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Question

A block and tackle system has the velocity ratio 3. Draw a labelled diagram of the system indicating the points of application and the directions of load L and effort E. A man can exert a pull of 200kgf. What is the maximum load he can raise with this pulley system if its efficiency is 60%?

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Solution

Part 1: Velocity ratio of a block and tackle system:
In a system of n pulleys, the velocity ratio is
V.R.=dEdL=ndd=n
Thus to get a block and tackle system of
V.R.=3, we will need 3 pulleys.
Part 2 : Diagram
The number of pulleys in the movable lower
block (=1) will be one less than the number
of pulleys in the fixed upper block (=2).
One end of the string is attached to the
hook of the lower block, so as to apply the
effort in the downward direction.
​​​​​​Part 3: Calculate the maximum load
Given:
Velocity ratio V.R.=n=3
Efficiency η=60%=0.6
So the mechanical advantage
M.A.=η×V.R.
=0.6×3
=1.8
Given:
Man can exert a pull of 200kgf
So 𝐸𝑓𝑓𝑜𝑟𝑡 E=200kgf=2000N
M.A.=1.8
M.A.=loadLeffortE
LoadL=E×M.A.=2000×1.8
=3600N=3600kgf

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