A steering shaft assembly being composed of an upper part and a lower part. The lower part is cylindrical in shape and is rotatably received by a cylindrical outer housing with the upper part fixedly secured thereabove on the outer housing. The lower part is integral with a steering shaft. An engaging block assembly is freely rotatable and is longitudinally slidable within the steering shaft assembly between an upper limit position and a lower limit position. The engaging block assembly comprises an engaging block and an actuating block. A number of balls are disposed between the steering shaft assembly and the engaging block assembly as well as between the engaging block and the actuating block for transmitting rotational movement. A steering wheel is fixedly mounted on the engaging block assembly by a lock assembly which further controls releasing and engaging between the engaging block assembly and the steering shaft assembly. The engaging block assembly, and therefore the steering wheel, is unable to drive the lower part when the engaging block assembly is in the upper limit postion, while the engaging block assembly rotates the lower part, and therefore the steering shaft, when the engaging block assembly is in the lower limit position.