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Jan., 1945 AIRPLANE VIBRATION RECORDER 45
is rarely necessary, because the multiple strands give sufficient friction without requiring excessive tension.
The 3-speed requirement was met by using change gears between the motor and a counter shaft. The flutter caused by gear disturbances is relatively high in frequency, and the amplitude is such that no trouble is experienced from this source. This, of course, partly results from the fact that more flutter may be tolerated at higher rates, as previously explained.
The film path is shown by Fig. 4. The film is held on the recording sprocket by 2 ball-bearing pad rollers, which are the only operating controls in threading. The upper roller has flanges which guide the film onto the sprocket and control film weave to within =*= one mil. The lower roller has a central section of rubber which causes the film to be pressed against the sprocket just ahead of the point where the film leaves the sprocket. This serves 2 purposes: It holds the film against the sprocket with sufficient friction so that displacements of the film owing to sprocket-tooth action tend to be attenuated by frictional damping. This action, combined with the proper choice of sprocket diameter relative to film pitch, keeps the generation of sprocket-hole flutter to a minimum.
A second advantage of this roller is that it makes a critical adjustment of the film take-up tension unnecessary even though this sprocket normally operates as a holdback sprocket. Each magazine provides a light tension on the feed roll, and it was found that as the diameters of the feed and take-up rolls interchange, a condition may arise where the film tension on either side of the sprocket may become approximately equal. Under this condition, it is obvious that considerable instability would be possible since the sprocket teeth are somewhat smaller than the sprocket holes. The action of the rubber roller is such that the film remains at all times in the position on the sprocket corresponding to normal operation as a holdback sprocket.
This film movement has the additional advantage of being extremely simple for threading in conjunction with the removable magazines. This film pulling arrangement has been found to be quite reliable and has operated satisfactorily in this application.
The flutter measured on this equipment has been found to be of the order of =*=0.1 per cent at rates below one cycle per second range, but in this range it is largely a function of the constancy of the supply voltage. From one to 10 cycles per sec, flutter is generally less than ±0.1 per cent and gradually climbs at higher rates to a maximum of