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Position Of A Floating Pulley

pulley tension

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#1 Jerry94



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Posted 27 February 2019 - 07:42 AM


I am studying the behaviour of a pulley that is attached to a sling. The situation looks like this:


There is a drum that can give or take cable. Then there is a floating pulley with a cable through it. The pulley can move on the circle with a radius the same as the length of the sling. At the end of the cable a load is attached. 

The following parameters are known:
- location of the drum
- length of the sling
- weight of the load (so you also know the gravity working on the system)
- location of the fixed point at the sling
- % friction in the pulley

The question is how the position of the floating pulley changes when a certain force is acting on the cable at the drum.

Anyone an idea on how to solve this problem?

#2 OceanBreeze



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Posted 27 February 2019 - 09:46 AM

Hi, welcome to the asylum


You haven’t posed a specific problem, so it is not possible to posit a specific answer.


Speaking in generalities, the greater the angle the suspended pulley moves from vertical, the greater the stress on the sling leg (dead lead) and the less the work load capacity.


The rig you have diagrammed will almost always move to an angle of 45 Degrees, under any reasonable work load. (It only takes common sense to see that)


The work load capacity is reduced by the sine of the angle, theta, the dead lead takes from vertical.

At 45 Degrees, the work load capacity is reduced by a factor of 0.707


Another way of looking at it, the tension in the dead lead increases by 1/sine theta for any given work load.


That’s why a rigging like you have diagrammed would probably never be used in practice.


Not on my ship, anyway.


#3 Dubbelosix



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Posted 27 February 2019 - 09:55 AM

Hi, welcome to the asylum