PURPOSE: To put a cover on or off without using tools by installing a protruding guide in parallel with the rack on a shaft guide peak part and by fitting an approximately L shaped slider with a pinion axis so that the slider freely slides.

CONSTITUTION: A pinion axis 6 is held by plural number of bearings 10a which are installed in the back portion of a cover 4 itself and the axis is freely rotatable. Then, a slider 14 is energized at the left and the right end of the pinion axis 6 by a spring 13 so that the slider is pushed against a pinion 5. The pinion 5 is then mounted on the pinion axis 6 by screws. The guide 8a formed on the cover peak part 7 and the fitting section 14a of the slider 14 which is mounted on the pinion axis 6 are fitted or removed by a sliding action. Thus, the cover is incorporated or removed without using tools.