Watch it work

Figure 1.32

The Maltese (or Geneva) Cross

The continuously moving gear mechanism, which is attached to the motor of the projector, initiates all movement involved in the passage of the filmstrip through the projection gate. If the sprocket wheels, which engage and move the filmstrip, were attached directly to the gear by means of a single drive shaft, the filmstrip would thus move continuously through the projector gate. As you can see, however, the gear is attached directly only to a cam, or pin wheel. Attached to the cam are a pin and a flat, moon-shaped disk called a cam band or shoulder, on which rests the Maltese cross, which is sometimes called the star wheel. When the pin enters the Maltese cross, only then does the cross move, turning counterclockwise as its concave face is pushed away from the cam band. While the pin is engaged in the slot, therefore, the filmstrip is moving. But when the pin is not inside a slot, the concave face of the cross continues to rest against the cam band, so that the cross—and the filmstrip—remain steady. This pattern of movement controls the drive shaft that turns the sprocket wheels. Without the cross mechanism, in other words, the sprocket wheels would move the filmstrip in a continuous motion through the projector gate; the effect of the cross mechanism is to render the movement of the filmstrip intermittent.

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