Single or dual purpose projector, which is more gentle?

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Pedro
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Post by Pedro » Wed May 22, 2002 12:36 am

Hi Moviestuff, I also really don´t want to offend, but I find the discussion interesting, and as I don´t want to go sleep right now, I will type in another reply :)

You are right, for constant pulling off the film from the roll, the biggest tension may occure towards the end. But this also depends on how the friction is designed! With a primitive constant friction, the faster turning reel causes more resistance, and the force to the film will rise.

But: only when the machine accepts bigger reels, it needs a more friction, in order to avoid free running of the full reel. For 50 ft, nearly no friction is needed.

When watching the reel movement of a sprocketless Projector (I once had a Bauer T182, with no upper sprocket), you note, that the upper reel is swining periodically: first it is pulled by the claw (really accellerating the complete weight of it) and it starts rolling, until it stops again, waiting for the next pulling impulse. It never rolls continuously. And with each pullin, the film in the upper film channel gets in tension and touches the film chanel or some plastic spring - and the projected image is "breathing" more than usual.

Excuse me, but this MUST be more force to the whole film (and claw), than an continuous linear sprocket pulling.

The camera design (Kodak 200 ft and Beaulieu 200 ft) also got back to sprocket again, as the sprocket, as an mechanical interface between linear movement and intermediate movement, holds anything away from the registration section what could mechanincally disturb.
So, the registration of both 200 ft solutions is considered to be better as that of the 50 ft cardriges. At least concerning the Beaulieu cardrige I can tell you that it is so.

Well, I keep my projector always in the best maintained state that is possible. The only danger related to sprockets during normal projection (not in threading mode) is, if a sprocket goes loosen and turns at a wrong speed, applying tension to the film. Or if the spring that holds the perforation to the sprocket teeth, goes off. But both situations are rather theoretical. THe biggest problem is the auto threading mechanism and a dirty gate, pinch roller or sound head presser.
Besides, most quality projectors are sound projectors, and I have never seen a totally sprocketless sound projector.

The only sprocketless projectors I know are toy projectors and the latest Bauer models ot the dieing amateur film area, where the upper sprocket only had been rationalized away. And this projectors are well known for a bad registration.
And yes, I PERSONALLY have seen these models for sale in normal ware houses and at photo dealers, before all the nice toys went over to the second hand sections of the shops.

Pedro

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wahiba
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Sprocketless Projectors

Post by wahiba » Wed May 22, 2002 9:21 am

Never had any problem with our original Kodak Standard 8 or Eumig 607 Dual Gauge. Only problem projector has been an older Eumig with sprockets, which do not make up for a dodgy claw mechanism.

Basically a sprocketless quality machine ie Eumig, Bolex etc. will beat a dodgy sprocket machine. As they were designed for Home Movies this is the job they do well.
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Post by MovieStuff » Wed May 22, 2002 3:20 pm

Pedro wrote:You are right, for constant pulling off the film from the roll, the biggest tension may occure towards the end. But this also depends on how the friction is designed! With a primitive constant friction, the faster turning reel causes more resistance, and the force to the film will rise.
Hi, Pedro!

Sorry but that makes zero sense. The physics does not change a bit whether you are using a sprocketed projector or a sprocketless projector. The simple fact is that the larger the reel of film, the less force is required to pull the film from the reel. Makes no difference whether it's being pulled by a sprocket or a claw.
Pedro wrote: When watching the reel movement of a sprocketless Projector, you note, that the upper reel is swining periodically: first it is pulled by the claw and it starts rolling, until it stops again, waiting for the next pulling impulse. It never rolls continuously.

Excuse me, but this MUST be more force to the whole film (and claw), than an continuous linear sprocket pulling.
No really. As you say, it depends on the design of the unit. On the one hand, you say that there must be more resistance but on the otherhand, you clearly indicate that the reel will keep turning due to a lack of back tension. The reason that any reel keeps spinning when pulled on slightly is because the reel is easy to turn! If it's so easy to spin that it keeps right on spinning then I see no problem with any type of resistance.

Is a sprocketed projector more likely to have better registration? Of course but the existence of sprockets is no guarantee. I have an Elmo ST1200HD and it works just great. But I also work a lot (obviously) with the cheaper GAF units and, frankly, they have just as good registration as my Elmo, which is in perfect condition.

But that isn't really the issue here. The issue is whether or not a sprocketed projector is more likely to damage film than a sprocketless projector. I say that if both units are working fine, then a projector with sprockets is as good a choice as any. But, if a projector is likely to "hiccup", I would much rather have my film in a sprocket less projector where the damage is confined to only a single frame.

