Geneva drive

The Geneva drive or Maltese cross is a gear mechanism that translates a continuous rotation into an intermittent rotary motion. The rotating drive wheel has a pin that reaches into a slot of the driven wheel advancing it by one step. The drive wheel also has a raised circular blocking disc that locks the driven wheel in position between steps.

Background

The name derives from the device's earliest application in mechanical watches, Switzerland and Geneva being an important center of watchmaking. The geneva drive is also commonly called a Maltese cross mechanism due to the visual resemblance.

In the most common arrangement, the driven wheel has four slots and thus advances for each rotation of the drive wheel by one step of 90°. If the driven wheel has n slots, it advances by 360°/n per full rotation of the drive wheel.



Because the mechanism needs to be well lubricated, it is often enclosed in an oil capsule.

Uses and applications

One application of the Geneva drive is in movie projectors: the film does not run continuously through the projector. Instead, the film is advanced frame by frame, each frame standing still in front of the lens for 1/24 of a second (and being exposed twice in that time, resulting in a frequency of 48 Hz). This intermittent motion is achieved using a Geneva drive. (Modern film projectors may also use an electronically controlled indexing mechanism or stepper motor, which allows for fast-forwarding the film.) The first uses of the Geneva...
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