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?

On Mon, 10 Feb 2020 03:57:46 -0800 (PST), 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?

Yes.

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

fred Wrote in message:
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?


Bell-cranks might be more suitable than pulley wheels.
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On 10/02/2020 12:36, Graham. wrote:
fred Wrote in message:
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?

Bell-cranks might be more suitable than pulley wheels.

That would depend on the length of the pull.

--
Max Demian

On Mon, 10 Feb 2020 12:43:46 +0000, Max Demian
wrote:

On 10/02/2020 12:36, Graham. wrote:
fred Wrote in message:
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?

Bell-cranks might be more suitable than pulley wheels.

That would depend on the length of the pull.

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

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

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"Max Demian" wrote in message
o.uk...
On 10/02/2020 12:36, Graham. wrote:
fred Wrote in message:
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?

Bell-cranks might be more suitable than pulley wheels.

That would depend on the length of the pull.

--
Max Demian

On Mon, 10 Feb 2020 12:36:42 +0000 (GMT+00:00), "Graham."
wrote:

fred Wrote in message:
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?


Bell-cranks might be more suitable than pulley wheels.

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

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.

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?

yes

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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?


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

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On Monday, February 10, 2020 at 12:53:44 PM UTC, Tim+ wrote:
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?


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

--
Please don't feed the trolls

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!

In article , fred
wrote:
On Monday, February 10, 2020 at 12:53:44 PM UTC, Tim+ wrote:
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?


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

-- Please don't feed the trolls

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.

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

--
from KT24 in Surrey, England
"I'd rather die of exhaustion than die of boredom" Thomas Carlyle

Just a firm pull really

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

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"charles" wrote in message
...
In article , fred
wrote:
On Monday, February 10, 2020 at 12:53:44 PM UTC, Tim+ wrote:
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?


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

-- Please don't feed the trolls

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.

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

--
from KT24 in Surrey, England
"I'd rather die of exhaustion than die of boredom" Thomas Carlyle

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

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"fred" wrote in message
...
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?

On Monday, February 10, 2020 at 8:33:01 PM UTC, Brian Gaff (Sofa 2) wrote:
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

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"fred" wrote in message
...
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?

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

On Mon, 10 Feb 2020 20:32:56 -0000, Brian Gaff \(Sofa 2\) wrote:

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.

--
Cheers
Dave.

On 12/02/2020 00:05, Dave Liquorice wrote:
4 or 5 pully's
pullEys..


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On Monday, 10 February 2020 11:57:49 UTC, 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?

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