A spline shaft that can suppress occurrence of stick slip by measures other than just simply making the surface roughness smaller. The spline shaft includes a first shaft with an outer-peripheral spline formed, and a second shaft with an inner-peripheral spline, which engages with the outer-peripheral spline peripherally and that is made slidable axially, formed. And, at least either one of the outer-peripheral spline and the inner-peripheral spline includes a substrate whose surface is formed to a predetermined surface roughness, and an amorphous carbon-system hard thin film, which coats and forms the surface of substrate.
1. A spline shaft for transmitting torque while making it possible to move relatively to each other in axial-line direction, the spline shaft comprising:
a first shaft being provided with an outer-peripheral spline, which is formed on its outer peripheral surface; and
a second shaft being provided with an inner-peripheral spline, which is formed on its inner peripheral surface, which engages with said outer-peripheral spline peripherally, and which is made slidable axially;
at least either one of said outer-peripheral spline and said inner-peripheral spline being equipped with:
a substrate whose surface is formed to a predetermined surface roughness; and
an amorphous carbon-system hard thin film, which coats and forms the surface of said substrate,
wherein said predetermined surface roughness is from Rz (10-point average roughness based on JIS B 0601: 1994) 0.1 μm or more to 5.0 μm or less.
2. The spline shaft set forth in claim 1 , wherein:
a friction coefficient between said outer-peripheral spline and said inner-peripheral spline is 0.15 or less when a relative sliding speed between said first shaft and said second shaft falls in an range of 0.1 m/s or less; and
a ratio of a static friction coefficient to a dynamic friction coefficient, between said outer-peripheral spline and said inner-peripheral spline, is 1.0 or less when said sliding speed is 0.1 m/s.
3. The spline shaft set forth in claim 1 , wherein a film thickness of said amorphous carbon-system hard thin film is from 0.4 μm or more to 10.0 μm or less.
4. The spline shaft set forth in claim 1 , wherein a hardness of said amorphous carbon-system hard thin film is 800 Hv or more.
5. The spline shaft set forth in claim 1 , wherein said amorphous carbon-system hard thin film contains either one of silicon, tungsten and titanium.
6. The spline shaft set forth in claim 1 , wherein an adhesion force of said amorphous carbon-system hard thin film to said substrate is 10 N or more.
7. The spline shaft set forth in claim 6 , wherein a treatment of undercoat, which comprises at least either one of nitrided layer, Si layer, Cr layer and CrN layer, is performed onto a superficial portion of said substrate of at least either one of said outer-peripheral spline and said inner-peripheral spline.
8. The spline shaft set forth in claim 1 , wherein said amorphous carbon-system hard thin film is formed by means of physical vapor deposition method or high-frequency plasma chemical vapor deposition method at 200° C. or less.
9. The spline shaft set forth in claim 1 , wherein said amorphous carbon-system hard thin film is formed on a superficial portion of said outer-peripheral spline.
10. The spline shaft set forth in claim 1 , wherein:
said spline shaft is a propeller shaft for transmitting a driving force of a power unit to a wheel of a vehicle;
said first shaft is connected to one of said power unit and said wheel at one of its opposite ends, and is provided with said outer-peripheral spline being formed on an outer peripheral surface on the other of its opposite ends;
said second shaft is connected to the other of said power unit and said wheel at one of its opposite ends, and is provided with said inner-peripheral spline, which is formed on its inner peripheral surface on the other of its opposite ends, which engages with said outer-peripheral spline peripherally, and which is made slidable axially.
11. The spline shaft set forth in claim 1 , wherein a lubricant is interposed between said outer-peripheral spline and said inner-peripheral spline.
12. The spline shaft set forth in claim 1 , wherein said predetermined surface roughness is from Rz (10-point average roughness based on JIS B 0601: 1994) 0.1 μm or more to 3.0 μm or less.
13. A method for manufacturing spline shaft for transmitting torque while making it possible to move relatively to each other in axial-line direction,
the spline shaft including:
a first shaft being provided with an outer-peripheral spline, which is formed on its outer peripheral surface; and
a second shaft being provided with an inner-peripheral spline, which is formed on its inner peripheral surface, which engages with said outer-peripheral spline peripherally, and which is made slidable axially;
said spline shaft manufacturing process comprising:
at least either one of said outer-peripheral spline and said inner-peripheral spline is subjected to coating and forming of an amorphous carbon-system hard thin film on a substrate surface after forming the substrate surface to a predetermined surface roughness,
wherein said predetermined surface roughness is from Rz (10-point average roughness based on JIS B 0601: 1994) 0.1 μm or more to 5.0 μm or less.
14. The spline shaft manufacturing process set forth in claim 13 , wherein said predetermined surface roughness is from Rz (10-point average roughness based on JIS B 0601: 1994) 0.1 μm or more to 3.0 μm or less.