continuously variable transmission mentioned above, since the residual austenite in the range of depth Z=1.0L from the surface of the rolling contact portion formed of the bearing steel is 15 wt % or less, and the hardness of the range is HRC 58-62, this can reduce the occurrence of a white structure, which is a factor of a short life. Thus, a long-life toroidal-type continuously variable transmission can be obtained at low cost. The inventors of the present invention found that adjusting the amount of residual austenite and the hardness of the predetermined depth from the surface of the rolling contact portion formed of the bearing steel can reduce the occurrence of a white structure based on their studies, and accomplished this invention.
It is preferable that an oxide particle size is less than 20 μm in the range of depth Z=1.0L, in the range of depth Z=1.0b, where b is the minor axis of the contact ellipse of the traction contact portion, or in the range of depth Z=0.4b-0.5b from the surface, of the rolling contact portion formed of the bearing steel, respectively. In this case, since the oxide particle size in the rolling portion is small, it is capable of reducing occurrence of flaking caused by the oxide particle as a source.
Another toroidal-type continuously variable transmission according to the present invention comprises an input disc having a concavely curved raceway portion in one side, an output disc having a concavely curved raceway portion opposing to the raceway portion of the input disc in one side, and a roller rotatably sandwiched between the raceway portions of the discs for transmitting torque between the discs by a traction force between the circumference of the roller and the raceway portion of each of the discs, wherein, at least rolling contact portion of at least one transmitting member of the input disc, the out put disc and the roller is formed of a bearing steel which contains C: 0.8-1.5 wt %; Si: 0.5-2.5 wt %; Mn: 0.3-2.0 wt-%; Cr: 1.9-2.5 wt %; Mo: 0.3-1.0 wt %; and a total of 1.0 wt % or more of Si and Mo; with the balance being iron and unavoidable impurities; wherein, a residual austenite in a range of depth Z=1.0L, where L is the major axis of a contact ellipse of the traction contact portion, from the surface of the rolling contact portion is 7-13 wt %, and the hardness of the range is HRC 59-61, and an oxide particle size in the range is 14-19 μm.
In such a toroidal-type continuously variable transmission, since the residual austenite in the range of depth Z=1.0L from the surface of the rolling contact portion formed of the bearing steel is 7-13 wt %, and the hardness of the range is HRC 59-61, and an oxide particle size in the range is 14-19 μm, this can reduce the occurrence of a white structure, which is a factor of a short life. Thus, a long-life toroidal-type continuously variable transmission can be obtained at low cost.
It is preferable that an oxide particle size is less than 20 μm in the range of depth Z=1.0L, in the range of depth Z=1.0b, where b is the minor axis of the contact ellipse of the traction contact portion, or in the range of depth Z=0.4b-0.5b from the surface of the rolling contact portion formed of the bearing steel, respectively. In this case, since the oxide particle size in the rolling portion is small, it is capable of reducing occurrence of flaking caused by the oxide particle as a source.
In both of the toroidal-type continuously variable transmissions mentioned above, the whole of the input disc, the whole of the output disc, and the whole of the roller can be formed of the bearing steel, and the toroidal-type continuously variable transmission is a full-toroidal-type. In this case, a long-life full-toroidal-type continuously variable transmission can be obtained at low cost.
A method for producing the torque transmitting member according to the present invention, comprises steps forming a predetermined shape of blank formed of the bearing steel; tempering the blank at a temperature of 250° C. or more after quenching; and finishing the heat-treated blank.
This method can provide the torque transmitting member with a residual austenite in a range of depth Z=1.0L, where L is the major axis of a contact ellipse of the traction contact portion, from the surface of the rolling contact portion is 15 wt % or less, and the hardness of the range is HRC 58-62 only by tempering a blank at the above temperature. Thus, this method can provide a long-life rolling contact portion at low cost.
An output portion 8 has output members 6 with sprocket gears formed on their circumferences and output discs 7 as the torque transmitting members supported by the output member 6 so as to be rotatable as one-piece. The out put portion 8 is provided in the middle of the input shaft 3 in the axial direction, and is rotatably supported relative to the input shaft 3. A concavely curved raceway portion 7a is formed in one side of each input disc 7 opposing to the raceway portion 4a of the input disc 4. The output shaft 10 of the variator 1 transmitting power to driving wheels is arranged in parallel with the input shaft 3, and has a pair of sprocket wheels 10a at positions corresponding to the output members 6. The output member 6 and the sprocket wheels 10a are engaged with a chain 9, and power is taken out to the output shaft 10.
The output disc 7 is assembled so as to allow small movements in axial direction relative to the output member 6. A back-up plate is arranged in the back of each output disc 7 with clearance. The clearance is sealed by a casing 12 and a seal (not shown). The output disc 7 is thrust toward the input disc 4 opposing thereto by applying hydraulic pressure to the clearance, and a predetermined axial end load is applied.
