What drive belt(s) ?

[user 39814]

Would you be prepared to educate me please?

I am trying to resurrect a vintage ( probably 1960) Bolex 18-5 cine projector on which the three "rubber" 2mm square section drive belts had perished and there is no way I can guess at their original diameter / length
The pulley sets are U-cross-section ( NOT square ) and 55 mm distance between the bottoms of the two pulley sets

I bought an advertised replacement for this model - see photo - an was supplied O-rings 1.5mm dia x 45mm ID - material unknown - see photos attached.

They slip instead of transmitting the power.

I am also told that O-rings are not designed for power transmission.

How can I obtain a suitable single belt - or group of 3 belts please?
I do not know how to attach pictures here.
The photos show
1. The state of the square belts as found - why square belts in a round pulley set?

2. the slightly too long set of 1.5 mm dia rings fitted at the moment

Can you please advise?

Many thanks

David Lewin

 

[Naiveambition]

I don't know much anything about projector style machine but did find one In a junk pile that worked when I was a kid. If I remember correctly it was a bell and Howell that used a metal drive quite similar to the drives on wilesco steam engines. It looks like a spring that locks in itself They are adjustable to fit your needs and may work in your situation.
With the belts being square I would think two things. If the belt is square on the bottom side then I would think that's where my drive force would be located as it pushes in it forces itself to the sides or forms to the round pulley adding additional traction. Or maybe it has just lapped itself so smooth it just needs say a 400 grit rough up. Lapping compound on a qtip comes to mind and you don't need too remove the pulleys.
I would look strongly into robot community building, they use many strong , small belts.. if they don't have what your looking for and you have machining skills it would be rather easy to make new set to fit a belt of your choosing. My personal thought would be a cross ribbed belt, they are pretty much in any size and can be custom made for you application. I do think Steve Howells v8, that he ordered it made to his specifications from some company. So maybe talk to him for more guidance. Make up two pulleys, mount, and use a flexible tape measure to measure the full distance, like in the wife's sewing kit. I would imagine there is some type of adjustment for slack On a side note I have heard there is a glue that was used to help the slipping of the old leather belts on the lathe. I believe it was sprayed on but could be incorrect. so may be another option for you
Sorry if your an experienced machinist and I said everything you already know. Good luck with your project
Mike

 

[user 39814]

Thank you for taking so much time and care!! I really appreciate that
I think my questions should be formulated a little simpler:
1. Why would there be a square-section drive belt in a round-section pulley?
2. As O-rings are designed for static use, sealing liquids and gasses - can they also be used as high-speed dynamic use as drive belts.
3. What material should a drive belt, transmitting the power in a film projector, be made off considering the speed it works at and the fact that it is probably warm in there.
4. If the distance between the bottom of the pulleys is 55mm, how much less should the dive belt be so that, when stretched across the pulleys it neither slips nor strains the pulley bearings.

 

[BaronJ]

Hi David,

In answer to your first question, since square cross section belts are common and I recall few pulleys with "V" shaped grooves to suit the belt, I would think that grip is the answer ! Since the belts are reasonably soft, the belt will conform to the shape of the pulley. I also think that the belt tension would be quite low in that application.

I've used "O" rings as drive belts several times, some types are very resistant to stretching past a certain point, but don't seem to want to break, others break very quickly under little stress. It really seems that the material is formulated for a particular application.

Number "4" is one of those "how long is a piece of string" questions ! I think that you are going to have to play about with this one. My guess would be a soft rubber with a low stretch ! Try 5% shorter and see how it goes.

 

[Longboy]

Maybe were sent the wrong part number. Bolex used both belt types. Match up the correct size O-rings for the drive from your replacement belts at the hardware store.
Refer to the following forum for further assists: :http://8mmforum.film-tech.com/cgi-bin/ubb/ultimatebb.cgi?ubb=next_topic;f=1;t=010437;go=newer

 

[user 39814]

Maybe were sent the wrong part number. Bolex used both belt types. Match up the correct size O-rings for the drive from your replacement belts at the hardware store.
Refer to the following forum for further assists: :http://8mmforum.film-tech.com/cgi-bin/ubb/ultimatebb.cgi?ubb=next_topic;f=1;t=010437;go=newer

Wow! How did you find that thread??? I had certainly failed to do that, (Google failed to bring it up) and they are discussing my precise problem of both machine and space!!

 

[Longboy]

Wow! How did you find that thread??? I had certainly failed to do that, (Google failed to bring it up) and they are discussing my precise problem of both machine and space!!

Whatever helps you Dave....you'll get my bill in the mail!