Steering system and method for manufacturing the same

1. A steering system comprising:

a steering operation shaft having rack teeth and configured to turn steered wheels of a vehicle by moving in an axial direction of the steering operation shaft;

a lower shaft having pinion teeth meshing with the rack teeth;

an upper shaft having one end to which a steering wheel is attached; and

an intermediate shaft disposed between the lower shaft and the upper shaft and connected to the lower shaft and the upper shaft via universal joints,

wherein the intermediate shaft includes:

a tubular member including a hollow portion; and

a shaft member disposed such that a part of the shaft member in a longitudinal direction of the shaft member is housed in the hollow portion, the shaft member and the tubular member being spline-fitted to be axially movable relative to each other and not to be rotatable relative to each other,

wherein an oil groove that retains lubricating oil in a form of a soft solid is provided in a predetermined range in an axial direction of the intermediate shaft on at least one of an outer peripheral surface of the shaft member at the part housed in the hollow portion and an inner peripheral surface of the hollow portion of the tubular member, and

wherein a sectional area of the oil groove in an upper shaft-side part of the predetermined range is larger than a sectional area of the oil groove in a lower shaft-side part of the predetermined range, the upper shaft-side part being located on a side of the upper shaft, and the lower shaft-side part being located on a side of the lower shaft.

2. The steering system according to claim 1 , wherein the oil groove has a helical shape in which a groove sectional area gradually increases from the lower shaft-side part to the upper shaft-side part.

3. A method for manufacturing the steering system according to claim 2 , the method comprising

emitting a laser beam for forming the oil groove to the outer peripheral surface of the shaft member, and moving the shaft member in an axial direction of the shaft member relative to a laser head that is emitting the laser beam while rotating the shaft member about a central axis of the shaft member.

4. The steering system according to claim 1 , wherein:

the oil groove includes a plurality of oil grooves provided in the predetermined range, and each of the plurality of oil grooves has an annular shape; and

a sectional area of each of the plurality of oil grooves is set such that the sectional area of the oil groove increases as a distance between the oil groove and the upper shaft decreases.

5. The steering system according to claim 1 , wherein the oil groove is provided on the outer peripheral surface of the shaft member.

6. The steering system according to claim 5 , wherein the shaft member has an oil hole extending in a direction intersecting a central axis of the shaft member, and the oil hole communicates with the oil groove.

7. The steering system according to claim 6 , wherein the oil hole extends through the shaft member, and both ends of the oil hole communicate with the oil groove.