Film-Tech Forum ARCHIVE: Shrinking top loop problem

This is topic Shrinking top loop problem - help needed in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Mike Jones (Member # 522) on 06-21-2001, 11:22 PM:

Got a slight problem that y'all might be able to help me with.
My top loop shrinks at most splices when they go through. I have a Century SA-TA projector and both the sprockets are tight in that area. Many films have played in there and all had good splices, so thats not the issue.

Anyone have any clue what would cause a loop to shrink if its not the splice and the sprocket or the lateral guide is not loose?

Thanks in advance.
-Mike

Film-Tech....the website that saves my ass over and over and over....

Posted by Randy Stankey (Member # 64) on 06-21-2001, 11:45 PM:

Have you checked the pad roller?

If it's not set correctly the film may jump off the sprocket when a splice goes through. This could cause you to "blow your loop".

Make sure the pad roller is set so that two layers of film will go through but three won't. Put three layers of film on the sprocket and close the pad roller. When you turn the roller with your fingers you should feel it dragging because of the contact with the film. When you take one of the layers of film off, the roller should turn freely again.
Also make sure the roller is set laterally. It sould be perfectly centered over the sprocket. It shouldn't have any "end play". (In and out movement.) It sould be as tight as you can make it, yet still allow free rotation.

Other things that can cause a blown loop are torn sprocket holes, crummy splices or otherwise damaged film. A poorly aligned film path, from the upper magazine roller through any digital penthouses then down to the sprocket would exacerbate any of the above problems.

Posted by Paul G. Thompson (Member # 655) on 06-22-2001, 12:58 AM:

In addition to what Randy said, check for a bent roller shaft or a lop-sided roller.

Posted by Mike Jones (Member # 522) on 06-22-2001, 02:22 AM:

Yeah, the pad roller is properly aligned and all is well there. There are no burrs or nicks on the sprockets and it is not bent.

the mystery continues...

Posted by Brad Miller (Member # 2) on 06-22-2001, 02:26 AM:

It's got to be the pad roller. Are you sure that 3 layers of film placed on the sprocket will not allow the pad roller to spin by hand (without the projector running), but 2 layers will? Make absolute sure of that first.

Only other thing I can think of right now would be the question, when you close your pad roller, does it snap shut strongly...or do you have to close it without there being a latching action?

Posted by Paul G. Thompson (Member # 655) on 06-22-2001, 02:36 AM:

Brad brought up a point that should be checked. There could be a problem with the entire pad roller assembly, or it may have "slipped" in the casting bore because a loose screw that retains the shaft in the bore.

Posted by John Pytlak (Member # 331) on 06-22-2001, 10:45 AM:

I agree to check the pad rollers to be sure they have the proper clearance, alignment and are closing completely.

But also check your splicing technique. When bent into a loop, the spliced film should form an even arc, with no kinks or tendency to fold up. Some people still make an overlap splice with tape over both sides of the overlap --- so you have two thicknesses of film plus two thicknesses of tape at the overlap, which is likely to "pop" a pad roller or get "hung up" in the gate. A "butt" tape splice on a properly adjusted splicer will usually be the most satisfactory.

Make a loop of film with an assortment of splices and see how well they go through the projector without jumping a sprocket or getting "hung up" somewhere.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
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

Posted by Ethan Harper (Member # 472) on 06-22-2001, 11:27 AM:

Are you using plastic pad rollers or metal? Are your pad rollers spinning? I would also guess that if your pad roller is not spinning then it would have shaved a flat spot on the roller itself which will cause your loop to shrink every time a splice runs through. Did you try replacing the pad rollers?

------------------
--"That's my story and i'm sticking to it!"--

Posted by Mike Jones (Member # 522) on 06-22-2001, 10:32 PM:

Everything with the pad roller checks out. I have done the three and two pieces of film test many times and its always good (which means it should never shed!

) It is not bent or misaligned.

I assembled a test loop of about seven splices and it shrank on only two of them, repeatedly. The thing is, there is nothing different with those splices than any other ones. My splicers are in perfect condition.

I've gone as far as taking the whole thing apart and putting it back together and there's nothing out of the ordinary. All the pad rollers (which are metal) are spinning with ease.

I'm wondering if there could be a problem is the gear shaft that is causing this?

Posted by Ethan Harper (Member # 472) on 06-22-2001, 10:44 PM:

Have you checked the belts for proper tension? Have you tried changing out the belts. Does it only happen while hitting a splice?
Are you having problems with the shutter timing? Have you tried replacing the pad roller with a new one or pad assembly?

------------------
--"That's my story and i'm sticking to it!"--

Posted by Ethan Harper (Member # 472) on 06-22-2001, 10:46 PM:

Are there any weird noises?

------------------
--"That's my story and i'm sticking to it!"--

Posted by Randy Stankey (Member # 64) on 06-22-2001, 11:08 PM:

So you say the pad roller is aligned correctly, but as Brad asked, does it shut tight and stay shut? There's a tiny little ball bearing and a spring inside the "hinge" at the base of the arm that holds the pad roller. That's the thing that locks the arm in place when you close it. If it's not engaging properly, or if the arm is loose on the hinge (stud) it won't lock. This could cause the roller to pop open for a split second and allow the film to slip. From your description I get the impression that you think it's okay but some confirmation on this issue would help.

The next thing I would do is to be sure that the feed sprocket isn't "stalling" momentarily.
The sprocket is driven by a gear in the back (off operator) side of the machine. It is a fiber gear that's prone to chipping if the machine gets jammed. (Usually because of a "brain wrap".) The strength of the polyester film can keep the sprocket from moving but the motor will try to keep turning it. The gear will get chips taken out of the teeth which will cause them to eventually strip out. If one of these "dead spots" on the gear hits at just the right moment it will stop turning for just a split second, just long enough to blow your loop.

Open the back of the projector. (And for God's sake, make sure the power is off!) Turn the machine over by hand and look/feel for any chips taken out of the gear. While you're at it, take a look at the vertical shaft/gear that drives it. After that, turn the motor on (but keep your fingers the hell out of the way) and look closely. Can you see any wobbling of the shaft or gears? (...Wooga-Wooga... )

Still, I'm leaning toward some inconsistency in the pad roller because you say it happens only when a splice goes through. If it were a problem in the drive train I'd think that it would happen in seemingly random spots, not just when a splice goes though.

Posted by Paul G. Thompson (Member # 655) on 06-22-2001, 11:28 PM:

I don't think he has any Wooga Wooga problems.

Looks like all areas have been covered. But nobody mentioned the sprocket itself. Is it "hooked"? Worn down to nubs?

Maybe it would be good at this point to ask if the picture stays in frame when the top loop gets smaller. That was never mentioned, either. Just scrambling for possibilities now.

Posted by Barry Hans (Member # 477) on 06-22-2001, 11:47 PM:

Here's a couple of dumb questions, but is the screw missing on either the sprocket or the fiber gear holding them to the shaft? Is the shaft well oiled and you have proper in/ out play and without the fiber gear does the shaft spin freely?

Posted by Paul G. Thompson (Member # 655) on 06-25-2001, 01:41 AM:

Mike, did you ever find out what the problem was? It would be nice for all of us to know so we can stuff it in out memory banks.