The steering system that transmits movement of an automobile’s steering wheel to the vehicle’s wheels includes parts such as the steering shaft and rack bar. These parts can be made hollow.
(1) Aim of creating hollow parts
The aim of creating hollow parts is not only to reduce weight but also to increase productivity and simplify production processes.
| Part | Aim of hollowing | Effect |
| Steering shaft | Weight reduction | Weight reduction rate: 15 – 55% |
| Rack bar | Process simplification | Weight reduction rate: 10 – 60% |
| Weight reduction |
(2) Approaches to the creation of hollow rack bars
| Type | Material | Part manufacturing method | Weight reduction |
|
| Cylinders | Rack gear cutting | |||
| Current | Round bar (S45C) | Gun drilling | Cutting out teeth | － |
| I | Cold finished seamless steel tube (S45C) | － | Same as above | 10% |
| II | Same as above | Cold press forming (thinner walls) |
Cold barrel shape forming Cutting out teeth | 40% |
| III | Electric resistance welded steel tube (0.30 – 0.40% C) | Same as above | Cold tooth profile forming | 60% |
(3) Representative examples of commercialization
| Part | Pipe dimensions | Pipe standards | Mechanical properties of material |
| Steering shaft (upper) |
21.7～31.8φ×2.6～3.2t | Equivalent to STKM15A | TS≒540～610N/mm2 |
| Rack bar | 25～40φ×3～5t | Sumitomo standards （0.30～0.35C） |
TS≒540～640N/mm2 |
The propeller shaft or drive shaft of an automobile is an important part that transmits the engine’s motive force to the wheels.
In the past, steel tube at the level of 500 N/mm2 was used for these parts, but demand for lighter vehicles and enhancement of output power has resulted in the use of high strength steel tube at the level of 700 N/mm2 or 800 N/mm2.
