Power transmission shaft, drive shaft, and propeller shaft

1. A power transmission shaft, comprising:

a torque-transmission teeth portion formed on an outer peripheral surface on an end portion side; and

a smooth portion formed on the outer peripheral surface adjacent to the torque-transmission teeth portion so as to have a stepwise reduced diameter, wherein:

a ratio of a minimum diameter of the smooth portion to a minimum diameter of the torque-transmission teeth portion is set to 0.9 or more and 1.02 or less;

a carbon content of a raw material of the power transmission shaft is set to 0.37 wt % or more and 0.44 wt % or less;

the torque-transmission teeth portion and the smooth portion undergo heat treatment so as to form a heat-treated hardened layer;

the heat-treated hardened layer increases in depth from a side of the torque transmission teeth portion to a side of the smooth portion;

at least a minimum diameter portion of the smooth portion undergoes full hardening for forming the heat-treated hardened layer from a surface to a central axis; and

an average prior austenite grain size of the heat-treated hardened layer is set to 8 μm or more and 35 μm or less.

2. A power transmission shaft according to claim 1 , wherein:

a ratio of a layer thickness of the heat-treated hardened layer of the torque-transmission teeth portion to an axial radius thereof is set to 0.4 or more and 0.65 or less; and

a surface hardness of the torque-transmission teeth portion is set to Hv 650 or more and Hv 690 or less.

3. A power transmission shaft according to claim 2 , wherein:

the surface hardness of the torque-transmission teeth portion is set to be lower than a surface hardness of the minimum diameter portion of the smooth portion; and

an upper limit value of a difference of the surface-hardness of the minimum diameter portion of the smooth portion and the surface hardness of the torque-transmission teeth portion is set to Hv 60.

4. A power transmission shaft according to claim 3 , wherein a hardness of an axial center portion of a fully-hardened part of the smooth portion is set to Hv 400 or more and Hv 600 or less.

5. A power transmission shaft according to claim 3 , wherein a surface compressive-residual stress of a fully-hardened part of the smooth portion is set to 400 MPa or more and 800 MPa or less.

6. A power transmission shaft according to claim 2 , wherein a hardness of an axial center portion of a fully-hardened part of the smooth portion is set to Hv 400 or more and Hv 600 or less.

7. A power transmission shaft according to claim 2 , wherein a surface compressive-residual stress of a fully-hardened part of the smooth portion is set to 400 MPa or more and 800 MPa or less.

8. A power transmission shaft according to claim 1 , wherein:

a surface hardness of the torque-transmission teeth portion is set to be lower than a surface hardness of the minimum diameter portion of the smooth portion; and

an upper limit value of a difference of the surface-hardness of the minimum diameter portion of the smooth portion and the surface hardness of the torque-transmission teeth portion is set to Hv 60.

9. A power transmission shaft according to claim 8 , wherein a hardness of an axial center portion of a fully-hardened part of the smooth portion is set to Hv 400 or more and Hv 600 or less.

10. A power transmission shaft according to claim 8 , wherein a surface compressive-residual stress of a fully-hardened part of the smooth portion is set to 400 MPa or more and 800 MPa or less.

11. A power transmission shaft according to claim 1 , wherein a hardness of an axial center portion of a fully-hardened part of the smooth portion is set to Hv 400 or more and Hv 600 or less.

12. A power transmission shaft according to claim 11 , wherein a surface compressive-residual stress of the fully-hardened part of the smooth portion is set to 400 MPa or more and 800 MPa or less.

13. A power transmission shaft according to claim 1 , wherein a surface compressive-residual stress of a fully-hardened part of the smooth portion is set to 400 MPa or more and 800 MPa or less.

14. A power transmission shaft according to claim 1 , wherein the power transmission shaft contains as materials: 0.02 wt % or more and 0.25 wt % or less of silicon; 0.6 wt % or more and 1.2 wt % or less of manganese; 0.02 wt % or less of phosphorus; 0.025 wt % or less of sulfur; 0.0005 wt % or more and 0.0035 wt % or less of boron; and 0.01 wt % or more and 0.05 wt % or less of titanium.

15. A power transmission shaft according to claim 1 , which is coupled to a constant velocity universal joint, wherein the minimum diameter portion of the smooth portion is formed at a part which approaches the constant velocity universal joint when the constant velocity universal joint forms a maximum operating angle.

16. A power transmission shaft according to claim 1 , which is applied to a stab shaft.

17. A drive shaft, comprising an intermediate shaft having both end portions to which constant velocity universal joints are mounted,

wherein the power transmission shaft according to claim 1 is applied to the intermediate shaft.

18. A propeller shaft, comprising an intermediate shaft having both end portions to which constant velocity universal joints are mounted through intermediation of stab shafts,

wherein the power transmission shaft according to claim 1 is applied to each of the stab shafts.