A space between the raceway portion 4a of the input disc 4 and the raceway portion 7a of the output disc 7 opposing each other is formed as a toroid-shaped space. Three disc-shaped rollers 13 (only two of them are shown) as the torque transmitting members, which rotate in press-contact with raceway portions 4a, 7a, are provided uniformly in circle in the toroid-shaped space. Each roller 13 is rotatably supported by a carriage 14. The axis of its rotation can be tilted by the carriage 14. The carriage 14 is actuated in the direction that roller 13 is thrust and pulled by applying a driving force of hydraulic pressure.
In the variator 1, a pair of input discs 4 transmits torque to the respective output discs 7 via six rollers 13. When transmitting torque, the rollers 13 are subjected to reaction. The driving force applied to the carriage 14 supports the reaction. If the reaction does not balance with the torque necessary to drive the output disc 7, the roller 13 changes its axis angle to balance them. For example, when a force which thrusts or pulls the carriage against the driving force of hydraulic pressure is occurred by a variation of driving load, acceleration or deceleration by an accelerator pedal, the axis angle of the roller 13 changes (see two-dot-dashed line in
Under the condition that the predetermined axial end load is applied, the traction contact portion A, which is a contact portion between each of the raceway portions 4a, 7a and the roller 13 via the oil film, has the form of a contact ellipse in the Y-Z plane as shown in
In the toroidal-type continuously variable transmission mentioned above, the input disc 4, the output disc 7, and each roller 13 are formed of bearing steel for semi-high temperature, respectively. This bearing steel contains C: 0.8-1.5 wt %; Si: 0.5-2.5 wt %; Mn: 0.3-2.0 wt %; Cr: 1.9-2.5 wt %; Mo: 0.3-1.0 wt %; and more preferably C: 0.9-1.05 wt %; Si: 0.9-1.1 wt %; Mn: 0.4-0.5 wt %; Cr: 1.9-2.5 wt %; Mo: 0.4-0.5 wt %; and a total of 1.0 wt % or more of Si and Mo; with the balance being iron and unavoidable impurities. The reason that the bearing steel for semi-high temperatures contains the contents with the above range of materials is described as follows:
1) C: 0.8-1.5 wt %
C acts as an element strengthening martensite in the base material by solid-solution. C is included to obtain strength after quenching and tempering, whereby a rolling fatigue life is improved. Thus, if C is less than 0.8 wt %, the amount of C of solid-solution in the base material is insufficient, so that it can not obtain hardness HRC 58 necessary for bearings. On the other hand, if the amount of C is over 1.5 wt %, large carbide is produced at casting, so that it can reduce a fatigue life or shock resistance.
2) Si: 0.5-2.5 wt %
Si effectively acts as an element increasing resistance to softening in tempering and the hardness after quenching and tempering to improve a rolling fatigue life. In addition, Si acts as an element for deoxidation at melting capable of low-oxidation of the steel. However, if the added amount of Si is over 2.5 wt %, this reduces machinability and tenacity. For this reason, it is preferable that Si is added in the range 0.5-2.0 wt %, and more preferably the range 0.5-2.5 wt %.
3) Mn: 0.3-2.0 wt %
Mn acts as an element improving heat treating characteristics of the steel. This can improve tenacity and strength of the base material, martensite, and can effectively improve a rolling fatigue life. However, if the added amount of Mn is over 2.0 wt %, this reduces machinability and tenacity.
4) Cr: 1.9-2.5 wt %
Cr acts as an element making carbide stable. This can reduce the amount of C of solid-solution in the base material and can reduce occurrence of a white layer caused by diffusion of C. Accordingly, this can improve a life. In order to achieve the effects, at least 1.9 wt % of added amount is required. On the other hand, if the added amount of Cr is over 2.5 wt %, this causes not only a problem that large carbide is produced but also a problem that free-machining characteristics is reduced or the like.
5) Mo: 0.3-1.0 wt %
Mo is an element improving heat treating characteristics. This can improve strength and further can improve a rolling fatigue life. In addition, Mo forms stable carbide with fine structure. This can reduce diffusion of C, so that reduction of the rolling life cause by a structural change is prevented. In order to achieve the effects, 0.3 wt % or more of Mo is required. However, if its content is over 1.0 wt %, the machinability of the steel is reduced.
6) A total of 1.0 wt % or more of Si and Mo
In order to obtain excellent hardness and high temperature rolling fatigue life characteristics after high temperature tempering, a total of 1.0 wt % or more of Si and Mo is required.
The raceway portion 4a of the input disc 4, the raceway portion 7a of the output disc 7, and circumferences 13a of the rollers contain a residual austenite (γ) in a range of depth Z=1.0L, where L is the major axis of a contact ellipse of the traction contact portion (see
Namely, the bearing steel is used under the condition after quenching and tempering, and the metal structure thereof is a mixed structure of martensite, a residual austenite and a sphere of carbide. When this material is used, the volume fraction of the residual austenite at the time of the completion of quenching and tempering is about 12%-19%. Generally, it is known that some amount of residual austenite improve a rolling fatigue life. However, regarding a toroidal-type continuously variable transmission, in the rolling member used under a high surface pressure condition, a residual austenite can be a source of a local structural change. This accelerates the occurrence of a white layer. For this reason, it is preferable that the residual austenite is 15 wt % or less, and more preferably 7-13 wt %.
