Elf ST/M - New motor belt = Faster sound speed!?!

[Can Sanalan]

Hello and a Happy New Year to you all,

I've come across a rather peculiar issue in regards to my sound speed after changing the motor belt that links the motor to the shaft.
I decided to replace my motor belt on my Elf ST/M, with a brand new one that is correct for this particular model, and upon watching one of my reels I noticed the sound was ever so slightly faster.
How is this possible?
The belt is on the correct pulleys for the 24fps.
I swapped the old one back on and the sound was running at normal speed.

I've attached photos of the two belts, the old one and new one.
The old belt is 4mm thick and measures 85mm across.
While the new one measures 5mm thick and 80mm across.

Could the current pulleys not be the original ones to the ST/M? That the originals have been replaced with ones from a different model? I can't tell.
I assumed the old belt was the correct one for this model but comparing it to the new one it clearly is not.

Any thoughts?
I'm leaning towards the pulleys are probably from a different model.

Here's a link to a video where you can hear the subtle difference in sound speed in the actor's voice (first clip new belt second clip old belt):

https://www.youtube.com/watch?v=0ol5...ature=youtu.be


The projector was salvaged from a school many years ago.

[Iain Warner]

I would think the change of belt thickness would cause the change of speed. For example if the nominal diameter of the motor pulley is 20mm and the cam box pulley say 60mm the ratio for drive speed would be 0.333.
If the effective diameters are increased to say 21mm and 61mm then the ratio would be 0.344 and thus cause the change in speed.
How about timing a known length of film to determine the actual projection speed with either belt. I have no idea what the original tolerance is on fps for these projectors.
Iain

[Maurice Leakey]

...
How about timing a known length of film to determine the actual projection speed with either belt...

I like Iain's idea.

I attach a Film Footage Indicator which will accept various entries, such a gauge of film, frames per second, length of film, running time of film as desired in seconds minutes hours, etc.

Scenesavers

By running, and timing, a known length of film, projector speed can be established. Note that many 16mm projectors do not run at 24fps for sound speed as they normally are around 25fps.

There is a possibility that the original belt may have expanded slightly with old age and that the new belt may be running at the correct speed anyway.


Maurice

[Nantawat Kittiwarakul]

The best possible approach is to use one of this.

https://www.amazon.com/dp/B00850ZCRI...NsaWNrPXRydWU=

Bought mine locally for about the same price as this. The best tool to resolve the speed issue of my projectors.

[Maurice Leakey]

[QUOTE=Nantawat Kittiwarakul;n24991]The best possible approach is to use one of these.[URL]

Here is a UK listing for the item recommended by Nantawat.

KKmoon Digital Photo Tachometer Handheld Non-Contact Tach Range 2.5RPM-99,999RPM LCD Display Motor Speed Meter with 3pcs Reflective Tape: Amazon.co.uk: DIY & Tools


Maurice

[Iain Warner]

Interesting, I guess for the application in question it would have to be aimed with target on one of the sprocket wheels and the result divided by the number of teeth.
Knowing the speed of actual electric motor would not help in this case.
Iain

[Iain Warner]

Engaging brain to think a bit more about it the result will have to be divided by 60 then multiplied by the number of teeth on the sprocket.
Iain

[Maurice Leakey]

Iain
I don't think we need to know the speed of the motor.
The net result will be the speed in frames per second of the projector with the two different belts.

Maurice

[Nantawat Kittiwarakul]

Although it's intended to be used with the reflective tape supplied with, but I've successfully done it by simply aiming it to the projected picture on the screen. The reflected light from the screen is more than enough to make it work.

Please be reminded that the measuring unit is RPM. So for "correct" sound speed with 2 - bladed shutter projector you should get 24*60*2=2,880 reading.

In practice anything between 2,820-2,940 (23.5-24.5 fps) would easily pass unnoticed to most audience. Don't forget that most films shown on PAL tv at 25fps, and virtually nobody notices.

[Iain Warner]

Thanks for clarifying that, it certainly will make the measurement easier just to point it at the screen. I will probably get one as I can think of several other applications where it would be useful.
Iain

[Can Sanalan]

Thank you gentleman for your replies. I've run the projector for a few hours with the new belt in the hope it may give a little and run at the correct speed but no luck.
I'll order the tachometer and test it with both belts and see what results I'll get.
I'm trying to remember where I had gotten the old belt from.

But anyway thank you again all and I'll post my results when I have them.

[Nantawat Kittiwarakul]

Great! Keep up your good work, I'd like to hear the outcome too.

My gut feeling tells me that your new belt will get you precisely (or very close to) the proper speed, which means it's been running a tad slow for the whole time.

By the way I currently have an Eike NT series. Even after doing every possible maintenance work (includes replacing the motor's ball bearings), it is still running at 23.9 fps. So you may now have an idea what to expect...

[Can Sanalan]

So the tachometer arrived today and the results are in.

The original, old belt (4mm x 85mm) measured at: 2963 rpm

The new belt (5mm x 80mm) measured at: 3067 rpm

and I tired another, new belt (4mm x 80mm) and that was at: 2944 rpm

Issue is resolved.
The new 4mm belt now runs the projector at the correct sound speed

Thank you every one for your advice and guidance.

[Dominique De Bast]

That's interesting : a French ninefiver told me he could get a 16fps on a 9.5 Buckingham (a former 16mm Elf/Eiki) projector by using another belt. When I told this to a repairman, he told me it's impossible. Well...

[Maurice Leakey]

Dominique
Not sure about 16 fps.
Looking at my Buckingham's Eiki ST/M Series instructions a silent speed of 18 fps can be obtained by slipping the motor drive belt to the alternative set of pulleys.
To do this, the inching knob is turned at the same time as guiding the belt first to the smaller motor pulley, and then onto the larger shutter pulley.

Maurice