The Geneva wheel or Maltese cross is a cam-like mechanism that provides intermittent rotary motion and is widely used in both low and high-speed applications.
Geneva mechanism is extensively used in automatic machinery where turret, spindle, or worktable must be indexed. The Geneva mechanism is popular for a long time by means of producing positive incremental motion.
Construction of Geneva Mechanism
The basic structure of the 4 slot geneva mechanism is shown in the figure below.
The system consists of a constantly rotating disk coupled with a slotted disk, which gives rise to the desired discrete motion.
A rotation of 2p radians of the former cause 2p/N radians of rotation of the latter, where N is the number of slots available on the slotted disk.
Thus, one complete rotation of the slotted wheel requires N complete rotations of the other disk, thereby also increasing the time period.
Working of Geneva Mechanism
Refer to the figure above. Pinwheel W rotates constantly about axis A and has a pin ‘a’ attached to it. This pin ‘a’ engages into the slots ‘s’ of the geneva wheel ‘G’ and rotates it as long as it is engaged with the slot. While the wheel rotates continuously, the Geneva wheel has a discrete rotation about axis ‘b’.
Wheel ‘G’ has a rotation time period of ‘tr’ when it is moving with disk ‘W’ and an idling time period of ‘ti’, when the pin ‘a’ is not inside one of the slots ‘s’ and is moving freely.
The three-quarter wheel ‘L’ is placed in order to prevent any unintentional rotation of wheel ‘G’ while it is idling.
For better understanding, please watch this video on the geneva mechanism.