In a message dated 9/4/01 7:49:51 AM Pacific Daylight Time,
alwilliams@olywa.net writes:
<<
> Picture a pulley up on your tower and you want to pull a 1,000 load
with
> a rope that goes through and down to the ground where you're going to
pull.
> The load on the pulley is 2,000 pounds - 1,000# on the load side and
1,000#
> on the pull side.
> >
A questions for the physics experts -- Is it true that in order to pull up
the 1000#
load that the pull will have to be 1000# plus the amount to overcome the
loss
due to the pulley efficiency? What is the efficiency for a typical 3 to 5"
pulley
of various bearing designs?
k7puc
>>
All question are worthy of some kind of an answer. I have to ask why the
efficiency of 3 to 5" pulleys of various bearing designs is a question?
Bearing drag percentage wise has got to be very small. I obtained some
pulleys from Boeing Surplus and made a series of block and tackles with up to
5" wheels. Many tried to buy them off me. I said "they were just for show
and tell."
Here is a case where wheel drag was important. I had a Hunter's block and
tackle I've elevated 25 elk on. I started using it on lifting the 20' mast
with a beam on top to get access to my PP motor or other heavy loads. It got
so that the pull on the rope with a 4/3 pulley was more than the weight of
the load--or seemed like it? I took it apart to find out why it had so much
drag. Over 40 years dirt had accumulated between the no bearing wheel and
the side plates so bad it was like a brake shoe with a fair friction. I
cleaned it all out and installed 3/4" bearings in each wheel. What a
difference! I could literally pull on the rope with 1 finger and lift a
heavy load. Here was a "worst case of wheel drag totally reversed."
I have an application for block and tackles outside permanently. I'm looking
for sealed bearings that will resist wind blown rain. Does anyone know if
that oil impregnated bearing bronze is available in tubes say for a 1/4"
bolt? It would make a great outside bearing for block and tackles. k7gco
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