In addition, since the rolling component used under rolling contact is generally used under a high stress such as thousands Mpa of contact surface pressure, the surface hardness is set to be HRC 60 or more. However, if hardness is over HRC 62, since the lattice strain of a material structure is high, and the density of dislocation, which is the lattice defect of the material, is also high, its structure becomes unstable. This tends to occur a white structure. On the other hand, if the hardness is lower than HRC 58, a normal rolling fatigue life is reduced. For the above reason, it is preferable that the hardness is HRC 58-62, more preferably HRC 59-61.
Further, in the case of rolling contact, it is known that the depth, in which the shear stress internally acting occurs, is 0.4-0.5b, where b is the minor axis of the contact ellipse of the contact portion. As a result of experiment, it is found that the depth, in which flaking caused by a white layer occurs, is shallower than 1.0b. Therefore, adjusting the amount of residual austenite can reduce white layer flaking.
Furthermore, it is preferable that the oxide particle size in the range of depth Z=1.0L from the surface of each of raceway portions 4a, 7a and each circumference of the roller 13a is less than 20 μm, more preferably 14-19 μm. Accordingly, decreasing the oxide particle size of the rolling contact portion can reduce the occurrence of flaking caused by the oxide as the source.
Next, the blank B obtained in the cutting mentioned above is subjected to heat treatment (
After the heat treatment, the flanks of gear teeth of the spline hole 4c of the blank B are subjected to shaving by the broach 21 (
A method for producing of the output disc 7 and the roller 13 is similar to a conventional method except that tempering temperature in the heat treatment is 250° C. or more.
Table 1 shows the experimental result of the amount of residual austenite in the range of depth Z=1.0L from the surface of the raceway portion, the hardness in the range, the maximum oxide particle size, and the life of the discs of bearing steel for semi-high temperature obtained by tempering at the temperature 250° C. (examples 1-4) and the discs of bearing steel for semi-high temperature obtained by tempering at the temperature 200-220° C. (comparative examples 1-6). In addition, in examples 3, 4 and comparative examples 4, 6, subzero treatment is performed for 60 minutes at −70° C. after the heat treatment. Moreover, the hardness is the value of HRC hardness calculated by the measurements of Vickers hardness.
The life-test conditions are as follows:
(1) The number of rotation of the input shaft 3 and the output shaft of the variator 1:2400 rpm (constant speed controlled by inverter control)
(2) Roller surface temperature: 120° C. (a constant temperature controlled by controlling the fluid temperature in an oil tank by measuring the surface temperature of the roller in proximity to the traction contact portion A with a thermocouple.)
(3) The output torque of output portion 8: 400 Nm
(4) Transmission ratio: 1
(5) Toroid radius Ra (
(6) Toroid radius Rb (
(7) Fluid (lubricating oil): SANTOTRAC50 of traction oil (Findett Corporation)
(8) Disc axial end load F1 (
(9) Roller driving force F2 (
(10) Outside diameter of the roller 13: 100 mm
(11) Radius of curvature of the end portion of the roller 13: 19 mm
The amount of residual austenite in the range of depth Z=1.0L from the surface of the raceway portion, the hardness in the range, and the maximum oxide particle size according to the examples 1-4, in which the tempering was performed at the temperature 250° C., were more than or equal to 7 wt % and less than or equal to 13wt %, HRC 59.0-61.0, and 14-19 μm, respectively. As shown in the table 1, it is obvious that the life of these examples is improved 2.5-4.0 times as much as the comparative examples 1-6. In addition, the evaluation of life was based on the increase of vibrations in the testing equipment caused by the surface damage of the discs or the rollers.
In the embodiment, the whole of the input disc 4, the out put disc 7 and the rollers 13 are formed of the bearing steel for semi-high temperature, respectively. However, only the raceway portion 4a, 7a of each of the discs 4, 7 and the circumference portion 13a of the roller 13, which are rolling contact portions, can be formed of the bearing steel for semi-high temperature, respectively. Additionally, at least one rolling contact portion of the raceway portions 4a, 7a and the circumference portion 13a can be formed of the bearing steel for semi-high temperature, in which the residual austenite in the range of depth Z=1.0L from the surface of the rolling contact portion is 15 wt % or less, more preferably 7-13 wt %, and the hardness in the range is HRC 58-62, more preferably HRC 59-61. Further, at least one roller 13, which is subjected to relatively higher load among the plurality of rollers, can be formed of the bearing steel for semi-high temperature. Moreover, although the toroidal-type continuously variable according to the above embodiment is described as a full-toroidal-type continuously variable transmission, it should be appreciated that the toroidal-type continuously variable according to the present invention can be applied to other toroidal-type continuously variable transmission.
| Number | Date | Country | Kind |
|---|---|---|---|
| 202962/2002 | Jul 2002 | JP | national |
