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Film Loading Mechanism


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Hi all,

Does anyone happen to have detailed info on how exactly the rapid film loading mechanism actually works from a technical point of view? I see a tulip-shaped thing, inside which there's a spring, then the camera bottom lid has a free-moving wheel that looks somewhat like a submarine hatch locking wheel.
But how do all of these work together? What's the idea?

Second question - what exactly is it that allows M-mount Leicas, be they rapid loading or not, to use a short film leader without any issue or risk, while pre-M3 ones needed a long leader? Yes, I know that some adventurous people claim to have used older Leicas without bothering to trim the leader, but that's not the point. As I understand, loading an LTM Leica without trimming the leader could (though not necessarily will) cause the film to snag. But somehow in M-mount Leicas it never does. Why is that?

You can tell I'm a geek, can't you? I like to know how things work, and why they do. 😊

Thank you.

Edited by Vlad Soare
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The two parts are meaning to work together when the base plate lock and film leader inside the "tulip".

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We see here that the film holes catched in sprockets of the "tulip outside".

Spring is there to push the film leader at the end, in right position as holes to be catched.

Edited by a.noctilux
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Pre-M3 Leicas do not have an opening back, so the film pressure plate is always tight (by spring force)  against the guiding rails. Trying to push the full width film up under the pressure plate and over the rails can easily kink (or fold back) the upper edge of the film. The long leader trimmed film only has to force under the lower rail, then pull the full width main film through from the end. Thus the full width film enters the channel over the inner rails and the outer guide rails that the pressure plate rests on. (Notice there are 4 film rails - the film rests on the inner rails which are shallower than the outer rails by about the thickness of the film, forming a channel where the film is free to move but held quite flat.)

I have a couple of Japanese copies of the old ltm Leica that do have either a fully hinged back (like other 35mm cameras) or a door flap like the M3. These can be loaded easily without special trimming of the leader.

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I see. I didn't think about the upper rail. I used to think the sprockets might be the problem, but then I noticed that even the short leader will easily clear the upper sprocket anyway, so why should it be a problem? But indeed, the rail does make sense.

So, let me see if I got this straight. When you first put the film into position it will clear the upper rail (thanks to the rear flap being open), but it will still sit a bit crooked. The purpose of the wheel is to push the tip of the film down (I'm saying "down" because we're holding the camera upside down at this point), so that the film becomes parallel with the rails, and the holes in its edge become perfectly aligned with the sprockets. But it needs to ensure that the tip of the film is pushed exactly as much as necessary, not even one millimetre more or less. That's where the spring comes in. The wheel pushes the film against the spring, whose elasticity is precisely calculated to offer exactly the right amount of resistance, no more, no less, so that the film ends up perfectly straight.
When you advance the film, it is actually the tulip that pulls it, not the sprockets. At least at first. Once the film has been pulled enough it will engage the sprockets, but this isn't actually necessary for the film advance. The sprockets merely keep the film from moving backwards, acting as a rewind lock.
In order to ensure that the film is caught securely and cannot escape during those first few centimeters when the sprockets are not yet engaged, the tulip rotates backwards (i.e. the opposite way of the winding crank). This causes a kink in the film leader that makes it impossible for the film to slip.
The tulip-like shape of the take-up spool doesn't play an actual role in the film advance but just makes it more comfortable for the user to place the film leader in there, providing enough space to insert it without having to fiddle with it too much. And the wheel is shaped as it is because it needs to fit around the "petals" when you put the lid back.

Is this correct? Have I got it right?

 

Edited by Vlad Soare
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Something like that,

when the loading is done, the two sprockets (each side of the rails we see when the rear door is open) take care of the film holes, advancing right amount for one frame.

So when close the door push the film holes in sprockets.

 

We see that to be safe loading we must follow the process ...

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