Film-Tech Forum ARCHIVE: stupid question: how do "dual 8" projectors work?

This is topic stupid question: how do "dual 8" projectors work? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Scott Norwood (Member # 30) on 02-20-2019, 03:46 PM:

As the subject says, this is likely a stupid question. I have limited experience with small-gauge film, but that has always involved projectors that were specifically designed for standard-8mm or super-8mm film. Putting the wrong format in the wrong type of projector destroys the film, which is why the reel shafts use different designs.

But what about the projectors that claim to be able to show both formats? Somehow they need to accommodate different reel shaft sizes, different perforation sizes and pitches, and different aperture dimensions. I've never seen one of these up close, but would assume that the user doesn't actually need to change out every sprocket and the pulldown claw when switching between formats.

No doubt, there is a simple and brilliant bit of engineering at work in these, and I am curious as to what it is.

Posted by Leo Enticknap (Member # 534) on 02-20-2019, 05:18 PM:

I had one Eumig dual 8 projector (had to give it away when I emigrated - couldn't afford to ship it). It had no sprockets apart from the intermittent claw in the gate. This, and the aperture plate, were swapped by operating a lever, to convert the machine between regular 8 and Super 8 operation. My memory is hazy, but I believe that a separate lever moved the lamp (a 12v halogen lamp with built-in reflector), too. An adapter could be put on, or removed from, the feed spindle as needed, and this completed the conversion.

The film was pulled through the magnetic audio head by a pair of pinch rollers on the exit side of it (which, needless to say, had to be kept spotlessly clean, or film scratching would result). It was possible to thread the projector manually, bypassing the mag head altogether, and when operated this way, the thing was remarkably kind to film.

Posted by Emiel De Jong (Member # 4225) on 02-21-2019, 03:55 AM:

The simple bit of engineering at work is that claw shape and movement can be made to work for both regular and super 8. The more expensive ones had switchable or changeable sprockets, the cheaper ones no sprockets at all. When projecting 8mm now in this age I would stay away from them and go for "dedicated" machinery...

Posted by Steve Guttag (Member # 268) on 02-21-2019, 07:16 AM:

We had an M95 projector (Kodak) that worked with both Super and Regular 8. It did VERY well, I might add. The supply spindle had an adapter to switch. One removed the cover that gained access to the lamp and there was a lever that switched modes. All of the little names/speeds that show on the controls would also switch.

Here is a link to a service instruction set someone came up with. It might give you some insights on the dual cam/claw system. It is pretty cool, actually:

http://www.cfargo.com/pdf/kodak/m-series.pdf

Posted by Stephan Shelley (Member # 8587) on 02-21-2019, 01:43 PM:

The ones I have seen have constant speed sprockets in the usual places. They just have the narrow teeth for super 8. Think fox hole sprockets for 4 track mag 35mm. The regular 8 runs fine on the narrow teeth just like reg 35mm will on the narrow fox hole sprockets.

Posted by Scott Norwood (Member # 30) on 02-21-2019, 03:32 PM:

I can imagine that sprocket teeth could be made to work woth both sizes of perfs, but the pitch is different, too. That would (should?) affect both the sprocket diameter and the stroke of the pulldown claw (and the distance between teeth on the claw). No doubt some of this depends upon the tolerances of the sprockets and the film. Certainly, 16mm camera film has a (slightly) different pitch from 16mm print film, and projectors will work with either type.

I have worked with various 35/70 and 16/35 projectors; those work because the formats are different widths, so the smaller format can run on an inner section of the sprockets with a smaller diameter (i.e. the effect is the same as changing out sprockets). The trick with the 8mm machines would be to make all of this work without needing to change sprockets and adjust the pulldown mechanism. Unlike a professional cinema projectionist, Aunt Millie is not going to change out half a dozen parts to watch her home movies.

I'll have to take a close look at the manual that Steve references, because there must be some magic here that I just don't understand.

Posted by Leo Enticknap (Member # 534) on 02-21-2019, 05:14 PM:

On the projector I had, there were two separate claws for regular 8 and Super 8 that protruded from cutouts in the gate chassis, and a plate behind the gate chassis that moved vertically, with apertures cut out of it for regular and Super 8.

Operating the lever in one direction retracted the regular 8 claw, extended the Super 8 claw, and positioned the aperture such that the Super 8 cutout was in the light path. Operating it the other way reversed that arrangement.

This machine was sprocketless (apart from the claws), so there was no swapping out sprockets to worry about.

I believe that briefly in the 1950s or '60s, Fumeo made a dual 16mm/9.5mm projector with the 9.5mm teeth recessed in the sprockets. It used a Maltese cross intermittent (no claw in the gate). There were two mag heads on a cylindrical shaft that rotated, and so turning the correct head into the film path and swapping the aperture plate were the only other steps you had to do to convert it.

Posted by Bill Brandenstein (Member # 7758) on 02-21-2019, 11:59 PM:

The Eumig 810 I use requires the changing of a pair of sprockets, the film gate pieces, a switch on the sound head (if sound is needed), and adjustment of the frame knob half a frame's worth. Plus the supply reel spindle, if necessary. The pull-down is a single little pin that works the common area between the two formats' holes as some of you have already described.

Posted by Emiel De Jong (Member # 4225) on 02-22-2019, 02:34 AM:

I wasn't aware of dual 8 projectors with separate claws for 8 / s8 until now. That is of course a much better design...

Posted by Monte L Fullmer (Member # 2797) on 02-26-2019, 02:46 PM:

I have a Kodak Moviedeck 265 Sound unit were it's a dual 8 unit.

Here, since the sprocket pitch is the same for both R8 and S8. the trap, having two apertures, slides per format and the shuttle cam adjusts to meet the hole placement.

Posted by Monte L Fullmer (Member # 2797) on 02-26-2019, 05:39 PM:

EDIT: I erred on the comment of the sprockethole pitch being it's the same. The claw adjusts to the different pitch of the sprockethole being the holes are different in frame spacing.

Posted by Steve Kraus (Member # 476) on 03-13-2019, 02:20 PM:

I picked up two B&H 8mm projectors for $25 off CL a few months back, one S8 only, and the one currently in my lap, a 456A which is dual format. When you move the format lever, the aperture is flipped over to another hole and one can see the claw move sideways. I have not studied the mechanism but it's likely something happens that changes the pitch of the pulldown.

But I have a much better question. This thing has NO sprockets! It has a single dual-rimmed rubber roller and when you push down the threading lever and feed it film, a dual-rimmed metal roller pinches against the rubber roller and together they pull film in and push it down the guide and down through the gate. Once you hear the claw drawing film, you let go, the claw takes over pulling film in and shoving it through the guides and around various rollers until it emerges at the back. I do not believe anything drives the film except the pulldown claw. Apparently it works but I am still driven to ask "How can that be?"

One more thing. There is a perforated metal plate right behind the aperture. The sort of thing you would expect to drop in only in still frame mode to keep from burning the film. But it does not move out of the way during normal operation. I wonder if that is a defect. Seems bright enough for normal home movie use but given that the tiny perforations in the plate are small and widely spread, I'd say it might be blocking 80% of the light.

Posted by Scott Norwood (Member # 30) on 03-14-2019, 12:14 PM:

Another question: with 16mm portable projectors, the claw normally has two or three teeth. How many teeth does the claw have on an 8mm projector? I have one in storage, but it is not easily accessible.

Posted by Steve Kraus (Member # 476) on 03-14-2019, 06:32 PM:

I just looked at the projectors. Both the 456A (R8/S8) and the other, which is a 482A (S8) use a two pin claw.