46 Chapter 2 • Statics


6. A liſting tackle has three pulleys on the upper and two pulleys on the lower block.To raise a body with amass of 85 kg, an effort of 240 N is required.Make a diagrammatic sketch of this arrangement and calculate for these conditions; 6.1 the velocity ratio, and 6.2 themechanical advantage of themachine.


7. A rope block has three pulleys on each of the blocks. If an effort of 350 N is required to raise a load of 1 300 N, determine for this load; 7.1 the velocity ratio, and 7.2 themechanical advantage of themachine.


(5; 3,47)


(6; 3,714)


8. A pulley system comprises five pulleys. Calculate the effort required to raise a load with amass of 180 kg if it is known that themechanical advantage at this load, is 4,25.


(415 N)


9. In order to lift a body with amass of 60 kg, an effort of 150 N is required with a rope block comprising six pulleys. Calculate for this load; 9.1 the velocity ratio, and 9.2 themechanical advantage.


(6; 3,92)


10. A pulley system comprising two blocks with two pulleys each, has amechanical advantage of 3,2 when raising a load of 1,3 kN. Determine for these conditions: 10.1 the velocity ratio, and 10.2 the effort required to lift the load.


Exercise 2.4


1. Make a neat, labelled sketch of a wheel and axle liſting machine. 2. With aid of the information obtained from question 1, derive the expression: VR = D


d


3. What is an important aspect to bear in mind when doing calculations involving a machine which has a crank handle?


4. Awheel and axle has diameters of 800mmand 250mmrespectively.When raising a bodywith amass of 50 kg, themechanical advantage was found to be 2,56.Calculate; 4.1 the velocity ratio, and 4.2 the effort required.


5. A simple winch has a handle with an effective length of 250mmwhich is fixed directly to the drum. If the drum has a diameter of 120mmand the winch is used to raise a load of 400 N with amechanical advantage of 3,59, calculate; 5.1 the effort required to liſt this load, and 5.2 the velocity ratio.


(4; 406,25 N)


(3,2; 191,4 N)


(111,4 N; 4,167)


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