Intermittent Mechanism

a device in motion-picture cameras, projectors, and copying equipment that imparts a periodic intermittent motion to the film passing through the film gate during filming, projecting, or copying. For a certain time period t_n, the film remains stationary while the photosensitive material is exposed—during filming or copying—or while the image is projected on a screen—during projection. The film is then advanced one frame during the time period t_g. In order to avoid a smearing of the image as the film is advanced, the light beam is interrupted by a shutter during t_g. The total operating period of an intermittent mechanism is T = t_n + t_g.

The two most widely used types of intermittent mechanisms are a Maltese cross and a claw mechanism. In the Maltese cross a gear drum mounted on the shaft of the cross moves the film. A claw mechanism has a film-traction tooth (or teeth) moving along a closed path in such a way that for a certain part of the path the tooth engages the film perforations and imparts motion to the film; for the remainder of the path the tooth is disengaged from the perforations. The necessary motion is transmitted to the tooth by means of a cam, crank mechanism, link gear, or other mechanism.

Intermittent mechanisms have a number of engineering characteristics. The kinematic characteristics describe the relations between time and the movement, speed, and acceleration of the film-traction teeth. The dynamic characteristics describe the relations between time and the force applied to the film perforations. The coefficient of efficiency

defines the light losses in the film apparatus during the time t_g. Another important characteristic is the stable positioning of successive frames with respect to the film gate. The spread in the positions of successive frames should not exceed 0.01–0.015 mm for filming and 0.025–0.03 mm for projection.