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THE PROJECTOR 221
(50) There must be no looseness between the shaft and the sprocket, nor should it fit over-tightly upon the shaft. If it is too loose the sprocket rim will not rotate in a true circle with relation to the shaft. It will wobble and result in a vertical movement of the screen image. If it is too tight the sprocket will be strained — forced out of true form — which may result either in side weave or an up-and-down movement of the screen image or both.
(51) In the event that the projectionist decides to install an intermittent sprocket himself the best procedure is as follows : Remove the intermittent movement from the projector and the sprocket and its shaft from the movement. Remove the taper pins (remember they are taper pins) holding the sprocket to the shaft. Then with the help of a clean piece of cloth grasp the shaft in one hand and the sprocket with the other, and pull it off the shaft with a twisting movement.
Or it can be done another way. Lay the end of the sprocket hub on wooden blocks placed on the jaws of a slightly opened iron vise, place a hardwood block upon the end of the shaft and tap upon it very gently, driving out the shaft. Be very careful not to let the shaft touch the jaws of the vise.
(52) Next clean the shaft thoroughly and lubricate it with a good grade of oil. Insert it in the, new sprocket and, using a twisting motion, work the sprocket slowly onto the shaft. If it sticks, remove it, wipe the shaft, re-lubricate and try again. Keep doing this until finally the sprocket slips into correct position.
Put the taper pins in very carefully and drive them in snugly, but not too tightly or you may strain the sprocket hub and force the whole thing out of true line.
In twisting the sprocket onto the shaft, remember that both shaft and sprocket are made with the greatest accuracy, so that it can only be necessary to wear off perhaps the thickness of 1/10,000 of an inch of metal by the twisting motion.
(53) In the event you do not care to purchase the pin ejector, you can make a V block as shown in Fig. 65. If