> > > >

1952 Theatre Catalog, 10th Edition, Page 297 (261)

1952 Theatre Catalog, 10th Edition
1952 Theatre Catalog
1952 Theatre Catalog, 10th Edition, Page 297
Page 297

1952 Theatre Catalog, 10th Edition, Page 297

ridges which may roughen edges of the film, particularly if the film is old and brittle. These ridges if deep enough, may even loosen the corners of poor splices. The guide roller assembly should be inspected frequently and, if necessary, removed for thorough cleaning. The guide roller tension springs should be checked occasionally to see that they have not become so weak that proper tension is not maintained. Weak springs might allow sidewise unsteadiness, particularly with a fluttering oversize loop.

Sprocket Pad Rollers or Idlers

Sprocket pad rollers or idlers are frequently set tooclose to the sprocket so that they ride the film and often cause marking and creasing. If they bind on the shaft, rubbing will occur at the side of the picture or possibly into the sound track area, particularly in the case of new prints. Such rub marks in the sound track area can seriously impair the sound reproduction quality. They generally cause frying noises and any picked-off spots which occur at intervals will result in ffiiiotorboatingf although it is also true that the latter effect might often be caused by printing defect. If roller spacing is too great, the film may also jump the sprocket, especially at a stiff splice. The generally accepted setting is at the point where the idler

FIG. 19. Cone-shaped springs for tension adjustment on newer type projectors.

can still turn freely with two layers of standard thickness film on the sprocket. To insure free turning, the rollers should be removed occasionally so that the shaft can be cleaned of any gummy deposit, then lubricated slightly. On reassembling and resetting, it is important to make certain that there is no binding between the roller and sprocket teeth. It is also


FIG. 17. Measuring gate or "trap" tension.

FIG. 18. Spring balance for tension testing.

advisable to check the lock nut on the adjusting screw periodically.

Trap or Gate Tension

One of the principal sources of film damage is excessive tension exerted on the film by the springs in the gate or trap. Some projectionists judge the tension by merely pressing on the pads, checking one side, then the other. As far as is known, there seems to be no generally accepted standard setting for any given projector. The results of tests made in the Kodak Research Laboratories on various projectors and over a long period of time, indicate that complete, equalized trap tension in excess of 16 ounces is unnecessary and only increases the wear on the film, while settings as low as 6 ounces appear, in some instances, to be sufficient to give a steady screen image. It would seem from these tests that the tension should fall somewhere within the range from 6 to 16 ounces.

The proper pressure will, of course, depend upon the projector used, the length of time it has been in service, and the type of surface treatment the film has received. The projectionist does not usually know what the tension might be for his projector, since he is not

ordinarily faced with the necessity of checking it when the equipment is in a smooth-running condition. He should, however, be familiar with the method of measurement.

Tension Testing

The proper method of making a tension test is illustrated in Figure 17. To determine the total trap tension, a full width piece of reasonably new film of standard thickness and free from oil is placed in the trap or gate of the projector. The test should be made on a cold projector to insure uniformity. Care should be taken to see that the perforations are not engaged by the intermittent sprocketiteeth and that the film is held properly in place by the tension shoe before proceeding. Using a small, graduated spring balance, such as'the one illustrated in Figure 18, which has been fastened to the film, an upward pull should be exerted slowly until the film just starts to move. If the film appears to stick at the start it should be moved slightly and the operations should be repeated. If the tension is found to be above 16 ounces, the tension should be reduced so that it falls within the range from 6 to 16 ounces, preferably near the lower end of the range. The projector should then be checked for steadiness of the screen image.

FIG. 20. Double-pronged finger-type springs used on older type prowctors.

Tension Adjustment

Tension adjustment on the newer type projectors has been greatly simplified. Centralized pressure exerted on the pads by cone-shaped springs (see Figure 19) has eliminated practically all of the difficulties previously experienced with double-pronged fingerstype springs (see Figure 20). Vertical unsteadiness may of
1952 Theatre Catalog, 10th Edition, Page 297