1. Field of the Invention
This invention relates to devices for automatic angular positioning of indexable rotary machine elements, especially for office machines, packaging machines and, more generally, machines in which it is necessary to adjust the angular position of one or a number of elements.
The invention is primarily applicable to automatic positioning of printing wheels in a postal franking machine, the printing drum of the machine being so designed as to constitute a rotary support for the printing wheels, indexing of which permits "value-setting" of the machine for each franking operation.
2. Description of the Prior Art
A machine of this type can comprise a frame, a main shaft rotatably mounted in the frame, a printing drum which is carried by said shaft and in which printing wheels are rotatably mounted on a cross-shaft and are adapted to carry toothed pinions each disposed in meshing engagement with a first corresponding toothed rack rigidly fixed to a bar which is mounted for longitudinal sliding motion within said main shaft whilst a second longitudinal toothed rack rigidly fixed to each slide-bar aforesaid is disposed in meshing engagement with one of a number of individual coaxial value-setting toothed wheels or so-called gear-wheels.
In some known devices and in particular the device described in U.S. Pat. No. 4,398,458, each slide-bar is actuated by a stepping motor. In order to ensure that it can readily be housed inside the machine, each stepping motor must be of small size and is therefore capable of producing only a low torque. This gives rise to a potential danger of interruptions caused by loss of step when the slide-bars and gear-wheels are hindered in their movement by the printing ink, thus entailing the need for frequent cleaning. Moreover, the axial arrangement of the slide-bars makes it necessary to provide complicated transmission systems between these latter and the printing wheels.
In other devices of the described, for example, in U.S. Pat. No. 3,965,815, the printing wheels are positioned by independent pinions which are engaged successively with a gear-wheel. The gear-wheel is rotatably mounted in a carriage which is capable of displacement in a direction parallel to the common axis of the pinions, said gear-wheel being driven by a single stepping motor. Displacement of the carriage is produced by two electromagnets which are energized either together or separately in order to cause pivotal displacement of levers connected to the carriage by means of a shaft which converts the movement of rotation to a movement of translation. In this design, there remain a large number of mechanical couplings, the inertia of which limits the speed of movements and makes it necessary to provide two electromagnets of large size. Finally, the number of positions of the carriage is limited to four positions representing the combinations of two electromagnets having two states.
In other devices of the type described, for example, in U.S. Pat. No. 4,050,374 (French Pat. No. 2,355,659), the slide-bars are actuated successively by a first stepping motor and the shaft of this motor slidably supports a carriage which is driven transversely by a second stepping motor. In spite of its simple design, the second stepping motor is both cumbersome and costly.