Drive system for projection platters

[Ethan Brake]

I was hoping someone could explain to me how the drive system on a platter works, pertaining to the film speed. Obviously, as the film empties off of one platter, it has to turn slower. What adjusts the motor to keep it turning at the appropriate speed so the film doesn't unravel onto the floor?

[John Salim]

You're correct Ethan, if you watch this YouTube video you'll see there's a variable speed control unit in the centre of the feed platter constantly adjusting the feed out speed ( at 1':35" )
You can guess what'll happen if the controller goes wrong !

 

John S ?

 

[Ethan Brake]

Thanks for the video, that's the best view of one of those I've been able to find. I'd like to know mechanically how it actually works. I've found a couple pdf manuals for the platter systems from Film-Tech, but they don't explain how that mechanism works.

[John Salim]

On simple designs, the moving feed arm is connected to a potentiometer controlling a variable speed DC motor.

John S

[Ethan Brake]

Thanks. Knowing that and watching the video again, I can see basically how it works. I am curious, what would a less simple design involve?

[John Salim]

I think that's as simple as it gets really.
Trying to do this mechanically would be more complicated - and a lot more expensive !

John S ?