The present invention relates to a key assembly for the keyboards of electric typewriters and similar machines and, more particularly, to a key assembly containing a plurality of vertically guided keyboard elements which, upon actuation, act upon domed contact beads of a contact mat arranged below the keyboard elements, with the contact mat, in turn, resting on a printed circuit board.
A keyboard utilizing a key assembly of the kind referred to above is disclosed in German patent application (DE-AS) 23 50 176. When used with an electronic pocket calculator, the keyboard elements are inserted into the upper part of the housing while the contact mat and the printed circuit board are arranged in the lower part of the housing. By a spring plate, the keyboard elements are pushed into their normal positions into which they are lifted by the beads of the contact mat.
This arrangement requires high dimensional accuracy in assigning the reference surfaces between the keyboard elements and the contact mat. Under certain circumstances, greater tolerances are responsible already within the relatively small key pad of an electronic pocket calculator, for noticeable differences in the key travel.
With regard to keyboards of electric typewriters and similar machines which, not only from the surface, but also from the number of keys are considerably more voluminous, it appears that the principle of the known type of construction cannot be applied thereto. Owing to the guide lines on the ergonomic design of keyboards, certain requirements are placed with a view to the key travel and the operating force, demanding an exact, narrow-tolerated assignment of the keyboard element to the respective contact bead.
It is, therefore, the object of the invention to provide a key assembly for electric typewriters and similar machines which may use a contact mat and meets the above requirements.