14-03-2017, 10:45 AM
The Geneva Wheel, or Maltese Cross, is a cam as the mechanism providing intermittent rotary motion and is widely used in low and high speed machinery. Although originally developed as a stop to prevent excessive winding of clocks, it is now widely used in automatic machinery, eg. Where a spindle, turret or workbench must be indexed. It is also used in film projectors to provide intermittent film advance. The basic design criterion of a Geneva wheel is that the central lines of the groove and crank are mutually perpendicular in the coupling and in the separation. The crank, which normally rotates at a uniform angular velocity, carries a roller to engage the grooves. During a revolution of the crank, the wheel of Geneva rotates a fractional part of the revolution, whose amount depends on the number of slots. The circular segment fixed to the crank effectively locks the wheel against rotation when the roller is not engaged and also positions the wheel for correct coupling of the roller with the next groove. Geneva mechanisms have long been popular as a means of producing positive incremental movement. This popularity is due in part to the simplicity of the mechanism, both in design and construction, which makes it an inexpensive indexing device. In addition, the mechanism inherently produces precise positioning movement which is necessary for many applications.
In applications where this mechanism is currently used, it has proven extremely trouble-free and reliable. In the future it is expected that this device can find many applications that require higher speeds. As higher speeds become necessary, the mechanism becomes less attractive as an incremental device because of its kinematic limitations. "For example, a severe limitation in these conditions can result from the maximum acceleration of the relative wheel with respect to its average acceleration. They cause excessive dynamic loads which, in turn, can cause severe wear of the pins and slots and / or Thus, the problem of analytical design in the case of high speed Geneva mechanisms, where inertial loads are dominant, is one in which the best combination of design variables is sought to reduce The kinematic limitations inherent in the mechanism.