Advertisements
Advertisements
Question
A ‘block and tackle’ system used 3 pulleys in the lower block and 4 pulleys in the upper block. What is the ‘velocity ratio’ of this system? If the load is to be lifted by a person capable of applying a maximum effort of 1000 N, what is the maximum load than can be lifted under ideal conditions?
The actual maximum load that gets lifted turns out to be 6300 N. What are the values of the actual M.A. and efficiency of the set-up?
Advertisements
Solution
There being 7 pulleys in all, the load is supported by 7 strands of string. Hence it gets lifted only through 1/7 th of the distance through which the effort is applied. Hence the velocity ratio of the setup has a value 7.
Under ideal conditions, M.A. = V.R. = 7
Hence, the maximum load that can be lifted under ideal conditions is:
Effort × 7 = 1000 N × 7 = 7000 N.
Under actual conditions, Load = 6300 N
and Effort = 1000 N
Hence, Actual M.A. =`(6300"N")/(1000"N")=6.3`
Also Efficiency =`"M.A."/"V.R."=6.3/7`
= 0.9 = 90%.
RELATED QUESTIONS
Explain how a gear system can be used to obtain gain in speed? Given one example.
Explain how a gear system can be used to obtain gain in torque? Given one example.
Define the following term in reference to a gear system for Gain in torque ?
The diagram below shows a pulley arrangement.
- Name the pulleys A and B.
- In the diagram, mark the direction of tension on each strand of string.
- What is the purpose of the pulley B?
- If the tension is T, deduce the relation between
- T and E, and
- E and L.
- What is the velocity ratio of the arrangement?
- Assuming that the efficiency of the system is 100%, what is the mechanical advantage?
A block and tackle pulley system has a velocity ratio 5. Draw a labelled diagram of this system. In your diagram, indicate clearly the points of application and the directions of the load and effort.
The diagram below shows a pulley arrangement.
(i) In the diagram, mark the direction of the forces due to tension, acting on the pulley P.
(ii) What is the purpose of the pulley Q?
(iii) If the tension is T newton, deduce the relation between T and E.
(iv) Calculate the velocity ratio of the arrangement.
(v) Assuming that the efficiency of the system is 100%, what is the mechanical advantage?
(vi) Calculate the value of E
Give reason for the following:
The efficiency of a pulley is always less than 100%
In the case of a block and tackle arrangement, the mechanical advantage increases with the number of pulleys. Explain.
Draw a diagram showing a block and tackle system of 4 pulleys.
(i) State, how many strands of tackle support the load?
(ii) Draw arrows to represent tension in each strand.
(iii) Find the mechanical advantage of the system, stating the assumptions made.
(iv) If the load is pulled up by a distance 1m, how much does the effort end move?
Name the type of single pulley that has an ideal mechanical advantage equal to 2. Draw a labelled diagram of the pulley mentioned by you.
