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386 J. C. SMACK [j. s. M. p. B.
four principal types of casing generally used are the fabric, the twowire metallic, the interlocking metallic, and the rubber covered. These casings all have special uses and will be taken up later.
For those interested in more detailed information on flexible shafting the S. S. White Dental Co. about two years ago published the Handbook on Flexible Shafts. This book has been so widely accepted and praised by engineers that a more elaborate handbook has been compiled giving complete engineering data on all sizes of flexible shafting, and in addition descriptions and photographs of actual applications in various industries. This handbook will be invaluable to all designing engineers and will be distributed gratis to those interested.
CAMERA DRIVES
Let us investigate the characteristics of those shafts most widely used in the motion picture industry, that is, the y4 in. and 5/i6 in. diameters. A flexible shaft, to be satisfactory for synchronized sound motion picture equipment, must be flexible transversely — must be-torsionally as stiff as practical, and still not be too heavy or cumbersome for general use. To this end the y4 in. diameter grade "H" left lay shaft was selected for driving the Bell & HowellWestern Electric cameras. This shaft has sufficient flexibility to allow the motor to be placed under the camera, to one side, or in whatever position is the most convenient for the operator. The safe allowable torque on this shaft at 1450 rpm., the speed of the camera drive shaft, is 13 inch-pounds — considerably more than the actual torque of the camera at any time. Extensive experimentation with this shaft indicated its altogether satisfactory performance, and it was adopted by the Western Electric Co. as standard equipment for their cameras. In the selection of a casing for this application, the y2 in. diameter fabric type was used, due to its lightness in weight and flexibility.
Further experimentation on various types of camera drives indicated that there were cameras which exerted a greater torque on the flexible shaft than others. Investigation showed that some cameras, in particular the new sound cameras, were considerably stiffer in operation than others. Some were so stiff that at times a wobble or fluctuation was experienced in the camera drive. A larger shaft, the 6/ie in. diameter, was substituted for the x/4 in and no further trouble was experienced on these cameras. With this shaft a 8/s in. diameter two-wire, black japanned metallic casing is used, or a 5/g in.