International projectionist (Jan-Dec 1956)

The Versatile Claw Intermittent By JOSE RUIZ A re-examination of this relatively simple and precise mechanism. Although the claw will not stand up under the continuous pounding taken by geneva movements in theatre projection, it still remains highly regarded for a variety of applications to motion pictures. SINCE the introduction of 16-mm motion picture film and equipment, the most favored mechanism to move the film intermittently has been without doubt the old and classical claw movement. This simple and reliable mechanism was the first practical means of accomplishing the illusion of real movement on the screen. One of the first machines to utilize the claw movement was the Lumiere Cinematographe camera-projector. This machine employed a mechanism of the cam and shuttle type. The movement is illustrated in Fig. 1. A heartshaped or three cornered cam (A) acting upon a shuttle (B) furnishes the vertical pulldown motion and a drunk cam (C) gives the shuttle claws (D) the motion of entering and leaving the film perforations. The path described at any point by the claws FIGURE 2 FIGURE 1 is shown in Fig. 2 and takes the shape of a rectangle parallel to film plane and claws. When activated by the cam, the claws move forward, down, backward, and up. The First Intermittents Many variants of the claw mechanisms were developed in the early days of cinematography. Fig. 3 shows one of the claw mechanisms used by Darlin. A mechanism employed by early English camera and projector manufacturers is illustrated in Fig. 4. These motions derived from the crank and connecting rod are of the type known as Williamson motions and were classical devices to change circular movement into alternative rectilinear motion. The path generated by any intermediate point between the crank-pin and connecting end rod is approximately an ellipse. By changing the gyration fulcrum point of the connecting rod end with a swing link, the motion generated becomes a D shaped motion. A projector mechanism employing the Williamson motion is shown in Fig. 5 along with the motion path generated by the claws. This movement was used in many English cameras and projectors. Although this mechanism is known as the Williamson motion, his mechanical principle is much older still and dates back to James Watt. An early English intermittent movement of the Prestwich type is is illustrated in Fig. 6. This intermittent produced a D-motion similar to the Williamson motion illustrated in Fig. 5. The connecting rod (B) is acuated by the crank-pin on the disk (D) and the alternative motion derived is constrained by the intermediate link (C), producing the motion shown in diagram at H. In order to get a straight path on the pulldown stroke, the claws (A) are secured on the flat spring (G), and the slotted strip (F) which is driven by the connecting rod end (B) at point (J), so when the claws enter the film gate slot (E), the claws are forced by the action of the spring (G) to follow a straight path and pull the film down toward the film gate (K), producing the motion of diagram (I). Faster Claw Movement A claw movement producing a rather more rapid shift, is the heart cam employed by Lumiere brothers in their Cinematographe in 1895. From then until the advent of the Armat geneva movement, many 35-mm projection FIGURE 3 16 INTERNATIONAL PROJECTIONIST • March 1956