Pulley wheels

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?

Yes.

How much it will be more may be difficult to predict / measure though. ;-)

Cheers, T i m

That would depend on the length of the pull.

Only if they are only used to change the direction of the 'pull' rather than to gain mechanical advantage as per a hoist etc?

Wouldn't a bell crank have the same rotational friction as a pulley (for the same diameter and rotation angle) and may also have the same additional frictions on the end links as the cord might have flexing round the pulley?

Interesting thought though. ;-)

Cheers, T i m

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.

yes

If you imagine reducing your pulley diameter to zero, then it becomes obvious that smaller pulleys will offer more resistance. There will be fairly rapidly diminishing returns for enlarging them beyond a certain point though.

Tim

And even with a short (sub 90 degree rotation even) pull, aren't bell cranks less efficient the further the line of pull goes from 90 degrees (giving 0% efficiency at 90 degrees)?

A conrod to crankshaft is such a link where you need a flywheel to take you past the two areas of no efficiency (of the linkage)?

Cheers, T i m

Thanks for all the input. The pulley wheels would be used to change the direction of travel of the cord by 90 degrees. A total of 4 pulleys would be involved. I'm looking at some 40mm diam. closed pulley wheels on ebay which should suit me. An explanation. THe electric roller shutter door has a corded release in case of a power failure. The garage has a side door adjacent to the motor but at a right angle. This door opens inwards, is rarely used and consequently is always blocked. I want to extend the cord pull to have it drop down alonG the side of the side door so that it could be accessed with minimal opening of the door.

The real answer would be an U.P.S with a fail safe to open the door in the event of power failure.

If only!

Have you worked out how much force you'd need to go manual?

Just a firm pull really

Depends on how stiff the cable you are pulling is. However remember if these wheels are not capped by a curved piece of something, the most likely failure mode is the cable jumping off the pulley completely. Brian

If we knew what he was trying to achieve it might help. I used to like those bell pull wires you used to get on a buss. It could be pulled anywhere along its length and the bell would sound. Brian

In this context, when it suddenly stops, it will jump off one of the pulleys due to the elasticity of the cord, in my view, so make sure this cannot happen if the tension reduces. Brian

Good point. I intend to use enclosed pulley wheels. The release on the door motor can be let go softly so that will help also

Theoretically yes if all other factors are the same, but normally they're not the same, and in reality the amount of friction is normally negligible.

NT

Seems it only needs tug so tension could be maintained with a curtain pull cord tensioner.

I'm doubtfull about how much force of the tug on the cord by the door will make it around 4 or 5 pully's to the release point. I suspect not a lot.

pullEys..