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Topic: Cheap, but effective failsafe for screening room

Mitchell Cope
Master Film Handler

Posts: 256
From: Overland Park, KS, United States
Registered: Jun 99

posted 02-01-2002 11:50 AM

I have a private screening room with a 35mm portable projector and a Christie AW3 platter system (circa 1970s). I have only had one brainwrap, but that was with acetate film stock. Running estar based film stock, I worry a lot about the damage if a brainwrap were to occur. I know nothing about failsafes, but is there a cheap and effective system that I could use that would mitigate my worry?

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 02-01-2002 12:29 PM

These devices sense if tension is building up between the platter feed and the projector. Normally, the film here has only a few ounces of tension, but a jam or wrap on the platter will quickly cause tension to build to the point of sprocket teeth ripping through the film perforations, bending roller shafts, or even causing equipment damage:
http://www.kodak.com/US/en/motion/newsletters/reel/may97/fail-safe.shtml

Tension-sensing devices can be as simple as the AVASK failsafe, which uses a spring-loader roller to actuate a microswitch when tension builds. The disadvantage is that even if the projector is shut down immediately, it's inertia will make it take some time to "coast" to a stop, possibly damaging a few feet of film.

Another approach is to mount a weighted or spring-loaded roller on a rail or "trolley" attached to the platter column. The film is threaded over this roller, and if tension builds, the roller starts to get pulled up, actuating a microswitch or magnetic proximity switch. Because there is some slack in the loop around the roller, the projector can coast to a safe stop:
http://www.neumade.com/platters.html
http://www.film-tech.com/manuals/ALPHAOPERATION.pdf

The Kelmar "Film Chopper" used a spring-loaded scissor blade to cleanly cut the film if tension became excessive, the thought being that it was better to have a clean cut that could be easily spliced, rather than loosing a length of film to stretching or ripped perforations.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

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