Pinch Rollers Whats going on with my model 1

[Don Warr]

I have 2 rollers of the same thickness (5mm) made of TPU98(soft rubber)... They are to move 8mm Supper 8 film pinched between them.. The Idler roller is 23mm diameter and can be forced against the main roller using a 3mm screw.. the main rollers diameter is larger at 30mm and is being driven by a stepper motor.. Problem is:: The main roller just slip's not driving the film. yet if I hand spin the Idler (smaller) roller it grips the film great and the film moves...
is this normal that the smaller roller should also be the driver having the stepper..??? If I "squish" the rollers together too much the friction will stall the stepper motor..
Any comments or suggestions would certainly be appreciated...

 

[Compositepro]

I guess that is why the standard configuration is a driven metal Capstan and a larger rubber pinch roller. Rubber deforms to conform to the metal. The metal is dimensionally stable and will determine the speed of the nip point.

 

[waross]

If your stepper is stalling,it may be several sizes too small.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Don Warr]

Thanks, Compositepro Your answer makes good sense,,, I'm an Electronics Engineer not mechanical, but I'm trying to make this work... I can easily change the design and TRY the metal stepper shaft, lowering parts count.. I just thought 2 rubber rollers would be better..(NOT),,, Speed is not a factor, as the movement is only 1 frame stops takes a picture and moves again 1 frame

 

[3DDave]

There are sprocket holes and well known mechanisms for advancing the film rather precisely. Using pinch rollers will do some damage to the emulsion. I see several for under $50 that could have that mechanism and the film guides harvested/repurposed.

 

[Don Warr]

Yes, "3DDave" I have 2 different diameter shafts with pins that are especially for the sprocket holes, and that works but have found they have no "give" and can and usually damage or break 40 year old Supper 8 film.. We had one Pinch roller prototype, like in the picture I posted, that worked great, unfortunately that design got deleted in pursuit of updating our prototype's and we haven't been able to recreate "move" the dam film since.... and thanks for your response..

 

[3DDave]

It would be a first guess that the pins weren't involute and didn't have enough clearance to accommodate small variations in hole-to-hole spacing. With the conventional feed mechanism there is no need for that spacing in the film to be perfect as the exposed frames are all registered to the leading edge of the holes. Even in 35MM still cameras there is a noticeable amount of clearance with the sprockets that pull the film through.


A metal pin will cause a sharp bend in the film. If the holes are too delicate to touch then just drive the take-up reel and sense when a frame is in position. Add a light friction drag, though the frame should be held flat enough by the film guides.

 

[Don Warr]

I may have used the wrong term "pins" but the "axel pin thing" I have, was designed by Philips to be used in the transport of the film using the sprocket holes... And it works, But badly for old film... There is a "HIGHLY expensive" machine made for Library's, who's main design is to protect old sensitive film, It uses a pinch roller system.. In My attempt, the motor roller turns great but the idler roller and film do not move, If I really pinch the rollers together
the large stepper will stall, that much pressure in unacceptable.. We had a design that worked, But it got deleted. We have no idea what the roller demention's were.. again, thanks for the input.....

 

[3DDave]

So, what is holding the film from moving? Are you using this to pull film from the reel? Is the pull force supposed to be taken as shear through the film and this is just spinning like a wheel in mud? Typical magnetic tape drives the friction is between the drive and the rubber and then from the rubber to the tape. But if contact with rubber isn't moving the film then it is also not driving the idler. You would know this if the idler moved without film.

Something to note is that the film has emulsion on one side and has a different friction coefficient which is likely why the one roller works and the other doesn't.

Anyway, it sounds like there is far too much drag on the film.