On 10/02/2020 11:57, fred wrote:
For various reasons I wish to run a thinnish pull cord via 4-5 pulley wheels. Does anyone know if smaller pulley wheels offer more resistance than big ones?


How long is a piece of string? It all depends on the details. Do you
mean you want the cord going through a zig-zag path defined by a wheel
at each angle, or that you want a block and tackle arrangement to give
you a big mechanical advantage?

I'm guessing you are thinking about small wheels of the type sometimes
used for old fashioned clothes airers or clothes lines. Here, you
usually have plastic wheels running on steel pins which gives reasonably
low friction. But think about wear if it is going to do a lot of movement.

How important is low friction? All other things being equal, if you keep
the bearing diameter the same but double the pulley size, you will halve
the friction. The losses in thin flexible cord should be low unless the
total angle is large.

With thin, flexible cord, say polyester or nylon, fairly small diameter
may be OK. You might need large pulleys with something stiffer, like
steel wire cable, especially if the angles are large.

Look for comparable "real world" examples. The thin, very flexible wire
cable used on deraillieur gears is OK at radii of around a centimetre.
Brake cable is thicker and has fewer strands, so would need larger pulleys.