I see no indication that a sprocketless projector is more likely to damage film over the long term than a sprocketed projector. BOTH types will damage film at some point; film jams are a fact of life when projecting. But a sprocketed projector will ALWAYS do more damage than a sprocketless projector when a jam occurs since the sprockets just keep on grinding and tearing the film, oblivious to the fact that only a single frame was the problem at the gate. On a sprocketless projector the problem is confined to only a single frame, which was usually already damaged or the unit would not have stopped in the first place!

Roger

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Post by wahiba » Thu May 23, 2002 11:01 am

Physics is flawed. The force pulling the film off the reel is constant. On a larger reel at the start this generates a greater torque (force x radius) and it is this torque that overcomes any resistance of the supply reel. So at the end the torque is less to overcome resistance. However the mass of the reel has now reduced as the film has gone so it is only the friction forces to overcome. If this is too much for the system then the projector needs sorting, possibly lubricating by cleaning or adding a lubricant, or more likely a bit of both.
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Post by mattias » Thu May 23, 2002 11:51 am

eep, the mass is proportional to the area of the film on the reel, which is proportional to the square of the radius, while the torque is propotional to the radius. do the math and you'll see that a larger reel is in fact harder to turn. i have no idea how projectors work, so this might not apply, but it's physics without flaws. ;-)

/matt

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Post by wahiba » Thu May 23, 2002 2:17 pm

Large wheel, same force, more leaverage, easier to turn? maths
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Post by MovieStuff » Thu May 23, 2002 3:39 pm

mattias wrote:eep, the mass is proportional to the area of the film on the reel, which is proportional to the square of the radius, while the torque is propotional to the radius. do the math and you'll see that a larger reel is in fact harder to turn.
A larger reel, indeed, has more mass but at the size we're talking about the larger diameter more than overcomes the minimal mass increase and that results in more leverage and an easier pull, no matter how you look at it.

If we were talking about a huge reel, say, 12 feet in diameter, where the resulting weight would be more than any projector advance (sprocketed or claw) could manage, then that would be an issue. However, we're talking about something that is WAY below that; insignficant, to be more precise. In fact, the mass difference between a 400 foot reel and a 50 reel is basically a distinction without a difference as it relates to this discussion. As such, both reels essentially have the same mass relative to the surplus of torque that both a sprocketed and non-sprocketed advance system would offer.

It would be like making a comparison between a car that weighed 2000 pounds and one that weighed 2001 pounds and wondering which would put more wear on a 350HP V8 engine. TECHNICALLY, the 2001 pound car would have more mass but in reality, the engine won't see any difference since it is over rated for both jobs. The same holds true for the difference between a 50 foot reel and a 400 foot reel. No appreciable difference in mass or weight that would make a difference in this situation.

Isn't it amazing the things we find to discuss on these forums? :)

Roger

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Tension shock absorbers?

Post by S8 Booster » Thu May 23, 2002 3:49 pm

Admittedly never seen a sprocketless projector but it would be logical that the film were transported over a tension dampener before the claw so it did not work directly against the reel. This would take a lot of stress of the claw advancement mechanics and would reduce the wear on both the projector & film? Simple device that should be.

RGDS
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Post by Guest » Thu May 23, 2002 5:03 pm

> A larger reel, indeed, has more mass but at the size we're talking about the larger diameter more than overcomes the minimal mass increase

minimal mass increase? are you using lead reels or what? i'd consider the mass increase huge. anyway:

(please follow, it's really not that hard)

t1 = resisting torque.
t2 = "pulling" torque.
ri = inner radius.
ro = outer radius.
kf = friction constant.
mr = mass of the reel
df = mass density of the film
ff = frictional force
fp = pulling force

t1 = ri * ff = ri * kf * (mr + ((ro^2 - ri^2) * pi * 8 / 2 * df))
t2 = ro * fp

let t1 = t2.

fp = (ri*kf*(mr+((ro^2-ri^2)*pi*8/2*df)))/ro

assume the reel is 100 g and the friction constant 0.001 N/g

fp = kf * mr = 0.001 * 100 = 0.1 N.

and lets say the inner diameter 20 mm, the outer 22 (a few winds of film) and the density of the film 0.005 g/mm3. (you'll get similar results with similar numbers)

fp(22) = 20*0.001*(100+((22^2-20^2)*3*8/2*0.005)))/22 = 0.02*(100+84*12*0.005)/22 = 0.095 N.

so, when the reel is almost empty, you're right, it gets easier with a few winds, but where's the breakpoint? we know from theory that the force approaches eternity when the radius does, so there has to be one. let's try a few more winds and fill the 200 mm reel (i don't feel like taking the algebra approach. too much thinking).

fp(100) = 20*0.001*(100+((100^2-20^2)*3*8/2*0.005)))/100 = 0.02*(100+9600*12*0.005)/100 = 0.14 N.

tada. an empty reel is harder to turn than a reel with a few winds of film on it, but a full reel is even harder. the breakpoint? solve the ro^2 equation with fp set to 0.1. hint: it seems to be very close to 70 mm.

