Geneva Mechanism: its history, function, and weaknesses

The Geneva mechanism is a timing device. According to Vector Mechanics for Engineers for Ferdinand P. Beer and E. Russell Johnston Jr.,  says, “ [It] is used in many counting instruments and in other applications where an intermittent rotary motion is required.” (945) Essentially, the Geneva mechanism consists of a rotating disk with a pin and another rotating disk with slots (usually four) into which the pin slides (see right).

According to Brittanica.com, the Geneva mechanism was originally invented by a watch maker. The watch maker only put a limited number of slots in one of the rotating disks so that the system could only go through so many rotations. This prevented the spring on the watch from being wound too tight, thus giving the mechanism its other name, the Geneva Stop. The Geneva Stop was incorporated into many of the first film projectors used in theaters.

In Optimum Design of Mechanical Elements, Ray C. Johnson makes many references to the use of the Geneva mechanism to provide an intermittent motion the conveyor belt of a "film recording marching." (13) He also discusses several weak points in the Geneva mechanism. For instance, for each rotation of the Geneva (slotted) gear the drive shaft must make one complete rotation. Thus for very high speeds, the drive shaft may start to vibrate. Another problem is wear, which is centralized at the drive pin. Finally, the designer has no control over the acceleration the Geneva mechanism will produce.

Also, the Geneva mechanism will always go through a small backlash, which stops the slotted gear. This backlash prevents controlled exact motion. (Picture at left from Optimum Design of Mechanical Elements.)

Below are models of the Geneva mechanism made with Working Model 2d v4.0. The second model shows velocity vectors for the slotted gear and the drive shaft. Velocity is the black arrow and acceleration is the green arrow.  Move the mouse over the mechanisms to start them running.

Here is a video clip of the Geneva mechanism modeled above:

Here are other sites about the Geneva Mechanism:

http://www.bath.ac.uk/Departments/Eng/ug/online/y3new/mech0048/99g2/messages/14.html

http://www.cabaret.co.uk/education/geneva.htm

http://www.brockeng.com/mechanism/Geneva.htm