Tuesday, 29 June 2010

The Intermittent Sprocket

The other day, a friend of mine brought to my attention the fact that I have never explicitly stated what, exactly, the intermittent sprocket is. So here you go:

In 35 mm and 70 mm projectors, there usually is a special sprocket immediately underneath the pressure plate, known as the intermittent sprocket. Unlike all the other sprockets in the projector, which run continuously, the intermittent sprocket operates in tandem with the shutter, and only moves while the shutter is blocking the lamp, so that the motion of the film cannot be seen. It also moves in a discrete amount at a time, equal to the number of perforations that make up a frame (4 for 35 mm, 5 for 70 mm). The intermittent movement in these projectors is usually provided by a Geneva drive, also known as the Maltese Cross mechanism.

Here's what it looks like:

Clear? No? Well, allow me to elucidate: The intermittent sprocket is a barrelly-looking cog-thing that holds the film in place on the aperture (a peep-hole at the front of the lamp-house that the bulb peers out of to watch the film it is projecting). It is synchronised with a shutter which blocks the light from the bulb at regular intervals (supposedly 24 times per second). The sprocket waits until the light is blocked and slyly yanks the film through to the next frame. If you ever see a movie in the cinema and the image appears to be smeared up or down the screen, this is because the intermittent and the shutter have gone out of sync, and the sprocket is yanking the film while the shutter is still open.

The Intermittent Sprocket is basically an important part of a larger machine which, when fully operational, makes your viewing pleasure possible in an entirely unnoticeable manner. You'd only be aware of the sprocket's effect on your movie if it ceased to do its job properly. It's like a tiny mechanical entertainment-ninja.

All factual information in this post was cribbed from Wikipedia (of course) and the above image was nicked without permission from www.film-tech.com where they also have this rather nifty guide to lacing film through a projector. Not the same model as the dreaded machines at my place, but it gives you a good idea.

So, now you know. And knowing is half the battle.


  1. I did wonder where the name came from, and now I know. Thanks. I've yet another bit of info I might be called upon to recall during Trivial Pursuit.

  2. I love it when you speak nerdinese.