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Material Notes:  URETHANE BELT
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Urethane round belts are O-ring-like devices used to transmit motion from one rotating object to another.  They are easy to work with and open up many design possibilities.

The urethane used for belts is a type of TPE  (ThermoPlastic Elastomer).  All thermoplastics can be melted and remelted, and this quality allows the easy creation of any length belt - just melt the ends together (described below) and clean up the join zone.  They have low hysteresis (meaning they maintain springyness), high abrasion resistance, low coefficient of friction, and they look nice.

A 3/16" diameter clear urethane round belt and 2 custom made pulleys are pictured to the right.  The small pulley mounts to a motor and the large one is the driven axis.  Belt is from McMaster-Carr.

There are several belt-like options for transmitting rotary motion. The most important are: chain (like bicycle chain), toothed belt (aka timing belt), V Belt and urethane round belt.  Each has pros and cons, summarized in the table below.  If perfect synchronization is not required, and some elasticity in the transmission is acceptable, then urethane round belts may be a good choice.
Toothed Belt
aka
Timing Belt
Buy. Expensive.
Limited sizes.
Straight Only
(parallel shafts)
NO
YES
One pulley must
have position
adjust
Minimal
Chain
Buy.  Bulky.
Limited sizes.
Cogging
Straight Only
(parallel shafts)
YES, easy
YES
One pulley must
have position
adjust
None
V-Belt
Buy or easy to
make
Straight, slight
misalignment OK
NO
NO
One pulley must
have position
adjust
Minimal
Urethane
Round Belt
Buy or easy to
make,
forgiving
Anything
goes
Yes, with melting
and sanding tools
NO
Pulleys need not
position adjust,
tension intrinsic
to belt
SIGNIFICANT
TYPE
TRANSMISSION GEOMETRY
PULLEYS
IN-SITU
CLOSURE ?
SYNCHRO-
NIZED ?
TENSION
ADJUSTMENT
ELASTICITY
works of  Carl C Pisaturo