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Author Topic: Differences between DKP70mm and RPG prints Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 09-01-2004 09:07 PM                     Stewart, I don't think that the majority of your stretching is happenning in rewind. I would look more closely at the take up tension as the film is comming out of the projector(still warm of course!). The splice is exposed to quite a bit of heat and that heat will easily soften up the adhesive on the tape. Then as its wound up the pull from take up stretches the splice a bit each time the film is run. Then rewinding each time will stress the splices a tiny bit more. And so it all adds up to a big headache! My feeling is that your main problems are: 1. The tape you are using is not adaquate enough for the job and that you don't re-do the tape splices at regular enough pre-determined intervals to see any real reliability. 2. You need to experiment to determine where the brunt of the problem lies and what aspect of the operation is causing them to stretch the most. Only after doing this can you arrive at any logical decision as to how often you need to re-do your splices. 3. Its a really easy experiment with some junk film to find out which mode of operation IS actually affecting the tape splices the most and you can easily calculate how often from those experimants based on the number of passes of the test film that you would need to re-do them. Of course this would have to be done before or after normal hours . Mark @ CLACO  |  IP: Logged Stewart Anderson Expert Film Handler Posts: 105 From: Sandy, Utah /United States of America Registered: Aug 2003 posted 09-02-2004 10:24 AM                        I think the take up tension (5 lbs) with the heat of freshly being exposed on screen is a factor. But when CDC was hear they ran all sorts of test to try and figure out the cause. They were just able to rule out the projector as being a cause. They did test the reel unit (kinda)... But I'm still leaning toward the tension on rewind since all of the splices that have stretched in our booth have been near the beginning of the film.  |  IP: Logged Bob Brown Expert Film Handler Posts: 146 From: Grand Rapids, MI Registered: Apr 2002 posted 09-02-2004 10:30 AM                        Well, after reading this thread, I am sooooooo glad that I have a QTRU I have inspected prints with 500 runs on them and most of the splices are still tight and did not need to be replaced. We here in my booth, we inspect each print after 125 runs, or if something gives us the reason to check it sooner. So far, the prints and splices have check out GREAT!!. BTW, I am scheduled to get the QTRU upgraded to 150 min decks real soon for Polar Express. IMAX is upgrading decks 2-5 to 150 minute capacity, with deck 1 at 90 min capacity. 71" diameter platters. Can't wait for this!  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 09-02-2004 03:57 PM                        quote: Stewart Anderson I think the take up tension (5 lbs) with the heat of freshly being exposed on screen is a factor. 5 pounds of tension is very high, even for 70mm film. For 35mm prints, SMPTE Recommended Practice RP106 says: quote: 2 Film tension Film tension under normal operating conditions shall be 1.7 N to 4.4 N (6 ozf to 16 ozf). 5 pounds of tension would be very likely to make a tape splice stretch, especially if it was under constant tension as head leader might be prior to starting.  |  IP: Logged Adam Martin I'm not even gonna point out the irony. Posts: 3686 From: Dallas, TX Registered: Nov 2000 posted 09-02-2004 04:11 PM                     Maybe he's referring to a 5-pound weight (or hydraulic arm) distributing the tension out over a cluster of rollers (similar to the take-up variac assembly on a SPECO LP-270). That would give you more than one strip of film to bear the tension.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 09-02-2004 06:40 PM                     Stewart, It wouldn't matter if its near the beginning or at the end because your system is one that has constant tension through out the wind process on take up. Rewind should also be constant tension if its setup correctly. Thats just the way the Baldor drive system works with the tension arm in your case. You should aquire a tension gauge and verify things yourself before you ruin a nice print! Mark  |  IP: Logged Stewart Anderson Expert Film Handler Posts: 105 From: Sandy, Utah /United States of America Registered: Aug 2003 posted 09-03-2004 12:02 AM                        Adam: that is exactly what I was refering to, everyone seems to have their own name for it (accumulator, dancer, pulley system...whatever you want to call it). CDC told me that is the normal tension for take up. Does anyone know what the tension is on the reel MK1? I would love to tell CDC "nope, that IS NOT RIGHT!". Mark: I think the Baldor VSDs do have the programmable capability for the left platter to release tension by "helping out" the right platter rewind (given the film is rewinding from left to right). This would keep the constant tension from beginning to end. Unfortunately, CDC has only been able to have the left platter increase tension (I think they did this to keep the film on the rewind roller, they did have some trouble with this when they were programming it). Right now it is programmed to have the most tension at the beginning of the rewind and then it releases tension toward the end of the rewind. So near the end of the rewind (at the beginning of the film) it will no longer increase the tension, but due to the circumference/RPM difference it is still adding a lot more tension then it should. In fact, a lot more then it would ever add at the beginning of the rewind. Do you have any ideas for measuring/calculating the tension on the rewind arm? I havn't been clever or creative enough my self. Out of boredom I also included this graphic for anyone reading this who hasn't worked with the MK1 design. Everyone: I greatly appreciate your input!  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 09-03-2004 10:08 AM                     On the MK1 the rewind brake is speced at 4lbs the projection mode breaks are spec at 3-6lbs Note that this is not the direct tension on the film but measure with a set of scales pulling on the flange of the reel drive plate  |  IP: Logged

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