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Hi Eric,

You are quite correct and they work very well. Simply order O-Rings from
a place like McMaster-Carr. 

These are great for small power applications and they are easy and cheap
to replace. You can get them a variety of lengths and pulleys are made
to run with them.

Now for an old-fashioned suggestion: Believe it or not, round leather
belting is still made. It's used on sewing machines and works well. The
best source is Page Belting in Concord, NH. They also make flat belting
which is very good in historical settings.

Of course, you can use v-belts and these work well except that they are
power hogs in hand-cranked situations. Keep the pulleys large in
diameter to lessen this problem and have good ways of tensioning them.

Multi-V belts (like those used on car fan-alternator serpentine systems)
are wonderful but it seems hard to find pulleys to match these. Anyone
out there know where to get these?

With regard to torque limiters, the ones McMaster-Carr sell are fine.
For smaller ones, go to a place called "small parts" They sell
instrument sized versions of these with 1/4 inch shafts, etc. They are
much more adjustable and work much better in small situations.

Then there is the matter of over-running clutches. These are VERY
IMPORTANT when the load being driven by the crank or pedals has inertia.
I've used all types of these. Bicycle ratchets work OK and you can run
No. 41 chain on them although it's not ideal. There are a number of
so-called "sprag clutches" that work very well. The basic parts are
cheap but complete assemblies are costly. Hub City makes (or made) a
very good type of ratchet over-running clutch used for tractor drives
that works well in hand-crank generators. 

While on the subject of power transmission, don't forget the obvious: be
sure you use adequate guards! 

John Bowditch
Exhibits Director
Ann Arbor Hands-On Museum


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For information about the Association of Science-Technology Centers and the Informal Science Education Network please visit www.astc.org.

Check out the latest case studies and reviews on ExhibitFiles at www.exhibitfiles.org.

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