On Mon, 10 Feb 2020 03:57:46 -0800 (PST), fred
1) Smaller will turn faster than bigger so there will be more turning
resistance (bearing friction).
2) Bending the cord though a greater amount will require more effort.
How much it will be more may be difficult to predict / measure though.
Cheers, T i m
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.
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
e than big ones?
Thanks for all the input. The pulley wheels would be used to change the dir
ection of travel of the cord by 90 degrees. A total of 4 pulleys would be i
nvolved. 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 ca
se 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 alo
nG the side of the side door so that it could be accessed with minimal open
ing 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.
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.
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.