/matt

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Post by mattias » Thu May 23, 2002 5:10 pm

> In fact, the mass difference between a 400 foot reel and a 50 reel is basically a distinction without a difference

well, we're obviously talking about the amount of footage on the reel, not the capacity? you won't get the benefit of the increased torque of the larger reel unless you fill it with film, right? a 400 foot reel probably has at least four times the mass of a 50 foot one. and actually, coming to think about it, this is probably true for the empty reel as well.

what do you mean? the only way you can escape this is by letting go of this whole mass/torque/radius thing, and claim that other factors are more imprtant, which i think they are for sure. ;-)

/matt

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Post by Guest » Thu May 23, 2002 6:33 pm

mattias wrote: well, we're obviously talking about the amount of footage on the reel, not the capacity? you won't get the benefit of the increased torque of the larger reel unless you fill it with film, right?
Hi, Mattias!

That is why I clearly wrote that the hardest that an advance system has to work is at the end of a roll and not the beginning where leverage is in abundance. The catalyst for this entire discussion is that Pedro claimed that film at the beginning of a larger roll is harder to pull than from a smaller roll. Clearly this is not the case as the larger roll has more leverage working for it. When the larger reel is nearly empty, there is more mass to over come than a smaller reel that is near empty but the difference that you point out in your (rather impressive!) math simply doesn't amount to an appreciable difference relative to film wear or the amount of power needed to keep the film moving. Is there more tension at the end of a larger roll? Of course! Does it make a difference. Not at all since, like the 350V8 I mentioned above, there is more power available than is necessary.

Roger

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Post by MovieStuff » Thu May 23, 2002 7:12 pm

Well, obviously I can't remember to log in each time I post. What a duffus.

Roger

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Post by mattias » Fri May 24, 2002 4:16 pm

> The catalyst for this entire discussion is that Pedro claimed that film
> at the beginning of a larger roll is harder to pull than from a smaller roll.
> Clearly this is not the case as the larger roll has more leverage working for it.

clearly? you don't need the math to figure this one out. just imagine an *huge* reel, with miles and miles of film on it. would that really be easier to turn than an empty one you think? i believe you're right for reels that are "small enough" though, so the remaining problem is to find the breakpoint where the mass increases beyond what the increased radius can compensate for in leverage, which is where my math came in.

my point is that you're both right. an almost empty reel is hard to turn because of the limited leverage available, and a full reel is hard to turn because of the increased weight (which is *not* compensated for by the increased leverage, as i've showed). the easiest reel to turn is one with around 70 millimeters of footage on it.


/matt

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Post by wahiba » Fri May 24, 2002 4:29 pm

As my Eumig projector is here on the desk beside me I thought I would have a lok. No sprockets, but one tension damper before the gate, and two after it. Two pull down sprockets, one after and one before the gate. As it has never given any trouble, or damaged film I asume the system works. 8O
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Post by MovieStuff » Fri May 24, 2002 5:12 pm

mattias wrote: clearly? you don't need the math to figure this one out. just imagine an *huge* reel, with miles and miles of film on it. would that really be easier to turn than an empty one you think?
Well, unfortunately for your position (and the rest of us shooting our mini-epics) super 8 cameras don't have mile and miles of film on them and many projectors don't even go beyond 400 feet, so while your argument is techinically accurate, it obviously requires going to extremes to muster any kind of appreciable difference. And you're right, you really don't need math to figure this one out. Within practical terms, big wheels turn more easily than small wheels. That's what leverage is all about. ;)
mattias wrote: i believe you're right for reels that are "small enough" though, so the remaining problem is to find the breakpoint where the mass increases beyond what the increased radius can compensate for in leverage, which is where my math came in.

my point is that you're both right. an almost empty reel is hard to turn because of the limited leverage available, and a full reel is hard to turn because of the increased weight (which is *not* compensated for by the increased leverage, as i've showed).
Ah, but it does and you did show it, as you readily admitted when you wrote:
mattias wrote: ...what the increased radius can compensate for in leverage.
The increased radius compensates for the minimal increase in mass by providing more leverage thereby making the film easier to pull off the reel at the beginning of a roll than at the end. That is how leverage and transmissions work. If an increased radius did not provide more leverage, then our cars would be reduced to Flintstone foot power as transmissions would not function.

There is nothing wrong with your math but the end results you are pointing out can only amount to an appreciable difference if taken to an extreme as it relates to this discussion, which is about film and claw wear due to detrimental tension during pulldown. And the simple fact is that a (normal sized) full reel DOES turn more easily. I can not count the number of times that a full reel has begun to unspool itself simply due to the tiny force created by the film hanging down to the floor. I'm sure I'm not the only person that has seen this effect and certainly the amount of force exerted by the weight of the film is far less than the power available from the claw or advance sprockets.

Roger

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