How can a "Dual" 8mm have only one set of sprocket

[jimhawley]

I am new to this so pardon my question if the answer is obvious.

I just found our old Chinon Whisper 8mm Dual projector and have been viewing some of our old super 8 movies. I then found an older regular 8 film and attempted to view that as well by moving the switch on the projector from S to R as instructed. The projector proceeded to eat the film, wrapping it many times around the first sprocket.

When I looked inside the projector, there was only one set of sprockets. This does not make sense to me. What is this S/R switch doing? Shouldn't it be switching between two sets of sprockets that match the corresponding films?

Thanks for the help,

Jim

[BigBeaner]

It switches the gates from the smaller Regular one to the larger Super one. I've used a projector like this all the time and it's fine.

[tim]

It doesn't need two sets of sprockets because of the design of the sprockets. The teeth are tapered both ways: both at both sides and from the inside, and there is a rim. R8 film sits right down on the sprocket teeth and inside the rim. S8 film sits on the top of the teeth, and on top of the rim. Very clever, really.

Several projector makes use this system which seems to work well for small sprockets, but not for larger diameters.

[jimhawley]

Are you saying that the tapering compensates for the necessity to have teeth spaced at different intervals for the two types of film. Don't R and S have a different spacing interval besides different size holes?

[Mitch Perkins]

I then found an older regular 8 film and attempted to view that as well by moving the switch on the projector from S to R as instructed. The projector proceeded to eat the film, wrapping it many times around the first sprocket.
Jim

Sometimes the last person to run the reel rewound the film incorrectly. Proper wind is emulsion out, (facing toward rear projector lens element), perfs on the side toward operator, or, y'know, on the tooth side of gears.
Emulsion side is somewhat duller than base, and if you have a length of black/clear film, put it lightly between your lips; the side that sticks is emulsion.
Also, if the wind is too "tight", w/too much curvature, the film might wind around the gear as you describe. Pull the first bit of leader head through your thumb and forefinger, much the same way you would remove curl of a piece of paper by "shoeshining" it on the edge of your desk.
Lastly, if the projector has a clipping device to round off the leader, make sure you use that.

Mitch

[Mitch Perkins]

Are you saying that the tapering compensates for the necessity to have teeth spaced at different intervals for the two types of film. Don't R and S have a different spacing interval besides different size holes?

R8/S8 perfs line up for approx. 3 perfs, which I guess is all you need.

Mitch

[tim]

Are you saying that the tapering compensates for the necessity to have teeth spaced at different intervals for the two types of film. Don't R and S have a different spacing interval besides different size holes?

That is the clever bit! Because the teeth taper in two planes, and because S8 film has smaller sprocket holes centred closer to the edge of the film than R8, the S8 film only goes part of the way down the teeth, and is pushed over onto the rim. Thus spacing is correct.

[jimhawley]

You guys are great....quick, informative, I wish I had this kind of response on all forums! Thanks again,

Jim